Journal of Cleaner Production 301 (2021) 126920

Contents lists available at ScienceDirect

Journal of Cleaner Production

journal homepage: www.elsevier.com/locate/jclepro

Non-food applications of natural dyes extracted from agro-food

residues: A critical review
Kim Phan a, Katleen Raes b, Veronique Van Speybroeck c, Martijn Roosen a,
Karen De Clerck d, Steven De Meester a, *
a
Laboratory for Circular Process Engineering (LCPE), Department of Green Chemistry and Technology, Ghent University, Campus Kortrijk, Graaf Karel de
Goedelaan 5, B-8500, Kortrijk, Belgium
b
Research Unit VEG-i-TEC, Department of Food Technology, Safety and Health, Ghent University, Campus Kortrijk, Graaf Karel de Goedelaan 5, B-8500,
Kortrijk, Belgium
c
Center for Molecular Modeling, Ghent University, Technologiepark 46, B-9052, Zwijnaarde, Belgium
d
Department of Materials, Textiles and Chemical Engineering (MaTCh), Ghent University, Technologiepark 70A, B-9052, Zwijnaarde, Belgium

a r t i c l e i n f o a b s t r a c t

Article history: Fruit and vegetables contain molecules that have particular colors, which can potentially be an envi-
Received 26 August 2020 ronmentally attractive substitute for their synthetic counterparts in (non-)food applications. The most
Received in revised form sustainable source for such natural colorants would be by the valorization of by-products from the fruit
26 March 2021
and vegetable industries, but qualitative and quantitative characteristics of food by-products for this
Accepted 27 March 2021
Available online 2 April 2021
purpose remain scarce. Natural dyes also show mediocre stability and affinity toward textile fibers, which
questions their potential feasibility for application and level of sustainability to overcome these issues.
Handling Editor: Prof. Jiri Jaromir Kleme
s This review describes three dye classes (i.e., anthocyanins, quinones, and carotenoids) along with their
occurrence, mass, and concentration in by-products that are generated from agricultural losses as well as
Keywords: the fruit and vegetable processing industries. To tackle the shortcomings of natural dyes on fibers, several
Natural dye application techniques were collected from the literature. A discussion on techno-economic potential
Anthocyanins and environmental sustainability is included. The latter is done by including a life cycle assessment (LCA)
Quinones to investigate the environmental impact of extracting anthocyanins, quinones, and carotenoids from fruit
Carotenoids
and vegetable processing by-products and their subsequent application to the dyeing process. The
Life cycle assessment (LCA)
mapping of by-products for each natural dye class illustrates the vast availability of agro-food residues
Organic waste valorization
(>0.1 Mt annually in the EU-28) with a natural dye content of up to 56 kg/t DW for anthocyanins, 18 kg/t
DW for quinones, and 593 kg/t DW for carotenoids. Metallic mordants are mostly favored for improving
the fixation of natural dyes but entail potential environmental issues. Greener approaches, such as
biomordants and enzymes, still show room for improvement, chemical modification methods might also
guarantee dye fixation, though questionable in environmental sustainability. The different valorization
scenarios of anthocyanins, quinones, and carotenoids from food waste, analyzed with LCA, showed the
environmental competitiveness of these natural dyes, applied as a crude extract, compared to synthetic
dyes. The valorization routes design shows that agricultural losses and food processing waste streams are
adequate sources of natural dyes, especially to be applied in niche scale applications.
© 2021 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Colorants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Quinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

* Corresponding author.
E-mail address: steven.demeester@ugent.be (S. De Meester).

https://doi.org/10.1016/j.jclepro.2021.126920
0959-6526/© 2021 Elsevier Ltd. All rights reserved.
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

3. Fruit and vegetable by-products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1. By-products generated within the food supply chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Content of natural colorants within fruit and vegetables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Quinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Natural dyes in practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Quinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.1. Benzoquinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.2. Naphthoquinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2.3. Anthraquinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. Carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. The valorization routes from agro-food residues to textile application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Valorization route for anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. Valorization route for quinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3. Valorization route for carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. LCA of naturally-dyed fabrics from agro-food residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. LCA results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2.1. Anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2.2. Naphthoquinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2.3. Anthraquinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.4. Carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.3. Sensitivity analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Declaration of competing interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1. Introduction et al., 2013). Natural dyes should be proven to be more sustainable

compared to their synthetic counterparts. Indeed, natural dyes are
Nature is full of colors. The use of natural dyes dates back to extracted from biomass, which typically has a significant environ-
3500 BC, when several civilizations used plants, vegetables, fruits, mental impact both in the agricultural and extraction phases. The
flowers, and insects to color natural textiles, wood, or clay type and concentration of the dye depend on the biomass source.
(Kadolph, 2008). In 1856, the synthetic dye, mauveine, was created When looking at vegetables and fruit as a primary source of natural
by Perkins, and by the end of 1910, most of the commercially dyes, they typically contain high concentrations of these colorants,
available textiles were synthetically dyed (Shahid et al., 2013). but extracting dyes from primary sources would be expensive and
Synthetic dye and pigments are well-established in industries could induce discussions on food security and the danger of
related to textiles, paints and coatings, paper, food and beverages, deforestation (RESFOOD, 2016).
plastics, printing inks, cosmetics and personal care, and other in- A more sustainable source of biomass might be food waste that
dustries (TBRC, 2019). The textile and paints industry account for occurs over the food industry’s life cycle. Thirty percent of all food,
62.2% of the synthetic dye and pigment market in 2018, the coating or about 1.3 Gt, is wasted worldwide (Gustavsson et al., 2011), the
industries about 50.7% (TBRC, 2019). The textile dyeing and fin- EU-28 generates an estimated 88 Mt of food waste per year
ishing industry is renowned as the largest polluter of clean water, as (Stenmarck et al., 2016). Among all food types, fruits and vegetables
it applies a vast range of chemicals that are toxic to human health, encounter the largest losses and waste generation, which can reach
indirectly or directly (Mirjalili et al., 2011). Global attention has up to 60% (Sagar et al., 2018). Fig. 1 illustrates only the European
shifted toward pollution control and environmental sustainability, amount of by-products that are generated during the different
which has caused a renewed interest in natural dyes. They are stages of fruits and vegetables’ life cycle (Gustavsson et al., 2011).
believed to be safe due to their non-toxic, non-carcinogenic, and The potential waste streams come from agriculture, where waste
biodegradable nature. Some of these dyes possess anti-UV and anti- occurs on the field due to harvest failure because of bad weather
microbial properties, and they neither cause pollution nor toxic conditions, diseases, or during post-harvest handling, which con-
wastewater effluents (Mirjalili et al., 2011; Singh et al., 2005). sists of crop grading and storage, awaiting distribution. The waste
The main application of natural colorants is for coloring food, that is generated from agriculture accounts for 10e20% and from
which accounts for 36% of the global food colorant market (Caro post-harvest handling 4e10%.
et al., 2012; Mortensen, 2006). The textile and other industries, in As the consumption of fruits and vegetables comes with health
their pursuit of more sustainable production, may also have an benefits, many of the raw materials undergo processing to obtain
interest in these natural dyes, although the competitiveness of various consumer-end products. The side streams that emerge from
natural dyes for synthetic dyes is complex in terms of price and the the production process typically contain high added-value com-
quality standards expressed as fastness properties. The dyeing pounds. Juice production, as well as the canning and freezing in-
process with natural dyes should be compatible with the techno- dustries, each generate around 5.5 and 6 million metric tons
logical equipment available in the modern textile industry (Shahid (Waldron, 2009). Wine production industries produce around 5 to

2
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Fig. 1. The food supply chain of fruits and vegetables and the number of waste streams generated, in weight percentages and at each stage of the chain (European values
(Gustavsson et al., 2011).

9 million metric tons of pomace (Schieber et al., 2001). Due to feed, anaerobic digestion, energy production, the landfill, or an
consumers’ demand for fresh, healthy, convenient, and additive- incineration facility for energy recovery (Roels and Vanhee, 2019).
free foods that are safe and nutritious, the European fresh-cut in- Extracting valuable components such as biomolecules (e.g., anti-
dustry has grown exponentially since the 1980s (FAO, 2010). The oxidants, phenolic compounds, and essential oils), polysaccharides,
production of fresh-cut iceberg salad generates at least 35.5% lignin, and proteins could be an interesting gain to increase the
(Plazzotta et al., 2017) of the waste; estimation of the UK alone added value of food waste/by-product management in a biorefinery
leads to 6.7 Mt of food waste (FAO, 2010). approach (Broeze et al., 2019). The European Union’s bio-based
There is also a significant difference in waste generation be- economy and circular economy initiatives with projects such as
tween industrialized and developing countries. Industrialized AWARENET (De las Fuentes, 2002), AGRIMAX (Fritsch et al., 2017),
countries limit fruit and vegetable losses to 2%, developing coun- AGROCYCLE (Patsios et al., 2016), FUSIONS (Stenmarck et al., 2016),
tries attain 20e25% losses (Gustavsson et al., 2011). For the latter, REFRESH (Wunder et al., 2020), etc. aim to prevent food waste in
this can be ascribed to the low level of handling techniques and and also stimulate the valorization of high added-value compounds
poor infrastructure/facilities (FAO, 2017; Sagar et al., 2018). The from unavoidable EU food production by-products. One of these
next stage of the supply chain is distribution at the retail store level, high added-value components is natural dyes, which come forward
where 10e17% of fruit and vegetable waste emerges (Gustavsson as a green alternative to synthetic dyes. When it comes to the
et al., 2011). The last actors in the food-loss chain are households. implementation of natural dyes in non-food applications, such as
This is influenced by households’ socioeconomic and demographic with textiles, research papers have assessed different types of ap-
characteristics (Close et al., 2019). Generally, in industrialized plications and their properties. There is a lack of general overview
countries with medium- and high-income households, large regarding in which waste sources the color molecules are present
amounts of human-edible produce are discarded (average 21 wt% and their concentrations and to what extent different dyeing
fruit and vegetable waste of the purchased products) compared to techniques are effective or demand more in-depth research to
developing countries (average 9 wt% fruit and vegetable waste) improve the natural dyes’ stability and affinity to different fibers.
(Gustavsson et al., 2011). Insufficient planning, improper storage, Natural dyes are not yet implemented in mainstream textile
and food spoilage, over-preparation, and confusion about expira- processes, as several technical challenges, such as cost-effectively
tion dates often lead to these food losses (Gunders and Bloom, improving extraction and application procedures, must be
2017). (Post-)consumer waste is commonly mixed and polluted, resolved before heading to a large (industrial) scale (Shahid et al.,
which generates difficulties for high-end applications (Wain et al., 2013). The cottage industry accounts for the largest share of
2019). The lockdowns in 2020 and 2021 in many countries due to naturally-dyed fabric, which is only 1% of the total textiles pro-
the COVID-19 pandemic have, from farm to fork, interrupted the duced (Muthu, 2014). For this reason, qualitative and quantitative
food supply chain and complicated interactions. Due to the close- characteristics of food by-products are one of the key features for
down of the catering industry, fresh produce remains on farms and designing valorization routes, as they indicate what types of fruit
is lost or wasted (Galanakis, 2020a). For these reasons, waste and vegetable processing waste streams would be most useful for
streams that are derived from agriculture or industry could be industrial-level applications (Garcia-Garcia et al., 2019). When
considered for the extraction of valuable components (Mirabella suitable valorization opportunities are identified and applied to
et al., 2014). This might also be interesting for other reasons, such dyeing techniques, a sustainability performance should be per-
as logistics or the fact that organic molecules tend to degrade due to formed to assess the environmental performance of novel path-
microbial activity when not properly handled, as is to be expected ways against current practices (Garcia-Garcia et al., 2019). The
with putrescible, post-consumer waste (Wain et al., 2019). number of studies on the environmental impact of deriving these
By-products from the food processing industries go to animal natural dyes from food waste and subsequent dyeing are scarce.

3
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Within the aforementioned context, this review aims to provide (Pg), peonidin (Pn), and petunidin (Pt). The distribution of these
more holistic insight into the potential of natural dyes that are anthocyanidins in fruits and vegetables is: Cy 50%, Dp 12%, Mv 7%,
extracted from food waste in non-food applications by: 1) Pg 12%, Pn 12%, and Pt 7% (Table 2) (Andersen, 2006; Castaneda-
providing detailed mapping of waste streams or by-products Ovando et al., 2009). Other types of anthocyanidins, such as 3-
derived from the fruit and vegetable processing industry in deoxyanthocyanidins, rare methylated anthocyanidins, and 6-
Europe, along with the concentration levels of anthocyanins, qui- hydroxyanthocyanidins, account for only 2e3% each of the total
nones, and carotenoids, as these are the main colorants of fruit and anthocyanidin and anthocyanin varieties (Andersen, 2006). The
vegetables; 2) investigating the performance of different applica- anthocyanin content is highly varied and depends on several var-
tion methodologies in terms of fastness properties of natural dyes iables, such as genetic factors, light, temperature, and agronomic
into textile fibers, with regard to overcome the dyes’ specific af- factors (Andersen, 2006).
finity and stability issues and identifying the potential emerging Anthocyanins, through reversible reactions, are known to un-
methods that require more in-depth research; 3) developing dergo structural changes at different pH values (Fig. S1). The most
valorization process flowsheets through the evaluation of various stable species in acidic aqueous solutions (pH 1e3) is the red fla-
conventional and emerging technologies for extraction, purifica- vylium cation (A in Fig. S1). Anthocyanins are known as diacids
tion and encapsulation; this includes the potential pitfalls with two pKa values at 4 and 7. Increasing the pH leads to isom-
encountered with each natural dye; and 4) analyzing the envi- erization via kinetic or thermodynamic pathways. The former in-
ronmental impacts of a naturally-dyed fiber by means of LCAs, duces subsequent deprotonation of the flavylium cation, which
using different scenarios that are based on the most optimal valo- yields purple-bluish neutral (B), monoanionic (B
), and dianionic
rization route and the influence of additional steps such as biomass quinoidal bases (B2
); the latter results in light-yellow chalcone
drying and different purification and encapsulation techniques. In isomers (C-E), as a consequence of the nucleophilic attack of a
the end, this study presents some examples of the most interesting water molecule at the C2-position of the flavylium cation (Ander-
valorization routes for each aforementioned natural dye, from fruit sen, 2006; Castaneda-Ovando et al., 2009).
and vegetable processing waste to the non-food application field. Anthocyanins are especially prone to pH fluctuations and also to
temperature. Other factors that can affect their stability are con-
2. Colorants centration, light, oxygen, solvents, presence of enzymes, proteins,
or copigments (e.g., phenolic compounds and metallic ions). The
The following section describes the characteristics of three latter results in self-stacking configurations by intermolecular
natural colorants’ anthocyanins in terms of their (chemical) prop- copigmentation and intramolecular copigmentation in the form of
erties, quinones, and carotenoids. Anthocyanins depict the most acylated anthocyanins (Giusti and Wrolstad, 2003; Malien-Aubert
colorful group among flavonoids and are responsible for red and et al., 2001). Intermolecular copigmentation exists via a weak as-
purple hues. Quinones, and especially anthraquinones, are, after sociation with alkaloids, amino acids, benzoic acids, coumarin,
azo dyes, the second-largest dye class of textile colorants. Unlike cinnamic acids, or other colorless (phenolic) compounds, such as
azo dyes, synthetic quinones have natural counterparts that are flavonols. The higher the molar ratio copigment/pigment, the more
mainly extracted from plants, nuts, and insects. This class of com- stable the anthocyanins become (Delgado-Vargas et al., 2000;
pounds displays various shades of brown, purple, red, orange, and Malien-Aubert et al., 2001). The acylated anthocyanins are known
yellow (Fouillaud et al., 2018). Carotenoids are found in many to be more stable in different pH, light, and heat conditions (Giusti
photosynthetic and nonphotosynthetic organisms and are and Wrolstad, 2003; Malien-Aubert et al., 2001). This is where an
responsible for their red, orange, and yellow color. aliphatic or aromatic organic acid is acylated on the saccharide unit,
mostly at the sixth position. Due to the free-rotatable bond of the
2.1. Anthocyanins saccharide unit at the C3 or C5 side, the planar aromatic acyl group
can overlap with the planar benzopyrylium ion due to hydrophobic
Anthocyanins are phenolic compounds that belong to one of six forces and p-p interactions. This protects the flavylium cation on
categories within the class of flavonoids (Fig. 2) (McNaught and both sides against nucleophilic attacks.
Wilkinson, 1997). They are mainly present in higher plants,
flowers, fruits, and vegetables and can be more specifically found in 2.2. Quinones
a cell’s vacuoles. Anthocyanins consist of a C6-C3-C6 backbone,
which mostly appears as a flavylium cation in acidic conditions and Quinones are organic compounds (C6H4O2) that have a fully
are glycosylated. In contrast, anthocyanidins are molecules that are conjugated cyclic dione structure (McNaught and Wilkinson, 1997).
without sugar moieties. At least 500 different anthocyanins have The simplest quinone molecules consist of a single benzene ring
been identified and isolated from plants (Andersen, 2006), 90% of and are called benzoquinones (Fig. 3). Due to the small size of the
all anthocyanins are derived from these six common anthocyani- chromophore, it is frequently substituted with alkyl groups.
dins: cyanidin (Cy), delphinidin (Dp), malvidin (Mv), pelargonidin Naphthoquinones, compared to benzoquinones, have an additional
benzene ring. Lawsone and juglone are the most frequently
occurring compounds.
Quinones that have a benzene ring on each side of the dione
structure are classified as anthraquinones; they represent the
largest fraction of naturally occurring quinones. In general, qui-
nones are more present within plants, especially the roots (e.g.,
aloe, madder, senna, and rhubarb), fungi, mushrooms, lichens, and
some insects rather than fruits or vegetables (Duval et al., 2016;
Sakakibara et al., 2003). The most frequently encountered anthra-
quinones are alizarin, catenarin, emodin, physcion, and rhein,
which are mostly glycosylated within the plant (Fig. 4, Table 3)
Fig. 2. The molecular structure of anthocyanin as a flavylium cation with its possible (Bechtold and Mussak, 2009; Duval et al., 2016). These compounds
substitution patterns. require hydrolysis before their dyeing applications. For dyeing
4
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Table 1
The percentage of by-products or waste streams generated at each stage of the fruit and vegetable supply chain and food in general (based on various studies).

Agricultural Storage and Processing and Distribution Consumption or References

Production Handling Packaging Households

Fruit and 20 5 2 10 (f), 2 (p) 19 (f), 15 (p) Gustavsson et al. (2011)

Vegetables 20 3 1 12 28 Gunders and Bloom (2017)
13 N/A 31.5 9 N/A Braekevelt and Vanaken
(2017)
a
Food 23 12 5b 9 52 Lipinski et al. (2013)
20 7.32 ± 5.32 2 4.87 ± 2.49 (f) 19 (f) Porter et al. (2016)
N/A N/A 39 19 42 Monier et al. (2010)
11 N/A 19 17 53 Stenmarck et al. (2016)
10.5 3.5 3.4 2.4 12.6 FAO (2017)
a
: food includes grain products, fruits and vegetables, fish and seafood, meat and milk.
b
by-products are not included; f: fresh; p: processed.

Table 2
An overview of the most naturally occurring anthocyanidins (Andersen, 2006; Delgado-Vargas et al., 2000).

Anthocyanidins R1 R2 R3 R4 R5 R6 R7

Most occurring anthocyanidins Pelargonidin (Pg) H OH H OH OH H OH

Cyanidin (Cy) OH OH H OH OH H OH
Delphinidin (Dp) OH OH OH OH OH H OH
Peonidin (Pn) OCH3 OH H OH OH H OH
Petunidin (Pt) OH OH OCH3 OH OH H OH
Malvidin (Mv) OCH3 OH OCH3 OH OH H OH
Rare methylated anthocyanidins 5-MethylCy OH OH H OH OCH3 H OH
Pulchellidin OH OH OH OH OCH3 H OH
Rosinidin OCH3 OH H OH OH H OCH3
Europinidin OCH3 OH OH OH OCH3 H OH
Capensinidin OCH3 OH OCH3 OH OCH3 H OH
Hirsutidin OCH3 OH OCH3 OH OH H OCH3
6-hydroxylated anthocyanidins 6-HydroxyPg H OH H OH OH OH OH
6- HydroxyCy OH OH H OH OH OH OH
6-HydroxyDp OH OH OH OH OH OH OH
3-deoxyanthocyanidins Apigeninidin (Ap) H OH H H OH H OH
Luteolinidin (Lt) OH OH H H OH H OH
Tricetinidin (Tr) OH OH OH H OH H OH
Carajurone H OH H H OCH3 OH OH
Carajurin H OCH3 H H OCH3 OH OH

purposes, Derksen et al. (2003) succeeded in hydrolyzing ruber- and xanthophylls (Fig. 5). The former’s molecules are comprised
ythric acid into the aglycone alizarin from madder. The aqueous only of carbon and hydrogen atoms, whereas the latter also con-
method that was used could circumvent the simultaneous forma- tains oxygen atoms. The extensive conjugated double-bond system,
tion of unwanted mutagenic lucidin that is derived from the hy- which acts as the light-absorbing chromophore, is responsible for
drolysis of lucidin primeveroside through endogenous enzymes. the red, orange, and yellow color in fruits, vegetables, fungi,
Natural anthraquinones often appear as colorless compounds, but flowers, as well as in birds, insects, crustaceans, and even fish
the color is provided due to the interaction with other molecules (Rodriguez-Amaya, 2001).
present within the matrix, or by transformation processes. The Frequently encountered carotenes are lycopene and z-carotene.
color is also affected by the number, types, and positions of the The former is mostly present in red-fleshed fruits and fruit vege-
substituents. Electron-withdrawing groups (i.e., carbonyl groups) tables, the latter is more omnipresent, but at low levels. Other
do not have an impact on the electronic spectrum, electron- carotenes that are ubiquitous in food are bicyclic b-carotene, a-
donating groups (i.e., amino groups) can vary the dye’s color from carotene, and g-carotene. For the carotenols or hydroxycarotenoids,
yellow, through red and blue, to green. The position of substituents lutein and zeaxanthin are present in leaves or green vegetables, b-
also affects the intramolecular-hydrogen bond and possible steric cryptoxanthin appears in most of the orange-fleshed fruits. Most
hindrance (Duval et al., 2016). Carminic acid, depending on the pH, carotenols in ripe fruit are esterified with fatty acids, which can be
can also change color (Fouillaud et al., 2018). hydrolyzed through saponification. Some carotenoids are uniquely
present in specific species. A few examples are capsanthin and
capsorubin in red pepper, bixin in annatto, and crocetin in saffron
2.3. Carotenoids
(Rodriguez-Amaya, 2001). The main disadvantage of carotenoids is
their susceptibility to isomerization and oxidation. This depends on
Carotenoids belong to the class of tetraterpenes and are linear
numerous factors, such as the presence of metals, enzymes, un-
molecules, which consist of eight isoprenoid units that have an
saturated lipids, pro-oxidants or antioxidants, light exposure,
inversion center. Carotenoids are mainly lycopene derivatives
temperature variations, type of carotenoid, extraction procedure,
(C40H56) that undergo reactions such as hydrogenation, dehydro-
packaging material, and storage conditions. Isomerization of the
genation, cyclization, oxygen insertion, double-bond migration,
more stable trans-isomer to the cis-isomer occurs by heating or
methyl migration, chain shortening or extension, or combinations
exposure to direct sunlight or ultraviolet light and may lead to the
of these processes (Delgado-Vargas et al., 2000; Rodriguez-Amaya,
photodestruction of carotenoids (Rodriguez-Amaya, 2001).
2001). Carotenoids can be divided into two subclasses: carotenes
5
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Fig. 3. (above) The common occurring naphthoquinones; (below) two naturally occurring benzoquinones.

data of Gustavsson et al. (2011) were taken into account to estimate

the amount of fruit and vegetable waste streams that are generated
from agricultural practices and processing side streams in Europe
(28 member states) (Table 1). Various authors also collected
representative ratios of the by-products that are generated for
specific fruits and vegetables (Gon~ i and Hervert-Hernandez, 2011;
Kips and Van Droogenbroeck, 2014) (Table S1). The amount of by-
products is derived from the percentage of waste generated
within the primary production (Monier et al., 2010); this can be
interpreted as a maximal value, as the entire crop production will
Fig. 4. The molecular representation of anthraquinones.
not be used for processing. The following section elucidates the
main by-product sources from which the aforementioned natural
3. Fruit and vegetable by-products colorants can be withdrawn.

3.1. By-products generated within the food supply chain 3.2. Content of natural colorants within fruit and vegetables

In the context of sustainable development, natural dyes are This section is an overview of the content present in each nat-
preferably extracted from waste biomass. In this section, we pro- ural colorant in specific fruits and vegetables. The concentrations
vide an overview of the amount of fruit and vegetable waste with are expressed in dry weight (DW), and fresh weight (FW) units
the expected concentrations of natural dye molecules. To under- were converted (US EPA, 2018) to DW by taking into account the
stand which fruit and vegetable by-products are interesting as a moisture content of the specific fruit and vegetables (Table S2)
source for natural dyes, an overview is given of the pre-consumer (USDA, 2019). This data can be coupled with the number of by-
generated waste for the most common fruit and vegetables, as products generated, which are illustrated in Fig. 6 and with in
well as the concentrations of anthocyanins, quinones, or caroten- Tables S3 and S4. In general, it can be stated that the composition
oids that are expected to be in these sources. Within this study, the and concentrations of natural colorants within fruits and
6
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Table 3
An overview of naturally occurring anthraquinone structures.

Anthraquinones R1 R2 R3 R4 R5 R6 R7 R8

1-methylamino-4-(amino)anthraquinone NHCH3 H H NH2 H H H H

1-methylamino-4-(hydroxyethylamino)anthraquinoe NHCH3 H H NH(CH2)2OH H H H H
Alizarin OH OH H H H H H H
Aloe-emodin OH H CH2OH H H H H OH
Carminic acid H OH COOH CH3 OH Glycosyl OH OH
Chrysophanol OH H CH3 H H H H OH
Danthron OH H H H H H H OH
Dermocybin OH OH OCH3 OH H CH3 H OH
Emodin OH H CH3 H H OH H OH
Laccaic acid A COOH COOH OH H OH OH Ph-2-(OH)-5-((CH2)2NHC(O)CH3) OH
Laccaic acid B COOH COOH OH H OH OH Ph-2-(OH)-5-((CH2)2OH) OH
Laccaic acid C COOH COOH OH H OH OH Ph-2-(OH)-5-(CH2CHNH2COOH) OH
Laccaic acid D CH3 COOH OH H H OH H OH
Laccaic acid E COOH COOH OH H OH OH Ph-2-(OH)-5-((CH2)2NH2) OH
Lucidin OH CH2OH OH H H H H H
Munjistin OH COOH OH H H H H H
Physcion OH H CH3 H H OCH3 H OH
Pseudopurpurin OH COOH OH OH H H H H
Purpurin OH H OH OH H H H H
Quinizarin OH H H OH H H H H
Rhein OH H COOH H H H H OH
Ruberythric acid OH Primeverosyl H H H H H H
Tectoquinone H CH3 H H H H H H
Xanthopurpurin OH H OH H H H H H

Fig. 5. Most frequently occurring carotenoids. Acyclic carotenes: A: lycopene and B: z-carotene; cyclic carotenes: C: a-carotene, D: b-carotene and E: g-carotene; carotenols: F:
lutein, G: zeaxanthin and H: b-cryptoxanthin; unique carotenoids: I: capsanthin, J: capsorubin, K: crocetin and L: bixin (Bechtold and Mussak, 2009; Rodriguez-Amaya, 2001).

vegetables depend on the cultivation condition, ripeness, variety or of cabbage and cauliflower show at least an anthocyanin content
cultivar, genotype, horticultural studies, geographic location, post- higher than 5 kg/t DW. Grapes abundantly consist of anthocyanins,
harvest storage conditions, processing method (machinery and/or especially within the skin. These compounds are only present
techniques), the purpose of the end-product, and the extraction within the blue, purple, or red varieties, white grapes contain little
methods (Arvayo-Enríquez et al., 2013; Freitas et al., 2015). It was to none. Red grape skin has the highest anthocyanin content along
also noticed that the largest content of these natural dyes is present with the highest available amount of by-products. Winery by-
within the outer layers (skin/peel) of the fruit and vegetables. These products such as red grape pomace and red wine lees are appro-
parts usually form the base of the pomace and are ideal sources for priate sources due to their many by-products as well as their high
obtaining these bioactive compounds. Based on the natural anthocyanin content (Braga et al., 2002).
colorant content and the number of by-products available, it can be Among different berry varieties, raspberries are a rich source of
concluded that red grape pomace is the most interesting source for anthocyanins (Horbowicz et al., 2008). As shown in Table S3, black
the extraction of anthocyanins, walnut by-products for naph- raspberries contain up to six times more anthocyanins compared to
thoquinones, rhubarb by-products for anthraquinones, and tomato the red variety, blueberries also contain around 33.6 kg/t DW
pomace for carotenoids. (Bobinaite_ et al., 2015; Horbowicz et al., 2008). The different
anthocyanin distributions within these berries are represented by
3.2.1. Anthocyanins Welch et al. (2008). The beverage industry can extract 75% of the
Fig. 6A reveals that anthocyanins are largely present within red, apple as juice, and the remaining by-products, after crushing and
black, or blue grape varieties, black raspberry, blackcurrant, and pressing, are solid apple pomace and apple pomace sludge (i.e.,
blueberry. For vegetables, onion red skin, rhubarb, and red varieties liquid waste). The pomace has the typical consistency of pulp (54%),
7
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Fig. 6. These plots illustrate the number of by-products that are generated by processing within Europe as a function of the anthocyanin (A), quinone (B), or carotenoid (C) content
within the different fruits and vegetables (Tables S3eS4). The term ‘raw’ indicates the concentration found in the primary raw material.

peel (34%), seeds (7%), the seed core (4%), and stem (2%), with the Welch et al., 2008).
peel containing the largest anthocyanin content (Table S4). Pome- Red leaf varieties generally contain higher flavonoid levels than
granates also possess anthocyanins, although in a six-fold smaller their green counterparts, although anthocyanins were only detec-
amount compared to apples (Zhao et al., 2013). Stone fruits such as ted in red cultivars (Mueller et al., 1999; Reif et al., 2013). Red
cherries, olives, plums, peaches, and nectarines are also rich in cabbage and purple cauliflower contain similar anthocyanin levels
anthocyanins. (Horbowicz et al., 2008; Volden et al., 2009). The flavonoid con-
Among stone fruits, cherries are the largest anthocyanin pro- centration of the by-products (e.g., leaves and stems) for the latter
vider (Horbowicz et al., 2008). Olives mainly accommodate cyani- is much higher in comparison to the edible parts where only trace
dins (4.4 kg/t DW) (Neveu et al., 2010). The two main waste streams levels are present (Llorach et al., 2003). Albishi et al. (2013) inves-
are crude olive cake and oil mill wastewater. The former is the tigated the anthocyanin content between onion varieties and their
remaining residue after the olive pressing process and consists of parts. Quantification of the content reveals a higher anthocyanin
olive pulp (30e50%), crushed stones (30e45%), olive skin (15e30%), level within the skin (63%) compared to the outer fleshy layer and,
water (~25%), and a small quantity of oil (4.5e9%) (Patsios et al., to a smaller extent, the edible part, which is limited to a single layer
2016). The composition of the latter varies along with the compo- of cells of the epidermal tissue (Albishi et al., 2013). The same trend
sition of the vegetation water, the olive oil extraction process, and was visible in the study of Gennaro et al. (2002), where the
the storage time. Red cultivars of plums attain a concentration of up anthocyanin content decreased from the outer parts until the core
to 5.3 kg/t DW within the peel, nectarines and peaches contain of the red onion: red dry skin > red outer fleshy layer > red edible
around 1.5e1.7 kg/t DW within peels (Toma s-Barberan et al., 2001; part. Despite the low anthocyanin content, the large amount of

8
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

available waste stream (2000e4000 kt) makes this worth of pumpkins mainly consists of b-carotene (>80%) (Seo et al., 2005).
mentioning. Only red- or purple-fleshed varieties of potatoes Shi et al. (2013) applied a conventional solvent extraction to obtain
contain anthocyanins. Purple tubers are more likely to have mal- a total carotenoid content of 12.9 kg/t DW from pumpkin.
vidin, petunidin, delphinidin, and peonidin, pelargonidin is more Carrots are also well-known sources of carotenoids, with b-
likely to be found in red potatoes (Table S3). During the recovery of carotene as the predominant carotenoid. The carrot peels (i.e.,
anthocyanins in potato waste, toxic glycoalkaloids occur in high pomace), generated after the production process, are mostly dis-
concentrations within the extracts, which can be circumvented by carded or used as fodder, as only 60e70% of the carrot is extracted
alkaline precipitation (Schieber and Saldan ~ a, 2009). Will and  
into juice (Cosi c et al., 2016; Wadhwa and Bakshi, 2013). The peel
Dietrich (2013) measured the anthocyanin content in rhubarb contains 204.5 g b-carotene equivalent/t DW, although thermal
(Rheum rhabarbarum L.) flesh, mash, skin, and juice. Five anthocy- processing such as hot-air drying could decrease the content by
anins were identified, and it was observed that the highest con- more than half (Chantaro et al., 2008). Perrin et al. (2016) investi-
centration was found in the skin. The mash contained only 379 g/t gated the carotenoid content in carrot leaves and roots and attained
DW, the juice contained 11.3e14.8 mg/L. In terms of by-product a maximum content of 4.6 kg/t DW in the leaves, with lutein as the
availability as well as anthocyanin content, winery by-products main carotenoid (36e49% of total carotenoid content). The carot-
such as red grape pomace and red wine lees are still the most enoid level found in the roots was about 31 times lower. Spinach,
recommended sources for valorization. red sweet pepper, and lettuce can still be considered as large
carotenoid content sources. Citrus fruits contain somewhat lower
3.2.2. Quinones carotenoid levels (Biehler et al., 2010) as Aravantinos-Zafiris et al.
The number of natural quinone sources that are derived from (1992) obtained 392 g b-carotene equivalent/t DW from citrus
fruits and vegetables appears to be limited in comparison with peels that attain a potential 2000 kt of by-products. Potato by-
anthocyanin sources (Fig. 6B). Walnut and its by-products (leaves products also come in high numbers, although potato peels only
and green husks) are a source of phenolic acids, syringaldehyde, contain 3.77 g carotenoids/t DW (Robles-Ramírez et al., 2016).
and juglone that, due to their anti-inflammatory and anti-
mutagenic properties, have been proven to positively enhance 4. Natural dyes in practice
human health and wellbeing (Colaric et al., 2005; Stampar et al.,
2006). Possible natural colorants that can be extracted from wal- The EU, after China, is the world’s second largest exporter of
nuts are naphthoquinone derivatives, such as 1,4-naphthoquinone, textiles and clothing (EURATEX, 2018). Many countries within the
juglone (5-hydroxy-1,4-naphthoquinone), 2-methyl-1,4- EU are top producers within the area of textiles and clothing, car-
naphthoquinone, and plumbagin (5-hydroxy-2-methyl-1,4- pets, cellulosic fibers, and technical textiles, with 99% of the com-
naphthoquinone) (Müller and Leistner, 1978; Shahidi and panies being qualified as SMEs (EURATEX, 2017). The EU published
Ambigaipalan, 2015). Colaric et al. (2005) quantified the juglone article 68(2) to restrict substances classified as carcinogenic,
content in several walnut cultivars and reported 123 g/t DW in mutagenic, or toxic for reproduction (CMR) in textiles for consumer
kernel and 3313 g/t DW in pellicle. Its by-products, such as leaves purposes and emphasize the development of strategies in sus-
and green husks, showed higher juglone concentrations tainability and a circular economy (EURATEX, 2018). Dyeing with
(Cosmulescu et al., 2011; Stampar et al., 2006). natural colorants, whether they originated from primary sources or
Anthraquinones could also be found within the Polygonaceae by-products, has been trending over the last decades and steadily
(rheum) family (Duval et al., 2016). Although many studies cover finds its way into non-food applications besides their use as food
the Chinese herb, rhubarb, Paneitz and Westendorf (1999) detected colorants. This part aims to review the studies performed for the
emodin, physcion, and chrysophanol within the Rheum rhabarba- aforementioned dye molecules on different fibers, including
rum petioles and leaves, with a respective share of 50%, 30%, and different attempts to improve affinity and stability.
20% of the total anthraquinone content. The concentrations found A literature review was performed by screening the databases,
in the petioles reach up to 610 g/t DW and leaves around 40 g/t DW SCOPUS and the Web of Science, for ‘“natural” AND “dye”’ within
(Paneitz and Westendorf, 1999). Kosikowska et al. (2010) obtained a the article title and a timespan of two decades (starting from 2000).
total anthraquinone content of 18.1 kg/t DW from rhubarb roots. The search results were refined on two criteria that consider: 1) the
Other natural anthraquinone sources were reported by Mueller presence of the natural dyes of interest (i.e., anthocyanins, qui-
et al. (1999), who investigated lettuce, beans, and peas for spe- nones, or carotenoids) within the source or raw material used for
cific anthraquinones: emodin (174 g/t DW), chrysophanol (342 g/t dyeing purposes; and 2) the application on textile substrates (i.e.,
DW), and physcion (21 g/t DW) (Mueller et al., 1999). It can be cotton, wool, silk, etc.). This resulted in 273 studies, of which 104
concluded that walnuts are the most interesting source for reported wash and lightfastness tests. Since natural dyes originate
obtaining naphthoquinones and rhubarb for anthraquinones. from natural sources and are not synthetically designed for non-
food applications, they are typically less stable and have less af-
3.2.3. Carotenoids finity. The screened literature reveals several techniques to over-
Carotenoids are known to be present within red, orange, and come the stability and affinity issues of anthocyanins, quinones
yellow fruits and vegetables. Tomato and pumpkin are the two (benzoquinones, naphthoquinones, and anthraquinones), and ca-
largest carotenoid sources (Fig. 6C). The main carotenoids found in rotenoids for different fibers (Table 4, Fig. 7). These approaches can
tomato waste are lycopene, which is the most abundant compound be grouped into four categories: mordants (metallic and bio)
(66e84% of the total content), followed by b-carotene (13e28%) (Shahid ul and Mohammad, 2015), enzymes, substrate modifica-
and lutein (<3e20%) (Strati and Oreopoulou, 2011b). Lycopene is tion, and dye modification.
more prominent within the water-insoluble fraction and the skin
(72e92%). As a result, extracts from the skin contain a five-fold  Metallic mordants rely on the formation of coordination com-
higher concentration of lycopene compared to pulp extracts (Kaur plexes between the substrate, dye, and metal ions. The substrate
et al., 2005; Papaioannou and Karabelas, 2012). Maximum carot- and dye, the ligands, donate their free electron pairs of oxygen
enoid content of 593 kg/t DW tomato waste was obtained through or nitrogen atoms to the free orbitals of the metal ion, which
soxhlet extraction (V
agi et al., 2007). Pumpkin is the second-largest acts as an acceptor. This leads to a strong coordination bond
carotenoid source, as illustrated in Fig. 6C. The carotenoid content with a covalent character. Metal salts also possess potential
9
 K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

toxicity and are not suitable for skin contact-related

applications.
Protein fibersa  Biomordants are possible alternatives. These are organic

20% (1/5)
molecules, which should form stabilizing intermolecular in-
The performance of each dye class, based on different dyeing methodologies, on cotton, protein and synthetic fibers has been requirements for general apparel (wash fastness (WF)  4 and lightfastness (LF)  5).

0% (0/1)

0% (0/1)
0% (0/7)

0% (0/1)
teractions with hydrogen bonds, p-p interactions, inclusion,
WF  4 & LF  5

or Van der Waals interactions.

Carotenoids (n ¼ 46)

e
e
 Enzymes are considered a safe and “green” technology. The
67% (2/3)
22% (2/9)
0% (0/6)

0% (0/2)
Cottone

enzymes can be reused, are biodegradable, and have high

chemo-, regio-, and stereospecificity; this means that fewer
e

e
e

e
e
side products are generated. The disadvantages of enzymes
39% (18/46)
33% (15/46)

are their instability toward extreme pH values, temperatures,

4% (2/46)
9% (4/46)
0% (0/46)
2% (1/46)
9% (4/46)
4% (2/46)
0% (0/46)
# studies

and solvents (Buchholz et al., 2012).

 The last two techniques involve more synthetic steps: sub-
strate and dye modification. The modification of the fiber al-
ters the fiber’s properties, such as improving the diffusivity of
100% (1/1)
100% (1/1)
100% (2/2)
Protein fibersa Syntheticd

33% (3/9)
25% (1/4)

the dye into the fiber by plasma treatment, graft co-

polymerization, gamma-ray irradiation, and ozonization, or
e
e
e
e

creating ionic bonds with the dye by cationization agents or

reactive group addition.
50% (11/22)

100% (3/3)
23% (7/30)

40% (2/5)

67% (2/3)
50% (1/2)
67% (2/3)
WF  4 & LF  5

4.1. Anthocyanins
e

e
Anthraquinones (n ¼ 112)

100% (1/1)
100% (2/2)
100% (2/2)
43% (3/7)

As shown in Fig. 7A, dyeing anthocyanins on textile substrates

0% (0/5)

0% (0/2)
0% (0/1)
Cottonc

without mordant show rather mediocre wash fastness and poor

e
e

lightfastness. Anthocyanins are predominantly combined with

metallic mordants. Typical metal mordants that are tried are:
41% (46/112)
31% (35/112)

1% (1/112)
4% (4/112)
4% (5/112)
5% (6/112)
2% (2/112)
5% (6/112)
6% (7/112)

Al(III)-, Ca(II)-, Co(II)-, Cr(VI)-, Cu(II)-, Fe(II)-, Fe(III)-, Mg(II)-,

# studies

Mn(II)-, Sn(II)-, Sn(IV)-, and Zn(II) salts; the salts of the most
frequent metal used alum (AlK(SO4)2) and ferrous sulfate (FeSO4).
The efficacy of the mordants depends on the applied substrate
Protein fibersa

and source, which is represented by the wash- and lightfastness

33% (4/12)
64% (7/11)

100% (3/3)
50% (1/2)

values. Stannous chloride is less commonly applied, but seems to

be the most effective metal mordant on cotton (Phan et al., 2020;
WF  4 & LF  5

Wang et al., 2016) and silk (Haddar et al., 2018; Shukla and
e
e
e
e
e
Naphthoquinones (n ¼ 64)

Vankar, 2013), increasing the wash and lightfastness by at least

100% (1/1)
40% (4/10)
50% (4/8)

50% (1/2)
0% (0/1)

0% (0/2)

three grades compared to the control (Wang et al., 2014). The

Cottonb

application of multiple mordanting procedures, such as metal/

e

e
e

metal (Patil and Datar, 2016) or metal/bio (Das et al., 2014; Uddin,
41% (26/64)
36% (23/64)

2014), also leads to excellent wash and lightfastness. Velmurugan

6% (4/64)
9% (6/64)
3% (2/64)
0% (0/64)
5% (3/64)
0% (0/64)
0% (0/64)
# studies

et al. (2017) investigated the effect of silver nanoparticles with

purple sweet potato dye. The addition of the nanoparticles
contributed only to the fabrics’ antibacterial activity and did not
Protein fibersa

significantly improve the properties.

The use of biomordants is less common and consists mainly of
10% (1/10)
36% (4/11)

75% (3/4)
0% (0/2)

tannins such as tannic acid or tartrate salts. The addition of gallic

WF  4 & LF  5

acid or tannic acid has been shown to improve the lightfastness

e
e
e
e
e

of anthocyanins, as they carry UV-protective properties (Chairat

20% (1/5)
Anthocyanins (n ¼ 50)

et al., 2008; Grifoni et al., 2011). This can be ascribed to the for-
0% (0/1)
0% (0/7)

0% (0/4)

0% (0/1)
0% (0/2)
Cotton

mation of a p-complex between the dye and biomordant, as

e
e

shown by various molecular dynamics simulations (Phan et al.,

2020; Trouillas et al., 2016). Tannic acid is often combined with
36% (18/50)
36% (18/50)

16% (8/50)
6% (3/50)

0% (0/50)
0% (0/50)
2% (1/50)
4% (2/50)
0% (0/50)
# studies

cotton, cotton/viscose blend, and lyocell

a metallic salt to improve the overall fastness. Two studies

involved enzymatic application: protease enzymes such as
acrylic, nylon, PLA and polyester.

trypsin, which hydrolyzes proteins at the C-terminal to arginine

cotton and cotton/viscose blend.

or lysine, and oxidoreductases such as laccase, which were used

Fiber Modification þ Mordants

for pretreating the textile substrate. The use of protease enzymes

did not result in any improvement compared to the control
Metallic þ Biomordants

Enzymes þ Mordants

cotton and viscose.

(Shukla and Vankar, 2013). The use of laccase is hampered by the

Fiber Modification
Metallic Mordants

optimal conditions wherein enzymes operate. The optimal pH,

Dye Modification

silk and wool.

for instance, for laccase is 5.5, which means that anthocyanins do

Methodology

Biomordants
No mordant

not show their red color as in an acidic environment. Laccase

Enzymes

aims to form a covalent bond through a radical-mediated reac-

Table 4

tion on polyphenolic compounds such as gallnut, but this

a

e
b
c
d

mechanism has not yet been proven to effectively work with

10
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Fig. 7. The fastness properties of anthocyanins (A), naphthoquinones (B), anthraquinones (C), and carotenoids (D) on various fibers (labels: Table S5). The wash fastness is based on
the color change on the grayscale, the light fastness is expressed via blue wool standards and the dotted lines represent the industrial-required fastness grades for general apparel.

anthocyanins (Bai et al., 2016). and stannous chloride on silk and cotton (Patil and Datar, 2016).
Coating films or thin-film sensors can be developed by mixing The use of, relatively effective, metallic mordants hardly fits in a
poly (acrylamide-co-acrylic acid) (Abidin, 2019) with anthocyanin ‘green story,’ even though 36% of the studies applied metal salts,
extract, or by adding an epichlorohydrin coupling agent for a only 16% worked with biomordants. The overall fastness for bio-
polyacrylonitrile application (Fares and Masadeh, 2018). Due to the mordants on protein fibers is two grades higher compared to cot-
halochromic properties of anthocyanins, nanofibrous wound ton. As such, the use of tannic acid on silk and wool (Haddar et al.,
dressings can also be created through a mixture solution of sodium 2018) or itaconic acid on silk (Das et al., 2014) seem more justified
alginate and polyvinyl alcohol (PVA) (Pakolpakcil et al., 2018). in terms of environmental impact and also meets the industrial
Printing purposes with anthocyanins are done by adding a thick- requirements for general apparel (i.e., WF  4 and LF  5 (Table 4))
ening agent such as gum tragacanth and guar gum, which results in (Richards, 2015).
the best printing silk, although with the addition of multiple
metallic salts (Yadav et al., 2010). Cyanuric chloride has also been 4.2. Quinones
modified with 4-amino-benzenesulfonic acid to create more
anionic sites that form ionic bonds with the flavylium cation, which The literature on naturally-derived quinones applied to textile
increases the overall fastness by three grades (Wang et al., 2017). In fibers, compared to anthocyanins and carotenoids, is more abun-
contrast to this, Gedik et al. (2013) and Nakpathom et al. (2018) dant. The applications of this dye class are categorized according to
used cationizing agents (e.g., Laucol RW (a polyamine derivative) their structure: benzoquinones, naphthoquinones, and anthraqui-
and 3-chloro-2-hydroxypropyltrimenthylammonium chloride nones. Anthraquinones, are, after azo dyes, the second-largest dye
(CHTAC)) in combination with a metal salt, which also seemed to class of textile colorants. The adequate properties of anthraqui-
improve the overall properties. Other protein-based applications nones have led to numerous profound studies.
for anthocyanins are hair dyes (Pipattanamomgkol et al., 2018; Rose
et al., 2018). Upon adsorption, hair provides a neutralization envi- 4.2.1. Benzoquinones
ronment, which shifts the equilibrium toward the anionic quinoidal The best-known benzoquinone-based dye is carthamin, which
base, resulting in a blue color (Rose et al., 2018). can be found in safflower (Pavli
si
c et al., 2011) (Fig. 3). By applying
Despite the hydrophilic property of anthocyanins, 54% of the metallic mordants combined with carthamin, excellent wash fast-
studies were applied to protein fibers (silk and wool) and 40% were ness properties (grades 4/5-5) were obtained for cotton (Jia and
applied to cotton. Forty-one percent of the studies with anthocy- Jiang, 2014; Zarkogianni et al., 2011), on silk it also resulted in a
anins applied to cotton and protein fibers without mordant show a lightfastness of grade 4/5 (Zarkogianni et al., 2011) (Fig. S10). The
wash fastness of 4 and lightfastness of 3 or higher (Fig. S9). Protein photofading property of carthamin was extended by the addition of
fibers show one to two higher grades compared to cotton. The UV absorbers, such as benzophenone-type containing a benzo-
highest properties were obtained with ammonium ferrous sulfate triazolyl group (Oda, 2012), nickel sulphonate derivatives of phenyl
11
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

esters (Oda, 2001), or naphthalene sulphonates derivatives (Oda, also applied on polyamide fabrics (Atav and Namirti, 2016); the
2001). Another type of benzoquinone, claussequinone, which can color strength increased without jeopardizing the wash fastness.
be found in African coralwood (Pterocarpus soyauxii), was dyed The literature shows that naphthoquinones are mostly applied on
with cotton, wood viscose, and bamboo viscose. It was shown that protein fibers (44%) and cellulose fibers (38%), with no large dif-
cotton is more appropriate for dyeing carthamin than the viscose ferences between the overall fastness of both types of fibers
types (Saha Tchinda et al., 2014). (Table 4). Only a single combination of copper sulfate and juglone
leads to the best possible fastness properties on cotton (Sharma and
4.2.2. Naphthoquinones Grover, 2011), though a more environmentally friendly procedure
The natural variety of naphthoquinones is quite limited in of applying naphthoquinones on wool is with tamarind seed coat, a
comparison to anthraquinones (Fig. 3). Nevertheless, the literature by-product generated from the tamarind pulp industry (Kumar and
reveals that three compounds are frequently reoccurring: juglone Bhattacharya, 2008) that is rich in polyphenolic compounds and
from various walnut materials, such as bark (Khan et al., 2016); condensed tannins (Prabhu and Teli, 2014).
shells (Coman et al., 2016; Mirjalili and Karimi, 2013); leaves (Onal
et al., 2004; Onal and Demir, 2009) or mill waste (Doty et al., 2016); 4.2.3. Anthraquinones
lawsone from henna leaves (Rahman Bhuiyan et al., 2018); and Anthraquinone dyes form the second-largest group of synthetic
alkannin from alkanet roots (Bahtiyari et al., 2017; Rekaby et al., dyes. For ecological purposes, naturally available sources from fruit
2009). Other less known quinones are lapachol (Sharma et al., or vegetables should be mapped to extract these dyes instead of
2013), shikonin, and acetylshikonin (Wang and Chen, 2009). using the industrial fossil-based approach. The most common
Al(III)-, Cu(II)-, Fe(II)-, and Cr(VI)- salts are the most commonly natural sources are madder roots (Sadeghi-Kiakhani, 2015; Yusuf
applied metal salts for naphthoquinones. Fig. 7B shows that et al., 2017), manjistha roots (Samanta et al., 2010; Vedaraman
applying naphthoquinones without mordant only acquires the in- et al., 2017), Rheum emodi (Sharma et al., 2011; Srivastava et al.,
dustrial fastness properties. Forty-one percent of the studies were 2011), Kerria lacca (Kamel et al., 2005), and cochineal
performed without mordant; within this category, 58% displayed (Ammayappan and Shakyawar, 2016; Sahinbaskan et al., 2018) in-
excellent wash fastness (4-4/5), 39% showed at least a lightfastness sects. The molecular substitution patterns of these compounds can
of grade 5 or higher (Fig. S11). The influence of adding metal salts, be found in Table 3. As illustrated in Fig. 7C, anthraquinones are
which account for 36% of the studies, is more of aesthetic value but mostly applied on wool and initially possess good wash and light-
is more negligible in terms of fastness properties. Fig. 7B also shows fastness properties and, in some cases, even very good to excellent
the small number of experiments conducted with biomordants: lightfastness. The adsorption behavior of cochineal dye on poly-
Indian gooseberry (Emblica officinalis G.) (Prabhu et al., 2011), amide revealed that dyeing at pH 3e4 is thermodynamically more
myrobalan (Terminalia chebula) (Chattopadhyay et al., 2018) or preferable compared to pH 6. This can be ascribed to the significant
tamarind seed coat (Sheikh et al., 2016). The combination of role of electrostatic interactions between the anthraquinone mol-
metallic mordant and biomordants as thickening agents yielded ecules and the polyamide or wool fiber due to the availability of
better wash fastness values for dyeing with ratanjot bark. Guar gum protonated amino groups, whereas only non-electrostatic hydro-
acts as the best natural thickener in terms of color yield and fast- phobic interactions are present at pH 6 (Mirnezhad et al., 2017;
ness property (Chattopadhyay and Pan, 2018). Sadeghi-Kiakhani et al., 2018).
Pretreating cotton with an enzymatic mixture (glucoamylase, Metal salts are also applied in large abundance, with alum
pectinase, and glucose oxidase) in combination with ultrasound (AlK(SO4)2), copper sulfate (CuSO4), and ferrous sulfate (FeSO4) as
resulted in a darker shade than when dyed with alkanet root or the most effective. This resulted in a half-grade increase until one
nutshell (Benli and Bahtiyari, 2015). The enzymatically pretreated grade in terms of wash fastness. This can be ascribed to the effective
cotton attained a dye uptake of 56% for diasterase, 49% for lipase, formation of metal-dye-fiber coordination complexes. Tin(II)chlo-
and 53% for protease/amylase, all fastness properties scored ride showed a marginal increase in color strength, but the light and
excellent (Vankar and Shanker, 2008). The dyeing of enzymatically wash fastness properties did not improve compared to the
pretreated polyamide, by trypsin or pepsin, with walnut bark did unmordanted wool fabric (Yusuf et al., 2015). Similar to anthocya-
not indicate any improvement compared to an untreated dyed nins, silver nanoparticles contribute to the antibacterial activity of a
sample (Bahtiyari and Benli, 2016). Khattak et al. (2015) applied fabric, but the formation of an insoluble anthraquinone complex
different synthetic additives to modify the cotton’s surface prop- improves wash fastness by one grade (Barani et al., 2017).
erties (cationizing agents, crosslinkers, and UV absorbers). The Studies about biomordants do exist, but they do not contain
quaternary ammonium compound yielded better color strength, complete fastness data. The types of biomordants are limited to
UV absorbers (UV-SUN and Rayosan C) and finishing agents tannic acid, chitosan (Liu et al., 2013; Yadav et al., 2019) or tannin
(Dicrylan) proved to be more effective during the post-mordanting containing matter such as pomegranate rind (Jahangiri et al., 2018;
procedure. Lin et al. (2019) added a glucose-based surfactant, ob- Teli and Ambre, 2018), myrobalan (Samanta et al., 2009a, 2010),
tained by reacting glucose, n-tetradecanol, and glycidyl ether, on sohkhu leaves (Banerjee et al., 2018), Acacia catechu (Sen et al.,
wool, which showed the highest color strength for the henna-based 2019; Yusuf et al., 2013), Indian gooseberry (Emblica officinalis G.)
dye (lawsone), due to adduct formation within the wool fiber. (Prabhu et al., 2011), eucalyptus (Jahangiri et al., 2018), chestnut-
Hebeish et al. (2006) obtained a 46% increase in color strength with leaved oak (Quercus castaneifolia) (Jahangiri et al., 2018), Aleppo
lawsone by modifying cotton with monochlorotriazinyl-b-cyclo- oak (Quercus infectoria) gall extract (Grifoni et al., 2011), green tea
dextrin. The application of lawsone and octa(aminophenyl) poly- leaves (Yavas et al., 2017), Sicilian sumac (Rhus coriaria) (Jahangiri
hedral oligomeric silsesquioxane or N-phenyl aminopropyl et al., 2018), black myrobalan (Terminalia chebula) (Khan et al.,
polyhedral oligomeric silsesquioxane nanoparticles on polylactic 2017; Srivastava et al., 2011), and gum Arabic tree (Acacia nilotica)
acid showed similar fastness properties compared to the control (Rather et al., 2019). Most biomordants are combined with a metal
sample (Baykus et al., 2017). Other synthetic fibers that were salt to improve the lightfastness properties of the dyed fabric,
modified are polypropylene, which was modified with a hyper- Acacia catechu seems to be effectively working as a single bio-
branched poly(ester amide) (HPEA) for dyeing juglone before the mordant for anthraquinones (Yusuf et al., 2016).
spinning process. This showed a slight improvement in wash fast- For the enzymatic approach, the pretreatment of wool by Savi-
ness (Davulcu, 2015). Ozonization, which is a mild treatment, was nase 16L, a protease (hydrolyzing peptide bonds), resulted in
12
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

increased absorption of madder and cochineal into the fabric additional presence of a 1-hydroxy group improves the photooxi-
(Nazari et al., 2014). The combination of potassium permanganate dative property of the compound by the formation of an
and an after-treatment with Savinase 16L and microbial trans- intramolecular-hydrogen bond with the 9-ketone group. Wu et al.
glutaminase showed inferior fastness values compared to ferrous (2019) also used alizarin as a platform molecule to graft alkyl and
sulfate (Cui et al., 2008). Vankar et al. (2007) studied the effect of hydroxylakyl groups with chain lengths from two to eight carbon
different enzymes with and without ultrasonication. It was atoms on the C2 position. It was observed that supercritical CO2
observed that lipase is the most suitable enzyme for dyeing cotton dyeing with the hydroxybutyl derivative showed the best perfor-
and silk, with emodin over diasterase and a protease-amylase mance. Fastness tests have indicated a higher preference for
mixture. Diasterase seems to be the more appropriate enzyme to hydroxyalkyl derivatives over alkyl derivatives of alizarin. The
apply for dyeing cotton with tectoquinone. The dye uptake studies showed a high application rate (61%) of natural anthra-
increased up to 28% with the use of ultrasonication compared to quinones on protein fibers. Eighteen percent of studies applied
conventional dyeing (Vankar and Shanker, 2008). The cavitation cellulose fibers and 15% used synthetic fibers (Fig. S12). Forty-one
effect of ultrasonic dyeing also resulted in a 47% increase in dye percent of the case studies did not use a mordant, 31% used
uptake, compared to conventional heating, for lac dye with metal salts, and barely 6% used biomordants. Naturally-derived
unmordanted wool (Kamel et al., 2005). A similar trend is observed anthraquinones applied on protein fibers, polyester, and PLA are
for microwave-assisted dyeing with cochineal dye on wool fabric able to meet the industrial fastness requirements for general
(Elshemy, 2011). apparel without the addition of any type of mordant (Table 4). The
Several cationizing agents were also applied to improve the addition of aluminum sulfate along with citric acid or potassium
affinity to anthraquinones: Solfix E, a polyaminochlorohydrin bitartrate can result in maximal fastness properties (Ammayappan
quaternary ammonium polymer with epoxide functionality for and Shakyawar, 2016). The use of black myrobalan (Terminalia
reaction on the cotton fiber (Kamel et al., 2009), 3-chloro-2- chebula) as a biomordant also improves the fastness properties by
hydroxypropyltrimethyl ammonium chloride (Kamel et al., 2011; two grades (Srivastava et al., 2011).
Zhou et al., 2011), polypropylene imine dendrimer-modified chi-
tosan (PPI) (Mehrparvar et al., 2016), chitosan-poly(amidoamine) 4.3. Carotenoids
dendrimer hybrid (Sadeghi-Kiakhani et al., 2019), chitosan-
cyanuric chloride (Safapour et al., 2019), polylactic acid with Even though carotenoids are largely present in tomatoes and
octa(aminophenyl) polyhedral oligomeric silsesquioxane or N- carrots, the literature shows that other sources are used to extract
phenyl aminopropyl polyhedral oligomeric silsesquioxane nano- carotenoids, such as annatto seeds (bixin and norbixin)
particles (Baykus et al., 2017), poly(ethyleneimine) (PEI) (cationic (Nakpathom et al., 2019; Savvidis et al., 2013), marigold (Tagetes
synthetic polymer) (Janhom et al., 2006), Alpsfix (polyamino erecta L.) flowers (lutein) (Jothi, 2008; Rashdi et al., 2019), and
compound) (Gulrajani et al., 2001, 2003), cetrimide, cetrimonium Gardenia (crocin and crocetin) (Park and Yoon, 2009; Zhou et al.,
bromide (CTAB), Sandofix-HCF (Samanta et al., 2009b), and even 2015). Fig. 7D shows that carotenoids possess good wash but
ovalbumin (Giacomini et al., 2020), a protein. poor lightfastness properties. It is no secret that exposure to light is
To overcome the hydrophobicity of wool, bentonite, an harmful to carotenoids, as it induces trans-cis photoisomerization
aluminum phyllosilicate, which is negatively charged, and and photodestruction (Rodriguez-Amaya, 2001). Once again, metal
bentonite modified with didecyldimethylammonium bromide salts based on Al(III), Cu(II), and Fe(II) are frequently used for textile
(DDAB), polyethyleneglycol (PEG400), and lecithin were applied to dyeing with carotenoids, which increases wash fastness by half a
create a more hydrophilic surface. The modified bentonite yielded grade and lightfastness by at least two grades. The effectiveness of
at least a 22% increase in color strength (Barani, 2018). Plasma these metal salts can be ascribed to the aforementioned caroten-
treatment involves surface modification through erosion, etching, oids. A similar trend in fastness properties is observed for rare earth
creation of microcraters, rounding of scale edges, or ablation (Lewis metal salts such as lanthanum chloride and neodymium chloride,
and Rippon, 2013). This resulted in better fastness properties for which are also able to form stable coordinative bonds between
dyeing cotton with madder roots (Haji, 2019). The dyeability of Gardenia dye and fiber (Zheng et al., 2011). The carotenoids found
cotton fibers with cochineal can also be improved by oxidizing the within the literature are xanthophylls (i.e., carotenoids containing
fabric with hydrogen peroxide followed by a chitosan biomordant oxygen atoms). This allows the metal ions to form effective metal-
treatment. The cellulose hydroxyl groups are oxidized to aldehyde dye-fiber coordination complexes. Several biomordants were also
functional groups, which react with the amino group of chitosan to investigated: tartaric acid (Cui et al., 2008; Swamy et al., 2015),
form amide bonds. This treatment increased rubbing and washing myrobalan (Chattopadhyay et al., 2018), pomegranate (Teli and
fastness by at least one grade (Ke et al., 2019). Ambre, 2018), and Indian gooseberry (Emblica officinalis G.) (Teli
Dye modification via the sol-gel process helps to immobilize and and Ambre, 2018).
stabilize the biomolecule or dye. The incorporation of (in)organic With enzymatic dyeing, the combination of pretreatment by
compounds occurs through hydrolysis and subsequent poly- using potassium permangate and Savinase 16 L and an after-
condensation with other organic or inorganic molecular precursors treatment of microbial transglutaminase did not render improved
to form a sol, a dispersion of colloidal particles or polymers within a fastness properties compared to using aluminum sulfate or ferrous
solvent. dos Santos et al. (2018) and Velho et al. (2017) encapsulated sulfate (Cui et al., 2008). Fiber modification involved the addition of
carminic acid through this process with tetraethylorthosilicate cationic dye fixing agents in the role of quaternary ammonium
(TEOS), aluminum chloride, and hydrochloric acid, which showed a compounds: 3-chloro-2-hydroxypropyl trimethyl ammonium
better wash fastness on bamboo viscose/PET/elastane, cotton, chloride (Yoo et al., 2011; Zhou et al., 2011), CTAB, cetrimide, or
polyester, and wool, other substrates such as polyacryl, polyamide, Sandofix-HCF in combination with aluminum sulfate and myr-
or PVC did not show any improvement. A more synthetic pathway obolan (Samanta et al., 2009b). Other chemicals were used to break
is shown by Drivas et al. (2011) who proved that alizarin and pur- bonds, more specifically disulphide bonds within wool by tris(2-
purin can be used as platform chemicals for improving the dyeing carboxyethyl)phosphine, which showed an increase in adsorption
properties of polylactic acid by mono-alkylation. Better results (Shen et al., 2014). Improving lightfastness properties were found
were obtained when an ethoxy, benzyloxy, or methyl-4-butanoate for annatto dyed silk in combination with a polydopamine (PDA)
group was added to the starting anthraquinone compound. The coating (Kesornsit et al., 2019), benzotriazole on jute (Samanta
13
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

et al., 2009b), and Rayosan PES resulted in an improvement for PET formation or encapsulation (Galanakis, 2015). The final purified
(Nakpathom et al., 2019). The gamma-ray pretreatment (30 kGy) of product is then applied to the textile fiber. Conventional and
the cotton fabric increases the negative potential of the cellulosic emerging technologies are discussed below, as the former have
fabric. The additional generation of carboxyl groups and a slightly technological and scale-up boundaries that restrict their efficiency
acidic pH (5.0) improved the wash and lightfastness properties of for commercial implementation. These barriers are increased en-
lutein by at least one grade (Adeel et al., 2017). ergy consumption for membrane processes, the higher operational
dos Santos et al. (2018) and Velho et al. (2017) encapsulated cost for chromatography systems, or reduced control over thermal
annatto seed compounds through the sol-gel process by using processes (Galanakis, 2013). Non-conventional technologies can
tetraethylorthosilicate (TEOS), aluminum chloride, and hydrochlo- shorten the extraction time, operate at a lower temperature and
ric acid, which showed a better wash fastness on bamboo viscose/ with reduced solvent consumption, and obtain a higher extraction
PET/elastane, cotton, and wool but worked less efficiently within a yield with lower energy consumption (Deng et al., 2015).
PVC matrix. In the end, 39% of the studies did not use any mordant,
33% used metal salts and 9% biomordants (Fig. S13). Only 44% of the 5.1. Valorization route for anthocyanins
purely applied carotenoids attained a wash fastness of 4, and 89% of
those unmordanted carotenoid fibers scored poorly on lightfast- The most interesting waste source for obtaining anthocyanins is
ness (below a grade of 3). The lightfastness property clearly red grape pomace, which is derived from the wine or juice
improved when applying a metal salt, as 73% of all metal mor- manufacturing industries, resulting in an estimated 15 Mt of by-
danted carotenoid fiber showed at least a grade 4 or higher. Forty- products in Europe alone. The pomace consists mainly of grape
four percent of the studies applied carotenoids on cellulose fibers, skins, which have an anthocyanin content of 56.4 kg/t DW (Ju and
33% applied carotenoids on protein fibers, though only a grade Howard, 2003). Other interesting sources are black olive pomace
difference was observed in the fastness properties for the no- and red onion skins, both of which have a minimum of two times
mordant method. The best fastness properties (WF ¼ 4/5 and lower anthocyanin content and by-product quantity. Anthocyanin-
LF ¼ 6-6/7) were obtained with a cotton/viscose mixture or just containing by-products are more prominent in southern parts of
cotton in the presence of a pomegranate rind as biomordant, which Europe, such as France, Spain, or Italy, where olives are cultivated
is a by-product generated by the pomegranate juice manufacturing and 74% of Europe’s vineyards are located (representing 32% of the
industry and is rich in polyphenolic compounds (Shahamirian et al., global vineyard surface area) (Eurostat, 2015). These three
2019). anthocyanin-containing by-products mainly emerge as side
The creation of new process inputs (e.g., natural dyes) to phase streams at manufacturing plants.
out hazardous substances is one of the four pillars for a new textile As these by-products contain at least 50% moisture (Table 5),
economy. Table 4 displays an overview of to what extent different dehydration of these residues is important to prevent chemical
application techniques for each natural dye achieved the industrial deterioration (oxidation) and microbial spoilage by yeasts and
fastness requirements for general apparel and which methods still molds (Galanakis, 2020b; Pedroza et al., 2017). The residual mois-
require more in-depth research and development. Metallic mor- ture content of a by-product depends on the fruit or vegetable
dants are mostly favored to improve the affinity and stability of variety, the processing conditions applied, and any depectinization
natural dyes but come along with potential environmental issues. of the mash (Galanakis, 2020b). Microwave and dielectric drying
The greener approaches, such as biomordants and enzymes, still systems could be recommended as they would be able to safeguard
indicate room for improvement. The former show that tannins the drying process of thermolabile materials without degrading the
positively contribute to wash and lightfastness properties, the latter product (Mujumdar, 2014). Dielectric heating typically attains an
induces coupling reactions; this creates covalent bonds through energy transfer of 60% from the generator to the product. The major
radicalization or increases the diffusivity of the fabric by breaking part of the emitted energy must be absorbed evenly by the matrix,
linkages, although possibly jeopardizing the fabric’s strength. The which is more difficult in by-products due to their varying matrix
other two modification methods for fiber and dye are more based composition (Galanakis, 2013).
on a synthetic nature that involves the addition of polymers or The process conditions for extracting anthocyanins should take
adding potentially toxic reactive groups (cyanuric chloride or into account that these compounds are susceptible to degradation
vinylsulfone) to the dye. This synthesis approach guarantees dye by light, pH, and temperature, resulting in pigment loss (Welch
fixation but might require more chemicals, which may not be et al., 2008). The degradation rate of anthocyanins increases pro-
preferable from a sustainability point of view. Encapsulating pro- portionally from 65  C onward, so a short time/high-temperature
cedures via the sol-gel process or with biopolymers are promising process is recommended for the best pigment retention
techniques, as they potentially protect the dye from undesirable (Markakis, 1982). As anthocyanins are water-soluble compounds,
environmental conditions. extraction can be simply performed with acidified water to avoid
potentially hazardous solvent residues, which may limit potential
5. The valorization routes from agro-food residues to textile commercial applications (Denev et al., 2010; Farooque et al., 2018).
application Acidification of the extract comes with several gains: maintaining a
low pH (<3.0) to secure stability and structural consistency, dis-
This section summarizes the main conclusions from sections 3 rupting cell walls to increase the accessibility of polyphenolic
and 4. In section 3, the number of by-products generated from fruit compounds and solvent transport, and inhibiting enzymes that
and vegetable processing industries and the adjoining anthocyanin, catalyze polyphenol decomposition (Farooque et al., 2018). Organic
quinone, and carotenoid content was evaluated. Section 4 provided solvents, mostly methanol or ethanol, are frequently added to
insight into the best possible application methods, assessed in water in various ratios to maximize the extraction yield (Ignat et al.,
terms of the wash and lightfastness properties for each of these 2011). Lapornik et al. (2005) revealed that the concentration was at
natural dyes on the different fiber types. The following sections least ten-fold higher with a 70% methanol or 70% ethanol mixture
evaluate the most interesting process chains for each of the natural compared to pure water. This can be ascribed to the less polar
dyes based on “5-stage universal recovery processing”, which in- character of methanol and ethanol compared to water, improving
cludes macroscopic pretreatment, macro-and micro-molecule the efficiency of extraction in cell walls and seeds (Lapornik et al.,
separation, extraction, isolation-purification, and product 2005). The use of methanol or ethanol also involves additional
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K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Table 5
The potential European production scale of naturally-dyed fabric that is derived from the most promising by-products containing anthocyanins, quinones, or carotenoids.

Amount of by-products (Mt) in Moisture content Concentration Amount of dye Amount of by- Estimated
Europe (%) (kt) products European
required for 1 m2 production of
naturally-dyed naturally-dyed
fabric (kg) fabric (km2)

Anthocyanins cotton wool cotton wool

Wet red grape pomace 14.6 81.7 (w) 20.5a (Ju and Howard, 2003) 299 1.16c,e 0.76c,e 12603e 19157e
Dried red grape pomace 3.3 4.5 (d) 106.8b 356 0.31d,e 0.21d,e 10663e 16207e
Wet red onion skins 3.1 58.2 (w) 9.2a (Gennaro et al., 2002) 28.4 2.59c,e 1.70c,e 1194e 1814e
Dried red onion skins 1.4 3.8 (d) 20.1b (Gennaro et al., 2002) 28.3 1.40d,e 0.92d,e 1010e 1535e
Wet black olive pomace 7.4 68.0 (w) 0.83a (Neveu et al., 2010) 6.1 28.62c,e 18.83c,e 258e 393e
Dried black olive 3.0 8.0 (d) 2.4b 7.1 13.53d,e 8.90d,e 218e 332e
pomace
Quinones
Wet walnut green husks 0.086 87.7 (w) 23.0a (Jakopic and Veberic, 2.0 1.03c,e 0.68c,e 84e 127e
2009)
Dried walnut green 0.018 8.0 (d) 172.0b 3.0 0.23d,e 0.15d,e 75e 114e
husks
Wet rhubarb roots 0.12 75.0 (w) 4.5a (Kosikowska et al., 2010) 0.56 0.84c,f 0.55c,f 148f 225f
Dried rhubarb roots 0.041 8.0 (d) 16.7b 0.68 0.31d,f 0.20d,f 133f 203f
Wet bean waste 0.42 90.32 (w) 36.0a (Mueller et al., 1999) 0.015 115.15c,f 75.76c,f 3.69f 5.60f
Dried bean waste 0.10 14.0 (d) 0.34b (Mueller et al., 1999) 0.034 30.32d,f 19.95d,f 3.32f 5.04f
Carotenoids
Wet tomato pomace 7.0 84.0 (w) 103.8a 723 0.23c,e 0.15c,e 30453e 46288e
Dried tomato pomace 1.9 10.7 (d) 579.2b (Vagi et al., 2007) 1078 0.067d,e 0.044d,e 27862e 42350e
Wet pumpkin pomace 1.0 91.6 (w) 1.1a 1.1 21.16c,e 13.92c,e 46e 70e
b
Dried pumpkin pomace 0.13 4.6 (d) 12.9 (Durante et al., 2014) 1.6 3.01d,e 1.98d,e 42e 64e
Wet carrot leaves 2.2 80.89 (w) 0.95a 2.1 24.96c,e 16.42c,e 89e 135e
Dried carrot leaves 0.60 8.0 (d) 4.6b (Perrin et al., 2016) 2.7 7.40d,e 4.87d,e 81e 123e

(w): wet, (d): dry.

a
kg/t FW.
b
kg/t DW.
c
mass wet by-product from scenario 1.
d
mass dried by-product from scenario 5.
e
dyed at 12.5% owf.
f
dyed at 2% ow.

evaporation or clean-up steps to remove the organic solvent, as fastness for biomordants on protein fibers is two grades higher
dyeing frequently occurs in an aqueous environment. Corrales et al. compared to cotton. The highest fastness properties were obtained
(2008) evaluated the anthocyanin extraction yield with three novel with ammonium ferrous sulfate and stannous chloride on silk and
extraction techniques: ultrasonication, high hydrostatic pressure cotton (Patil and Datar, 2016) but the use of metal is generally not
(HHP), and pulsed electric fields (PEF). It was shown that PEF is the preferred. The use of tannic acid on silk and wool (Haddar et al.,
preferred technique, as the total anthocyanin content increased by 2018) or itaconic acid on silk (Das et al., 2014) also meets the in-
58% compared to the control sample. The high voltage electric dustrial requirements for general apparel (Richards, 2015). Tannins
pulses induce the formation of pores in the membrane (electro- also carry UV-protection properties, which are certainly of added
poration), which increases the cell permeability, allowing mole- value for these light-prone anthocyanins (Grifoni et al., 2011). In
cules to be released into the extraction medium (Baiano, 2014). The general, applications with anthocyanins in the form of the red
highest extraction yield from red grape skin is obtained through stable flavylium cation can be deployed in an aqueous environ-
pressurized liquid extraction (PLE). This technique allows the total ment, provided that alkaline substances should be avoided and are
anthocyanin content to reach up to 41.3 kg/t DW with acidified relevant within short life cycle products (e.g., biodegradable
water at 50  C and increases by 22% with an acidified 60% ethanol products, pH indicator, paper-based coffee cups, etc.) (Phan et al.,
mixture and 37% with an acidified 60% methanol mixture (Ju and 2020). With the amount of red grape pomace generated in
Howard, 2003). Europe, the woven fabrics produced there could be dyed approxi-
The crude extract can be used in two ways: apply it on the mately five times (Table 5). The amount of by-products required for
substrate of interest or include an additional purification step dyeing 1 m2 of naturally-dyed fabric was considered (vide infra).
usually performed with solid-phase extraction (SPE) (Denev et al., Anthocyanin dyes produced from black olive pomace would be able
2010). This preparative method can clean up the matrix (i.e., to dye 10% of the woven fabrics produced in Europe, red onion skins
sugars, acids, proteins, pectin, etc.). The anthocyanins are concen- account for 48%.
trated through elution with an organic solvent (e.g., ethanol)
(Denev et al., 2010). This requires an additional drying step to 5.2. Valorization route for quinones
obtain a dye powder or liquid concentrate. The following three
drying methods are commonly used: spray drying, vacuum drying Quinones are not widely found in fruits and vegetables but are
(e.g., rotary evaporator), and freeze drying (Muthu, 2014). Freeze more frequently encountered within plants and insects. The main
drying comes with high equipment and operating costs but is the sources of quinones are walnuts, which specifically contain naph-
most recommended strategy for heat-sensitive dyes (Muthu, 2014), thoquinones, rhubarb and beans contain anthraquinones. The
although 80e90% of anthocyanin encapsulates are spray-dried juglone content within walnut green husks attained 15.0 kg/t DW
(Mahdavi et al., 2014). (Stampar et al., 2006). Rhubarb parts (e.g., petioles, leaves, and
Despite the hydrophilic property of anthocyanins, the overall roots) showed the presence of anthraquinones (e.g., emodin,
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K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

physcion, and chrysophanol). Anthraquinones concentrations and processing large sample volumes that allow for an industrial-
found in the petioles are 610 g/t DW, leaves 40 g/t DW, and roots scale-up (Duval et al., 2016). The drawback of this technique is
18,100 g/t DW (Kosikowska et al., 2010; Paneitz and Westendorf, that the stationary phase is still a liquid, which is difficult to
1999). The amount of quinone-containing by-products differs by maintain inside the column with the mobile phase flowing through
two orders of magnitude compared to anthocyanin-and carot- it (Berthod et al., 2009). The use of the entire extract after drying
enoid-containing by-products. Quinone by-products are more the organic solvent can be seen as the most practical solution.
likely to be found through agricultural losses or post-harvest Naphthoquinones are mostly applied on protein fibers (44%) and
handling; only rhubarb is further processed to become juice but cotton (34%), leaving a dark reddish to brown color. Forty-one
is rather new on an industrial scale (Will and Dietrich, 2013). Nuts percent of the studies were performed without mordant; within
that have fallen to the ground before harvesting are dryer this, 58% displayed excellent wash fastness (4-4/5), 39% showed at
compared to the less mature nuts that must be shaken from the least a lightfastness of grade 5 or higher. In essence, naph-
trees. They also require drying to a storage safe moisture content of thoquinones met the industrial fastness requirements for both
8% to prevent mold development (Khir et al., 2013). There are no protein and cotton fibers, without the addition of any mordant. In
restrictions on drying techniques, as 1,4-naphthoquinones are the meantime, natural anthraquinones show a high application rate
thermally stable up to 600  C (McWhinney et al., 2013). The ther- on protein fibers (61%), 17% were applied on cotton. When these
mal decomposition of emodin starts at 281  C, chrysophanol 189  C, anthraquinones were applied to protein fibers, polyester, and PLA
and physcion 200  C (Fouillaud et al., 2018). The most suitable without any mordant, the industrial fastness requirements for
solvents for extracting phytochemicals from walnut green husk and general apparel were met. As emodin, chrysophanol, and physcion
leaves are water, methanol, and ethanol (Jahanban-Esfahlan et al., are the main natural anthraquinones found in rhubarb and bean by-
2019). It has also been noticed that the phenolic content in the products, the fibers will display yellow to orange colors (Caro et al.,
extract increases by optimizing the ratio of the aforementioned 2012). The anthraquinones from rhubarb waste are suitable for
solvents (Jahanban-Esfahlan et al., 2019). A 60% ethanol-water dyeing only 6% of Europe’s entire woven fabric production, bean by-
mixture at a temperature of 60  C for 30 min. Ultrasonication products only account for 0.15% and naphthoquinones from walnut
seems to yield the highest total phenolic content from the walnut husks are good for 3% (Table 5).
green husk (Tabaraki and Rastgoo, 2014). Nour et al. (2016) opti-
mized the extraction conditions of walnut leaves by using ultra- 5.3. Valorization route for carotenoids
sonication in a hydroalcoholic medium. The optimized parameters
were 62.82% (v/v) ethanol concentration, a solid-liquid ratio of 4.96 Tomatoes are among the world’s most popular vegetables. They
v/w, and an extraction time of 48.75 min. The concentration and are consumed fresh and are also frequently processed into paste,
ratio are the most determining factors for extraction efficiency. juice, ketchup, or puree (Broeze et al., 2019). This results in many
Anthraquinones from rheum species are frequently extracted by-products, estimated at 7 Mt in Europe per year (Fig. 6C). The
through ultrasonication and a 50e80% ethanol-water mixture pomace, mostly consisting of skins and seeds, is also a rich source of
(Kosikowska et al., 2010; Wang et al., 2008). The anthraquinone carotenoids, which reach up to 593 kg/t DW via soxhlet extraction,
extraction yield depends on the type of solvent, which is highest for with a 96 (v/v %) ethanol-hexane mixture (Va gi et al., 2007). There
acetone > acetonitrile > methanol > ethanol. Acetone displayed the is a difference in consistency between wet pomace (i.e., 33% seed,
best results in both maceration and ultrasonication. This can be 27% skin, and 40% pulp) and dry pomace (i.e., 44% seed and 56%
ascribed to its structural similarity with anthraquinones (i.e., pulp and skin), which could lead to various extraction profiles
carbonyl functional group) (Duval et al., 2016; Wang et al., 2008). (Barbulova et al., 2015; Sogi and Bawa, 1998). Carrot leaves and
Generally, a compromise should be found between yield and the pumpkin by-products are also two viable carotenoid sources,
environmental impact of the extraction procedure. For ultra- though the waste streams are at least three times lower in quantity.
sonication, Duval et al. (2016) recommend a temperature between Carrot leaves mostly emerge after post-harvest handling, tomato
55 and 67  C for 30e60 min. Other extraction techniques for an- pomace and pumpkin are more likely derived from processing side
thraquinones are microwave-assisted extraction (MAE), PLE, or streams. Drying is certainly mandatory for these thermolabile
supercritical fluid extraction (SFE), each with its advantages and carotenoid-containing by-products, as they rapidly spoil due to
drawbacks in terms of extraction time, ease of use, solvent con- high moisture and nutrient content (Table 5) (Broeze et al., 2019).
sumption, operation cost, etc. (Duval et al., 2016). The extract Hern andez-Ortega et al. (2013) retained 2.5 times more b-carotene
should be subjected to mutagenicity studies, as 1,3- in carrot pomace that was microwave-dried compared to hot-air-
dihydroxyanthraquinones having a methyl (rubiadin) or hydrox- dried.
ymethyl (lucidin) group on the C2 position can form covalent ad- As most carotenoids in tomato are lipid-soluble, extraction
ducts with nucleobases (Bechtold and Mussak, 2009). The procedures depend on the use of organic solvents (e.g., hexane,
engineering of efficient power ultrasonic systems (e.g., generators acetone, ethanol, ethyl acetate, ethyl lactate, chloroform, petroleum
and reactors) for industrial processing can be a possible issue, ether, or mixtures of these aforementioned solvents) (Strati and
although ultrasonic systems are frequently and successfully applied Oreopoulou, 2014). Extraction with ethanol releases more xan-
in the food sector (Fritsch et al., 2017). thophylls and a ten-fold lower contribution of lycopene (V agi et al.,
An additional fast and straightforward purification step for 2007). It is noteworthy to remark that ethyl lactate, which is an
quinones is not yet available, as flash chromatography based on environmentally friendly solvent that is derived from the fermen-
silica gel is the only method applied for naphthoquinones tation of carbohydrate feedstocks available from corn and soybean
(Mathiyazhagan et al., 2018; Zhang et al., 2015). Countercurrent industries, produces good results in the carotenoids’ extraction
chromatography (CCC) or centrifugal partition chromatography (Strati and Oreopoulou, 2011a), but its high boiling point (150  C)
(CPC) is the preparative method of choice for separating and pur- makes solvent recovery more costly compared to lower boiling
ifying anthraquinones from the crude extract (Duval et al., 2016). solvents such as ethyl acetate and hexane (Broeze et al., 2019;
This chromatographic technique is based on two liquid phases, as Fritsch et al., 2017). Besides obtaining carotenoids via soxhlet
one is kept stationary by a centrifugal force and the other acts as a extraction, other conventional and more unconventional extraction
mobile phase that flows in the opposite direction. The main ad- methods were applied, such as ultrasonication, MAE, PEF, PLE, SFE,
vantages are its high sample recoveries, large loading capacities, and enzyme assisted extraction (EAE), which makes use of
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K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

hydrolytic enzymes (cellulase and pectinase) for disrupting the cell anthocyanins, naphthoquinones, anthraquinones, and carotenoids
wall (Saini and Keum, 2018). After extraction, when chromato- to investigate the influence of various extraction and purification
graphic analysis is desired, saponification is carried out as an pathways (Fig. 9) (Finnveden et al., 2009).
additional step. The purpose of this procedure is to remove xan-
thophylls that are acylated with saturated and unsaturated fatty  Scenario one consists of a simple extraction from the wet waste
acids along with chlorophylls and lipids. This will give rise to an stream and a subsequent batch dyeing procedure at a single site.
interference-free chromatographic analysis, but this would be  Scenario two involves a drying step of the biomass at the
redundant for application purposes, as this extra step induces manufacturer, as this would reduce the transport costs before
quantitative losses, artefact formation, and carotenoid degradation moving the by-products to the extraction and dyeing site.
(Saini and Keum, 2018). The purification of carotenoids from the  The other three scenarios include the impact of an additional
matrix can be simply performed with SPE (Shen et al., 2009). When purification step of the crude extract in combination with one of
carotenoid dye powder or concentrate needs to be obtained, three common drying methods:
concentrating carotenoid extracts by rotary evaporation is possible o Spray drying (scenario 3);
at reduced pressure and a temperature below 40  C (Rodriguez- o Vacuum drying (e.g., rotary evaporator) (scenario 4);
Amaya, 2001), although freeze drying is the most advisable tech- o Freeze drying (scenario 5).
nique due to the thermolability of carotenoids.
In terms of applications, only 44% of the purely applied carot- The natural dye is converted into a batch of dye powder or liquid
enoids attain a wash fastness of 4, and 89% of those unmordanted concentrate for long-term storage and ease of transport, and
carotenoid fibers score poorly on lightfastness (below a grade of 3). guaranteeing shade uniformity (Muthu, 2014). To protect these
Thirty-nine percent of the studies applied carotenoids to cotton sensitive compounds against environmental conditions, malto-
fibers, 33% applied them to protein fibers. Carotenoids show the dextrin was considered as a drying agent during the spray and
best fastness properties (WF ¼ 4/5 and LF ¼ 6-6/7) on a cotton/ freeze-drying step. This compound increases the glass transition
viscose mixture or cotton in the presence of a pomegranate rind as temperature, reduces the hygroscopicity of the powder, and assures
a biomordant containing polyphenolic compounds that are pro- pigment retention through encapsulation (Costa et al., 2015). As
tecting the carotenoids from photodestruction. The dyed fabric will natural dyes show lower substantivity to textile fibers, a dye
vary in color from yellow/orange to red, depending on the polarity amount of 12.5% on weight fiber (owf) was calculated as the worst-
of the solvents used during extraction. Xanthophylls are mostly case scenario (except for anthraquinones) (Phan et al., 2020). Tan-
yellow-orange colored pigments, b-carotene orange and lycopene nic acid (50% owf) was also taken into account as a biomordant in
red (Khoo et al., 2011). Due to the vast amount of tomato by- both natural anthocyanins (Phan et al., 2020) and carotenoids as
products and their large carotenoid content, the woven fabrics well as synthetic dyeing processes. For the latter, the additional
produced in Europe could be approximately dyed 12 times, carrot contribution of the biomordant can also be interpreted as an
leaves only allow dyeing 4% of the entire production and pumpkin approximation for the additional chemical modification of the
pomace just 2% (Table 5). synthetic 9,10-anthracenedione building block. The zero-burden
approach was applied in all cases, as the environmental impact of
6. LCA of naturally-dyed fabrics from agro-food residues the waste stream was not allocated. The solid residue after
extraction can still be applied to other integrated or coupled pro-
As shown in the previous section (e.g., Fig. 8), the main chal- cesses, which allows the subsequent recovery of each residue. The
lenge with finding a suitable extraction procedure for application, residue can also be deployed after the required post-treatments
and to improve dyes’ stability and fastness by one level, offering the with a desolventizer and dryer to remove the solvent residue, in
same product service compared to synthetic dyes, is not straight- low economic valorization purposes depending on its chemical and
forward. It is not easy to match the performance of synthetic dyes; functional properties (Broeze et al., 2019). Due to these complex
the main driver to choose natural dyes will be the potential envi- factors, neither contribution was taken into account in this study.
ronmental performance. Bio-based products are not necessarily Ecopoints are calculated with the ReCiPe endpoint method as the
‘greener.’ We have, by LCA, determined the environmental impact sum of the impact categories: resources, human health, and eco-
of suitable natural dye scenarios, starting from the waste stream systems. The results are illustrated in Fig. 10.
until the dyed end-product.
6.2. LCA results
6.1. Methodology
6.2.1. Anthocyanins
The goal of the studied systems is to determine how environ- Scenarios one and two (Fig. 10A) differ regarding whether the
mentally friendly it is to dye one m2 of cotton or wool fabric with a red grape pomace has been dried before extraction or not. The
natural dye (Functional Unit) that is derived from the waste stream, aqueous extraction from the dry pomace showed a lower impact
compared to dyeing one m2 with a synthetic anthraquinone dye compared to the wet pomace. The drying of the biomass accounts
(i.e., 9,10-anthracenedione building block displaying a yellow co- for 55% of the total environmental impact (Fig. 10A). In the
lor). Data for the production chain was collected from the literature following scenarios, which include a purification step, biomass
and comprises the natural dyes’ concentration, waste’s moisture drying still accounts for at least 40% of the total impact. Afterward,
content, transportation logistics (±115 km), extraction conditions, to obtain a dye concentrate or powder, three drying procedures,
filtration process, purification method, recovery losses, drying after purification of the crude extract, were compared to each other.
methods, dyeing methods, and wastewater treatment. Comple- As shown in Fig. 10A, drying in a vacuum has the lowest environ-
mentary data, as well as data of the entire production and dyeing mental impact, followed by spray drying and freeze drying. Even
process with the synthetic anthraquinone dye, were collected from though spray drying might degrade temperature-sensitive com-
the ecoinvent version 3.1 database, using OpenLCA software pounds, up to 90% of the anthocyanins encapsulated are spray-
(Wernet et al., 2016). The full data inventories can be found in dried (Mahdavi et al., 2014). Freeze drying guarantees more care-
sections S13eS20. Different strategies can be applied to obtain ful handling of temperature-sensitive compounds but comes with
naturally-dyed goods. Five explorative scenarios were studied for higher operating costs and a 15% higher environmental impact. Due
17
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Fig. 8. Examples of potential valorization routes for each natural dye class are summarized.

Fig. 9. A general scheme of the different explorative scenarios for the LCA analysis of the extraction of and dyeing with natural dyes.

to the green extraction method, the total environmental impact of wide pores can efficiently separate macro- and micro-molecules.
dyeing 1 m2 of wool with an anthocyanin crude extract is better The molecular cut-off becomes more stringent for narrower
compared to the synthetic dye. membrane pores, and the separation is mainly based on com-
pounds’ solubility, membrane hydrophobicity, and polarity resis-
6.2.2. Naphthoquinones tance (Galanakis, 2015). This technique is competitive with
Fig. S15 depicts the five different scenarios for obtaining natural conventional high energy-consuming methods such as distillation,
quinone from walnut green husks with ultrasound-assisted extraction, or drying, as it is proven to be more economical, safe,
extraction (UAE). As dyeing frequently occurs in an aqueous envi- and eco-friendly (Verhoef et al., 2008). This leads to a 21% lower
ronment, pervaporation, a membrane separation technique, was impact for scenario one compared to the synthetic dye reference.
applied to separate binary azeotropic mixtures. Membranes with The drying of biomass shows yet again a large contribution to the
18
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

Fig. 10. The environmental impact of dyeing one m2 wool with anthocyanins (A), naphthoquinones (B), and anthraquinones (C), and one m2 cotton with carotenoids (D). The
horizontal bar is the synthetic dye reference.

overall impact (55e71%) (Fig. 10B). The only adequate preparative 6.2.4. Carotenoids
purification method for naphthoquinones is flash chromatography The LCA scenarios of a naturally-dyed carotenoid cotton from
(Mathiyazhagan et al., 2018), which is not the most practical pro- tomato pomace are shown in Fig. S17. Despite this by-product’s
cedure and shows an average contribution of 13% to the total high carotenoid content, the environmental impact is at least 72%
impact. In terms of the drying method, vacuum evaporation is again higher than the synthetic anthraquinone dye (Fig. 10D), which
the most efficient method, which is allowed due to the great could be ascribed to the use of organic solvents and the required
thermostability of 1,4-naphthoquinones. In the end, dyeing with clean-up step afterward. The additional SPE purification procedure
crude extract produces a better environmental impact compared to was taken from Shen et al. (2009). Due to the use of many organic
the synthetic dye. solvents during purification, the environmental impact of this step
takes about 28%e47% of the total impact. When carotenoid dye
powder or concentrate is desired, concentrating carotenoid extracts
by rotary evaporation is possible at reduced pressure and a tem-
6.2.3. Anthraquinones
perature below 40  C (Rodriguez-Amaya, 2001). Spray drying is not
This is the most interesting case, as a one-to-one comparison is
recommended as this would degrade the carotenoids by both
possible. Natural anthraquinones are more present in plant- and
increasing residence time and temperature (Tang and Chen, 2000).
insect-based sources. For this reason, only rhubarb by-products
The stability of the spray-dried carotenoid powder granules may
form the base of this LCA. Due to the good fastness properties of
decline at high processing temperatures (Tang and Chen, 2000). For
these dyes, calculations were based on a 2% owf. This resulted in a
these reasons, freeze drying is the most advisable technique to
small number of by-products and lower amounts of consumables.
obtain a stable carotenoid powder, even though freeze drying is the
The direct application of the extract, after pervaporation, on wool
costliest drying technique and contributes to a larger environ-
fiber showed a 21% lower impact on the reference synthetic
mental impact (Fig. 10D). Broeze et al. (2019) performed a techno-
anthraquinone dye (Fig. 10C). The scenario involving the drying of
economic analysis of tomato seed oil’s production costs, which is
biomass is 2.5 times higher in impact compared to scenario 1. This
further refined to carotenoids from tomato by-products. The reve-
could be ascribed to the drying process, which accounts for 81% of
nues and production costs are nearly break-even on a small scale,
the total impact. The purification method with CPC is the most
on a ten times larger production scale, these production costs could
applied method for separating anthraquinones from the crude
decrease by 40%.
extract (Duval et al., 2016). This purification system consists of a n-
The valorization routes were designed with the natural dye as
hexane/ethyl acetate/methanol/water mixture and comprises up to
the target compound, but by-products also contain other high
46% of the total impact (Skalicka-Wo zniak and Garrard, 2015). Af-
added-value substances. To remain competitive in processing costs,
terward, the purified anthraquinones could be spray-dried, as these
the EU RESFOOD project advises the co-production of multiple
dyes only start to decompose from 189  C onward (Fouillaud et al.,
compounds. This means that by-products should be valorized to-
2018) and this method is 43% lower in total impact compared to
ward a major bulk compound (e.g., protein, fat, carbohydrate, or
vacuum evaporation. The application of crude extract is the most
fiber), the minor compounds can be separated from the crude
environmentally friendly approach for dyeing with natural an-
extract afterward (RESFOOD, 2016). Even though dyeing with the
thraquinones. Agnhage et al. (2017) performed an LCA analysis
crude extract is in all cases the most competitive and environ-
from the cultivation of the madder plant, dye extraction, and dyeing
mentally friendly approach, this is recommended only when the
of 1 kg of polyester with 3% owf concentration. The results showed
extract is depleted of the main valuable substances or their
that the dye extraction phase (solvent and energy use) and the
extraction is not economically viable. Otherwise, purification is
dyeing phase (liquor ratio) have the largest environmental impact
justified; the purified dye extract can be processed to a concentrate
of the entire process, but this study lacks a comparison with the
via vacuum evaporation. This has the lowest impact on the three
synthetic dye equivalent.
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K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

types of drying techniques. For a dye powder, spray drying should account the natural dye’s maximal extraction content) is shown in
be balanced against freeze drying, depending on the natural dye’s Fig. S20eS21B. Convergence is observed at the lowest dye amount,
properties, cost efficiency, and, for further application purposes, although this amount might be too low for proper coloration.
encapsulates its quality. Despite the high environmental impact of Again, scenarios one and two were more competitive to the syn-
these purified natural dyes, there is still room for optimization, just thetic dye (2% owf) reference impact compared to the scenarios
as synthetic dyes were chemically modified and optimized that included a purification and drying step. For anthocyanin dyed
throughout the years. The aspect of post-consumer textile waste wool, scenario two revealed a similar environmental impact
also needs to be considered, as in lower-value applications only 13% compared to the synthetic dye at 3.2% owf and scenario 4 reached
of the material used for clothing ends up being recycled and even the same point at 2.4% owf. Naphthoquinone dyed wool fabrics
less than 1% results in a similar quality application (closed-loop showed a better impact with scenario one and exhibited identical
recycling) (Morlet et al., 2017). The goal during the chemical recy- environmental impacts to the synthetic dye, at 7.2% owf for the
cling of textiles is to obtain pulp that is free from impurities such as crude extract method with the drying of the by-products and 6.1%
additives and synthetic dyes. The removal of these compounds is owf for the purified dye concentrate. The carotenoid crude extract
crucial, as this increases the fiber’s solubility as well as simplifies dyeing on cotton displayed an equal impact at 3.4% owf. For an-
and stabilizes the regeneration process (Wedin et al., 2018). From thraquinones, the environmental impact of one 1 m2 of naturally
this point of view, natural dyes can contribute to an easier recycling anthraquinone dyed wool outperformed the synthetic analog when
process, leading to a higher recycling rate and quality. using the crude extract (scenario 1). If the drying step of the bio-
products (scenario 2) was included, the impact matched the syn-
6.3. Sensitivity analyses thetic dye at 0.9% owf, and the spray-dried natural anthraquinone
(scenario 3) only became competitive at 0.27% owf.
Sensitivity analyses were conducted to explore the variables The effect that transportation has on the overall impact depends
that can be encountered during the process chains. The LCA ana- on the natural content of the by-product and its density. For ca-
lyses mentioned above were based on the maximal extractable rotenoids, only small increments in the impact were observed due
amount of dye from the by-products; the largest amount of dye that to the large carotenoid content in tomato pomace, which results in
can be used for dyeing (12.5% owf) to have a comparable shade lower amounts of by-products required for dyeing 1 m2 of fabric
depth, with respect to the synthetic dye, was also analyzed. The (Fig. S20eS21C). Red grape pomace (anthocyanins) and walnut
concentration of natural dye obtained depends largely on the type green husks (napthoquinones) contain proportionally lower
of by-product and extraction technique. For the same amount of amounts of natural dye and show a linear increase in impact at
dye required for the application, a smaller extracted dye content larger transport distances. The capacity of a semi-trailer for food-
would lead to a higher amount of by-products and require more stuff logistics (30,000 L) would contain lower loads of by-products
consumables. The influence of the natural dye content extracted on for anthocyanin or naphthoquinone dye production compared to
the environmental impact of a 1 m2 naturally-dyed fabric is dis- tomato pomace, despite their varying densities. This sensitivity
played in Fig. S20A for wool and S21A for cotton. This case could analysis is also related to the regional availability of the fruits and
also refer to the use of other by-products that contain smaller vegetables and the derived by-products across the different EU
natural dye concentrations (Fig. 6). A general trend is visible be- member states. To feed a large-scale plant, sufficient supply from
tween the direct application of the crude extract and the purified regional waste streams is crucial (Broeze et al., 2019). The envi-
dye. The increase in the environmental impact at a lower extraction ronmental impact and production costs will increase when larger
content is higher for scenarios that involve a purification step than distances need to be covered between the processing industry unit
those with the direct crude extraction application. The plots show and biorefinery (Fig. 11B). The average cost of road freight transport
an increase of at least 14% in the environmental impact when the by truck is 0.14 EUR/tkm (Schade et al., 2006).
natural dye content extracted for anthocyanins drops in half. A The extraction of high added-value bioactive compounds and
200% increase was found for napthoquinones and anthraquinones, their application should be encouraged to be maximally exploited
and 75% for carotenoids. at locations where a strong agricultural production of specific fruits
The shade depth can vary depending on the type of application and vegetables exists, leading to potential agricultural losses and
and its desired quality. A lower amount of dye can be used, which where large amounts of by-products arise to minimize the number
leads to lower amounts of consumables (Fig. 11A). This environ- of steps in the valorization route. This is in line with the concept of
mental impact as a function of the required dye amount (taking into the distributed bio-based economy (Luoma et al. (2011). This

Fig. 11. These 3D surface plots display the influence of (A) the concentration of natural anthraquinone extracted and the amount of dye (% owf) required for dyeing and (B) the
concentration extracted and transportation of the by-products from waste source to biorefinery on the environmental impact of dyeing 1 m2 wool with natural anthraquinone crude
extract (scenario 2). The synthetic dye reference is indicated by the gray surface.

20
K. Phan, K. Raes, V. Van Speybroeck et al. Journal of Cleaner Production 301 (2021) 126920

concept suggests the simultaneous development of a global bio- financial interests or personal relationships that could have
based economy and distributed production models at a local appeared to influence the work reported in this paper.
level. This “glocal” economy emphasizes the proximity between
sites where raw materials are obtained and production and con- Acknowledgments
sumption sites; this leads to closed-loop value networks of local
plants that produce and use by-products from neighboring plants The authors would like to thank the Research Board of Ghent
in different processes (Luoma et al., 2011; McCormick and Kautto, University (BOF), Belgium and the grant number is BOF-
2013). STA2017001101 for their financial support.

7. Conclusions Appendix A. Supplementary data

Agro-food residues are traditionally converted into a product Supplementary data to this article can be found online at
with a relatively low economic value (e.g., fodder). To increase the https://doi.org/10.1016/j.jclepro.2021.126920.
value of these by-products, novel valorization routes should be
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