5.

Valorisation of Citrus Waste for Bio-Cleaners

Valorisation of citrus waste involves converting by-products from the citrus industry, like peels, pulp, and seeds, into
valuable bio-cleaning products. Citrus fruits, such as oranges, lemons, and grapefruits, produce large amounts of waste
during juice extraction, creating an opportunity for sustainable repurposing. These wastes are rich in beneficial
compounds, including limonene, pectin, and flavonoids, which exhibit natural degreasing, antimicrobial, and aromatic
properties, making them ideal for bio-based cleaners.

5.1 CONCEPT OF BIO CLEANERS AND ECO-FRIENDLY CLEANING AGENTS

Bio cleaners, or eco-friendly cleaning agents, are natural, biodegradable cleaning solutions derived from organic
materials like plant extracts, microbial enzymes, and agricultural by-products. Unlike conventional chemical cleaners,
which often rely on synthetic and potentially harmful ingredients, bio cleaners harness biological processes or natural
substances to break down dirt, stains, and grease effectively. They use enzymes—such as protease, amylase, and
lipase—or beneficial bacteria to decompose proteins, fats, and starches, converting them into harmless by-products
like water and carbon dioxide.
Formulated with biodegradable, non-toxic ingredients, bio cleaners are safe for humans, pets, and the environment,
producing no harmful residues or pollutants. By sourcing from renewable materials and agricultural waste, they
support waste reduction, promote a circular economy, and offer a sustainable alternative that reduces air and water
pollution. Many bio cleaners also include natural antimicrobial properties, providing effective cleaning power without
the ecological footprint or health risks associated with traditional chemical cleaners.

5.2 EXTRACTION OF LIMONENE AND OTHER ESSENTIAL OILS FROM CITRUS PEEL

Essential oils are highly concentrated, volatile compounds derived from plants, widely utilized in industries such as
pharmaceuticals, cosmetics, and aromatherapy. The extraction of these oils is critical to preserving their purity, aroma,
and therapeutic properties. Extracting these oils is a vital process to preserve their purity, aroma, and therapeutic
benefits. Among the most valuable sources of essential oils are citrus peels, which are rich in aromatic compounds,
including limonene, the predominant component in citrus oils. Peels from fruits such as oranges, lemons, and
grapefruits contain oil glands that store these volatile compounds. The most common extraction method is cold
pressing, where mechanical pressure ruptures the oil glands, releasing the essential oils. This method is favoured for
citrus oils as it avoids heat, preserving both the natural fragrance and the chemical integrity of limonene. Alternatively,
steam distillation can be used, but it is less suitable for citrus oils as high temperatures may alter the properties of
volatile components like limonene. Known for its fresh, citrus aroma and multifunctional benefits, limonene
constitutes 90–95% of the extracted oil and finds applications in industries ranging from food flavouring and perfumery
to pharmaceuticals and eco-friendly cleaning products, thanks to its antimicrobial, anti-inflammatory, and solvent
properties. Recent advancements, such as supercritical CO₂ extraction, have further improved the efficiency and
quality of limonene extraction. This technique preserves the volatile nature of the compound without requiring heat
or harsh solvents, ensuring a high-quality yield. By refining extraction methods, essential oils from citrus peels,
particularly limonene, have become integral to sustainable, eco-friendly product innovation and development.

5.3 Production of Biodegradable Cleaning Agents from Citrus Waste

The growing emphasis on sustainability and eco-friendly practices has led to the innovative use of citrus waste such as
peels, pulp, and seeds in the production of biodegradable cleaning agents. Citrus waste, a by-product of the juice and
food industry, is rich in a variety of natural compounds, including limonene, organic acids, flavonoids, pectin,
polysaccharides, and essential oils, all of which enhance its cleaning potential. Repurposing this waste not only reduces
environmental pollution but also provides a cost-effective, renewable resource for manufacturing green cleaning
solutions. A key active ingredient derived from citrus waste is limonene, a powerful natural solvent found in the
essential oils of citrus peels. Known for its strong degreasing capabilities, antimicrobial properties, and refreshing citrus
aroma, limonene is widely used in surface cleaners, degreasers, and dishwashing liquids. Alongside limonene, organic
acids such as citric acid play a vital role in cleaning formulations by offering descaling properties, removing hard water
stains, lime-scale, and soap scum. Additionally, flavonoids from citrus waste contribute antioxidant and antimicrobial
properties, enhancing the stability and efficacy of cleaning agents. Pectin, isolated from the leftover biomass, serves
as a natural thickener and stabilizer, improving the texture and usability of the product. Fermentation of citrus waste
further produces enzymes and bio-surfactants, which significantly improve grease-cutting and stain-removal
efficiency. Polysaccharides found in the peel and pulp can also act as emulsifiers, enhancing the overall formulation.
Moreover, oils extracted from citrus seeds contain bioactive compounds that can be used for mild surfactant
properties in specialized cleaning solutions. Biodegradable cleaning agents made from citrus waste offer numerous
benefits. They are non-toxic, safe for humans and pets, and decompose naturally, leaving no harmful residues in the
environment. These eco-friendly products are increasingly favoured in both household and industrial settings, aligning
with global efforts to promote sustainable alternatives. By utilizing the full spectrum of valuable metabolites in citrus
waste, this approach minimizes waste disposal issues, supports a circular economy, and advances sustainability in the
cleaning industry.

5.4 Efficacy and antimicrobial properties of citrus- based bio-cleaner

Citrus-based bio-cleaners exhibit significant antimicrobial properties, making them effective alternatives to
traditional cleaning agent’s Citrus-based bio-cleaners are effective and eco-friendly alternatives to
conventional chemical cleaning agents, leveraging a variety of natural compounds derived from citrus waste.
These cleaners utilize a combination of limonene, organic acids (e. g., citric acid), flavonoids, pectin, and
essential oils, which work synergistically to deliver excellent cleaning and antimicrobial performance.
Organic acids, such as citric acid, are key contributors to the antimicrobial efficacy of citrus-based cleaners.
Citric acid disrupts microbial cell walls and biofilms, effectively neutralizing bacteria, fungi, and viruses.
Additionally, its descaling properties make it ideal for removing hard water stains and lime-scale. Flavonoids,
another group of active compounds in citrus waste, offer both antimicrobial and antioxidant properties,
helping to inhibit bacterial growth and extend product shelf life. Pectin and other polysaccharides extracted
from citrus peels enhance the cleaning formulation by stabilizing emulsions and improving product
consistency. These compounds also aid in removing dirt and grime by improving the physical interaction
between the cleaner and the surface. Essential oils, beyond limonene, contribute to antimicrobial activity,
with compounds like citral and linalool playing important roles in neutralizing pathogens while adding
pleasant natural fragrances. Research has shown that citrus-based bio-cleaners are effective in breaking
down oils, grease, and microbial contaminants. They perform comparably to, and in some cases better than,
chemical cleaners while being safer for humans, pets, and the environment. Their natural composition
ensures biodegradability and minimizes harmful residues, making them suitable for diverse applications in
households, industries, and institutions. By harnessing the full range of metabolites found in citrus waste,
these bio-cleaners combine cleaning efficacy with robust antimicrobial properties, contributing to the global
movement toward sustainable, non-toxic cleaning solutions.
REFERENCES ( 5.4 )
Organic Acids and Antimicrobial Activity: -Davidson, P. M., Taylor, T. M., & Schmidt, S. E. (2013). Chemical preservatives and natural antimicrobial
compounds. Food Microbiology: Fundamentals and Frontiers, 4th ed. American Society for Microbiology Press

Kato, Y., & Shibamoto, T. (2001). Citric acid and its effects on microbial activity. Journal of Agricultural and Food Chemistry, 49(1), 374-378.

Flavonoids as Antimicrobials: Cushnie, T. P., & Lamb, A. J. (2005). Antimicrobial activity of flavonoids. International Journal of Antimicrobial Agents, 26(5),
343-356.

Daglia, M. (2012). Polyphenols as antimicrobial agents. Current Opinion in Biotechnology, 23(2), 174-181.

Pectin and Polysaccharides in Cleaning Formulations: Thakur, B. R., Singh, R. K., & Handa, A. K. (1997). Chemistry and uses of pectin—a review. Critical
Reviews in Food Science and Nutrition, 37(1), 47-73. Munir, M. T., et al. (2016). Pectin and its role in cleaning formulations: Stability and application. Food
Hydrocolloids, 57, 238-250.

Essential Oils and Antimicrobial Efficacy:- Burt, S. (2004). Essential oils: Their antibacterial properties and potential applications in foods. International Journal
of Food Microbiology, 94(3), 223-253.

Bakkali, F., Averbeck, S., Averbeck, D., & Idaomar, M. (2008). Biological effects of essential oils–a review. Food and Chemical Toxicology, 46(2), 446-475.

General Effectiveness of Citrus-Based Cleaners: Muthaiyan, A., & Martin, E. M. (2017). Green cleaning: The science and impact of citrus-based bio-cleaners.
Journal of Environmental Sciences, 45(4), 241-248. Jain, S., & Singhal, P. (2016). Applications of citrus peels in eco-friendly cleaning formulations: A review.
Sustainable Chemistry and Pharmacy, 3, 70-77.
6. Technological Approaches for Citrus Waste Valorisation
Transforming citrus waste into valuable resources through modern technologies presents a promising path toward
sustainable resource management. This involves methods such as pre-treatment, advanced bioprocessing, integrated
bio-refineries, and waste-to-energy systems, all working together to ensure citrus by-products are used efficiently and
effectively.

6.1 Overview of Pre-treatment Methods (Chemical, Thermal, Enzymatic)

Citrus waste valorisation involves a variety of pre-treatment methods aimed at breaking down complex biomass and
recovering valuable bioactive compounds. It involves pre-treatment methods such as chemical method which involves
acid hydrolysis, thermal processes and enzymatic treatment these methods are described below:
Acid hydrolysis is an essential pre-treatment step in bio-refinery processes, used to break down complex lingo-
cellulosic biomass, such as fruit and vegetable wastes (FVW) and corn Stover (CS), into fermentable sugars. (Rodríguez-
Valderrama et al., 2020). The process involves treating the biomass with diluted hydrochloric acid (0.5% HCl) at
elevated temperatures (120°C for 120 minutes). (Rodríguez-Valderrama et al., 2020). During this treatment, cellulose
and hemicellulose are converted into monomeric sugars like glucose, xylose, and arabinose, which serve as carbon
sources for further biochemical processes. (Rodríguez-Valderrama et al., 2020). The resulting liquid hydrolysates, rich
in reducing sugars (RS), are separated from solid residues and detoxified using over-liming to reduce inhibitory
compounds such as furfural, HMF, and phenolic compounds. (Rodríguez-Valderrama et al., 2020). While over-liming
improves fermentation efficiency, it may cause minor sugar loss. (Rodríguez-Valderrama et al., 2020).in addition,
Thermal processes, such as microwave and ultrasound-assisted treatments, are advanced techniques that significantly
enhance biomass conversion efficiency in bio-refinery systems. (Joglekar et al., 2019). These methods offer notable
advantages over conventional approaches, including lower energy consumption, shorter processing times, and
improved product yields. (Joglekar et al., 2019).Microwave-assisted treatment is particularly effective in increasing the
extraction efficiency of essential oils, pectin, and other valuable compounds from biomass. (Joglekar et al., 2019). It’s
faster processing time and reduced energy requirements lead to a significant decrease in environmental impacts, such
as global warming potential (GWP), acidification, and eutrophication. (Joglekar et al., 2019). For instance, microwave-
assisted extraction achieved a 24% pectin yield in just 3 minutes, compared to only 18.3% obtained over 120 minutes
with traditional methods. (Joglekar et al., 2019). Similarly, ultrasound-assisted treatment enhances the extraction of
phenolic compounds and antioxidants while consuming less energy and lowering environmental indicators. Studies
have shown that ultrasound-assisted extraction can reduce environmental impacts by nearly 48% compared to
conventional techniques. (Joglekar et al., 2019). Overall, both microwave and ultrasound-assisted treatments provide
sustainable and efficient alternatives for biomass processing, making them valuable tools in modern bio-refinery
systems. (Joglekar et al., 2019).
The enzymatic treatment of citrus peel is an innovative and sustainable method aimed at extracting high-value
bioactive compounds, improving their bioavailability, and promoting efficient waste utilization (Panwar et al., 2021)
.This process involves enzymatic hydrolysis, where enzymes such as pectinases, cellulases, and hemi-cellulases are
used to break down complex polysaccharides like pectin, cellulose, and hemicellulose into simpler, more functional
molecules (Panwar et al., 2021) .Through this method, the recovery of valuable compounds, including pectins,
essential oils, and flavonoids like hesperidin and naringin, is significantly enhanced (Panwar et al., 2021). These
compounds exhibit notable antioxidant, antimicrobial, and cosmetic properties, and enzymatic treatments further
boost the yields and functional quality of polyphenols and flavonoids (Panwar et al., 2021). Compared to traditional
chemical extraction methods, enzymatic processes are environmentally friendly, as they minimize the production of
toxic by-products (Panwar et al., 2021). The treated citrus peel can subsequently be utilized in various applications,
including as functional food ingredients, nutraceuticals, and bio-adsorbents for wastewater treatment (Panwar et al.,
2021). This approach supports circular economy principles by transforming citrus peel waste into valuable resources
while reducing environmental impact (Panwar et al., 2021).

6.2 Advanced Bioprocessing Technologies

Advanced bioprocessing technologies, including fermentation methods and bioreactor systems, are vital for
transforming citrus waste into valuable bio-based products, contributing to sustainable bio-refineries and circular
economy goals.
Fermentation methods like submerged fermentation (SmF) and solid-state fermentation (SSF) are widely utilized for
citrus waste valorization. SmF involves processing citrus biomass in a liquid medium, enabling precise control over
parameters such as pH, temperature, and oxygen levels, making it particularly effective for producing bioethanol, citric
acid, and enzymes like pectinase. In contrast, SSF uses solid substrates with low moisture content, providing an
economical solution ideal for developing countries, and is highly efficient for producing enzymes, organic acids, and
single-cell proteins, especially using fungi like Aspergillus species. (Gervasi & Mandalari, 2024)
Bioreactor systems play a central role in optimizing fermentation processes. Stirred-tank reactors (STRs) are highly
efficient for aeration and agitation, ensuring maximum product yields. Pneumatic bioreactors, such as airlift systems,
offer low shear mixing, making them suitable for sensitive microbial cultures and effective for co-producing ethanol
and fungal chitosan. Rotating drum bioreactors (RDBs) enhance solid-state fermentation by improving substrate
aeration while minimizing shear forces that could inhibit fungal growth. By integrating these advanced technologies,
citrus waste can be efficiently utilized to produce valuable products, reducing environmental impact and promoting
sustainable industrial practices. (Gervasi & Mandalari, 2024)

Gervasi, T., & Mandalari, G. (2024). Valorization of Agro-Industrial Orange Peel By-Products through Fermentation
Strategies. Fermentation, 10(5), 224. https://doi.org/10.3390/fermentation10050224

6.3 Integrated Bio-Refinery Approaches for maximum utilization

Bio-refinery integration approaches focus on combining multiple processes to efficiently convert biomass into various
value-added products and biofuels. This integration can include a combination of pre-treatment, extraction, and
conversion technologies. Integrated bio-refinery approaches offer a sustainable and efficient way to utilize citrus
waste, particularly in regions with substantial citrus production. These techniques integrate biomass conversion
processes to generate valuable products like biofuels, bio-chemicals, and bioenergy, while also addressing
environmental concerns. Citrus waste, abundant in bioactive compounds such as essential oils, pectin, and
polyphenols, can be converted into high-value resources, benefiting both the environment and the economy. The
process begins with the extraction of essential oils, including D-limonene, which can hinder composting and biofuel
production if not removed. Following this, pectin and polyphenols are extracted, and the residual waste is repurposed
for biofuel production or composting. While biofuels derived from citrus waste show great promise, scaling up
production remains challenging as most efforts are still confined to laboratory studies. To overcome these obstacles,
advancements in green extraction methods, improved resource efficiency, and thorough life cycle analyses are crucial.
Citrus waste valorisation aligns with the principles of a circular bio-economy, reducing waste and enabling applications
across industries such as food, agriculture, and pharmaceuticals. Transitioning from traditional disposal methods, like
landfilling, to integrated bio-refinery systems not only mitigates environmental impacts but also decreases operational
costs for the citrus industry. However, industrial-scale implementation faces challenges related to market demand,
public acceptance, and technological feasibility. Ongoing research and development are vital to enhance the cost-
effectiveness of these processes, optimize the recovery of valuable compounds, and boost energy production. These
efforts will play a key role in advancing global sustainability and securing energy resources for the future. (Lee et al.,
2024).

Lee, S., Park, S. H., & Park, H. (2024). Assessing the feasibility of biorefineries for a sustainable citrus waste

management in Korea. Molecules, 29(7), 1589. https://doi.org/10.3390/molecules29071589

6.4 Waste-to-energy technologies (anaerobic digestion, pyrolysis, etc.)

Citrus waste valorisation through technological approaches such as anaerobic digestion and pyrolysis presents
significant opportunities for sustainable waste management and energy production. These methods not only mitigate
environmental impacts but also convert waste into valuable resources, aligning with circular economy principles. The
following sections detail the key technologies and their benefits

6.4.1 Anaerobic digestion

Anaerobic digestion involves two key stages: hydrolysis and methanogenesis. In hydrolysis, complex organic
compounds degrade into simpler molecules, producing volatile fatty acids (VFAs). (Guerrero-Martin et al.,
2024).Methanogenesis further processes these VFAs to generate biogas, primarily methane (CH4). (Guerrero-Martin
et al., 2024). A two-phase anaerobic digestion system improves process efficiency by separating these stages into
distinct reactors. (Guerrero-Martin et al., 2024). The first phase employs a packed bed reactor to degrade citrus
residues and produce VFAs, maintaining optimal acidity. (Guerrero-Martin et al., 2024).The second phase utilizes an
Up-flow Anaerobic Sludge Blanket (UASB) reactor to convert VFAs into biogas, achieving methane concentrations
exceeding 54% with yields of 0.51 L CH4 per gram of volatile solids. (Guerrero-Martin et al., 2024).This setup reduces
process limitations like pH imbalances and microbial inhibition common in single-phase systems. The approach not
only mitigates the environmental impacts of citrus waste—such as greenhouse gas emissions and landfill overflows—
but also supports renewable energy goals by producing biogas with neutral emissions. (Guerrero-Martin et al., 2024).
Furthermore, the resulting digestate is a valuable by-product for sustainable agriculture, enhancing soil fertility and
reducing reliance on chemical fertilizers. These advancements in anaerobic digestion highlight its role in circular bio-
economy models, transforming waste into energy and agricultural inputs while reducing environmental footprint.
(Guerrero-Martin et al., 2024). Combining hydrothermal carbonization (HTC) with anaerobic digestion (AD)
significantly boosts methane production and accelerates degradation rates, achieving more than 90% removal of
chemical oxygen demand (COD) (Vallejo-Cantú et al., 2023). Proper waste management is essential for reducing
pollution, especially from improper disposal methods like dumping waste in landfills. (Guerrero-Martin et al.,
2024).This can lead to soil contamination, unpleasant odor , and health issues. (Guerrero-Martin et al., 2024) .By using
bio-refinery systems, we can transform citrus waste into valuable products such as biofuels, essential oils, and pectin,
which helps tackle the waste problem in a more sustainable way. (Guerrero-Martin et al., 2024) .In addition, anaerobic
digestion (AD) plays a key role in managing citrus waste by producing biogas, a renewable energy source that helps
cut down greenhouse gas emissions. (Guerrero-Martin et al., 2024) .AD also significantly reduces organic waste—up
to 90%—while generating methane, which can replace fossil fuels. The leftover digestate from the process is a rich,
organic fertilizer that nourishes the soil, reduces the need for chemical fertilizers, and supports more eco-friendly
farming practices. (Guerrero-Martin et al., 2024) .This circular approach not only minimizes waste but also transforms
it into valuable resources for both energy and agriculture. (Guerrero-Martin et al., 2024)

6.4.2 Pyrolysis

Pyrolysis is a process where biomass is thermally decomposed in an oxygen-free environment, resulting in the
production of three key outputs: solid (charcoal), liquid (bio-oil), and gases (fuel gases). This process is widely regarded
as a sustainable method for converting biomass into energy and valuable chemical compounds. The overall reaction
for pyrolysis can be represented as:

Biomass → Charcoal + Volatile Compounds (bio-oil and gases)

The volatile compounds consist of bio-oil, which includes condensable vapours, and permanent gases such as carbon
dioxide (CO2), hydrogen (H2), and methane CH4. Pyrolysis unfolds in three distinct stages based on the decomposition
of biomass components. Hemicellulose is the first to break down at temperatures between 470–530 K (197–257°C),
producing CO2 acetic acid (CH3COOH), water, and char. For example:

C5H8O4 → CO2 + CH3COOH + H2O + Char

Next, cellulose decomposes at 510–620 K (237–347°C), resulting in levoglucosan, tar, and gases such as CO and H2.
Finally, lignin breaks down over a broader temperature range of 550–770 K (277–497°C), yielding tars and char with
high aromatic content.

The distribution of pyrolysis products is influenced by operating conditions. High-temperature settings (850–1300 K)
promote gas production, while moderate temperatures (550–950 K) maximize bio-oil yield. Lower temperatures (300–
600 K) favour char production. Based on these parameters,

Pyrolysis is classified into three main types.

Conventional pyrolysis, conducted at 550–950 K with slow heating rates (0.1–1 K/s) and long residence times (450–
550 seconds), generates a balanced mix of char, bio-oil, and gas.

Fast pyrolysis, typically performed at 850–1250 K with heating rates of 10–200 K/s and residence times of 0.5–10
seconds, is optimized for bio-oil production.
 Flash pyrolysis, involving temperatures of 1050–1300 K, extremely rapid heating rates (>1000 K/s), and residence
times under 0.5 seconds, predominantly produces gases.

This versatility makes pyrolysis a highly efficient and environmentally friendly technology for converting biomass into
energy-rich products and useful chemicals.

6.4.3 COMPARATIVE STUDY OF PYROLYSIS AND ANAEROBIC DIGESTION

Anaerobic digestion (AD) is an effective and sustainable way to process wet organic materials, such as citrus waste and
sewage sludge. By using advanced pre-treatment methods like hydrothermal carbonization, it’s possible to improve
the breakdown of tough materials like orange peels, which often contain inhibitors like limonene. AD produces biogas,
a mix of methane and carbon dioxide that can be used for generating electricity heating, or even as vehicle fuel.
Alongside biogas, it also yields a nutrient-rich digestate that works great as a fertilizer. This makes AD an excellent
choice for handling moisture-heavy organic waste, with pre-treatments offering a way to further boost bio-methane
yields.

On the other hand, pyrolysis is ideal for dry biomass, such as wood and agricultural leftovers. Before processing, the
feedstock usually needs some preparation, like reducing its size, to ensure even heating and efficient conversion.
Pyrolysis results in three valuable products: solid charcoal, which can serve as a fuel or soil amendment; liquid bio-oil,
a potential energy source or chemical ingredient; and syngas, which is used for power generation or making chemicals.
One of the great things about pyrolysis is its flexibility—by tweaking the process settings, you can adjust the
proportions of these products to suit your needs. Bio-oil can even be upgraded into transport fuels, and syngas can
help power the system itself, making pyrolysis a versatile and energy-efficient technology.

For high-moisture waste: AD is more energy-efficient. For dry biomass: Pyrolysis produces a wider range of energy
products and has a higher energy output potential but with greater energy input requirements.

7. Economic and Environmental Benefits

The economic feasibility of citrus waste valorisation is increasingly recognized as a sustainable approach to managing
agricultural residues. By employing bio-refinery techniques, citrus waste can be transformed into valuable products,
enhancing both economic and environmental outcomes. This process not only reduces waste but also generates
revenue through the production of biofuels, essential oils, and other bioproducts.

7.1 Economic feasibility of citrus waste valorisation