Industrial Crops & Products 162 (2021) 113274

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Industrial Crops & Products

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

Circular bioeconomy and integrated biorefinery in the production of

xylooligosaccharides from lignocellulosic biomass: A review
César D. Pinales-Márquez a, Rosa M. Rodríguez-Jasso a, *, Rafael G. Araújo a,
Araceli Loredo-Treviño a, Debora Nabarlatz b, Beatriz Gullón c, Héctor A. Ruiz a, *
a
Biorefinery Group, Food Research Department, Faculty of Chemistry Sciences, Autonomous University of Coahuila, 25280, Saltillo, Coahuila, Mexico
b
Interfase, School of Chemical Engineering, Industrial University of Santander, Bucaramanga, AA678, Santander, Colombia
c
Department of Chemical Engineering, Faculty of Science, University of Vigo (Campus Ourense), As Lagoas, 32004, Ourense, Spain

A R T I C L E I N F O A B S T R A C T

Keywords: Biorefineries are an operational processing strategy similar to classic petroleum biorefineries in the production of
Prebiotic energy and products. However, the biorefineries use renewable sources (biomass) as feedstock. In these days, the
Hemicellulose biorefineries of second generation from lignocellulosic biomass are a promising strategy in the production of high
Pretreatment
added value compounds as the xylooligosaccharides (XOs) from hemicellulose fraction of lignocellulosic
Xylan
biomass. The XOs can be applied in different areas as food, energy, materials, promoting the sustainability of the
Hydrothermal processing
Autohydrolysis biorefinery model in terms of a circular bioeconomy. This review provides the latest advances in the processing
of biomass; production, general properties and structural characteristics, advances in industrial scale and com­
mercial potential of XOs. Also, the production of XOS in continuous processing operation as an important
strategy at pilot and industrial level for biotechnological application is presented.

1. Introduction community and industry have made efforts to produce XOs from various
raw materials, and it has been proven it is possible to produce XOs from
The circular bioeconomy in the production of high added value lignocellulosic biomass (Hong et al., 2019).
compounds and biofuels in recent years has attracted attention due to Lignocellulosic biomass is mainly composed of cellulose, hemicel­
new policies of several countries in the reuse, valorization, sustainability lulose, lignin and other components; it means a significant potential for
of biomass as raw material, and the biorefinery concept is the platform the use of agro-industrial waste since it is the most abundant natural
in the processing of biomass. Therefore, the integration of circular bio­ component of the earth (Schneider et al., 2018), providing different
economy with the concept of biorefinery incorporate an interesting types of cheap raw materials with wide industrial applications, such as
technological strategy in the use of biomass for the sustainable pro­ food additives, chemical compounds with high-added value, rheology
duction compounds of industrial interest (Ruiz et al., 2013a, 2017; modifiers, and finally for the production of different types of bioenergy
Lara-Flores et al., 2018; Ubando et al., 2020; Ruiz et al., 2020; Qin et al., (Benito-González et al., 2018; Huang et al., 2020). In this sense, the
2021). hemicellulose fraction and its main chain – xylan are of great importance
According to European commission (Spekreijse et al., 2019), there in the production of XOs. From xylan fraction in the biorefinery context,
are a large number of mature bio-based products and biobased in­ high added value compounds can be produced to be applied in the food,
gredients derived from biomass, in this sense the oligosaccharides has energy and materials area. Ruiz et al. (2013a) showed a biorefinery
gained attention as functional ingredients due to the positive impact that scheme in the production of these compounds as ethanol, xylitol, lactic
has been shown on human health (Vázquez et al., 2000; Dávila et al., acid, XOs, furfural, xylose, arabinose,
2019a; Ávila et al., 2020a). An example of these are xylooligo­ Finally, this review aims to provide a complete perspective on the
saccharides (XOs), considered promising compounds to be applied in potential for xylooligosaccharides production through a second-
different areas such as food, cosmetics, materials and bioenergy (Nakasu generation biorefinery model produced by agro-industrial wastes
et al., 2017; Alves-Ferreira et al., 2019). Nowadays, the scientific (lignocellulosic biomass) in terms of bioeconomy.

* Corresponding authors.
E-mail addresses: rrodriguezjasso@uadec.edu.mx (R.M. Rodríguez-Jasso), hector_ruiz_leza@uadec.edu.mx (H.A. Ruiz).

https://doi.org/10.1016/j.indcrop.2021.113274
Received 26 October 2020; Received in revised form 10 January 2021; Accepted 13 January 2021
Available online 29 January 2021
0926-6690/© 2021 Elsevier B.V. All rights reserved.
C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

2. Importance of prebiotic foods consumption in human health 2.3. Relationship between the intestinal microbiota and health care

2.1. Generalities of probiotics Inside the gastrointestinal system of humans and different mammals,
there are more than 1000 species of microorganisms that represent a
Probiotics are a group of live bacteria that, due to their correct oral total average population of 1014 microorganisms, exceeding ten times
intake, represent a benefit for the host’s health (Lin et al., 2019). These the number of somatic cells (Wang et al., 2020a). These intestinal or­
organisms and their benefit were indirectly observed through various ganisms (intestinal microbiota) supplies to the host with essential nu­
fermented foods throughout history (Sen and Mansell, 2020). These trients such as vitamins, metabolize bile acids and undigested
beneficial bacteria are mainly composed of Bifidobacterium (Bifido­ compounds, defend against the invasion of pathogens into the human
bacterium bifidum), Lactobacteria (Lactobacillus rhamnosus, Lactobacillus body, forming part of the intestinal immune system, and play an
acidophilus, Lactobacillus Plantarum), Enterococcus, Propionobacteria, and essential role in maintaining the function of the intestinal barrier (De
Peptostreptococci, among others. Furthermore, these bacteria restrict Filippo et al., 2019; Leclercq et al., 2019). Recently, it has been shown
opportunistic bacteria such as Bacteroides, Clostridia, Bacilli, Fuzobac­ that the intestinal microbiota influences brain functions, such as myelin
teria, Enterobacteriaceae, Eubacteria, Actinobacteria, Peptococci, Staphylo­ synthesis, neuroinflammatory responses, and the blood-brain barrier
cocci, Streptococci, yeasts, and many others, all of which cause adverse permeability, as well as the host’s mood, this is a result of the different
effects on gut health (Joshi et al., 2018). synthesized molecules, metabolites, that produce bacteria from carbo­
hydrates and proteins, dietary or endogenous sources (Leclercq et al.,
2019).
2.2. Prebiotics: definition, sources and the role of XOs among these One of the most notable examples of this mechanism is the produc­
substances tion of Short-Chain Fatty Acids (SFCA), which are a group of carboxylic
acids, including acetate, propionate, butyrate, valerate, and caproate.
Prebiotics were defined by the International Scientific Association of Through the synthesis of endogenous fermentation of intestinal bacteria,
Probiotics and Prebiotics (ISAPP), as "a substrate used selectively by host non-digestible carbohydrates (dietary fiber) are transformed into energy
microorganisms that confer health benefits." The symbiotic relationship that promotes bacterial growth and produces SCFA (Xu et al., 2020).
formed between probiotic foods, prebiotics, and intestinal flora, pro­ Finally, these organic acids have essential functions in different organs,
motes the survival of beneficial bacteria showing a positive effect on the such as the liver, skeletal muscle, adipose tissue, and pancreatic islets,
bacteria of the large intestine (Raddatz et al., 2019). Some substances they can also directly affect carbohydrate metabolism and lipid use.
have been relevant for studying prebiotic foods, such as inulin, a soluble Indirectly regulate energy metabolism through the parasympathetic
fiber highly known for its prebiotic activity. Another similar example is nervous system, they are also crucial for energy homeostasis. They can
the case of resistant starch (hi-maize), which are also less studied pre­ alleviate the symptoms of obesity and diabetes and ultimately limit
biotics but of relevance in research. Another example is rice bran, which inflammation and inhibit the growth of tumor cells (Xu et al., 2020).
also has prebiotic value, and it has also shown positive effects on human The bacterial population that lives in the human gastrointestinal
health in terms of protection against gastrointestinal cancer (Raddatz tract of more than 500 bacterial species that belong to four leading
et al., 2019). groups: Firmicutes and Bacteroidetes, which represent almost 80–90 %,
There are various sources of prebiotics, some of them come from while Proteobacteria and Actinobacteria about 10–20 %; then there is a
natural sources, such as milk, honey, onion, barley, Jerusalem artichoke, small part of the methanogenic Archaea (mainly Methanobrevibacter
rye, chicory, and salsify, among others, in which prebiotic concentration smithii), eukaryotes (especially yeasts) and viruses (one example of that
range between 0.3 and 6% (w/w) of fresh weight. Fructooligo­ is phages) (De Filippo et al., 2019). Due to the significant impact on the
saccharides (FOs) are also considered as prebiotics such as oligofructose, physiology of the host, the intestinal microbiota can be considered as a
these substances are present in honey, banana, barley, tomato, aspar­ microbial organ made of microorganisms (Cao et al., 2019), For this
agus, sugar beet, garlic, wheat, mushrooms, and rye, among others. reason, the compositional alterations in the microbiota, as well as the
Also, substances like glucooligosaccharides, glycooligosacharids, and reduction or unbalance of the microbial life of the intestine (also known
maltooligosaccharides are considered as prebiotics. Another group of as Dysbiosis), are important causes for the development of different
them is made by galactooligosaccharides (GOs), which are naturally diseases, such as allergies, diabetes, obesity, metabolic syndrome, irri­
found in bovine and human milk. Finally, xylooligosaccharides (XOs) table bowel syndrome, and colon cancer (García-Mantrana et al., 2016;
and arabinoxylooligosaccharides (AXOs), and these can also be seen Wang et al., 2020b).
from natural sources such as a variety of fruits, vegetables, dairy, Some of the factors that can badly affect the intestinal flora are:
bamboo shoots, and honey, but, they can be produced from agro- overuse or misuse of antibiotics (Li et al., 2019), different eating habits,
industrial waste, having low price because of the availability of the an example of this is that thin people with low-calorie balanced diets and
raw material needed for their production, however, currently FOs and physical activity have higher levels of intestinal flora (Codella et al.,
GOs are the ones with the most significant global participation in the 2018); caesarean-sections (C-section) deliveries (Mitselou et al., 2018),
market of prebiotics (Aguedo et al., 2015; Ashwini et al., 2019; embryonic development and human food in the first years of life
Khangwal and Shukla, 2019; Ricke, 2015). (Tanaka and Nakayama, 2017), emotional factors such as stress, phys­
The consumption of XOs selectively increases the intestinal amount ical activity and external factors such as altitude, temperature, toxic
of Bifidobacterium, preventing the growth of other bacteria and because pollutants, among others (Gubert et al., 2020). Due to the wide variety of
of this reason, it is possible that these sugars prevent constipation, factors that can result in the deterioration of intestinal flora and human
diarrhea associated with geriatric diseases, and cancer (Cao et al., 2020). health, prebiotics and probiotics represent elements that can boost the
Another benefit of consuming XOs is the increase in the functionality in bacterial life of the intestine (Liu et al., 2019).
the digestion and absorption of nutrients, especially in infants with a
weak digestive system, it is also reported that the intake of syrup with 3. Development of biorefinery models facing the food and
4.2 g of XOs can relieve symptoms of constipation in pregnant women environmental concern
without any side effects (Cao et al., 2020). XOs which has 2–9 depoly­
merization of degree (DP) (DP or 2–9 units of xylose) stimulate the 3.1. Biorefinery concept
growth of gut bacteria, but for food applications, XOs with 2–4 DP are
selected because of its properties and its rapid assimilation to the Due to the global need to mitigate the environmental concerns of
digestive system (Reque et al., 2019; Singh et al., 2019a). today, it has caused a growing search for technologies by the world

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C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

scientific community, and this has evoked towards the generation of agents, among others. Finally, hemicellulose, which is considered a
energetic compounds made from biomass or renewable sources, also great source for the production of compounds with high added value,
called biofuels, an example of that are bioethanol, biodiesel, bio­ such as XOs, which are compounds with a potential for the application of
hydrogen, biogas, vegetable oils, bio-oil, biosynthetic gas, bio-char, the food and pharmaceutical industry (Fig. 1) (Aguilar et al., 2018;
among others (Ruiz et al., 2013a, 2017; Ruiz et al., 2020; Carrillo-­ Baptista et al., 2020; González-García et al., 2018; Pino et al., 2018).
Nieves et al., 2019; Olguin-Maciel et al., 2020). In this sense, the bio­ There are a wide variety of co-products that can be extracted through
refinery concept represents an example of the conventional model of the different processing branches applied to lignocellulosic materials. In the
refinery, using different types of biomass through various process routes, case of cellulose, they are formic acid, ethylene glycol, acetic acid, lactic
primarily environmentally friendly processing, to create biofuels and acid, sorbitol, glycerol, glycolic acid, and, of course, bioethanol. For
also a wide range of useful compounds that try to promote a mayor use lignin, quinones, phenol benzene, syringaldehyde, pyruvate, and
of renewable raw materials and thus decrease the environmental impact different lipids. With the xylan from hemicellulose, it is possible to
caused by the excessive use of hydrocarbons (Aguirre-Fierro et al., 2020; obtain XOs, xylose, xylitol, furfural, hydroxymethylfurfural (HMF),
Islam et al., 2020). levulinic acid, pentane, among others. This wide variety of products
However, the term biorefinery also includes the generation of high depends on the type of processing, the main goal for the biorefinery and
added value products, linking the production of fuels and a wide range social necessity (Islam et al., 2020).
of compounds of interest with a variety of technological applications Exist several causes that prevent the deconstruction of LCM. These
(Ruiz et al., 2013a). With the consideration of the available amount of materials have complex structural and chemical mechanisms that form a
biomass, it is easy to perceive the great opportunity of taking advantage natural barrier for their conversion; this is called biomass recalcitrance
of the low costs of the raw material for the generation of chemical (Huang et al., 2020). The cellulose crystallinity and its polymerization
commercial products or compounds that represent a necessity for hu­ degree are factors that increase the resistance for enzymatic attacks and
manity (Ruiz et al., 2013a). Through sustainable processes, it is pro­ acid hydrolyzation. Another natural barrier is the lignin, which can
posed to replace fossil fuels and form a bioeconomy based on the use of absorb or inhibit the enzyme, the amount and the union cellulose-lignin
biological and renewable resources, resulting in reduced environmental significantly contributes to the recalcitrance of the LCM. Hemicellulose
impacts, greenhouse gases, enriching food production and materials. also influences the inhibition of the enzyme since it makes a natural
Within the future bioeconomy, biofuels play an essential role. Never­ barrier between the hydrolyzation process and the material (Dávila
theless, the viability of these systems requires the right choice of pro­ et al., 2019b), but it is not as important as the contribution of cellulose
duction methods, raw materials, and the generation of associated and lignin. Finally, the acetyl groups of the biomass, its porosity, particle
co-products (Bose et al., 2020). size are factors that can affect the yield of the process in a biorefinery,
that is one of the most important reasons to select carefully the processes
3.2. Biorefinery generations that convert the biomass into biofuels and byproducts. One example of
this is the use of hydrothermal pretreatments which actively improves
The use of different biomasses resulted in the specialized develop­ the biomass availability during all the rest of the processes (Kumar et al.,
ment of processes for each raw material, coupled with the interest in the 2020; Ruiz et al., 2011).
production of several specific compounds, it has caused a marked sep­ The second-generation of biofuels contemplates the elimination of
aration or branching between processing models; as well as their ob­ soil competition because instead of allocating crops for the creation of
jectives and economic approaches, resulting in the creation of different biofuels and bioproducts, it intends to use residues or lignocellulosic
types or biorefinery generations (Moncada et al., 2015; Singh et al., biomass created from agriculture, forestry, and food industry, gener­
2019b). ating chemical compounds from residues composed of cellulose, hemi­
cellulose, and lignin (Bryngemark, 2019)
3.2.1. First generation
Biofuels, in its first stage, were primarily bioethanol and biodiesel 3.2.3. Third generation
production using starch and sucrose or oil content crops (Correa et al., The use of algae as an alternative for creating third-generation bio­
2017). According to the Renewable Fuels Association (2020), during fuels has gained popularity because its treatment can result in the cre­
2019 global bioethanol production reached 15,776 millions of gallons, ation of multiple compounds. Algae are organisms whose growth
The United States contributes 54 % of the ethanol produced worldwide, depends on photosynthesis, which allows the microorganism to develop
and in this same year 98 % of corn grains were used and 2% of sorghum in aquatic environments with ease and low absorption of nutrients. This
for the production of bioethanol and coproducts, approximately 5,323, supposes multiple benefits since this biomass could be used to satisfy the
749 corn and 106,670 sorghum 000bu, respectively. The uses of edible energy demand without generating competition from agricultural soils,
sources of raw material make an important problem because it creates a as would happen in the first generation (Khoo et al., 2019; Rose­
land competition between energy and food production lands, also the ro-Chasoy et al., 2021; Velazquez-Lucio et al., 2018).
growing demand for energy will contribute to the overexpansion of Algae can be classified into microalgae (microorganisms with a cell
crops, causing degradation erosion of soil, loss of biodiversity systems size between 2− 200 μm) or macroalgae, due to their morphology and
and an increase of water consumption (Correa et al., 2017). way of growing in the aquatic environment, the components of biomass
are lipids carbohydrates, and proteins, which can generate various types
3.2.2. Second-generation of biofuels and a variety of high added value compounds (Khoo et al.,
The term of the second-generation biorefinery is because the use of 2019; Kumar and Singh, 2019; Velazquez-Lucio et al., 2018).
lignocellulosic biomass as raw materials, this feedstock is mainly Nowadays, technological advances and research offer an approach to
composed of cellulose, hemicellulose and lignin, and this biomass rep­ third-generation biorefineries in which their viability will depend on the
resents a great opportunity (Arai et al., 2019; Xu et al., 2019), due to its integration of high added value compounds into their process (Aparicio
wide spectrum of products such as biochemicals, biomaterials or bio­ et al., 2020). The fractionation of the raw material into various products
fuels, capable of being formed from this biomass (Ajao et al., 2018). makes this type of biorefinery one more analog to conventional bio­
Cellulose is composed of glucose monomers, so through enzymatic hy­ refineries. Currently, there are two perspectives for the solution of this
drolysis and fermentation, it is possible to divide and ferment the cel­ problem: The direct cascade, where the highest value bio-products are
lulose glucan to form liquid biofuels, such as bioethanol. Lignin is a the first focus of the process and the biofuels are with their residues; in
biopolymer that can be used to produce aromatic compounds, syngas the second case, inverse-cascade, which prioritizes satisfying the de­
products, heavy metal sequestrant, for the formation of antimicrobial mand for biofuels first and reverses the roles in the production (Lara

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Fig. 1. Structure of lignocellulosic materials adapted and modify from Lomovsky et al. (2016).

et al., 2020). 3.3. Importance of second-generation biorefineries against of global

Finally, the biorefinery from micro and macroalgae offers perspec­ environmental issues
tives to contribute to the generation of biofuels, and the integration of
high added-value compounds production, such as polyunsaturated fatty The Paris Agreement of the United Nations has among its objectives
acids such as agar, carrageenans DHA or EPA, antioxidants, proteins, to limit climate warming from 2.0 ◦ C to 1.5 ◦ C this issue is becoming
carotenoids, glucuronoxylorhamnans, glucuronoxylorhamnogalactans, worrying for the scientific and social community (Li et al., 2020). The
or xyloarabinogalactans, alginate, among others. Third-generation bio­ increase in the temperature of the earth is highly caused by the emis­
refineries provides an alternative that eliminates dependency on fossil sions released into the atmosphere by the excessive use of fossil fuels, the
fuels and the cogeneration of bioproducts with a wide variety of appli­ burning of these fuels produces large amounts of CO2, CO, NO(x), CH4,
cations (Lara et al., 2020; Li et al., 2015). which has demonstrated its ability to produce an effect greenhouse
capable of raising the temperature or causing climatic changes on the
3.2.4. Fourth generation surface of the earth, due to this the ratification of the Kyoto Protocol was
The fourth-generation biorefineries are based on searching for the made, as a worldwide effort to reduce the release of gases that cause the
ideal biomass, which focuses on finding higher yields, higher percent­ greenhouse effect by developing new technologies and methodologies
ages of bioconversion, and less energy for its growth (Yang et al., 2015). (Lozhkin et al., 2018; Özener and Özkan, 2020). However, the increase
Fourth-generation biomass is defined as those plants or microorganisms in the consumption and burning of fossil fuels has generated not only
with a high carbon sequestration capacity from the atmosphere or soil, environmental problems but also health problems for humans, since
including those genetically modified living plants with better charac­ there is a relationship between 2.9 million premature deaths from lung
teristics and organic wastes for biofuels and high added-value com­ disease, which are caused by various diseases such as lung cancer,
pounds (Kumar et al., 2020; Yang et al., 2015). Also, this generation is ischemic heart disease, respiratory tract infections and type 2 diabetes
based on the balance between biomass, carbon use, and the production mellitus and saturation of polluting gases in the atmosphere, especially
of biofuels, it is considered the ideal solution to a biorefinery model when it causes particulate matter with dimensions equivalent to or
(Yang et al., 2015). These fourth-generation biomass transformation greater than 2.5 micrometers (PM 2.5) (Özener and Özkan, 2020).
processes use bioengineering techniques and high technology, including Given the growing environmental and social problems, the concept
parameters like the metabolism of microorganisms and photosynthetic of biorefinery poses a potential partial or total solution through an
phenomena to produce biofuels and biomaterials with negative carbon economic and social system as a result of technological development for
production, including the use of clean energy generation, like solar en­ the creation of sustainable economic alternatives that allow to reduce
ergy, for the process (Ale et al., 2019; Ziolkowska, 2020). However, this the impacts on the environment and public health, promoting the use of
model is still in research and development, searching for new alterna­ the responsible use of renewable resources, such as biomass, however, it
tives and increasingly efficient processes (Ale et al., 2019). is necessary to highlight that the development of a biorefinery requires
the technological, governmental and private industry integration to
establish an economic chains that generate both economic and social

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gains (Ubando et al., 2020). light transmittance and low oxygen and aroma permeability, that makes
The incessant search for energy and food resources is caused directly xylan an exciting substrate for various applications; the products pro­
by the increase in population, this situation causes the need to develop duced from the depolymerization of xylan, it is possible to produce
technologies and policies that regulate the use of soils that avoid the prebiotic compounds such as xylobiose and xylotriose (DP = 2, 3) or
conflict between food and energy resources; however, due to this xylose, which can be used for the production of xylitol, a low-calorie
overproduction, large quantities of waste or non-edible raw materials sweetener used in food and pharmaceutical industry (Banerjee et al.,
for biorefineries for the production of biofuels and high added-value 2019; Zhang et al., 2019). Besides, different processing stages can be
compounds, such as different straws of cereals, bagasse of sugarcane, adapted to allow coupling by-products from cellulose, hemicellulose,
perennial pastures, corn stubble, agricultural and forest biomass wastes, and lignin, promoting a complete process in search of the best use of
among other industrial organic wastes (Nizami et al., 2017). Approxi­ biomass (Islam et al., 2020). In the recent years, some review articles
mately 1.3 billion tons of lignocellulosic biomass are produced annually have been published on XOs (Gullón et al., 2017a; Amorim et al., 2019;
worldwide. For all this biomass, only 3% of all this biomass is currently Poletto et al., 2020; Santibáñez et al., 2021), these reviews are focused
used to produce bioproducts. Although this material has the advantage on the production, application and purification. In addition to updating
of not competing with the land provided for the production of food and the above-mentioned points, in this review is focuses on showing and
takes advantage of the waste produced by this industry, the develop­ discussing the new advances of the XOs, continuous production and
ment of second-generation biorefineries involves the integration of commercial potential in terms of circular bioeconomy and biorefinery
bioproducts and the production of biofuels so that it becomes a sus­ concept (Fig. 2).
tainable economic model (Ubando et al., 2020). However, the use of The use of agro-industrial waste represents an advantageous oppor­
lignocellulosic biomass for a second-generation biorefinery model still tunity for the production of compounds with high added value, due to its
represents a technological challenge since the materials contain highly quantity and availability, for this reason, important environmental
variable percentages of structural polysaccharides, which are also benefits are generated, because due to production using agro-industrial
complexly bound, making these characteristics an impediment to the waste as raw material is creates a more complete and organized man­
efficient formation of fermentable sugars and compounds with high agement of the millions of tons of waste produced annually. However,
added value, this being the main obstacle to their transformation and there are still technological opportunities for increasing performance in
full use (Hassan et al., 2019). For this reason, the use of lignocellulosic processing stages and the use of enzymes or towards more environ­
biomass revolves around the fractionation of biomass through processes mentally friendly environments in the polymerization stages, elimi­
that are increasingly sustainable and friendly to the environment, which nating the tendency of chemical treatments (Amorim et al., 2019).
must also favor the division of each of the components produced to Finally, the global demand for biobased chemicals in the biorefinery
reduce the costs of the process on an industrial scale (Satari and Jaiswal, concept and circular bioeconomy, such as polymers, fibers, and fuels,
2021). sets the global investment in biochemicals in US$ 2.6 to US$ 5.8 trillion
dollars in the periods from 2025 to 2030 (McIntosh et al., 2019); and
specifically, XOs for the food industry have an estimated value of US$
3.4. XOs in the circular bioeconomy model of Biorefinery
130 million dollars in the prebiotic food market until the year 2023 is a
great alternative for economic development (Poletto et al., 2020).
Xylan is the most abundant component of hemicellulose, a fraction
According to global prebiotic ingredients market, the compound
that due to its amorphous nature, favors the extraction and production
annual growth rate (CAGR) is forecasted to register 12.2 %, during the
of high added value compounds, hemicellulose is also found in LCM
period of (2019-2024) and it is valued at 3.85 billion USD (Research­
from 20 to 50 % depending on the type of raw material, for that reason,
anmarkets ™, 2019), that increases interest in the industrial sector, due
it converts into high added value compounds (i.e. xylitol, XOs, etc) for so
to the great opportunity to apply compounds produced from xylan,
many different types of industries, especially for food, pharmaceutic and
especially with XOs.
biopolymers production (Banerjee et al., 2019). The xylan present in the
hemicellulose is a biopolymer with vast potential applications due to its
hydrophilic affinity, it allows to create matrix permeable to oleophilic
substances, this biopolymer also has different properties such as high

Fig. 2. Circular Bioeconomy and integrated biorefinery concept from lignocellulosic biomass.

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C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

4. General properties and structural characteristics of XOs The XOs have a relative sweetness. According to Samanta et al.
(2015), the recommended daily intake in adults is 8–12 g/day for
Although xylobiose relative sweetness is approximately 30 % with healthy adults, and that involves multiple benefits, as it is a low-calorie
respect to sucrose, the possibility of having a sweetening substance with sweetener (1.5 kcal/g) with prebiotic content. Hence, it is suitable for
antioxidant and prebiotic activity and its low energy content (1.5 kcal/ diabetic patients, also has antioxidant and cytoprotective activity. XOs
g) supposes several advantages for the enrichment of foods (Samanta are generally recognized as GRAS for food and drugs and animal feed;
et al., 2015; Shi et al., 2012). Structurally, XOs are composed of 2–10 however, the excess dose of XOs could generate distension of the in­
xylose monomers which are linked by β-(1,4)-xylosidic bonds, and more testine, nausea, flatulence, and diarrhea.
importantly, these compounds stand out for their overall thermal and
pH stability since these oligomers are stable at temperatures above 100 5. Potential sources for the XOs production
◦
C and are capable of withstanding pH variations between 2.5–8 (Gullón
et al., 2017b). Finally, this results in an ingredient available for various Xylans can be classified in different ways: as homoxylans, arabi­
applications and processes for the food industry (Amorim et al., 2019). noxylans, glucuronoxylans, and arabinoglucuronoxylans. These classi­
In the depolymerization process, xylan is converted into high mo­ fications, arabinoxylan stands out, because is the most abundant
lecular weight xylooligomers, branched or linear XOs, low and high constituent of the cell walls of plants, and also in grains such as wheat or
molecular weight xylooligomers, xylose, or be degraded to furfural. That different cereals, this polymer is made up of a backbone made by xylose
depends on several factors such as the xylan nature, the process to which units and has arabinose residues linked to its O-2 or O-3 (Costa et al.,
it is subjected, whether chemical or enzymatic (Ruiz et al., 2013b; Wu 2015; Singh et al., 2019c).
and Lin, 2011). Due to the great variety of plants in which xylan is found. Xylan as a substrate can be found in different forms in nature;
A common structural feature is a backbone formed by xylopyranosyl however, the use of agro-industrial waste to produce XOs represents
residues (Xylp) with β-1,4 linkages. In many cases, xylan also has acetic multiple environmental and economic advantages for the process. In
acid, which esterifies the Xylp backbone residues. Also, branched xylan Table 1, it is shown various potential sources of LCM capable of serving
avoids the crystallization of xylan. These factors are very relevant since as a raw material in food-grade XOs production (Antov and Đorđević,
they are responsible for the hydration, solubility of xylan, and the pro­ 2017).
tection of xylopyranosyl bonds from hydrolysis by microbial enzymes
(Arai et al., 2019).
6. Fundaments of the XOs production and extraction from
The molecular formula of the XOs can be reduced as C5*nH8*n +
lignocellulosic biomass
2O4*n+1 for polymerization grades from 2–6. Therefore, the molecular
mass corresponds to 282–810 Da, respectively. The names for these
The biomass fractionation starts with drying and grinding, because it
compounds are: Xylobiose (DP = 2), Xylotriose (DP = 3), Xylotetraose
directly affects the crystallinity of the material and improve its contact
(DP = 4), Xylopentose (DP = 5) and Xylohexose (DP = 6) and it is a solid,
surface, then this type of operations become in a preliminary step for the
colorless, although this is depending on the source obtained and the
following pretreatments; the operations that are usually used for this
process of purification and drying (Samanta et al., 2015). Finally, the
pre-stage are dry crushing, chipping, ball milling, and compression
XOs used appropriately can result in an agent that improves the nutri­
(Kumar et al., 2020). Despite their high utility, these preliminary pre­
tional and sensory properties of food (Antov and Đorđević, 2017).
treatments are incapable of separating the components of the

Table 1
Potential lignocellulosic organic biomasses to produce xylooligosaccharides (% dry basis).
Type of biomass Biomass % Cellulose % Hemicellulose % Lignin References

Hardwood Acacia 49.0 13.0 32.0 (Kim et al., 2016)

Oak 43.2 21.9 35.4 (Yu et al., 2017)
Softwood Pine 45.6 24.0 26.8 (Yu et al., 2017)
Spruce 45.5 22.9 27.9 (Yu et al., 2017)
Agave bagasse 20.0 – 30.0 12.0 – 20.0 17.0 – 19.0 (Aguilar et al., 2018; Pino et al., 2019)
Food Agroindustrial residues
Almond shell 27.0 30.0 36.0 (Á lvarez et al., 2018)
Hazelnut Shell 30.0 23.0 38.0 (Á lvarez et al., 2018)
Nutshell 37.0 22.0 36.0 (Á lvarez et al., 2018)
Pine nutshell 31.0 25.0 38.0 (Á lvarez et al., 2018)
Barley straw 35.4 28.7 13.1 (Hassan, et al., 2018)
Empty fruit bunch from oil palm 19.7 25.3 13.11 (Acosta et al., 2018)
Coconut Shell 29.58 27.77 31.04 (Gonçalves et al., 2015)
Coffee grounds 33.1 30,0 24.5 (Hassan et al., 2018)
Corn residues 31.8 19.04 14.7 (Aguilar-Reynosa et al., 2017b)
Hemp stalk 52.0 25.0 2.0 (Kim et al., 2016)
Rice husk 40.0 16.0 26.0 (Hassan et al., 2018)
Rice straw 38.14 31.12 26.35 (Hassan et al., 2018)
Wheat straw 37.4 29.4 23.6 (Ruiz et al., 2011)
Sugarcane bagasse 35.2 - 43.0 28.0 – 33.1 13.8 - 24.0 (Acosta et al., 2018; Zhou and Xu, 2019)
Cotton stalk 67.0 16.0 2.0 (Kim et al., 2016)
Non-food agroindustrial residues
Cotton gin 30.0 7.6 34.4 (McIntosh et al., 2019)
Grasses and weeds Amur silver-grass 42.0 30.2 7.0 (Raud et al., 2016)
M. saccharflorus (leaves) 31.7 26.0 16.9 (McIntosh et al., 2019)
M. saccharflorus (stalks) 36.6 26.9 16.3 (McIntosh et al., 2019)
M. sinensis (leaves) 28.5 25.1 17.9 (McIntosh et al., 2019)
M. sinensis (stalks) 35.6 19.5 19.5 (McIntosh et al., 2019)
Crofton weed stem 37.6 22.4 16.4 (Kumar et al., 2020)
Reed 49.4 31.5 8.7 (Raud et al., 2016)
Rye 42.8 27.8 6.5 (Raud et al., 2016)
Eichhornia crassipes 18.2 48.7 3.5 (Kumar et al., 2020)
Lantana camara 45.1 17 27.3 (Kumar et al., 2020)

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agro-industrial waste, which can mean a considerable increase in the 2020). The use of ultrasound has reduced extraction times and tem­
cost of production and the generation of multi-stages for their trans­ peratures. However, a problem in this type of pretreatment is that it
formation. However, these previous mechanical procedures are neces­ causes the cleavage of glycosidic bonds and easily forms degradation
sary to obtain a particle size and moisture content for the following compounds, so a correct selection of parameters will help to preserve the
pretreatments (Kumar et al., 2020). molecular and structural properties of the molecules (Naidu et al.,
The complex union of the components makes necessary the next 2018).
stage, the pretreatment, it improves the biomass availability to subse­ Kawee-ai et al. (2016) reported that the biomass fractionation pro­
quent processes. That is the reason why the pretreatment is considered cessing by Fenton reaction assisted with ultrasound and TiO2 improved
as the critical step, which has a significant impact on the cellulose di­ the yields of lignin extraction and enzymatic hydrolysis for XOs pro­
gestibility (Mazlan et al., 2019). It strongly influences downstream costs duction. The raw material used was corncob and it was treated by
involving detoxification, enzyme loading, waste treatment demands, adding 1 g/L of TiO2 to the sample solution with Fenton catalysts,
and other variables, such as the processing cost, because the pretreat­ including Fe◦ 7.61 mg/L, Fe2+ 9.89 mg/L, Fe3+ 14.27 mg/L, and H2O2
ment constitutes 40 % of the total processing cost (Sindhu et al., 2016). 376.88 mg/L were added to 10 % (w/v); The reaction was sonicated for
These pretreatments can be physical (ultrasound and microwave), 12 h with an ultrasonic bath at a frequency of 37 kHz and an output
physicochemical (hydrothermal processing as steam explosion, liquid power of 340 W at 35 ◦ C. With this, the lignin concentration level was
hot water), biological (through the use of microorganisms), and chem­ 1.03 g/L, which also means a lignin dissolution level of 80.25 %. In the
ical (acid, alkali) (Kumar et al., 2020). However, due to its utilities, it is case of XOs, the production was 4.645 g/100 g of the substrate.
preferred to create XOs with low DP and these pretreatments can only
produce a wide range of oligosaccharides (DP = 2–20) (Chen et al., 6.2. Physicochemical pretreatments for LCM
2017). Then, the application of enzymes or acid treatments can increase
the production of xyloses or XOs with a lower degree of polymerization, 6.2.1. Hydrothermal processing - steam explosion
around 2–6 units (Poletto et al., 2020). The steam explosion process is an autohydrolysis process, in this
treatment, the biomass is pretreated to high pressures and temperatures
6.1. Physical pretreatments for lignocellulosic material to produce XOs using steam over some time, after that the material is released abruptly,
which causes the expansion of the cellulose matrix, resulting in an
6.1.1. Microwave process alteration in the cell walls of the biomass. That implies several benefits;
The microwave heating process is an alternative to conventional one of them is because of the heating of the biomass with steam, then it
heating because it reduces the use of solvents or auxiliary chemicals that is possible to obtain higher concentrations of sugars, due to the excess of
favor product separation and also this technology improves the energy water on the biomass is eliminated; another benefit is because the
transfer due to its rapid heating capacity. This combination of factors temperature and time range needed in these treatments are short, for
creates a technology capable of redefining reactions where heat transfer example, it can be 220 ◦ C during 1 min or at lower temperatures, such as
or temperature effect plays an important role (Aguilar-Reynosa et al., 190 ◦ C, for 10 min (Lara-Flores et al., 2018; Ruiz et al., 2020; Singh
2017a, b). Nowadays, microwave irradiation treatment is formed from et al., 2015).
the exposure of electromagnetic waves with wavelengths from 1 mm to Generally, this process is fascinating because it solubilizes the
1 m and with a frequency between 300 and 300,000 MHz. Currently, hemicellulose and, due to fast expansion, it is possible to deform the
microwave treatment is used in lignocellulosic biomass of 2 ways: the structure of cellulose and lignin, which improves the susceptibility for
first, microwave-assisted solvolysis, which reaches temperatures below enzymatic hydrolysis. However, the severity of the treatment itself also
200 ◦ C has the objective to depolymerize the structure of the biomass represents a challenge, since it can quickly form degradation compounds
and produce high added-value compounds. the second form is (HMF and furfural), or low yields in obtaining sugars and also do not
microwave-assisted pyrolysis of lignin without oxygen with tempera­ reach a complete breakthrough of lignin (Pino et al., 2019; Singh et al.,
tures above 400 ◦ C or convert biomass to bio-gases or bio-oil (Hassan 2015).
et al., 2018). Álvarez et al. (2017) studied the production of XOs from wheat straw
An example of these applications was made by Wang and Lu (2013) using steam explosion pretreatment to separate hemicellulose from
who produced XOs from wheat bran by microwave-assisted enzymatic cellulose and lignin. The pretreatment was carried out in a 10 L reactor
hydrolysis. The experiment consisted of whether exposure of the wheat at 200 ◦ C, with a 50 s heating for an isotherm of 4 min and after
bran powder exposed to microwave radiation favored the production of expansion, a cooling that occurred in 2− 3 min. These conditions
XOs from wheat bran powder. The dry bran powder at a solid-to-liquid managed to solubilize the hemicellulose and extractives, allowing
ratio (SLR) of 1:5 was exposed to 1400 W microwave for 120 s. After this enrichment of the liquid phase in cellulose and insoluble lignin of 55.6 %
treatment, xylanase was added at a rate of 0.5 g/100 g of the substrate and 28.5 % by dry basis weight, respectively. Finally, the endo-1,
(2500 units of bacterial xylanase / 100 g of the substrate) and reacted at 4-β-xylanase and β-glucosidase enzymes were used, reaching 91 %
55 ◦ C and 100 rpm for 24 h. The XOs were purified using the ethanol conversion of high molecular weight XOs to XOs with a DP = 2–6,
precipitation method and the activated carbon adsorption method. Then generating 8.9 g of XOs per 100 g of raw material.
the samples were lyophilized to remove the ethanol and thus dilute the
XOs in water and quantify them by HPLC. After following this process, it 6.2.2. Hydrothermal Processing - liquid hot water
was possible to obtain 6.4 g of XOs with a DP of 2–4 from 100 g wheat Liquid hot water (LHW) is also a hydrothermal pretreatment (also
bran powder. known as autohydrolysis) is an operation which is performed by
disposing water and LCM at high temperatures (150− 230 ◦ C) and
6.1.2. Ultrasound process pressures (4.9− 20 bar) at different times of residence (10–50 min),
The use of ultrasound in combination with chemical treatments for which seeks to solubilize hemicellulose and concentrate cellulose and
the solubilization of hemicellulose has resulted in improvements to the lignin in the solid fraction. The autohydrolysis is achieved through the
conversion process of lignocellulosic biomass. That effect is because of autoionization of water, which is caused by the severe pretreatment
the absorption of ultrasonic waves in the plant matter causes the phe­ conditions that cause the formation of OH− and (H3O) + ions that act as
nomenon of cavitation bubbles, which means that when impacting the catalysts and induce the release of xylan acetate and hydrolysis. of the
pressure wave on the material, it generates bubbles that, when glycosidic bonds of hemicellulose; subsequently, a decrease in the pH of
imploded, cause microfractures in the matter causing the material swells the medium that promotes hydrolysis reactions occurs, and finally, the
or increases its porosity achieving higher mass transfer (Kumar et al., combination of these events results in the depolymerization of

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C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

hemicellulose (constituted mostly by xylan) and causes non-significant advantages and disadvantages to conventional NaOH treatment, such as
hydrolysis to glucan (formed by cellulose) (Kim et al., 2016; Neto the cost of the process in the case of Ca(OH)2 treatment or the positive
et al., 2020; Ruiz et al., 2013b, 2017; 2020; Zanuso et al., 2017). effects on enzymatic treatments caused by Na2SO3. This treatment
An example of hydrothermal pretreatment was made by Surek and causes the raw material to widen, increasing its contact surface and
Buyukkileci (2017) where hazelnut shell was used as biomass for the breaking the structural bonds of cellulose, hemicellulose, and lignin,
production of XOs. This process was carried out with dry and ground which creates a decrease of lignin in the biomass (Dávila et al., 2017).
hazelnut shell with a size <2 mm in a 600 mL stainless steel reactor with These changes in the biomass occur in several conditions, with the
propeller stirring; temperatures ranged from 150− 200 ◦ C and heating application of high temperatures, pressures, and short residence times,
was performed non-isothermally and isothermally with times of (5–30 for example, 121 ◦ C at 15 psi for 1− 10 min or with longer times and
min). The maximum concentration of XOs reached in this experiment pressures like 24 h at room temperature (Singh et al., 2015).
was 10.1 g/L, and this was obtained at the conditions of 190 ◦ C for 5 For this reason, the pretreatment above is considered as a first option
min. because alkaline reagents are selective for separating lignin from the
material, and that lets to obtain lignin extracts with high purity for
6.3. Biological processing as a pretreatment for LCM several applications (Dávila et al., 2017). For this reason, the pretreat­
ment above is considered as a first option because alkaline reagents are
Biological treatments are carried out by microorganisms applied selective for separating lignin from the material, and that lets to obtain
directly, or that the enzymes produced by them affect the biomass. These lignin extracts with high purity for several applications (Dávila et al.,
pretreatments are selective and are intended to eliminate the addition of 2017). However, this method allows the solubilization of hemicellulose
external chemicals, reduce the energy used, and, due to their low since it manages to break the hydrogen bonds between cellulose and
severity, do not produce degradation compounds as furans (inhibitors) hemicellulose and the ester bonds that join hemicellulose with lignin
for hydrolysis or fermentation. For this reason, biological pretreatments (Santibáñez et al., 2021). On the other hand, solubilization through
are considered as green processes, since they do not require chemicals or alkaline reactions has disadvantages such as corrosivity and contami­
their separation and also do not emit toxic or dangerous compounds into nation of water effluents and the fact that they can cause toxic com­
the environment (Sindhu et al., 2016). pounds, especially at high pressures and temperatures (Santibáñez et al.,
The pretreatment of the raw material through fermentation has 2021).
gained relevance because it improves the biomass availability. These Regarding the production of XOs, it is essential to mention that acetyl
treatments depend on the enzymes produced by the organisms, which groups are cleaved under this pre-treatment type. This can be advan­
will promote the biomass digestibility. The microorganisms that have tageous in the production of XOs through enzymatic hydrolysis since it
been used in these pretreatments are bacteria such as Bacillus, Actino­ eliminates the xylan substituents, avoiding acetyl esterase enzymes
mycetes, and Streptomyces, and fungi species like Aspergillus or Candida (Qian et al., 2020; Santibáñez et al., 2021). However, it can also cause a
(Ummalyma et al., 2019). These microorganisms can produce a wide drop in xylan’s solubility, reducing the hemicellulose extraction yields
variety of types of enzymes like cellulase, β-glucosidase, xylanase, and (Santibáñez et al., 2021). Finally, this processing route allows obtaining
amylase, which are responsible for the degradation of polysaccharides biomass rich in cellulose that can be transformed by fermentation into
into sugars (Thapa et al., 2020; Ummalyma et al., 2019). However, some biofuel (Ruiz et al., 2020) and minimize the degradation of XOs (Singh
species can also produce proteases and lipases, which are responsible for et al., 2019c).
the degradation of amino acids and fat of the biomass. Finally, these Singh et al. (2018) experimented with the extraction of xylan from
processes need the control of different variables like the temperature, areca nut husk through a two-stage alkaline process. The biomass was
stirring, pH, the isolation and reproduction of the microorganisms, to first incubated in alkali using NaOH at five different temperatures 25, 50
keep of the CO2 and O2 levels into the reactor, making these processes to and 65 ◦ C, with alkali concentrations of 5%, 10 %, 15 % and 20 % (w/v),
look sophisticated (Kim et al., 2016). But the fermentation using or­ the incubation periods were 8, 16 and 25 h to determine the xylan
ganisms are a potential to improve material availability and reduce the extraction. The second stage consisted of determining the best-operating
cost of using commercial enzymes for the process; they become an conditions for an alkali-assisted hydrothermal treatment, working at
essential alternative for the development of biorefineries (Ummalyma 121 ◦ C at different residence times 1, 1.5, and 2 h at different alkali
et al., 2019). concentrations 5 and 10 % (w/v). Incubation of the peanut shell with 10
Pereira et al. (2018) studied solid-state cultivations of genetically % w/v NaOH at 65 ◦ C for a period of 8 h, followed by hydrothermal
modified strains of Aspergillus nidulans A773 to produce xylanase and treatment at 121 ◦ C for one h, was determined to aid recovery of ~ 94 %
arabinofuranosidase with soybean fiber as a substrate. Subsequently, (w/w) of xylan. Then, an enzymatic hydrolysis process was carried out
these enzymes were used to produce XOs from the same agro-industrial for the recovery of XOs with DP range 2–4, it was found that the opti­
waste. From the optimization of the conditions for the culture of the mized condition was 50 ◦ C, pH 4, and enzyme dose 10 U with the
fungus, the best option was selected to obtain the highest concentrations enzyme endo-1, 4-β-Xylanase M1 for 24 h. Finally, the process resulted
of crude enzymes. Also, the application of xylanase in soybean fiber was in a mixture of XOs with the following composition, xylobiose: 25.0
evaluated to obtain xylooligosaccharides (the best condition was 50 ◦ C g/100 g of xylan, xylotriose: 9.2 g/100 g of xylan and xylotetraose: 0.9
and 117 U/g of soybean fiber). The highest yield of xylooligosaccharides g/100 g of xylan.
obtained was 28 % of the total xylan (w/w). Final concentrations of
xylobiose, xylotriose, and xylotetraose were 138.36, 96.96, 53.04 mg/g 6.4.2. Diluted acid treatments
of xylan, respectively. Acid pretreatment has been the most widely used and studied for the
transformation of lignocellulosic biomass because it is capable of solu­
6.4. Chemical pretreatments bilizing and hydrolyzing hemicellulose and improves the availability of
biomass. The application temperature range is from 120 to 200 ◦ C,
6.4.1. Alkaline pretreatment depending on the severity of the treatment. The most used acids are
Alkaline pretreatment has been extensively studied because it pro­ sulfuric and hydrochloric acids, however, various organic acids (such as
motes the biomass availability for the enzyme attack. Some compounds acetic, phosphoric and oxalic) and inorganic acids (nitric, maleic or
that are frequently used in this treatment are NaOH, which has received nitrous) have also been studied. The selection of the acid and the
attention for its vast capacity for delignification, its high reaction rate, severity of its application are fundamental parameters since, in very
and because it does not form inhibitory compounds. However, com­ critical treatments, they can produce furfural and HMF (Fig. 4), which
pounds like Ca(OH)2 or Na2SO3 are also considered, which have are inhibitors for stages such as fermentation, and although these

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C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

compounds are also classified as value-added compounds, the use this (Generally recognized as safe) microorganisms, such as the case of
type of pretreatment requires reactors with particular characteristics, xylanases produced by Humicola insolens (syn. S. thermophilum and now
such as being anticorrosive, and also complicate stages such as post- Mycothermus thermophilus), which were approved by the FDA and they
process water treatment (Kumar et al., 2020). are currently used in the food industry (for the beer and starch extrac­
Han et al. (2020) studied with the use of the corncob, the tion industry from wheat) and feed industry. Nowadays, H. Insolens
co-production of XOs, and glucose from the gluconic acid pretreatment. GH10 xylanases are commercialized as Shearzyme by Novozymes
Hydrolysis with gluconic acid achieved a maximum yield of 56.2 % (Chadha et al., 2019).
when 0.6 mol/L of acid was used at 154 ◦ C for 47 min in a 50-mL XOs can be produced from the hydrolysis of xylan using chemical or
stainless steel tube reactor. The results indicated that gluconic acid enzymatic methods; however, both processing routes have advantages
was an effective solvent for biomass production since it managed to and disadvantages compared to each other. Hydrolysis by chemicals
produce 180 g of XOs per 1 kg of pretreated biomass. such as acids is cheaper than enzymatic hydrolysis today; nevertheless,
improper treatments can lead to the degradation of monosaccharides
6.5. Enzymatic treatments for XOs production and produce toxic compounds (Fig. 3), that is why enzymatic hydrolysis
is so relevant; also, the use of enzymes means much more specific and
Enzymatic treatments in the production of XOs have gained rele­ easy to control processes, especially because it can be carried out in mild
vance within the scientific community (Amorim et al., 2019). However, conditions, so they do not require special equipment, in addition to
the costs of the enzyme and reaction times raise the value of the process, being considered an environmentally friendly treatment (Acosta-­
prevent the formation of undesirable compounds, such as furfural or Fernández et al., 2020; Antov and Đorđević, 2017).
phenolic acids, such as ferulic and p-coumaric, which are produced by Furthermore, there are efforts regarding the work with enzymes for
the esterification of oligomers such as arabinose, from the acid hydro­ the continuous production of XOs. Such is the case of the work reported
lysis of hemicellulose (Goldbeck et al., 2016; Huang et al., 2020; Poletto by Acosta-Fernández et al. (2020) who experimented with the produc­
et al., 2020). tion of XOs from coffee parchment xylan. This biopolymer was first
Hemicellulolytic enzymes used in these processes are found as extracted from by autohydrolysis (LHW) with a 3.75 L reactor with
enzymatic cocktails. These cocktails are generally composed of endo- operating conditions of 180 ◦ C in a non-isothermal regime and a
1,4-β-xylanases (which divide the β-1,4-xylose bonds inside the heter­ water-biomass ratio of water 8:1 (w/w). The liquid fraction was subse­
oxylan skeleton, E.C.3.2.1.8) and β-xylosidase (which is responsible for quently filtered and concentrated on a rotary evaporator, then the
breaking the bonds between xylobioses releasing monomeric xylose, E. sample was frozen and finally lyophilized. Enzymatic hydrolysis was
C.3.2.1.37). Besides, these enzymatic cocktails can contain accessory performed with an endo-1,4-β-xylanase enzyme in four different con­
enzymes, which can be hydrolases or esterases, which act on the side figurations, two of them in 1 tank reactor stirred with the free enzyme
chains of xylan or in the leading chains of different types of xylan; and immobilized by magnetic nanoparticles and similarly in a mem­
Finally, to improve the production of XOs, it is necessary to take into brane bioreactor with the enzyme-free and immobilized in the same
account two considerations. The first one is that the activity of β-xylo­ way. The enzyme concentration was 0.11 mg/mL, and the substrate
sidase must be low to release the least amount of monomers possible. varied between 1, 5, and 10 mg/mL, and 40 ◦ C of temperature. Finally, it
Consequently, the endo-xylanases must release XOs with a small poly­ was confirmed that the membrane reactor with the free enzyme had a
merization degree, making the selection of these the susceptibility of the higher performance in the continuous output of oligomers with a DP =
substrate crucial parameters (Michelin et al., 2017: Acosta-Fernández 1–20 with 89.6 % of conversion, and an operational time of 2 h.
et al., 2020; Poletto et al., 2020).
Xylanases are widely used in different industries, technological ap­ 6.5.1. Immobilized xylanase systems XOs production
plications, creation of biofuels, improving food quality, wood pulp To achieve the industrial implementation of enzymatic systems, it is
bleaching, in the baking industry, in the textile industry, as an additive necessary to implement various operational strategies, such as searching
in detergents, and in the clarification of fruit—juice with a combination for more stable reactors or a continuous synthesis of chemical products.
of cellulase and pectinase. Xylanases are produced from fungi and bac­ One of the most notable efforts in this regard is enzyme immobilization,
terial species, such as Aspergillus spp, Bacillus spp, Trichoderma spp, a key technology for driving biocatalysis and flow chemistry. Immobi­
Streptomyces spp, and Bacillus spp (Collins et al., 2005; Uday et al., lization methods are based on making heterogeneous reactors capable of
2016). Also, the production of hemicellulases enzymes (xylanase and maximizing activity and stability to produce a catalytic enhancement of
β-xylosidase) from XOs as carbon source has been studied. Michelin et al. enzymes because the support where the enzyme is attached promotes
(2012) produced xylanase and β-xylosidase from hydrothermal pre­ greater efficiency in terms of surface phenomena, creating better reactor
treatment (200 ◦ C/30 min) liquor (XOs) of corncob as carbon source and flow mechanisms (Aggarwal et al., 2020; Shi et al., 2012). For those
Aspergillus ochraceus as microorganism, obtaining 605 total U of xyla­ reasons, immobilized enzymes were considered one of the most prom­
nase and 56 total U of β-xylosidase. In a recent study, Michelin et al. ising methods for the catalysis of lignocellulosic biomass and algal
(2018) studied the biorefinery integration in the production of xylanases biomass (Liu et al., 2020).
using XOs (10.6 g/L) as substrate. They reported the production and Traditional enzyme immobilization technology is achieved by the
activity of xylanases 10 IU/ mL. random confinement or localization of the enzyme in a support material
The xylan present in the cell wall have great structural diversity; for through physical or chemical interaction such as hydrophobic, van der
this reason, there is a great diversity of enzymes created by organisms Waals forces, affinity bonding, hydrogen bonding, ionic bonding with
and microorganisms for their degradation (Collins et al., 2005; Srivas­ retention of its catalytic activities (Ariaeenejad et al., 2020). This can
tava et al., 2020). The enzymes responsible for the hydrolysis of xylan inhibit the enzymatic activity because the carrier material is often not
belong to the glycosyl hydrolases (GH) family (Poletto et al., 2020). To specific, and it does not promote selective action on the substrate (Liu
date, the carbohydrate-active enzymes (CAZy) database presents 168 et al., 2020). For systems with enzymes and polysaccharides as sub­
families (http://www.cazy.org). GH xylanases are also grouped into strates, the traditional immobilization method is not very efficient. At
clans, which are 5, 7, 8, 9, 10, 11, 12, 16, 26, 30, 43, 44, 51 and 62. present, the materials that fix the enzymes must create a system with
These clans are divided into two types. The clans 16, 51, and 62 are sufficient active sites and pore size to guarantee efficient flow mechanics
bifunctional enzymes since they contain two catalysts. This means that (Ariaeenejad et al., 2020; Liu et al., 2020). One of the most used ma­
clans 5, 7, 8, 10, 11, and 43 have a truly different catalyst domain with terials for this task is the magnetic nanoparticles of Fe3O4, porous ma­
endo-1,4-β-xylanase activity (Srivastava et al., 2020; Uday et al., 2016). terials, and treated wool (Liu et al., 2020).
The xylanases used in the food industry are produced by GRAS An example related to this topic was reported by Shivudu et al.

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Fig. 3. Comparison between enzymatic and acid production of XOs with a low degree of polymerization, adapted and modified from Poletto et al. (2020) and Zanuso
et al. (2017).

(2020), who used the enzyme endo-1, 4-β-D-xylanase (XynC) of B. showed that the immobilized systems had greater recovered activity.
subtilis KCX006 and a nanometric material composed of carbon, silica, Regarding pH, both systems had an optimal activity with a pH of 6.6.
and zirconia to create a XOs production system. The recovered activity However, the immobilized endoxylanase system had a broad optimum
of the endoxylanase immobilized in the nanometric material was in the temperature range (50–65 ◦ C) for catalytic activity. The endoxylanase
range of 52 a 92 %. A comparison was also made between the free/­ immobilized in nanoparticles did not lose activity after five consecutive
soluble endoxylanase system and the immobilized system, which batches. Finally, the results show that endoxylanase immobilized in

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C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

Fig. 4. General diagram of a second-generation biorefinery with continuous production of XOs, adapted and modified from Ruiz et al. (2020) and Pérez-Pimienta
et al. (2020).

nanoparticulate carbon, silica, and zirconia matrices is useful for the et al., 2020). Finally, these factors change the enzyme activity
production of XOs. throughout the reaction and making immobilized enzyme systems have
Even though in the present, the processes use enzymes in free-form. more promising technological applications (Fopase et al., 2020).
Enzymes in this form are more susceptible to salts, surfactants, alkalis, The Table 2 shows the selection of pretreatment at different LCM
trace levels of inhibitory substances, which causes a clear advantage in with the combination of enzymatic processing which results in a wide
the use of immobilized systems over free-form enzyme systems (Fopase variety of XOs with different depolymerization of degrees.

Table 2
Pretreatment fractionation and production of XOs.
Type of biomass Biomass Treatment or pretreatment Enzymes used Hemicellulose XOs yield XOs profile Reference
employed Conversion reported mg
(XOs)/g
(Xylan)

Agroindustrial Wheat LHW with 180 ◦ C for 40 min Endo-β-1-4-xylanase 33% 229 54.4 % XOs (Huang et al.,
wastes straw (DP=2) 20.2 % 2017)
XOs (DP=3)
12% XOs (DP=1)
0.2 % of Phosphoric acid at 180
55.35 195 40% XOs (DP=2)
◦
C, for 5min
48% XOs (DP=3)
A mixture of recombinant
Sugar cane 4% XOs (DP=1) (Goldbeck
endo-xylanase and feruloyl
bagasse Glacial acetic acid 8.74 M and 10% XOs (DP=2) et al., 2016)
esterase
Hydrogen peroxide 2.6 M, during 21.36 356 16% XOs (DP=3)
7h at 60 ◦ C >50% XOs
(DP>3)
42.24 % XOs
Ionic liquid 1-ethyl-3-methylimi­
(DP=1) 15.33 %
dazolium acetate ([Emim][Ac]),
Sugarcane α-L-arabinofuranosidase XOs (DP=2) (Ávila et al.,
purchased from Iolitec (Germany) 48.64% 338
bagasse and endo-1,4-xylanase 19.15 % XOs 2020b)
(98% purity), at 100 ◦ C for
(DP=3) 14.16 %
30 min.
XOs (DP>3)
LHW, 190 ◦ C for 13 min with 1.8 Two endo-β-1,4-xylanases XOs (DP=3 - (Arai et al.,
Corncob 50% 494
Mpa (GH 10, 11) DP=5) 2019)
Hydrothermal assisted alkaline 83.7 % of XOs
Pineapple Endo- 1, 4-β-Xylanase M1 (Banerjee
extraction with NaOH 15 % (w/ 41.4% 257 (DP=2) 16.0%
peel (Megazyme) et al., 2019)
v), 121 ◦ C for 1.5 h XOS (DP=3)
Endo-xylanase from
Almond LHW, 200 ◦ C for 15 min with 18- XOs (DP=2, (Singh et al.,
Thermomyces lanuginosus 54.0% 545
Shell 20 kg/cm2 DP=3) 2019c)
(expressed in A oryzae)
< 10 % XOs
Hard wood Mahogany 52.70% 572 (DP>3) 50-55 %
Mild Sodium hydroxide (0.05 N) Native xylanase from XOs (DP =2) (Rajagopalan
at 121 ◦ C for 15 min Clostridium strain BOH3 < 1 % XOs (DP et al., 2017)
Mango tree 43.40% 504 =1) 35-40 % XOs
(DP=2)

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C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

6.6. Separation and purification XOs in the context of biorefinery hypertension and diabetes, tested in vitro. The addition of XOs showed
higher antioxidant activity and a notable increase in the viscosity of the
Chen et al. (2016) performed a study that XOs produced using the final product, and this is because of the action of XOs capturing water.
autohydrolysis of Miscanthus × Giganteus. A first separation was made Despite that, there was a good acceptance of the product in sensory
using activated carbon from the hydrolysate of the pretreatment, using analysis, proving that XOs are an alternative for the enrichment product.
the liquors and activated carbon 10 % (w/v) at 60 ◦ C with manual Finally, Amorim et al. (2019) reported that in North America, the
stirring − 15 min, then filtered powder with high XOs content using a FDA catalogs the XOs as GRAS has two notifications about the use of this
vacuum pump. The enriched carbon cake was washed with deionized ingredient for adult food GRN 458 (FDA, 2013) and GRN 343 (FDA,
water, and then eluted using ethanol 95 % (v/v). Then, the ethanol was 2010), the latter for AXOs obtained from wheat bran.
evaporated and the recovery XOs calculated. Finally, three connected
ion exchange columns were used to purify XOs, obtaining a product with 7.1.1. Antioxidant effect of XOs
a concentration of 89.1 % (w/w). In last years, have a great interest to find biocompounds with anti­
Choi et al. (2016) reported using simulated moving bed continuous oxidant activity to substitute the synthetic compound that has presented
method for separation of XOs, which consists of multiple absorbent evidences of negative health effects (Zhang et al., 2019). The antioxidant
columns with Dowex-50WX4 resin, connected in series at 65 ◦ C, using activity of XOS was associated to the presence of ester-linked hydrox­
distilled deionized water as a desorbent. In this study, the separation of a ycinnamic acid derivates as caffeic, ferulic, syringic and coumaric acid
solution of XOs with a concentration of 103 g/L was carried out. The residues and methyl glucuronic acid ramifications on the xylan chain.
principal objective was to purify the xylobiose, and this solution con­ Also, the presence of some sugars with acetyl and uronyl groups have a
tained: 34.3 g/L of xylobiose, 29.65 g/L of xylotriose, 18.91 g/L of positive effect in antioxidant activity (Ávila et al., 2020b; Poletto et al.,
xylotetraose, 18.99 g/L of xylopentose and xylohexose, and lower con­ 2020).
centrations of glucose, xylose, and arabinose. Finally, the xylobiose re­ A recent studied showed that XOS produced by enzymatic hydrolysis
covery was 90 %. of glucuronoxylan with a family GH30 xylanase (Xyn30D) produces acid
Singh et al. (2019c) reported a study of low polymerization XOs from XOS with higher antioxidant activity when compare with XOS produced
almond shells. The liquor with a high content of XOs came from a pre­ by xylanase of family GH10 (Xyn10A), a mixture of acid and neutral
treatment by autohydrolysis at 200 ◦ C for 10 min and enzymatic hy­ XOS. However, XOS produced by autohydrolysis showed higher anti­
drolysis using an endo-xylanase from Thermomyces lanuginosus. The XOs oxidant activity than XOS produced enzymatically, probably due the
separation process was performed with molecular weight cut-off mem­ presence of some amount of lignin and the presence of long XOS. The
branes of 1 kDa and 250 Da (47 mm). Finally, it was possible to get 69.1 length of XOS was correlated with antioxidant activity, a very long XOS
% (w/w) of produced xylooligosaccharides with those membranes. have a high antioxidant activity (Valls et al., 2018). Bouiche et al. (2020)
showed the antioxidant activity differences between glucuronox­
7. An overview of industrial production and applications of XOs ylooligosaccharides (UXOS) and arabinoxylooligosaccharides (AXOS)
as functional ingredient generated by enzymatic hydrolysis, showing the more than 80 % of in­
hibition to UXOS and more than 30 % to AXOS. This difference can be
7.1. Advances in the use of XOs for human food explained by the negative charge of methylglucuronic acid decorations
of UXOS that was suggested the negative charge increase the antioxidant
Nowadays, there exist several industries that produce XOs for con­ activity (Valls et al., 2018). The XOS have an important antioxidant
sumption, for example, Longlive, Kangwei, HFsugar, Henan, Shengtai, activity, from 70 to 92 % of inhibition (Bian et al., 2013; Bouiche et al.,
YIBIN YATAI, HBTX, YuHua and ShunTian are the most important 2020; Saleh et al., 2020) and other bioactivities or proprieties, which
producers worldwide. One of the products available is PreticX®, a non- generates a great interest in producing and applying XOS in different
GMO corncob-derived XOs prebiotic from AIDP Inc., which is manu­ food applications to generate bioactive and nutritional health foods.
factured by LongLive, and it has a GRAS status given by the FDA, con­
sumers can purchase prebiotic supplement on nutritional websites for ≈ 7.2. Advances industrial scale biorefineries and XOs production
12.5–22 € per 100 g of product (Michelin and Teixeira, 2020).
Due to the interest in the uses of XOs, the scientific community has In Denmark there is a biorefinery demo-plant. Its process includes
made efforts towards the use of this novel ingredient for its bioactive hydrothermal pretreatment, the use of enzymatic hydrolysis, and a
properties, flavor, and physicochemical properties to apply it in various mixed fermentation of pentoses and hexoses using advanced yeast. This
human food sources (Cao et al., 2020). demo-process uses wheat straw as a raw material; its contributions of
Ayyappan et al. (2016) studied the evaluation of the enrichment of energy are 1.8 million gallons of ethanol and 13,000 tons of biopellets
cookies added with XOs. It was necessary to evaluate the change in the per year (Hossain et al., 2017; Rasmussen et al., 2017)
physicochemical parameters, the nutritional value, and the general Also, in the city of Alagoas in Brazil, there is a second-generation
characteristics of cookies: weight, height, diameter, texture, humidity, biorefinery that uses Sugarcane straw and bagasse, the production of
and properties of storage, XOs retention capacity of cookies, and pre­ this company is 21.6 million gallons of ethanol from per year. This
biotic activity. Commercial XOs and local ingredients were used to company combines pretreatment, enzymatic hydrolysis, and fermenta­
produce the cookies replacing 5 %, 10 %, and 15 % (w/w) of the sugar tion technologies to create biofuel (Grassi and Pereira, 2019; Hossain
with XOs. The cookie of control cookie’s initial dietary fiber was 3.13 et al., 2017).
g/100 g. This variable was increased to 4.2, 4.9, and 7.0 g/100 g, In the United States, specifically in Kansas City, there was an industry
respectively, by the addition of XOs. Another change was in color, hu­ process that used wheat straw and agricultural wastes. The process of
midity, and texture; the addition of the XOs caused darker colors and this plant involves a chemical pretreatment, enzymatic hydrolysis to
softer cookies, being cookies with 5% (w/w) sucrose substitution the convert C5 and C6 in fermentable sugars, and then die fermentation to
most comparable to the control ones. Finally, the storage period was 21 ethanol and distillation. This commercial scale process produced 25
days, and the enriched cookies retained 74 % (w/w) of the XOs millions of ethanol gallons and 18 MW of power per year (Hossain et al.,
incorporated. 2017; Soccol et al., 2019).
Souza et al. (2019) reported a study related to the use of XO in
strawberry-flavored whey drinks. It was found that the addition of XOs 7.3. Research trend in the production of XOs
have a positive effect on the activity of the whey since it presented a
higher capacity of inhibition to the enzymes, which are related to The development of continuous processes is undoubtedly a necessary

12
C.D. Pinales-Márquez et al. Industrial Crops & Products 162 (2021) 113274

part to be developed at present since it has been reported that contin­ untreated biomass. For the liquid phase produced in this continuous
uous processing is a technique compatible with the intensification of processing method, it was found that the XOs were the main components
sustainable methods (Rogers and Jensen, 2019). In the bioprocessing of the liquors of the three residence times evaluated, with values that
area, the model proposed by continuous processing promotes the prin­ ranged from 9.5–38.1 g/kg of biomass.
ciples of green chemistry (Romero-Fernández et al., 2018). Neverthe­ Finally, the Fig. 4 shows the scheme of process and operations to
less, the majority of experiments reported have been in laboratory and perform the conversion of lignocellulosic material to ethanol, but also
bench scales (1− 10 g/h), overall, in batch mode. The development of integrates the separation of solid and liquid streams after pretreatment
continuous processing and productivity is now the focus to rise the with the help of a centrifuge that would manage to separate both
processes to industrial scales (Pérez-Pimienta et al., 2020). Finally, the continuous streams, after two continuous stirred tank reactor (CSTR),
following information is to show some of the advances in the develop­ the first of them will continue the conversion of the solid phase for the
ment of continuous processing in second-generation biorefineries. conversion of bioethanol with the solid phase and the second reactor
will perform the conversion of high molecular weight XOs to polymer­
7.3.1. Continuous processes implementation for the development of ization grade XOs (DP = 2–5), to prioritize those who have an applica­
biorefineries in the production of XOs tion to food due to its novel prebiotic and antioxidant properties
Bhatia et al. (2020) studied pilot-scale XOs production, using Mis­
canthus as raw material and steam explosion as a pretreatment; the 8. General perspectives and conclusions
parameters were 200 ◦ C at 15 bars for 10 min. With those conditions,
obtaining XOs yields over 52 % (w/w of initial xylan) in the hydrolysate The use of XOs from lignocellulosic biomass in the food industry is a
fraction. The authors found hydroxycinnamate residues in the liquor; potential for nutritional improvement as a prebiotic source. In this sense,
these substances are known due to their physicochemical properties agro-industrial waste represents an ideal source for obtaining XOs. In
known for their high prebiotic effects and antioxidant activity in nu­ addition to implying an economic benefit to the process, its use repre­
traceutical foods. Finally, the enzymatic hydrolysis of XOs-rich hydro­ sents an advantage due to the low cost and availability of these raw
lysate using commercial endo-xylanases resulted in the obtention of materials. However, the complex and recalcitrant chemical structure of
380–500 g/kg of initial xylan. this biomass remains a challenge in processing. The development of
Rodríguez et al. (2019) reported other advances made in the search biomass fractionation technologies should be focused on seeking more
for continuous processes. This work describes the use of a stainless-steel sustainable processes, easy to operate and preferably continuous pro­
tubular reactor for pilot scale pretreatment. The reactor, which had an cesses in terms of circular bioeconomy. Hydrothermal processes are the
extruder screw that propelled wet biomass and steam from the inlet most promising pretreatments at pilot and industrial level for the pro­
system to the reactor’s end. After, the pretreatment matter is released duction of XOs, therefore more research and scaling strategies are
abruptly, like what would happen in a steam explosion system. Finally, necessary, as well as new forms of heating sources in the pretreatment to
using wheat straw, the highest production of xylooligosaccharides reduce costs in the process.
(approximately 70 %) was achieved without degradation at 37 min of This review analyzed the most important trends in the conversion of
residence time. lignocellulosic biomass into high added value compounds as XOs, and
Romero-Fernández et al. (2018) developed a work focused on the therefore we can conclude that selection of the pretreatment for biomass
design of continuous flow xylan hydrolysis reaction for the production of fractionation in the production of XOs it is very important to find
XOs based on the use of packed bed reactors using xylanase immobilized different applications in areas as food, energy and materials. On the
on supports based on methacrylic polymers. The experiment consisted of other hand, the trend in the development of new applications of the XOs
developing and optimizing a packed bed reactor (PBR) for the produc­ will continue to expand, and in this case in the food area it is the most
tion of XOs. Three different supports were made of three methacrylic promising for the next few years, and the of XOs with a low degree of
polymers, which were activated with glyoxal groups, were morpholog­ polymerization prove to be the most important for this area of appli­
ically characterized and selected for multipoint covalent immobilization cation due to its prebiotic and antioxidant activity.
of xylanase (Xys1Δ). In choosing the polymer, its physical and me­
chanical properties, thermal stability, and maximum protein load were
considered. The best polymer was chosen to be attached with the Declaration of Competing Interest
enzyme to produce XOs with xylan extracted from and corncob. The
reaction was carried at in an 8 mL FPLC column reactor. Finally, the The authors declare that they do not have any competing financial
operation of the reactor was maintained, conserving >90 % of its initial interests or personal relationships that could have appeared to influence
activity for 120 h in a continuous process with the specific productivity the work reported in this paper.
for XOs at 10 mL/min and a flow rate of 3277 g(XOs)/(g(Enzyme)*h).
Pérez-Pimienta et al. (2020) used a pilot-scale continuous tubular
Acknowledgments
reactor (PCTR) to treat four different lignocellulosic materials: wheat
straw, agave bagasse, corn stover, and sugarcane bagasse, with three
This project was funded by the Secretary of Public Education of
residence times (20, 35, and 54 min). The main objective was to study
Mexico - Mexican Science and Technology Council (SEP-CONACYT)
and improve the simultaneous saccharification and fermentation (SSF)
with the Basic Science Project-2015-01 (Ref. 254808). César D. Pinales-
at high solid loading (20 %) to produce bioethanol. The process, in
Márquez also thanks the National Council for Science and Technology
general, consisted of different stages: A hopper-shaped feeder with a first
(CONACYT, Mexico) for his Master Fellowship support (grant number:
screw to compress the biomass into a densified plug to feed the pre­
1001882).
treatment reactor, with a steam injection that heated the biomass to 180
◦
C and a conveyor screw driven by a 1 HP variable speed motor, due to
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