**3. New biomaterials**

Cellulose is the most abundant polymer in the world. It is a linear polymer of β-d-glucose molecules linked by β(1 → 4) bonds. Due to this bond, each molecule has the ability to rotate 180° with regard to the previous one, forming long linear chains that are stabilized by the presence of hydrogen bonds and join chains to others. The cellulose micelle is made up approximately from 60 to 70 cellulose chains, and the union of 20 or 30 cellulose micelles achieves a semicrystalline packing and the formation of microfibrils. However, the morphology, size, and other characteristics depend on the cellulose origin, and according to the above, cellulose microfibrils (MFC)/nanofibrils (NFC), cellulose nanocrystals (CNCs), and bacterial nanocellulose (BNC) can be obtained [78].

### **3.1 Micro/nanofibrillated cellulose (MFC/NFC)**

Microfibrillated cellulose (MFC) is obtained with the longitudinally disintegration of cellulose fibers by multiple mechanical shearing actions; in this way, a three-dimensional network of cellulose microfibrils (10–100 nm) is achieved, which has a higher surface area than conventional cellulose fibers. Due to its structure, MFC has the ability to form gels. Different mechanical treatment procedures have been reported to obtain MFC (high-pressure homogenization and grinding for example) and various pretreatments to facilitate the mechanical treatment (enzymatic, acid hydrolysis, mechanical cutting pretreatments, etc.) [79]. The mechanical properties of MFCs are higher compared to lignocellulosic fibers because they have a more homogeneous structure. The main application of MFCs is in the packaging industry due to its excellent mechanical and barrier properties, which are required in this sector [80]. Adel et al. [81] obtained micro/ nanofibrillated cellulose from lignocellulosic residues (rice straw, sugarcane bagasse, cotton stalk) and botnia softwood Kraft pulp. First, the lignocellulosic residues were subjected to an alkaline pretreatment to eliminate the lignin, and later, the mechanical treatment was applied to them using a mill. According to their results, the crystallinity index of MFC increased and the length of the fibers that correspond to lignocellulosic residues decreased compared to the fibers of the pulp. And they concluded that the MFC obtained have optimal mechanical and optical properties; therefore, they can be used as reinforcement in the paper-making industry. Nanofibrillated cellulose (NFC) is obtained by delamination of wood pulp (wood, sugar beet, potato tuber, hemp, flax, etc.) by mechanical pressure before and/or after chemical enzymatic treatment with a diameter between 5 and 60 nm and its length in several micrometers. It exhibits amorphous and crystalline domains and high specific surface area. Nanofibrillated cellulose (NFC)/polyvinyl alcohol (PVA) nanocomposites are prepared by dispersion of nanofibers obtained from several biomass sources, normally at low contents (1–10%), into PVA aqueous solutions typically followed by solvent casting. Frone et al. [82] also used cellulose nanofibers obtained from microcrystalline cellulose by ultrasonic treatment as reinforcement (at lower 1–5 wt%) dispersed in PVA. In summary, these materials exhibit a high aspect ratio and specific surface area, excellent flexibility and strength, low thermal expansion, high optical transparency, and barrier properties. Consequently, they can be used to form strong transparent films and aerogels, as a rheology modifier and strength additive in the paper-making industry, like a constituent of food packaging and in different biomedical applications (drug delivery) [79].

#### **3.2 Bacterial cellulose (BC)**

Bacterial cellulose is produced by bacteria such as *Acetobacter xylinus* or *Gluconacetobacter xylinus* [78]*.* Its structure is similar to the original cellulose but

**45**

*Getting Environmentally Friendly and High Added-Value Products from Lignocellulosic Waste*

with an ultrafine three-dimensional network of nanofibers with an average diameter 100 times thinner than that of common plant fibers [79]. BC has high water retention due to the fact of being very hydrophilic and having high crystallinity, is relatively inexpensive to produce, and is widely used in biomedical applications (carriers for drug delivery, artificial skin and blood vessels, tissue engineering, etc.); hence, it promotes physical interaction with microorganisms and other active compounds because of its high porosity and surface area [83]. Azeredo et al. [84] explored the possibility of using BC as a raw material in the food and packaging industry applications, and they concluded that the use of this material is increasing and therefore its production cost is decreasing. However, research in this area continues to develop.

Cellulose nanocrystals are obtained by enzymatic hydrolysis and have the following characteristics: elongated, less flexible, cylindrical, and rod-like nanoparticles with 4–70 nm in width, 100–6000 nm in length, and 54–88% crystallinity index [85]. Gopi et al. [86] used hydrochloric acid to carry out the hydrolysis of cellulose and obtained an improvement in the thermal stability of the CNCs but with a significant agglomeration of the crystals. Park et al. [87] demonstrated a facile and green method of CNC extraction that uses only an high-pressure homogenization (HPH). The obtained CNCs presented rod-like shapes with a size distribution of 4–14 nm for width and 60–20 nm for length. Nanocrystalline cellulose (CNC) was dispersed in an alginate matrix for film application by Huq et al. [88]. They observed that with a small amount of CNC (approximately 5% wt), the mechanical and barrier properties of the films made were improved by comparing with an alginate film. According to the results obtained by infrared spectroscopy (FTIR), they concluded that there was a molecular interaction between the CNC and the alginate through hydrogen bonds. In summary, the morphology and size of cellulose nanocrystals vary according to the kind of lignocellulosic biomass, extraction method, and manufacturing conditions. Nanocellulosic materials can be characterized by employing a variety of techniques [89]: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), helium pycnometer, differential scanning calorimetry (DSC), thermogravimetric analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and atomic force (AFM) among others. On the other hand, cellulose nanocrystals not only consist of primary reactive sites (i.e., hydroxyl groups) but also they possess higher surface area to volume ratio, making CNC highly reactive and easy to be functionalized. The most common surface modifications of CNCs are sulfonation, TEMPO-mediated oxidation, esterification, etherification, silylation, urethanization, amidation, polymer grafting, etc. The applications having the greatest potential due to the high available amount of volume on cellulose nanomaterials are placed in the following industries: automotive (body components, interiors), construction (air and water filtration, insulation, and soundproofing), packaging (fiber/plastic replacement, filler, coating, film), paper (filler, coatings), personal care (cosmetics), textiles (clothing), aerogels, aerospace (structural, interiors), industrial (viscosity modifiers, water purification), paint, sensors (medical, environmental,

*DOI: http://dx.doi.org/10.5772/intechopen.93645*

**3.3 Cellulose nanocrystals (CNCs)**

and industrial), electronics, photonic structures, etc. [90].

**4. Recovery of chemical compounds of industrial interest**

Diverse processes can be used to release lignin as the main product for the revaluation of different biomasses with high-value applications. Each process uses respective chemical agents to extract and obtain different materials from lignocellulosic biomass and produces other materials with different compositions and

*Getting Environmentally Friendly and High Added-Value Products from Lignocellulosic Waste DOI: http://dx.doi.org/10.5772/intechopen.93645*

with an ultrafine three-dimensional network of nanofibers with an average diameter 100 times thinner than that of common plant fibers [79]. BC has high water retention due to the fact of being very hydrophilic and having high crystallinity, is relatively inexpensive to produce, and is widely used in biomedical applications (carriers for drug delivery, artificial skin and blood vessels, tissue engineering, etc.); hence, it promotes physical interaction with microorganisms and other active compounds because of its high porosity and surface area [83]. Azeredo et al. [84] explored the possibility of using BC as a raw material in the food and packaging industry applications, and they concluded that the use of this material is increasing and therefore its production cost is decreasing. However, research in this area continues to develop.

#### **3.3 Cellulose nanocrystals (CNCs)**

*Biotechnological Applications of Biomass*

rial nanocellulose (BNC) can be obtained [78].

**3.1 Micro/nanofibrillated cellulose (MFC/NFC)**

and in different biomedical applications (drug delivery) [79].

Bacterial cellulose is produced by bacteria such as *Acetobacter xylinus* or *Gluconacetobacter xylinus* [78]*.* Its structure is similar to the original cellulose but

**3.2 Bacterial cellulose (BC)**

Cellulose is the most abundant polymer in the world. It is a linear polymer of β-d-glucose molecules linked by β(1 → 4) bonds. Due to this bond, each molecule has the ability to rotate 180° with regard to the previous one, forming long linear chains that are stabilized by the presence of hydrogen bonds and join chains to others. The cellulose micelle is made up approximately from 60 to 70 cellulose chains, and the union of 20 or 30 cellulose micelles achieves a semicrystalline packing and the formation of microfibrils. However, the morphology, size, and other characteristics depend on the cellulose origin, and according to the above, cellulose microfibrils (MFC)/nanofibrils (NFC), cellulose nanocrystals (CNCs), and bacte-

Microfibrillated cellulose (MFC) is obtained with the longitudinally disintegration of cellulose fibers by multiple mechanical shearing actions; in this way, a three-dimensional network of cellulose microfibrils (10–100 nm) is achieved, which has a higher surface area than conventional cellulose fibers. Due to its structure, MFC has the ability to form gels. Different mechanical treatment procedures have been reported to obtain MFC (high-pressure homogenization and grinding for example) and various pretreatments to facilitate the mechanical treatment (enzymatic, acid hydrolysis, mechanical cutting pretreatments, etc.) [79]. The mechanical properties of MFCs are higher compared to lignocellulosic fibers because they have a more homogeneous structure. The main application of MFCs is in the packaging industry due to its excellent mechanical and barrier properties, which are required in this sector [80]. Adel et al. [81] obtained micro/ nanofibrillated cellulose from lignocellulosic residues (rice straw, sugarcane bagasse, cotton stalk) and botnia softwood Kraft pulp. First, the lignocellulosic residues were subjected to an alkaline pretreatment to eliminate the lignin, and later, the mechanical treatment was applied to them using a mill. According to their results, the crystallinity index of MFC increased and the length of the fibers that correspond to lignocellulosic residues decreased compared to the fibers of the pulp. And they concluded that the MFC obtained have optimal mechanical and optical properties; therefore, they can be used as reinforcement in the paper-making industry. Nanofibrillated cellulose (NFC) is obtained by delamination of wood pulp (wood, sugar beet, potato tuber, hemp, flax, etc.) by mechanical pressure before and/or after chemical enzymatic treatment with a diameter between 5 and 60 nm and its length in several micrometers. It exhibits amorphous and crystalline domains and high specific surface area. Nanofibrillated cellulose (NFC)/polyvinyl alcohol (PVA) nanocomposites are prepared by dispersion of nanofibers obtained from several biomass sources, normally at low contents (1–10%), into PVA aqueous solutions typically followed by solvent casting. Frone et al. [82] also used cellulose nanofibers obtained from microcrystalline cellulose by ultrasonic treatment as reinforcement (at lower 1–5 wt%) dispersed in PVA. In summary, these materials exhibit a high aspect ratio and specific surface area, excellent flexibility and strength, low thermal expansion, high optical transparency, and barrier properties. Consequently, they can be used to form strong transparent films and aerogels, as a rheology modifier and strength additive in the paper-making industry, like a constituent of food packaging

**3. New biomaterials**

**44**

Cellulose nanocrystals are obtained by enzymatic hydrolysis and have the following characteristics: elongated, less flexible, cylindrical, and rod-like nanoparticles with 4–70 nm in width, 100–6000 nm in length, and 54–88% crystallinity index [85]. Gopi et al. [86] used hydrochloric acid to carry out the hydrolysis of cellulose and obtained an improvement in the thermal stability of the CNCs but with a significant agglomeration of the crystals. Park et al. [87] demonstrated a facile and green method of CNC extraction that uses only an high-pressure homogenization (HPH). The obtained CNCs presented rod-like shapes with a size distribution of 4–14 nm for width and 60–20 nm for length. Nanocrystalline cellulose (CNC) was dispersed in an alginate matrix for film application by Huq et al. [88]. They observed that with a small amount of CNC (approximately 5% wt), the mechanical and barrier properties of the films made were improved by comparing with an alginate film. According to the results obtained by infrared spectroscopy (FTIR), they concluded that there was a molecular interaction between the CNC and the alginate through hydrogen bonds. In summary, the morphology and size of cellulose nanocrystals vary according to the kind of lignocellulosic biomass, extraction method, and manufacturing conditions. Nanocellulosic materials can be characterized by employing a variety of techniques [89]: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), helium pycnometer, differential scanning calorimetry (DSC), thermogravimetric analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and atomic force (AFM) among others. On the other hand, cellulose nanocrystals not only consist of primary reactive sites (i.e., hydroxyl groups) but also they possess higher surface area to volume ratio, making CNC highly reactive and easy to be functionalized. The most common surface modifications of CNCs are sulfonation, TEMPO-mediated oxidation, esterification, etherification, silylation, urethanization, amidation, polymer grafting, etc. The applications having the greatest potential due to the high available amount of volume on cellulose nanomaterials are placed in the following industries: automotive (body components, interiors), construction (air and water filtration, insulation, and soundproofing), packaging (fiber/plastic replacement, filler, coating, film), paper (filler, coatings), personal care (cosmetics), textiles (clothing), aerogels, aerospace (structural, interiors), industrial (viscosity modifiers, water purification), paint, sensors (medical, environmental, and industrial), electronics, photonic structures, etc. [90].

#### **4. Recovery of chemical compounds of industrial interest**

Diverse processes can be used to release lignin as the main product for the revaluation of different biomasses with high-value applications. Each process uses respective chemical agents to extract and obtain different materials from lignocellulosic biomass and produces other materials with different compositions and

properties. There are distinct chemical processes of biomass hydrolysis, which use acids, bases, or enzymatic hydrolysis and others (other processes can be used, but their description would come out of the focus of this chapter) whose choice mainly depends on the material structure and characteristics desired for the products to be recovered. However, various sources of lignocellulosic materials need to be considered separately since they have different compositions of cellulose, hemicellulose, and lignin. Against all odds, the depolymerization process of the lignocellulosic biomass is a common goal for all different feedstocks for the production of all types of chemicals [91]. In particular, polyphenolic acids are a group of chemical compounds that are widely distributed in plant biomasses. Those compounds are important antioxidants that efficiently interact with biomolecules such as DNA, RNA, lipids, proteins, enzymes, and other cellular molecules to produce desired results. Due to the benefic effects, that can be useful for preventing the oxidation in foods, and therapeutic human disorders [92], all of them can be used with potential applications in the pharmacy, food, cosmetic, and nutraceutical industries.

### **4.1 Chemicals derived from alkaline-based methods**

Alkaline pretreatment is one of the most intensively studied technologies for biomass delignification [93], and the application of alkaline liquid with NaOH into the bagasse to obtain a black liquor that contains value-added chemicals has been investigated. This procedure is useful for the releasing of chemical compounds in different biomasses; particularly, this method has been commonly used for the processing of the switchgrass (*Panicum virgatum*), corn stover (*Zea mays*), and forestry biomasses. Due to their abundance and availability, the use of the process can produce a different number of high-value fine chemicals such as sugars, vanillin, isoeugenol, guaiacylpropanol, guaiacylethanol, ferulic, p-coumaric, and syringic acids [94, 95]. For example, different woody species of *Quercus* and *Robinia* were subjected to alkaline hydrolysis, and liquors were analyzed by GC-MS. The authors recovered and identified specific bioactive phenols for each woody species such as gallic acid, coniferyl alcohol, vanillic acid, syringaldehyde, and traces of epicatechin and catechin [96]. For the optimum alkali treatment concentration in sweet sorghum bagasse, different types of phenolic species were determined with the use of alkali treatments between concentrations of 3.0 and 6.0 M NaOH, resulting in high concentrations of phenol, 4-ethylphenol, and guaiacol [97].

#### **4.2 Chemicals derived from acid-based methods**

The acidic pretreatment is a contemporaneous method for the processing of different cereal straws. Nowadays, acidic and alkaline methods are used especially with other methods such as enzymatic hydrolysis for the production of fermentable sugar and polyphenols. Dilute sulfuric acid pretreatment was used on corn stover feedstock and storage for 3 months, resulting in nonobservable microbial infestation. The cellulose content was stable while the hemicellulose content exhibited a slight decrease in furfural and oligomers, and the concentration of chemical compounds such as *O*-glucose and *O*-xylose was also constant [98]. In recent years, the focus has been on the use of other types of biomasses of fruit, for example, apple pomace, citrus, bananas, and mango among others [99]. In that aspect, different solutions of sulfuric acids were used for the valorization of apple pomace and the production of fermentable sugars and organic acids; the hemicellulose of the biomass was hydrolyzed, and the obtained liquor contained different concentrations of sugars such as glucose, xylose, arabinose, rhamnose, and galacturonic acid [100]. Those are new examples of the use of the acidic digestion of new biomasses with new co-products with a high application mainly in food industries.

**47**

*Getting Environmentally Friendly and High Added-Value Products from Lignocellulosic Waste*

Hot water, also known as autohydrolysis, hydrolyzes hemicellulose to release acetyl chemical groups and diverse polyphenols and removes lignin, making cellulose fibers more accessible [101]. The hot water method is very extreme, due to the fact that this method uses water at high temperatures usually between 170 and 230°C [102]. The resulting liquor contains different concentrations of sugars and chemical constituents such as polyphenols. Polyphenol compounds are covalently attached to the cell wall constituents such as cellulose, hemicelluloses, lignin, pectin, and structural proteins [103]. For example, hydroxycinnamic and hydroxybenzoic acids form ether linkages with lignin through their hydroxyl groups in the aromatic ring and ester linkages with structural carbohydrates and proteins through their carboxylic group [104]. Therefore, the recovery of the polyphenols can be made by selective extraction with ethyl acetate, purified and cleaned with resins to obtain a high yield of polyphenols with a direct use in food industries [105]. Ares-Péon et al. characterized phenolic compounds from liquors of stems maize (*Zea mays*) and *Eucalyptus globulus* with the use of hot water. Those authors found high recoveries of different polyphenols such as vanillin, ferulic, coumaric, sinapinic, hydroxybenzoic acids, guaiacol, and others. In addition, strong antioxidant activities have been reported in oligosaccharides esterified with polyphenols compounds derived from cell wall of diverse biomasses subjected to hot water methods. For example, in heteroxylans, such as arabinoxylan or glucuronoxylan, the main and predominant component is the hemicellulosic chain polymer, found in hardwoods, brans, and other softwoods [106], which can link some esterified phenolic acids to the oligosaccharides chain. In that sense, Rivas et al. [107] analyzed, by autohydrolysis, samples of liquor from rice husks, *Eucalyptus globulus* wood, and *Pinus pinaster* and found high amounts of hemicellulose-derived saccharides with esterified polyphenols. The samples displayed higher antiradical activities against strong antioxidants such as DPPH, ABTS, and ferric-reducing power; in addition, the polyphenol samples exerted high antioxidant protection to β-carotene-linoleic emulsions. The authors concluded that those antioxidant activities were mainly due to the esterification of polyphenols such as ferulic, syringic, and vanillic acids found in these polymers. Autohydrolysis has been used to extract polysaccharides and polyphenolic compounds from different biomass sources such as coffee, *Eucalyptus* and hazelnut

*DOI: http://dx.doi.org/10.5772/intechopen.93645*

**4.3 Chemicals derived from hot water methods**

shells among others, with high antioxidant activities [108, 109].

Different enzymes have been involved in the lignin break down in order to release value-added chemical compounds, with different uses in the food industries. It is important to note that alkaline and acidic methods can support the delignification of the biomasses residues to support the use of enzymatic digestion and obtain mainly sugars, polyphenols, and organic acids. Biomasses such as sugarcane, maize, agave, and sweet sorghum bagasse are widely used for the sugar and phenol extractions [110]. There are other nonconventional biomasses that can use this type of acidic or alkaline pretreatments for the degradation of hemicellulose and therefore obtain fermentable sugars and release antioxidant molecules. For example, biomasses such as corn cobs, orange, and pomegranate peels produced high yields of glucose and reduced sugars employing alkaline and enzymatic treatments [111]. Pomegranate biomass contains a high concentration of fermentable sugars that can be used in ethanol production and secondary polyphenols derived from the chemical hydrolysis. Pomegranate biomass contains a high concentration of fermentable sugars that can be used in ethanol production and secondary polyphenols derived from the chemical hydrolysis, due to this fact, pomegranate peels were subjected to acidic hydrolysis,

**4.4 Chemicals derived from enzymatic-based methods**

*Getting Environmentally Friendly and High Added-Value Products from Lignocellulosic Waste DOI: http://dx.doi.org/10.5772/intechopen.93645*

### **4.3 Chemicals derived from hot water methods**

*Biotechnological Applications of Biomass*

properties. There are distinct chemical processes of biomass hydrolysis, which use acids, bases, or enzymatic hydrolysis and others (other processes can be used, but their description would come out of the focus of this chapter) whose choice mainly depends on the material structure and characteristics desired for the products to be recovered. However, various sources of lignocellulosic materials need to be considered separately since they have different compositions of cellulose, hemicellulose, and lignin. Against all odds, the depolymerization process of the lignocellulosic biomass is a common goal for all different feedstocks for the production of all types of chemicals [91]. In particular, polyphenolic acids are a group of chemical compounds that are widely distributed in plant biomasses. Those compounds are important antioxidants that efficiently interact with biomolecules such as DNA, RNA, lipids, proteins, enzymes, and other cellular molecules to produce desired results. Due to the benefic effects, that can be useful for preventing the oxidation in foods, and therapeutic human disorders [92], all of them can be used with potential applica-

tions in the pharmacy, food, cosmetic, and nutraceutical industries.

Alkaline pretreatment is one of the most intensively studied technologies for biomass delignification [93], and the application of alkaline liquid with NaOH into the bagasse to obtain a black liquor that contains value-added chemicals has been investigated. This procedure is useful for the releasing of chemical compounds in different biomasses; particularly, this method has been commonly used for the processing of the switchgrass (*Panicum virgatum*), corn stover (*Zea mays*), and forestry biomasses. Due to their abundance and availability, the use of the process can produce a different number of high-value fine chemicals such as sugars, vanillin, isoeugenol, guaiacylpropanol, guaiacylethanol, ferulic, p-coumaric, and syringic acids [94, 95]. For example, different woody species of *Quercus* and *Robinia* were subjected to alkaline hydrolysis, and liquors were analyzed by GC-MS. The authors recovered and identified specific bioactive phenols for each woody species such as gallic acid, coniferyl alcohol, vanillic acid, syringaldehyde, and traces of epicatechin and catechin [96]. For the optimum alkali treatment concentration in sweet sorghum bagasse, different types of phenolic species were determined with the use of alkali treatments between concentrations of 3.0 and 6.0 M NaOH, resulting in high

The acidic pretreatment is a contemporaneous method for the processing of different cereal straws. Nowadays, acidic and alkaline methods are used especially with other methods such as enzymatic hydrolysis for the production of fermentable sugar and polyphenols. Dilute sulfuric acid pretreatment was used on corn stover feedstock and storage for 3 months, resulting in nonobservable microbial infestation. The cellulose content was stable while the hemicellulose content exhibited a slight decrease in furfural and oligomers, and the concentration of chemical compounds such as *O*-glucose and *O*-xylose was also constant [98]. In recent years, the focus has been on the use of other types of biomasses of fruit, for example, apple pomace, citrus, bananas, and mango among others [99]. In that aspect, different solutions of sulfuric acids were used for the valorization of apple pomace and the production of fermentable sugars and organic acids; the hemicellulose of the biomass was hydrolyzed, and the obtained liquor contained different concentrations of sugars such as glucose, xylose, arabinose, rhamnose, and galacturonic acid [100]. Those are new examples of the use of the acidic digestion of new biomasses with new co-products with a high application mainly in food industries.

**4.1 Chemicals derived from alkaline-based methods**

concentrations of phenol, 4-ethylphenol, and guaiacol [97].

**4.2 Chemicals derived from acid-based methods**

**46**

Hot water, also known as autohydrolysis, hydrolyzes hemicellulose to release acetyl chemical groups and diverse polyphenols and removes lignin, making cellulose fibers more accessible [101]. The hot water method is very extreme, due to the fact that this method uses water at high temperatures usually between 170 and 230°C [102]. The resulting liquor contains different concentrations of sugars and chemical constituents such as polyphenols. Polyphenol compounds are covalently attached to the cell wall constituents such as cellulose, hemicelluloses, lignin, pectin, and structural proteins [103]. For example, hydroxycinnamic and hydroxybenzoic acids form ether linkages with lignin through their hydroxyl groups in the aromatic ring and ester linkages with structural carbohydrates and proteins through their carboxylic group [104]. Therefore, the recovery of the polyphenols can be made by selective extraction with ethyl acetate, purified and cleaned with resins to obtain a high yield of polyphenols with a direct use in food industries [105]. Ares-Péon et al. characterized phenolic compounds from liquors of stems maize (*Zea mays*) and *Eucalyptus globulus* with the use of hot water. Those authors found high recoveries of different polyphenols such as vanillin, ferulic, coumaric, sinapinic, hydroxybenzoic acids, guaiacol, and others. In addition, strong antioxidant activities have been reported in oligosaccharides esterified with polyphenols compounds derived from cell wall of diverse biomasses subjected to hot water methods. For example, in heteroxylans, such as arabinoxylan or glucuronoxylan, the main and predominant component is the hemicellulosic chain polymer, found in hardwoods, brans, and other softwoods [106], which can link some esterified phenolic acids to the oligosaccharides chain. In that sense, Rivas et al. [107] analyzed, by autohydrolysis, samples of liquor from rice husks, *Eucalyptus globulus* wood, and *Pinus pinaster* and found high amounts of hemicellulose-derived saccharides with esterified polyphenols. The samples displayed higher antiradical activities against strong antioxidants such as DPPH, ABTS, and ferric-reducing power; in addition, the polyphenol samples exerted high antioxidant protection to β-carotene-linoleic emulsions. The authors concluded that those antioxidant activities were mainly due to the esterification of polyphenols such as ferulic, syringic, and vanillic acids found in these polymers. Autohydrolysis has been used to extract polysaccharides and polyphenolic compounds from different biomass sources such as coffee, *Eucalyptus* and hazelnut shells among others, with high antioxidant activities [108, 109].

#### **4.4 Chemicals derived from enzymatic-based methods**

Different enzymes have been involved in the lignin break down in order to release value-added chemical compounds, with different uses in the food industries. It is important to note that alkaline and acidic methods can support the delignification of the biomasses residues to support the use of enzymatic digestion and obtain mainly sugars, polyphenols, and organic acids. Biomasses such as sugarcane, maize, agave, and sweet sorghum bagasse are widely used for the sugar and phenol extractions [110]. There are other nonconventional biomasses that can use this type of acidic or alkaline pretreatments for the degradation of hemicellulose and therefore obtain fermentable sugars and release antioxidant molecules. For example, biomasses such as corn cobs, orange, and pomegranate peels produced high yields of glucose and reduced sugars employing alkaline and enzymatic treatments [111]. Pomegranate biomass contains a high concentration of fermentable sugars that can be used in ethanol production and secondary polyphenols derived from the chemical hydrolysis. Pomegranate biomass contains a high concentration of fermentable sugars that can be used in ethanol production and secondary polyphenols derived from the chemical hydrolysis, due to this fact, pomegranate peels were subjected to acidic hydrolysis,

and after an enzymatic process with cellulase there were released different fermentable sugars, moreover, bioethanol in presence of ethanol-producing microorganisms was produced. High concentrations of different sugars were released, with acid hydrolysis, such as glucose, xylose, cellobiose, arabinose, and fructose, with a range of ethanol production between 4.2 and 14.3 g/L [112]. Similarly, Talekar et al. [113] incorporated hydrothermal processing in combination with acid and enzymatic hydrolysis in pomegranate peels to recover pectin, phenols, and bioethanol. They recovered pectin ranges of 19–21% and phenolic compounds between 10.6 and 11.8%.

### **5. Pellets elaboration**

Pellets are a type of biomass fuel, that is made from different agroindustrial biomasses; as an example, pellets are a derivative of forest biomass such as wood, sawdust, fruit shells, and kernels as well as agricultural remains derived from straw, corn stove, rice husk, and additionally from plant species with energetic potential such as *Jatropha* and *Ricinus communis* [114], which serve as a source of energy; therefore, it is a good way to use and recycle agricultural surpluses. However, the pellet production is not only focused on using them in the energy industries as solid fuel and thus avoid the use of nonrenewable energy resources such as coal, natural gas, nuclear energy, and oil [115]. Nowadays, the high cost of fossil fuels has led to a high consumption of energy pellets, mainly, since some biomasses are capable of producing a similar calorific index than the oil. Hence, the use of biomass as a heating fuel had an increase in the last decade [116]. Besides, biomass is considered as a carbon-neutral fuel due to the fact that there are no additional carbon dioxide concentrations like fossil energies [117]. However, for the pellets to be used in restaurant kitchens and home kitchens, the biomass must be treated to avoid toxic pollutants for health. For example, it is known that after the consumption of biomass pellets, these produce ashes, which in their contents have high concentrations of chlorides, sulfides, carbonates, and silica among others that can be toxic to the health [118]. Different authors have pretreated the biomass with methodologies such as alkaline hydrolysis and heat treatment to obtain liquors rich in ashes, sugars, and other chemicals. In that sense, Retsina and Pylkkanen (2014) [119] used different treatments of the feedstock to produce an extract liquor that contained different chemicals such as soluble ash, hemicellulosic oligomers, acetic acid, dissolved lignin, and cellulose; the authors produced low-ash biomass ready to be transformed into energetic pellets. One of the most important parameters in the pellet production is its durability and is given by the pellet durability index (PDI). In order to achieve those parameters of PDI, strategies have been implemented to remove lignocellulose and sugars efficiently with the use of alkaline hydrolysis. Those molecules influence the final PDI of the pellet and its energetic capacity. For example, Tang et al. (2018), evaluated the release of lignin, soluble sugars, and whole particle size on the PDI of the untreated and treated Poplar (*Populus* spp.) wood sawdust, with a combination of alkaline and acid pretreatments and steam. The authors presented that PDI increased with those treatments, more specifically, with acidic pretreatment.
