Applications of Brewer’s Spent Grain Hemicelluloses in Biorefineries: Extraction and Value-Added Product Obtention

Abstract

A circular economy is imperative for environmental sustainability. In this context, biorefineries stand out as a means of production able to reduce the carbon footprint and the impact of global warming. Biorefineries may employ lignocellulosic biomass from various plant sources to produce bioproducts with the potential to replace fossil derivatives through synthesis by microorganisms without competing with food crops. Brewer’s spent grain (BSG), the residue of the brewery production process, is an option with potential for use, being a cheap raw material highly available throughout the year. The chemical composition of this biomass is quite variable, with significant amounts of hemicellulose, mainly consisting of xylose and arabinose monomers that can be technologically converted into value-added products such as xylooligosaccharides, xylitol, second-generation ethanol (2G ethanol), biofilms and furfural. To this end, catalysts are unusual in making biorefineries increasingly competitive in the market, selectively optimizing reactions and reducing the environmental impact of the production processes of these bioproducts. The present review addresses the primary methods for extracting and processing hemicelluloses from BSG using either biocatalysts (enzymes) or homogenous (acids, alkali, and salts) and heterogenous catalysts (solid acids and metal oxide) that can be used to pretreat the biomass and obtain the preferred byproducts. The state of the art of optimized catalysis mechanisms is also presented.

1. Introduction

The need for a circular economy and the consequent search for materials of sustainable origin to replace the extensive use of petroleum products has led to the development of techniques for reusing and recovering waste that previously seemed destined for disposal. This has led to biorefineries using lignocellulosic biomass from various plant sources to produce bioproducts. The catalysts are mainly acids and microorganisms or their enzymes in these circumstances. Interestingly, besides their potential to replace fossil derivatives, these bioprocesses do not compete with food crops [1,2].

Despite being made on a large scale by various sectors of the economy, today, much of this waste is destined for burning, use for animal feed, or for landfills. It is estimated that the global production of lignocellulosic biomass is around 181.5 billion tons per year [1], emphasizing sugarcane bagasse, which accounts for an annual output of 279 million tons [2]. However, the use of biomass in bioprocesses depends on its chemical constitution. The lignocellulosic material is composed mainly of cellulose, hemicellulose, and lignin, arranged in a highly stable plant crystalline structure. For them to be useful for biorefinery purposes, a pretreatment approach is required to isolate the fractions of interest and make them available for further processing [3]. Cellulose is a long-chain polymer (with a polymerization degree ranging from 1000 to 15,000) that is very resistant to hydrolysis and consists of glucose units linked by β-1,4-glycoglycoside bonds. Cellulose molecules are linear and tend to form inter- and intramolecular hydrogen bonds, giving them high tensile strength and making them insoluble in most solvents. Lignin is a hydrophobic substance derived from irregularly repeated phenylpropanoid units, originating in the dehydrogenative polymerization of coniferyl alcohol. It can inhibit fermentative processes if not satisfactorily removed from the carbohydrate composition before processing [4]. Hemicellulose is a heteropolysaccharide formed by hexoses (D-glucose, D-galactose, and D-mannose), pentoses (D-xylose and L-arabinose), and branches with acetyl groups, which give rise to acetic acid, an essential inhibitor of fermentative processes when hydrolyzed [5]. Hemicellulose is bound to cellulose by hydrogen bridges and lignin by covalent bonds (mainly α-benzyl ether bonds [6].

Catalysts are crucial for biorefining lignocellulosic biomass, from pretreatment to downstream processing. These molecules lower the activation energy of the reaction while remaining unconsumed, which means that they can be separated and reused. Beyond that, waste generation is considerably lower when compared to the traditional use of stoichiometric reagents, which are consumed during the reaction, and usually generate unwanted byproducts. In this sense, solid catalysts are preferred due to the recovery facilitation. These catalysts can be categorized into four groups according to their characteristics: micro and mesoporous materials, metal oxides, supported metal catalysts, and sulfonated polymers [7]. The utilization of catalysts is also necessary to achieve high selectivity towards the desired products, enabling efficient cleavage of C–C and C–O bonds, and thus facilitating the depolymerization of the lignocellulosic structure and recovery of the holocellulosic portion for further processing [8]. Recently, solid catalyst systems have been developed for the optimized conversion of biomass and other raw feedstocks into value-added chemicals and fuels.

Brewer’s spent grain (BSG), the residue of the brewery production process, has emerged as a feedstock option for use in biorefineries. It is estimated that for every 100 L of beer produced, ~20 kg of BSG is generated [9]. This byproduct has animal feed as its primary destination (which presents a low valuation). In addition to its high availability, which is independent of seasonality, another attraction of this biomass is its chemical characteristics. BSG is rich in carbohydrates, mostly xylose (coming from hemicellulose), which through biotechnological processes, can be converted into products of higher added value, going far beyond the current applications [10].

For this review, we initially searched the Web of Science (WoC), Science Direct, Scopus, Wiley, and Springer databases with the terms “brewer’s spent grain + biorefinery”, “brewers spent grain + hemicellulose + hydrolysis”, “brewers spent grain + catalyst”, and “brewers spent grain + hemicellulose + catalyst”. Then, to broaden the scope, we added the terms “hemicellulose + biorefinery + catalyst” and “catalysis + mechanism + hemicellulose” to the search, exploring the possibility of methods applied to other biomasses of a similar structure applying to BSG hemicelluloses processing (Figure 1).

At the end of the bibliographic search, more than 20,000 articles (research or review) were found in different databases (

Table 1. Result of the number of articles found in each database with the search terms related to the subject of the review.

Database	Brewers Spent Grain + Biorefinery	Brewers Spent Grain + Hemicellulose + Hydrolysis	Brewers Spent Grain + Catalyst	Brewers Spent Grain + Hemicellulose + Catalyst	Hemicellulose + Biorefinery + Catalyst	Catalysis + Mechanism + Hemicellulose
Scopus	61	35	21	3	136	145
Web of Science	103	47	12	1	196	134
Science Direct	328	561	381	253	3441	4720
Wiley	148	367	1062	226	1476	3183
Springer	127	188	157	73	1235	1632

). We also considered that some could be included in multiple databases. The material thus collected was used to prepare this review.

2. Productive Chain and Physicochemical Composition of Brewer’s Spent Grain

In 2020, worldwide beer production was close to 182 billion liters, and was especially high in North and South America (61.6 billion liters), Asia (55.1 billion liters), and Europe (50.1 billion liters). Considering that every 100 L of beer produced generates approximately 20 kg of dried brewer’s spent grain, 36.4 billion kg of this biomass is available annually [11].

Being a cheap raw material (approximately 0.04 USD/kg) [12] with high availability, this lignocellulosic biomass has a high potential for conversion into value-added products as part of a circular economy approach focused on the search for energy alternatives [13]. Around 70% of brewer’s spent grain is used for animal feed, just over 10% is intended for biogas production, and the remainder is disposed of in landfills without processing. Currently, the cost of transportation, especially when wet (the bagasse leaves the brewery with 70–80% humidity), the drying process, and the need for pretreatment to access the carbohydrates are some barriers to increasing its use. This issue delays the effective use of brewer’s spent grain and many other biomasses in second-generation biorefineries [11,12]. An interesting approach would be to install these platforms near the waste generation areas, on a smaller and modular scale, within a cooperative and decentralized system commanded by the producers, as already happens with some plants that combine electricity and biogas production. This could reduce transport and investment costs, benefiting the local economy’s circularity [1].

Brewer’s spent grain represents about 85% of the residues produced by the brewing process, obtained after mashing and filtration of the wort. It is composed of bark, pericarp, and seed. These parts are not extracted during the brewing process. It represents a rich source of fiber (40–70%), proteins (19–30%), and lipids (10% on average), as well as minerals (2–5%), vitamins, and phenols (0.7–2%) [13]. It also contains many amino acids, such as threonine (3.5 g/100 g), isoleucine, valine, phenylalanine (5.5 g/100 g), lysine, and leucine. The phenolic compounds present in the highest concentration are ferulic (336 mg/100 g), p-coumaric (65 mg/100 g), synaptic (42 mg/100 g), caffeic (10 mg/100 g), and syringic (7 mg/100 g) acids, which confer an essential antioxidant capacity [14]. The high content of carbohydrates, proteins, and moisture makes BSG susceptible to microbial deterioration (from 2 to 7 days), so it must be quickly processed [15].

The chemical composition of this biomass is quite variable (

Table 2. Chemical composition of different brewer’s spent grain (BSG) samples and comparison with sugarcane bagasse (SCB), pearl millet (PM), and rice husk (RH) (on a dry basis; g/100 g).

Component	BSG [16]	BSG [17]	BSG [18]	BSG [19]	SCB [20]	PM [21]	RH [22]
Cellulose	15.99 ± 0.88	19.2 ± 1.40	15.2 ± 0.50	19.24	39.52 ± 0.66	40.99 ± 2.50	31.12
Hemicellulose	29.92 ± 1.60	18.4 ± 3.70	25.1 ± 0.70	53.09	25.63 ± 0.44	20.90 ± 2.00	22.48
Xylan	25.22 ± 1.46	11.3 ± 1.20	16.9 ± 0.70	-	-	-	-
Arabinan	4.71 ± 0.14	-	6.6 ± 0.30	-	-	-	-
Total Lignin	20.80 ± 0.42	-	12.5 ± 0.80	8.53	30.36 ± 0.13	18.3 ± 1.80	22.34
Soluble Lignin	3.02 ± 0.01	9.9 ± 1.40	5.5 ± 0.30	-	3.60 ± 0.90	18.20 ± 1.80	-
Insoluble Lignin	17.78 ± 0.41	-	7.0 ± 0.50	-	26.40 ± 0.20	0.06 ± 0.01	-
Starch	-	4.8 ± 0.46	5.3 ± 0.20	-	-	-	-
Lipids	-	5.2 ± 2.10	-	-	-	-	-
Proteins	21.16 ± 0.61	26.6 ± 0.38	21.2 ± 0.20	17.25	-	-	-
Ashes	3.76 ± 0.05	2.7 ± 0.07	2.3 ± 0.10	3.68	1.45 ± 0.21	5.96 ± 0.50	13.87
Extractives	8.33 ± 0.76	-	18.5 ± 1.00	-	3.04 ± 0.17	19.30 ± 1.40	2.33

“-”: data not available.

), depending on factors such as the type of barley used in malting, the stage and manner in which it was harvested, the quality of the malt, and the additives applied during the mashing process (e.g., gum, iron, zinc, and polypeptides, used to optimize foam formation and stability, but which also cause the hydrolysis of β-glucans, removal of oxalic acid, and degradation of polyphenols). In addition, storage conditions can influence the physical–chemical characteristics of bagasse, with studies demonstrating that frozen samples had a higher protein and fat content compared to lyophilized and air-dried samples, but had a lower carbohydrate content, especially arabinose [11,13].

Despite the high availability, low cost, and useful chemical composition, which is rich in carbohydrates and aromatic compounds, the application of this biomass is still very restricted to research, and it is necessary to expand its practical use in second-generation biorefineries [23]. To achieve this, however, it is essential to implement sustainable technologies for converting brewer’s spent grain and other lignocellulosic biomasses products of technological interest, which should be the focus in the coming years [24,25].

3. The Use of Brewer’s Spent Grain in Biorefineries

A bibliometric analysis by Sganzerla et al. [24] showed that brewer’s spent grain has been the object of study in scientific research since the 1970s, with the first paper being published in 1976. Since 2004, scientific interest in this biomass has increased, possibly due to the demand for alternatives to oil products and concern about climate change. While initially focused on agriculture engineering, studies began to diversify, opening space for research with fuels, biotechnology, chemical engineering, environmental sciences, and applied chemistry. The last decade produced 75% of the knowledge about processing and characterizing brewer’s spent grain. This paper also established Brazil as the country that publishes the most about this biomass, with about 14% of all publications. However, Europe stands out in the practical application of this resource in producing renewable energy, being the pioneer in discussions about the concept of the circular economy.

The processing of BSG is characteristic of the “Second-generation Biorefineries”, where agro-industrial waste composed of lignocellulosic biomass is processed to obtain bioproducts which are later reintroduced into the production and consumption chains. One of the potential products is biomethane (biogas). After an optional pretreatment step (which increases digestibility and productivity), biomass undergoes a series of transformations through the action of microorganisms in syntrophy during an anaerobic digestion process consisting of the following steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. This microbial catalytical synergy forms carbon dioxide and methane, which may be used for energy production or as fuel. Moreover, the residue of this process (the digestate) can still be used as a biofertilizer [26,27,28].

The lignocellulosic biomass presents a complex and naturally recalcitrant structure that requires a series of treatments to access their components, with the subsequent synthesis of bioproducts. For this, several pretreatment methods can be adopted, either catalyzed or not, depending on what it is intended to produce, to optimize the recovery of each fraction [4]. Fortunately, several strategies can be used in the solubilization and recovery of hemicelluloses, and the most prominent are listed below.

3.1. Pretreatments for Extraction of Hemicelluloses from Brewer’s Spent Grain

Brewer’s spent grain can have more than 50% hemicellulose in its composition, which consists mainly of xylose and arabinose monomers [19]. The solubilization of this fraction can be acheived using several approaches, including physical, chemical, or combined pretreatments (Table 3), depending mainly on the desired characteristics of the hydrolysate [29]. For example, suppose the next step to pretreatment is a conversion by microorganisms, such as xylitol production. In that case, it is essential to consider the formation of fermentation-inhibitory compounds such as furfural (product of degradation of pentoses) and acetic acid (product of hydrolysis of acetyl groups present in the branches of hemicelluloses). If these substances are present, a detoxification step of the hydrolysate may be required [30]. Physical pretreatment, such as drying, is essential to increase the microbiological stability of this biomass, which has a high moisture content (80% average) and facilitates transport to processing sites. Usually, this is the first pretreatment to which this biomass is subjected, regardless of whether it is used for animal feed or biorefineries [31].

Pretreatments with diluted acid catalysts (in concentrations < 5%, usually sulfuric acid) and high temperatures (between 120 and 210 °C) are often used for the solubilization of BSG hemicelluloses, with satisfactory recovery results [32]. In the presence of the acid catalyst, the hydrogen bonds between hemicellulose and cellulose break, and the hydrolysis of hemicellulose occurs [33]. Rojas-Chamorro et al. [18] reached hemicellulosic sugar recoveries greater than 80% in all tested conditions with diluted H₂SO₄, with the concentration of pentoses (xylose + arabinose) reaching 30 g/L on average. A process with two-step acid-catalyzed hydrolysis using diluted sulphuric acid was developed by Bedő et al. [34], wherein the first acidic hydrolysis that rendered high arabinose yield (76%) was achieved under optimized conditions (90 °C, 1.85 w/w % H₂SO₄, 19.5 min) and an arabinose/arabino-xylooligomer-rich supernatant was obtained. The remaining xylan was solubilized in the second acidic hydrolysis, resulting in a 90% xylose yield. The main disadvantage of this pretreatment method is that it forms byproducts, such as 5-hydroxymethylfurfural (HMF), furfural, and acetic acid (Figure 2). This is mainly related to the high processing temperatures, which may jeopardize an eventual next step involving non-tolerant microorganisms [35]. These molecules, however, can also be desirable and are considered essential building blocks for a biorefinery. Furfural, for example, can be transformed into valuable products such as furfuryl alcohol [36,37]. Organic acids, such as oxalic, formic, and maleic acid, are also being investigated as homogenous catalysts for hemicellulose solubilization, presenting promising results while leading to a lower formation of inhibitory compounds [38,39,40]. However, the use of organic acids in the solubilization of BSG hemicelluloses needs more studies to evaluate the feasibility of its use.

Alkaline hydrogen peroxide (AHP) can also solubilize the hemicellulosic portion, which can be recovered with high purity by using ethanol precipitation [41]. This can be done at room temperature, although it takes longer than at higher temperatures, which results in less or no inhibitory compound formation. This method uses a combination of hydrogen peroxide and sodium hydroxide solutions at pH 11.5, which is effective for the delignification and solubilization of hemicellulose while using a lower concentration of the alkali reagent [42]. Hydrogen peroxide is highly reactive and exhibits solid oxidizing ability, dissociating to produce hydroperoxide anions that react with hydrogen peroxide in an alkaline medium to form hydroxyl radicals and superoxide anions that oxidize lignin. In the next step, hydrophilic (carboxyl) groups are introduced into the structure, and interunit bonds are cleaved, resulting in the dissolution of lignin and hemicellulose [43]. A study by Fernández-Delgado et al. [44] using AHP (5% H₂O₂, pH 11.5, 50 °C, 150 rpm) in comparison with alkaline and ozone pretreatment obtained hydrolysates with significant recovery (80% maximum) of hemicellulose monomers (13.0 g/L of xylose and 5.2 g/L of arabinose), and concentrations of furfural and HMF under the detection limits (<1 mg/L), which made it suitable for enzymatic and microbiological conversion.

Another pretreatment method commonly applied to BSG is hydrothermal (also known as liquid hot water or autohydrolysis), in which the biomass is hydrolyzed for a time that can vary between minutes and hours only using water under high temperature and pressure, between 150 and 230 °C and approximately 50 bar, respectively [45]. The high temperature promotes the release of acetic and uronic acid in the medium due to the hydrolysis of acetyl and uronic groups present in the xylan, acting as biocatalysts and increasing the xylan hydrolysis rate. For example, Parchami et al. [46] achieved a 76% rate of BSG hemicellulose solubilization, performing this pretreatment at 180 °C for 30 min, observing that the yield improved with the temperature. This reaction is very similar to the pretreatment with dilute acid. This pretreatment is advantageous since it does not require chemical, corrosive, or toxic reagents, preventing equipment corrosion and the need for chemical recycling. Additionally, the costs of this pretreatment are lower compared to others. However, it demands an efficient control of temperature and pressure, which can cause the thermal degradation of hemicellulose [47,48].

Recently, new routes were developed by the Institute for Research in Bioenergy (IPBEN/UNESP), with two patent registrations. The first is a modification of the organosolv process, allowing the obtention of lignin combined with the recovery of hemicellulose in the same method, which mainly comprises adding ethanol to precipitate the hemicellulose/xylan and subsequent acidification to precipitate the lignin of the liquid fraction [49]. The second, by Felipuci et al. [50], focuses on the extraction of xylan from lignocellulosic biomass using the association of biological pretreatment with a chemical treatment to increase the extraction of xylan from biomass. This biological pretreatment employs a fungus that causes changes in the composition of lignin and/or material and a consequent decrease in biomass recalcitrance, resulting in higher yields in hemicellulose extraction.

Table 3. Brewer’s spent grain hemicellulose extraction methods, yield, and the final product obtained.

Method of Extraction	Yield/Recovery	The Product Obtained from BSG Hemicelluloses	Reference
Diluted acid (o.c. 1: 130 °C, 1% H2SO4, 26 mi)	34 g/L, average	Xylose/arabinose monomers (subjected to fermentation to obtain bioethanol)	[18]
Two-step acidic hydrolysis (o.c.: 90 °C, 1.85% H2SO4, 19.5 min)	1° hydrolysis: 29.9% total xylose yield and 69.7% total arabinose yield. 2° hydrolysis: 89.7% xylose yield, complete arabinose solubilization.	Xylose/arabinose monomers and arabinoxylan-oligosaccharides	[34]
Ultrasound-assisted extraction w/1.5 M NaOH	58.47 ± 3.17% max.	Arabinoxylan solution w/increased emulsifying properties	[51]
Direct fermentation (o.c.: T. reesei, 20 g/L of BSG, pH 7.0, 30 °C, 72 h)	326.2 mg XOS/g max.	Arabino-xylooligosaccharides (AXOS) w/degree of polymerization from 2 to 5.	[52]
Steam Explosion (o.c.: 180 °C, 10 min, 25% initial dry matter)	>73.1%	Xylooligosaccharides (XOS)	[53]
Nixtamalization + thermal treatment w/CaO, 100  °C	6.43%	Arabinoxylan chains	[54]
Hydrothermal pretreatment (160 °C for 20 min)	20 g/L max.	Xylitol and 2G ethanol	[55]
Thermal extraction (100 °C for 30 min) followed by NaOH 0.5 mol/L solution (25 °C 100 rpm, 8 h)	-	Thermoplastic films	[56]
Alkaline extraction (4 M KOH + 5 mM Na2S2O5) followed by ethanol precipitation and enzymatic hydrolysis	63.6% max.	Xylose monomers	[57]
SSF w/Fusarium oxysporum (o.c.: BSG extruded, 48 h fermentation)	63.28%	Soluble Arabinoxylans	[58]
Microwave-assisted alkali pretreatment (o.c.: 172 °C, 0.38 M NaOH) followed by ethanol precipitation/enzymatic hydrolysis	133 kg AX/t BSG	Arabino-xylooligosacharides (AXOS) and biobutanol	[59]
Organosolv pretreatment (o.c.: 180 °C, 10 min, 50% ethanol)	50% average	Arabino-xylooligosaccharides (AXOS)	[60]
Subcritical water hydrolysis (174 °C, 60 min and 5% (w/v) of dry BSG)	>80%	Arabinoxylan-rich hydrolysate	[61]
Peroxide alkaline pretreatment (o.c.: 5% H2O2, pH 11.5, 50 °C, 150 rpm) followed by enzymatic hydrolysis	30% max.	Arabino-xylooligosaccharides (AXOS) and monosaccharides	[44]

¹ optimal condition.

Table 3. Brewer’s spent grain hemicellulose extraction methods, yield, and the final product obtained.

Method of Extraction	Yield/Recovery	The Product Obtained from BSG Hemicelluloses	Reference
Diluted acid (o.c. 1: 130 °C, 1% H2SO4, 26 mi)	34 g/L, average	Xylose/arabinose monomers (subjected to fermentation to obtain bioethanol)	[18]
Two-step acidic hydrolysis (o.c.: 90 °C, 1.85% H2SO4, 19.5 min)	1° hydrolysis: 29.9% total xylose yield and 69.7% total arabinose yield. 2° hydrolysis: 89.7% xylose yield, complete arabinose solubilization.	Xylose/arabinose monomers and arabinoxylan-oligosaccharides	[34]
Ultrasound-assisted extraction w/1.5 M NaOH	58.47 ± 3.17% max.	Arabinoxylan solution w/increased emulsifying properties	[51]
Direct fermentation (o.c.: T. reesei, 20 g/L of BSG, pH 7.0, 30 °C, 72 h)	326.2 mg XOS/g max.	Arabino-xylooligosaccharides (AXOS) w/degree of polymerization from 2 to 5.	[52]
Steam Explosion (o.c.: 180 °C, 10 min, 25% initial dry matter)	>73.1%	Xylooligosaccharides (XOS)	[53]
Nixtamalization + thermal treatment w/CaO, 100  °C	6.43%	Arabinoxylan chains	[54]
Hydrothermal pretreatment (160 °C for 20 min)	20 g/L max.	Xylitol and 2G ethanol	[55]
Thermal extraction (100 °C for 30 min) followed by NaOH 0.5 mol/L solution (25 °C 100 rpm, 8 h)	-	Thermoplastic films	[56]
Alkaline extraction (4 M KOH + 5 mM Na2S2O5) followed by ethanol precipitation and enzymatic hydrolysis	63.6% max.	Xylose monomers	[57]
SSF w/Fusarium oxysporum (o.c.: BSG extruded, 48 h fermentation)	63.28%	Soluble Arabinoxylans	[58]
Microwave-assisted alkali pretreatment (o.c.: 172 °C, 0.38 M NaOH) followed by ethanol precipitation/enzymatic hydrolysis	133 kg AX/t BSG	Arabino-xylooligosacharides (AXOS) and biobutanol	[59]
Organosolv pretreatment (o.c.: 180 °C, 10 min, 50% ethanol)	50% average	Arabino-xylooligosaccharides (AXOS)	[60]
Subcritical water hydrolysis (174 °C, 60 min and 5% (w/v) of dry BSG)	>80%	Arabinoxylan-rich hydrolysate	[61]
Peroxide alkaline pretreatment (o.c.: 5% H2O2, pH 11.5, 50 °C, 150 rpm) followed by enzymatic hydrolysis	30% max.	Arabino-xylooligosaccharides (AXOS) and monosaccharides	[44]

¹ optimal condition.

Catalysts are an essential component of any reaction. These substances can be classified as homogeneous (where the active sites are in the same phase as that of the reactant, as in most acid and alkaline hydrolysis), heterogeneous (where the catalyst and the reactant are in different stages), or enzymatic (where an enzyme acts as a catalyst in the reaction) [61]. Heterogeneous catalysts (HTC) have the advantage of being quickly recovered at the end of the process, mainly because they are in solid form and constitute an up-and-coming field of study in green chemistry [62]. Solid acid catalysts (SAC), such as zeolites and metal oxides, comprise an exciting option to substitute mineral acids in the solubilization of hemicelluloses, maintaining the advantages of acid hydrolysis while increasing selectivity towards the glycosidic bonds, leading to higher sugar yields, with the possibility of using a continuous flow process [62]. Recently, a novel SAC was synthesized from biomass-derived glucosamine HCl and used as a catalyst for a one-step conversion of corn stover into furfural and HMF, showing promising results [63]. Additionally, some studies have satisfactory demonstrated that hemicelluloses can be selectively hydrolyzed into sugar monomers using HTC with cation-exchange properties [64]. Even so, the direct conversion of hemicelluloses into value-added products using HTC is still poorly reported in the literature, mainly due to the poor selectivity of such catalysts. In addition, more studies need to be carried out to develop HTC that can optimize the recovery of monomers of hexoses from cellulose and pentoses from hemicellulose in the same process and in a viable way [65].

3.2. Obtaining Value-Added Products from Brewer’s Spent Grain Hemicelluloses

In a biorefinery environment, the heterogeneous structure of hemicelluloses can be transformed into many bioproducts with diverse properties, ranging from simple carbohydrates to complex molecules such as oligosaccharides, lactic acid, xylitol, ethanol, biofilms, and enzymes. The most prominent products will be addressed below.

3.2.1. Xylooligosaccharides

Xylooligosaccharides (XOS) link two to seven xylose molecules through β-1,4-glycosidic bonds. These are emerging molecules with a huge market potential in the food, feed, health, and cosmetics industries due to their biological and physicochemical properties, especially prebiotics. XOS present a moderate degree of sweetness, stability of pH and temperature, and do not exhibit toxicity or negative effects on human health, characteristics that make them suitable compounds to be incorporated into several edible products [53]. The global XOS market is expected to exhibit a compound annual growth rate (CAGR) of 7.57% from 2019 to 2024 and to reach 130 million USD by 2023 [66,67].

The production of XOS typically involves two stages: a chemical or hydrothermal pretreatment to solubilize the xylan of the lignocellulosic material, followed by xylan hydrolysis by xylanolytic enzymes (Figure 3) [66]. Using commercial xylanase from Aspergillus oryzae, Sajib et al. [67] used BSG to obtain a hydrolysate with 98% of XOS (mainly X2-X5) and only about 2% (w/w) of the xylose present as monosaccharides after 5 h of hydrolysis. Methods using the direct steam explosion of screw-press-dried BSG [53] and extremely low acid (ELA) catalysis in liquid hot water (LHW) hydrothermal treatment (HTT) [68,69] achieved yields of 73.1% and 76.4%, respectively, showing that an enzymatic step is not always needed for XOS production.

3.2.2. Xylitol

Xylitol is a polyol of the pentitol type, and is also called sugar alcohol or polyalcohol. It is the sweetest of the polyols, equivalent to sucrose in sweetness, but with fewer calories and a lower glycemic index. Another advantage is that its two main absorption pathways (liver and intestinal flora) are independent of insulin, which makes xylitol an appropriate sweetener for people with diabetes. In addition, due to its low-calorie load, it can be considered a suitable sweetener for carbohydrate-controlled diets. The most common application of xylitol worldwide is in sugar-free chewing gums. It is very convenient due to its organoleptic properties, good solubility, and controllable crystallization, which is very useful in providing dental coating benefits. The consumption of xylitol exerts a prebiotic effect by decreasing the fecal pH. Due to its emollient and humectant properties, it can also be used in cosmetic applications [70].

In biorefinery platforms, xylitol is considered one of the most important products derived from carbohydrates. It can be produced by chemical or biological conversion (Figure 4) of hemicellulose [71]. Several studies demonstrate that xylitol is the main product potentially derived from the hemicelluloses in a BSG biorefinery [24]. Chemical routes traditionally carry out the large-scale conversion of biomass to xylitol through the hydrogenation of purified xylose (from the hydrolysis of hemicellulose) using chemical catalysts at high pressures and temperatures. Some of the catalysts successfully used for this purpose include Ru/(NiO–TiO₂) [72], Ni₃Fe₁ [73], silica-supported monometallic cobalt (Co/SiO₂) [74], and Ni-Re bimetallic nanoparticle catalysts (Ni-Re/AC) [75]. However, for safety and environmental reasons, the biotechnological route employing microorganisms and enzymes is being explored and optimized [76].

Many microorganisms can use xylose as a carbon source and convert it into xylitol, generally mediated by enzymatic catalysis by xylose reductase (XR). Xylitol production is obtained by reducing xylose to xylitol as the first step of xylose metabolism with NAD(P)H-dependent XR. Yeasts are the preferred biocatalysts for this process due to high rates of pentose assimilation in some species and stable levels of XR expression, resulting in high xylitol yields [77]. Candida guilliermondii [78,79], Debaryomyces hansenii [80], Komagataella pastoris [81], and Pachysolen tannophilus [55] are some of the yeasts used in research to obtain xylitol from BSG hemicelluloses efficiently.

3.2.3. 2G Ethanol

The combustion of fossil fuels contributes to CO₂ emissions and global warming, making the energy transition to low-carbon fuels a worldwide necessity to tackle climate change. With the growing global demand for energy, fossil fuel resources on our planet are anticipated to become depleted. In the past decade, several fuels have emerged as a promising alternative to fossil fuels, with second-generation bioethanol being the most prominent option [82]. The United States and Brazil are the leading producers of 2G ethanol, which uses lignocellulosic biomass (residues from crops such as sugarcane bagasse, corn stover, and BSG) as feedstocks to produce this biofuel (Figure 5) [83].

At first, only the cellulosic part of the biomass (predominantly glucose) was subjected to fermentation due to the incapacity of the traditional industrial yeasts (usually Saccharomyces cerevisiae industrial strains, such as PE-2) to convert pentoses (such as xylose and arabinose, the main constituent monomers of hemicellulose) into ethanol once they do not harbor an efficient xylose assimilation pathway [84]. However, the use of native yeasts capable of fermenting xylose, such as Scheffersomyces stipitis and Scheffersomyces shehatae (that can ferment both glucose and xylose) [55], and the genetic engineering of S. cerevisiae strains to include this xylose assimilation pathway, as well as fermentation inhibitor tolerance [85], have made it possible to use pentoses from lignocellulosic biomass in 2G ethanol production, which includes BSG, in addition to possibly reducing costs associated with the removal of the detoxification step of the hydrolysates.

One condition for the metabolization of xylose that requires more attention is oxygen supplementation. Xylose metabolization by yeast occurs through a redox process where, initially, xylose is reduced to xylitol by the enzymatic action of XR. Subsequently, xylitol dehydrogenase (XDH) catalyzes the oxidization of xylitol into xylulose, which is then phosphorylated to xylulose-5-phosphate by xylulokinase. Finally, xylulose-5-phosphate enters the pentose phosphate pathway (PPP), which leads to ethanol formation [86]. Some yeasts achieve higher yields regarding conversion from xylose into ethanol due to recycling coenzymes (NADH/NAD+) in the subsequent reactions catalyzed by XR and XDH, respectively. However, when XR uses NADPH as an electron donor and XDH uses NAD+ as an electron acceptor, there is a redox imbalance which leads to xylitol accumulation to the detriment of ethanol production. In the presence of oxygen, however, such inequality may be overcome [5]. Indeed, Da Silva et al. [55], using hemicellulose liquor BSG as substrate, noticed that the yeast Pachysolen tannophilus preferentially produced xylitol when oxygen was limited. In contrast, ethanol was preferentially produced with higher concentrations of this gas in the medium.

The use of bacteria to produce 2G ethanol is another possible pathway. In bacteria, the conversion of xylose to xylulose occurs directly by the catalysis of the xylose isomerase enzyme. Through the PPP, xylulose-5-phosphate turns into glyceraldehyde-3-phosphate and fructose-6-phosphate, leading to ethanol formation via the Embden–Meyerhof Pathway [5]. Rojas-Chamorro et al. [87] used Escherichia coli SL100 to ferment a mixed sugar solution obtained after phosphoric acid pretreatment and enzymatic hydrolysis of BSG. The obtained hydrolyzate, containing hemicellulosic sugars and starchy glucose, without previous detoxification, yielded 0.40 g of ethanol per gram of sugar, which represents an overall yield of 17.9 g of ethanol per 100 g of raw BSG.

3.2.4. Enzymes

Enzymes have proven to be one of the most exciting and well-researched potential products of BSG fermentation, as they have multiple applications in the food, pharmaceutical, and chemical industries. Filamentous fungi are particularly appropriate for the production of enzymes for later extraction, because many of their plant cell-wall-degrading enzymes are secreted extracellularly into the substrate medium, allowing for their extraction without disrupting the fungal cells. Fungi are known to produce extracellular enzymes at higher concentrations, and thus their yield can be more significant on an industrial scale [88].

Due to the necessary amount of hemicellulose in its structure, xylanases and hemicellulases are among the enzymes that can be obtained using BSG as substrate. After eight days of growth on BSG, Humicola grisea var. thermoidea, and Talaromyces stipitatus reached maximal xylanase activities, with the first producing the highest level of the enzyme (16.90 ± 0.59 U/mL) [89]. Penicillium janczewskii also expressed a substantial amount of xylanase, with 15.81 U/mL in submerged culture with BSG 2% for 7 days [90] and in solid-state fermentation with 40% (α-L-arabinofuranosidase) and 50% (xylanase and β-xylosidase) moisturized BSG cultures [91]. Another study conducted with Moesziomyces aphidis PYCC 5535T grown on BSG reached a remarkable 518.20 U/mL of xylanase volumetric activity, promising for biotechnological applications [92].

Typically, many enzymes exhibit enhanced activity at higher temperatures, inhibiting unwanted microbial growth in these conditions. However, many xylanolytic enzymes produced by fungi are not heat stable [93]. In general, most microbial enzymes used in industrial processes are mesophilic and are used from 35 to 60 °C, which can be a challenge if they are to be used for industrial purposes. Moreover, many of these studies have only proven the viability of lab-scale processes, and the scale-up of many of them may be unviable, requiring more studies to optimize its use on a larger scale.

3.2.5. Biofilms and Bioplastics

The concept of biorefinery can support the production of bioplastics by the industry since a single raw material can yield several products of high value. The integration of the various processes involved would reduce production costs. However, a higher implementation of biodegradable bioplastics by the industry requires the development of new technologies for production to become viable enough to compete with conventional plastics [94]. BSG contains significant amounts of hemicellulose (mostly arabinoxylans) in its composition, making it an essential raw material for bioplastics. Oligosaccharides obtained from the hydrolysis of water-soluble arabinoxylans have prebiotic effects, making them suitable as functional ingredients in encapsulating active compounds for active/edible food packaging [95].

Hemicellulose is a polymer with a significant potential application for the development of bioplastics. For example, a bioplastic based on xylan and blended with gelatin was biodegraded entirely after 15 days of conditioning, being considered 100% biodegradable since the sample could not be recovered for weighing [96]. BSG biofilms also demonstrate an essential role in the manufacturing of active food packaging. A nanocomposite biofilm prepared with BSG arabinoxylans showed good thermal stability (230 °C max), besides antioxidant activity up to 90% (DPPH activity), antibacterial activity against Staphylococcus aureus and Escherichia coli, and antifungal activity towards Candida albicans [97]. Pérez-Flores et al. [98] also developed a biofilm using the arabinoxylans of BSG, with glycerol as a plasticizer, and applied it as a release matrix for caffeine. The biofilm showed acceptable mechanical and morphological properties, and a diffusion control mainly released the caffeine with minimum residue.

The challenges for applying hemicellulose-based biofilms are their heterogeneity, low mechanical properties, difficulty forming homogeneous bioplastics, and hydrophilic properties, which demand more focused studies to achieve feasible production [99]. Chemical changes in hemicellulose and blends with other polymers are alternatives for surpassing the difficulties in its use. Gordobil et al. [100] demonstrated that adding nanocellulose to xylan-based bioplastics can increase its mechanical properties and hydrophobicity. Gluten [101], micro cellulose [102], chitosan [103], and plasticizers such as polypropylene glycol [56], glycerol, xylitol, and sorbitol [104] were also reported as promising alternatives for improving the mechanical properties of hemicellulose-based bioplastics.

3.2.6. Furan Derivatives (Furfural and Furfuryl Alcohol)

Furan derivatives are important platform compounds for preparing biofuels, polymers, and pharmaceuticals. They are the main chemicals industrially obtained from hemicelluloses, with furfural being the most appealing due to its versatility. The furfural molecule has an aldehyde group and a dienyl ether functional group, so it can be hydrogenated, oxidized, chlorinated, nitrated, and condensed to produce more than 1600 derivatives. Due to its versatility, the global furfural market size is expected to reach USD 954.36 million by 2030, registering a CAGR of 7.0% from 2023 to 2030 [96]. Downstream products derived from furfural include furfuryl alcohol (a chemical building block for cleaning composites, drug synthesis, viscosity condenser for epoxy resins, paint strippers, and wood modification), furan, tetrahydrofuran, succinic acid, maleic anhydride, and levulinic acid. Among these, about 60–70% of furfural is converted to furfuryl alcohol through catalytic hydrogenation, which makes furfuryl alcohol the main industrial green chemical produced from hemicelluloses [97]. The global furfuryl alcohol market size is estimated to reach USD 821.6 million by 2028, according to a new report by Global Information, Inc. (2023) [105], and expected to expand at a CAGR of 7.2% from 2021 to 2028.

The dissolution and conversion of hemicellulose into furfural occur under harsh conditions. Employing a catalyst during the first step can significantly improve the reaction rate of hemicellulose hydrolysis and subsequent pentose dehydration to furfural, contributing to increased product purity and yield [106]. The obtention of furfural via catalytic systems can be achieved using homogeneous or heterogeneous catalysts (

Table 4. Application of heterogeneous and homogeneous catalysts in furfural production from different substrates (adapted from Ye et al.) [110].

Substrate	Catalyst	Type of Catalyst	Yield (%)	Reference
Cornbob	H-ZSM-5	Heterogeneous	71.7	[111]
Corn stover	MSPFR 1	Heterogeneous	50.3	[112]
Wheat straw	CrPO4	Heterogeneous	67.0	[113]
Bagasse	H-USY	Heterogeneous	55.0	[114]
Xylan	GO-SO3H 2	Heterogeneous	87.0	[115]
Xylose	FePO4	Heterogeneous	88.7	[116]
Bamboo	HCl	Homogeneous	46.0	[117]
Eucalyptus	AlCl3	Homogeneous	70.3	[118]
Xylose	NaCl	Homogeneous	76.7	[119]
Tung shell	H3NSO3	Homogeneous	92.2	[120]
Switchgrass	H2SO4	Homogeneous	84.0	[121]

¹ p-hydroxybenzenesulfonic acid-formaldehyde resin acid catalyst. ² Sulfonated graphene oxide.

). Current industrial furfural production mainly involves the use of a homogeneous catalyst such as sulfuric acid in low concentrations (3–6%), in batch or continuous reactors, and using high constant temperatures (between 180 and 230 °C) [107]. This technology has some drawbacks, including the limited furfural yields (55% average) due to undesired side reactions—such as resinification, condensation (cross-polymerization), and fragmentation reactions—the high steam consumption, and the equipment corrosion or requirements for high-cost materials due to the use of hot mineral acid [108]. Among organic acids, formic acid is mainly used to produce furfural. In addition, formic acid is produced during furfural production via decomposition reactions of hemicellulose and furfural, leading to an auto-catalyst system and reducing production costs [109].

Heterogeneous catalysts would be beneficial because of the principles of green chemistry. Compared with homogeneous catalysts, they are reusable, which solves the problems of separation and recycling. Several have been widely studied, including metal oxide, metal salts, hetero-poly acids, natural clay minerals, and ion exchange resins. The stability and recycling performance of solid catalysts will tend to be the focus of researchers in the coming years [106].

The formation process of furfural can occur in one or two steps (Figure 6). The one-step (“one-pot”) process involves the hydrolysis of arabinoxylans to xylose monomers and simultaneous dehydration to furfural. Rozefelde et al. [122] performed a catalytic hydrothermal treatment using aluminum sulfate (Al₂(SO₄)₃). They reported that the increase in this salt catalyst leads to the formation of high furfural and glucose yields, showing that it is possible to directly convert hemicelluloses into value-added products while enhancing the glucose content of the media for further transformations. The two-step process involves hydrolysis of pentosans under mild conditions and subsequent dehydration of xylose into furfural. The two-step process produces a higher furfural yield than the one-step process [109].

Despite rising demand for furfural, current manufacturing procedures are costly, energy-intensive, and not environmentally friendly [123,124]. The use of lignocellulosic biomass, such as BSG, more specifically its hemicellulosic portion, despite being a good alternative towards green chemistry, has not yet been shown to be sufficient to make the process viable. Additional research into scaling-up challenges, potential biological implements in furfural production, and optimization of catalysts are critical topics of research for the coming years.

3.3. The Use of Catalysts in Hemicellulosic Biorefinery: What’s Next?

The extensive use of BSG hemicelluloses in biorefineries is still very limited to a few products, such as XOS and xylitol, and is linked to scientific research that seeks to make such processes viable for industrial use. Developing possible catalysis mechanisms targeting the heterogeneous characteristics of different biomasses is a challenge for implementing this technology on a larger scale [125].

Homogeneous catalysts, such as minerals and organic acids, are well-established strategies used in the industry to increase the productivity and consequent competitiveness of products derived from various types of biomass. Minerals and diluted organic acids are often used as catalysts to obtain hemicellulose monomers (xylose, arabinose) and valuable products. In contrast, metal salts (Lewis acids) recently showed potential to be used for this purpose [8]. However, despite having excellent yields for production, homogeneous catalysts have the drawbacks of being difficult to recover and generating undesirable reaction byproducts, which leads to environmental concerns. Thus, the employment of chemical and biocatalysts (enzymes) in the processing of lignocellulosic biomass is a field of study in constant development.

Therefore, using heterogeneous catalysts has several advantages, such as the easy recovery of the catalyst, high reuse potential, and low waste generation. However, regarding the hydrolysis of polysaccharides, mass transfer limitation between the solid polysaccharides and SAC is the most significant limiting factor for extensive use since this implies a lower product yield. In addition, the polysaccharide chains in hemicellulose are quite heterogeneous and may be constituted by glucuronoxylan, arabinogalactan and other galactans, mannans, and xylans [126], which means that a catalyst designed for one biomass may not be fit for use with another. The design of SAC for hydrolysis, such as for polysaccharides, is challenging, demanding characteristics such as solid binding sites for adsorption, strong acid sites for degradation, and high specific surface areas to enhance the catalytic activities [7,8]. Another aspect that needs to be considered when developing HTC is the necessity to use aqueous-phase processes to convert hemicellulose into bioproducts. Hence, the catalyst must be chemically and structurally stable under this condition, especially its surface acidity [127].

Enzymes are biocatalysts utilized mainly in the industry for the most diverse purposes. These molecules are biocompatible, biodegradable, safe, and derived from renewable resources. The hydrolysis of hemicellulose demands the activity of several enzymes simultaneously, including endoxylanase, xylan 1,4-b-xylan esterases, ferulic and p-coumaric esterases, α-1-arabinofuranosidases, α-glucuronidase, α-arabinofuranosidase, acetyl xylan esterase and α-4-O-methyl glucuronosidases xylosidase. The use of biocatalysts in biorefineries has many advantages, such as mild reaction conditions, fewer effluents, low energy demand, and high specificity. However, it requires long reaction times, usually days, so it is rarely used on an industrial scale for this purpose. Another drawback related to biocatalysis is that enzymes are water soluble and thermo-degraded, making it difficult to recover them from the high-temperature aqueous media generally used in lignocellulosic biorefineries.

Although enzymatic immobilization solves problems related to recovery and stability, the technique has not been widely applied because immobilization can impact the catalytic performance of the enzyme [128,129]. Consequently, most enzymes are currently involved in single-use processes, which is unsuitable for environmental and cost-related reasons. Currently, the use of nanomaterials, such as magnetic/carbonaceous materials and metal–organic frameworks, bioprospecting of native yeasts strains, and techniques of genetic engineering (such as directed evolution, gene editing, and heterologous expression) are being explored to improve thermal and pH stability, resistance to inhibitors, regenerative capacity, activity, and the reusability of different enzymes, showing positive and promising results [8,130,131].

4. Conclusions

The residue of the brewing process, BSG, is a lignocellulosic biomass with great potential for use in second-generation biorefineries. Its structure contains an essential amount of hemicelluloses, a polysaccharide of heterogeneous form consisting mainly of xylose, arabinose, and uronic acids that can be transformed into value-added bioproducts such as xylooligosaccharides (XOS), xylitol, second-generation ethanol, enzymes, biofilms, and furan derivatives, such as furfural and furfuryl alcohol though catalyzed bioprocesses.

The use of catalysts is crucial to improve the feasibility of biorefinery processes. Homogeneous catalysts, such as mineral and organic acids, are well-established and widely used strategies. However, methods involving heterogeneous and biocatalysts (enzymes) are gaining more attention due to their advantages related to the growing environmental concern surrounding industrial processes. Despite this, studies still need to advance to make these methods viable on a large scale, especially regarding these molecules’ recovery, reuse, and stability.

Lignocellulosic-based biorefineries are ecologically sustainable alternatives that have been the subject of studies in the diverse areas of science for years now. Currently, one of the main challenges for the actual implementation of this concept is to enable its processes to become competitive against the traditional industry so that we can finally achieve a circular economy and secure the sustainable future of this planet.