A Review of Chemical and Physical Analysis, Processing, and Repurposing of Brewers’ Spent Grain ^†

Abstract

Beer production produces significant amounts of brewers’ spent grain (BSG), a lignocellulosic by-product with important environmental and economic impacts. Despite its high moisture content and rapid microbial breakdown, BSG has a stable, nutrient-rich composition, especially high in protein, fiber, and polyphenolic compounds. While its perishability limits direct use in food systems, BSG is often repurposed as livestock feed. Recent advances in bioprocessing and extraction technologies have expanded their use across different sectors. This review explores the composition of crude BSG and evaluates innovative valorization methods, including recovering bioactive compounds with pharmaceutical and nutraceutical value, and converting them into biofuels such as biogas, biodiesel, and bioethanol. Special focus is given to methods involving enzymatic hydrolysis, fermentation, and chemical extraction to isolate proteins, peptides, amino acids, sugars, and polyphenols. By analyzing emerging applications and industrial scalability challenges, this review highlights BSG’s growing role within circular economy models and its potential to promote sustainable innovations in both the brewing industry and the wider bioeconomy.

1. Introduction

1.1. Bachround

The Reinheitsgebot, the beer “purity law” initiated in Bavaria in 1516, defined beer as consisting only of malted barley (Hordeum vulgare L.), hops (Humulus lupulus L.), and water [1,2,3,4]. This strict limitation of ingredients and other regulatory stipulations in this ruling has had widespread ramifications for the production of beer, especially in Germany and worldwide into the present day. Of course, if the biological nature of yeast (primarily Saccharomyces spp., especially S. cerevisiae, but also other taxa) had been recognized at that time, over a century before the invention of the compound light microscope, it almost assuredly would have been addressed and included as an essential ingredient, as it was in later iterations of the law [3,4]. Beyond the native German brewing industry, even in neighboring European countries and indeed, in the U.S. currently and worldwide, the use of un-malted barley and other grains (wheat, oats, rye, rice, maize, millet, sorghum, etc.) as well as cane sugar and corn syrup and the addition of other herbs and spices besides hops is now commonplace. Even though the Reinheitsgebot has had an undeniable role in setting the stage for what is normative for beer, its production on the ground both before and after the early 16th century has always varied and will remain complex.

Fortunately for those interested in beer waste streams, it is important for scientists to recognize the variation in biological, chemical, and physical properties from brewery to brewery [5,6,7,8]. These minor differences are not enough to hinder large-scale use of brewers’ spent grain [7,8]. In fact, there is plenty of waste available. As of 2022, global beer consumption reached approximately 192.1 million kiloliters, a 2.9% increase from the previous year, indicating recovery after the pandemic slump [9,10,11]. Recent data shows that the European Union produces approximately 3.5 million metric tons of brewers’ spent grain (BSG) annually, slightly more than the previously reported 3.4 million metric tons [9,12], with the United Kingdom alone contributing over 0.5 million metric tons each year. Beer production is a significant industry within the food and beverage sector. In 2013, EU-28 beer production was around 383.6 million hectoliters [9]. In 2002, Brazil, the world’s fourth-largest beer producer at 85 million hectoliters per year, generated approximately 1.7 million metric tons of spent grain [9,13]. Based on 2011 global beer production of 1.93 billion hectoliters, the worldwide annual generation of brewers’ spent grain is estimated at 38.6 million metric tons [14]. Globally, barley ranks as the fourth most important cereal after wheat, maize, and rice, mainly grown for beer production or as animal feed [13,15,16]. Although there are chemical and nutritional differences between brewers’ spent grain and raw barley, spent grain is primarily repurposed as animal feed due to its high dietary fiber and protein content [17,18]. These qualities that make brewers’ spent grain attractive as animal feed have also spurred research into upcycling this waste stream for health foods for humans [17,18,19]. Currently, spent grains are often used as cattle feed and as human nutritional supplements [11,20]. Only in recent years have people seriously started considering spent grain as a source for biofuel, driven by decreasing profits from silage, rising energy costs, and policies promoting renewable energy [20]. A recent bibliometric analysis on brewers’ spent grain found that several countries studied its valorization, with Brazil being the most prolific, mainly in biofuel production. Meanwhile, interest in developing other value-added products is also increasing, though logistical issues related to biomass pretreatment and industrial-scale processing still need to be addressed through research and practical use cases [21]. The potential for growth in this field is substantial, and research into brewers’ spent grain continues to expand and evolve, focusing on new applications and innovations.

1.2. Origin and Composition of Brewers’ Spent Grains

Brewers’ spent grain is one of the organic residues produced by breweries, which can also include brewery wastewater, spent hops, trub, and yeast [22,23]. However, spent grain is the main waste type generated by breweries, often accounting for (up to) 85% of all waste from beer production [24,25,26,27]. This waste stream contributes 30–60% of the biochemical oxygen demand and suspended solids at any given brewery [9,28].

Between 60% and approximately 75–80% of the mass of brewers’ spent grain is found to be attributable to its water content [25,29,30]. This makes it critical, regardless of the application, that this moisture is largely eliminated, that the material is treated, and/or that it is utilized immediately in such a way that its degradation does not begin, as brewery spent grains’ shelf life in warm climates is 7–10 days owing to their high-water content [13,31]. Spent grain retains 31% of the original malt weight, representing approximately 20 kg per 100 L (hL) of beer produced [13,24]. This material, not including water content, in the spent grain is mainly lignocellulosic (70% fiber, cellulose, hemicelluloses, lignans, etc.), with the remainder accounted for by protein content (20%) as well as lipids (~10%) [32,33,34]. Scattered literature reports indicate that brewers’ spent grains contain a significant amount of protein (18–30 wt.%). However, the protein content entirely depends on BSG raw ingredients, additives, and brewing processes.

Table 1. Composition of brewers’ spent grain reported from various peer-reviewed sources by compound class/primary type of material (% dry weight basis).

Paper	Lignin	Cellulose	Hemicellulose	Starch	Phenolics	Lipids	Protein	Ash
Kanauchi et al., 2001 [38]	11.9	25.4	21.8	—	—	10.6	24.0	2.4
Carvalheiro et al., 2004 [39]	21.7	21.9	29.6	—	—	—	24.6	1.2
Silva et al., 2004 [21]	16.9	25.3	41.9	—	—	—	—	4.6
Mussatto and Roberto, 2006 [25]	27.8	16.8	28.4	—	—	—	15.2	4.6
Jay et al., 2008 [15]	20.0–22.0	31.0–33.0	—	10–12	1.0–1.5	6.0–8.0	15.0–17.0	—
Xiros et al., 2008 [40]	11.5	12.0	40.0	2.7	2.0	13.0	14.2	3.3
Robertson et al., 2010b [7]	13.0–17.0	—	22.0–29.0	2.0–8.0	—	—	20.0–24.0	—
Niemi et al., 2012a [41]	19.4	46.7 *		2.8	—	7.8	23.3	4.9
Sobukola et al., 2013 [42]	9.2 ± 0.1	±0.3		—	—	6.2 ± 0.1	24.4 ± 0.5	2.5 ± 0.1
Kemppai-nen et al., 2016 [43]	19.6	45.0 *		—	—	—	20.3	4.1
Yu et al., 2020 [44]	—	51.0 ± 0.7 *		—	—	9.4 ± 0.1	23.4 ± 0.2	4.1 ± 0.1
Naibaho and Korzeniowska, 2021a [5]	—	50.7 ± 0.4–60.2 ± 1.7 *		—	—	9.5 ± 0.5–13.1 ± 0.3	22.2 ± 0.1–30.2 ± 0.1	3.3 ± 0.1–4.3 ± 0.1

* = Total carbohydrates (cellulose and hemicellulose etc.). Adapted in part from Jackowski et al., 2020 [45].

shows the composition of the brewery’s spent grains, including proteins from the literature. Among them, amino, proline, and glutamic acids are the most prevalent proteins found in BSG. Extracting proteins from BSG is a viable idea to increase economic value. Although several extraction methods have been reported in the literature, the alkaline extraction followed by precipitation is the most common method for protein extraction [17,34,35,36,37]. Additionally, these protein-rich renewable sources could be used as N-doped precursors during the thermochemical process, which adds additional value in the circular economy [2,34]. Furthermore, converting these protein-rich wastes into pellets via a thermochemical process enhances soil health and improves the modern agro-farming system [2,37]. Additionally, BSG is also a source of bioactive compounds, which include phenolic compounds as well as insoluble dietary fiber or proteins.

During the germination of barley, the stage of the malting process after steeping, much of the cell walls of the endosperm are enzymatically broken down as the embryo develops and the rootlets, collectively known as chit, appear. The reason why barley has been historically preferred, out of all the cereals, in the beer brewing process is that barley grains anatomically have two, three, or even several cell-thick layers of aleurone cells, whereas all other economically significant cereals only have a single cell-thick aleurone layer [3,23,24]. These cells contain distinctive, protein-rich aleurone grains, which bear enzymes capable of hydrolyzing starch into smaller polysaccharides, oligosaccharides, and simple sugars. This unique architecture involving a multilayered aleurone cell band within the barley grains results in more enzymes being produced and released during malting and translates into greater diastatic power to convert starches into fermentable sugars. Steeping and germination are the two initial steps of malting that allow this diastatic power to develop [25]. Kilning, the final step, dries out the germinated grains, halts their further development, and depending on oven temperature and other factors, may produce new desirable flavors through caramelization and Maillard reactions as well as alter the malt’s diastatic power [3,25]. Figure 1 shows the malting and different stages of the brewing process [13,14].

Once malted barley reaches the brewery, it is milled and then mixed with water in the mash tun, the large container where the mashing process occurs [3,25]. The temperature of this initial mash is gradually increased to catalyze the breakdown of its components, particularly starches, but also other compounds such as proteins, (1,3;1,4)-β-D-glucans, and arabinoxylans from malted barley and other parts of the grain bill [3,14,32]. The grain bill is essentially the list and quantities of pre-boil malts and adjuncts (and occasionally hop additions at this stage) used in mashing. During this process, starch is converted into fermentable sugars (mainly maltose and maltotriose) and non-fermentable sugars (dextrins), while proteins are partially broken down into polypeptides and amino acids [3]. This mashing stage, which depends on the diastatic power of the malts and other factors like temperature and pH-parameters that significantly influence enzyme activity—produces a sweet liquid called wort [14]. The insoluble components of the wort are allowed to settle and form a bed in the mash tun, after which the liquid containing the hydrolysates is filtered through this bed, a process known as lautering [3,14]. Sometimes, a separate lauter tun vessel is used for this stage. The beer is then created by fermenting the wort with yeasts and other micro-organisms [3,14]. After lautering, the leftover material from the grains—mostly or entirely barley—consists of the collective aleurone cell layers, pericarp (i.e., the bran), and any remaining insoluble endosperm material, collectively called brewers’ spent grain [13]. In summary, the total solids and water remaining after mashing and lautering comprise the brewers’ spent grain.

By the time the mashing and lautering processes have run to completion, the chemical and physical nature of brewers’ spent grain is dominated by the combined husk/pericarp/seed coat layers that constituted the exterior tissues of the original barley grain, which are most of the remaining tissues; mashing selectively solubilizes mostly what is necessary to produce the wort and ultimately the beer, leaving behind hydrophobic proteins and some lipids as well as the components of cell walls [13,46,47,48]. The evenness of the malting process or other factors may also determine whether or not and to what degree some adhering aleurone cell layer and starchy endosperm, especially remnants of cell wall materials and potentially a small number of leftover starch granules, may form part of the brewers’ spent grain observed [13]. Depending on the desired beer style and the grain bill used to produce its wort starting material, spent grain consists either entirely of the remains from malted barley or of an even more heterogeneous material derived from malted barley and additional adjuncts (malted and un-malted sources of fermentable sugars), including un-malted barley, wheat, oats, rye, rice, and maize [3,13].

2. Analyzing the Physical and Chemical Composition of Brewers’ Spent Grain

2.1. Nutritional Contents

Brewers’ spent grain is typically assumed to contain minimal starch [13,49]. However, research by Jay et al. (2008) revealed that appreciable amounts of starch can remain in material [15]. In their study, the spent grain was separated into five particle-size fractions using vibratory sieving and milling. The coarser fractions—captured on 500-, 250-, and 150-µm sieves—primarily consisted of arabinoxylan-rich outer tissues like palea and lemma. Meanwhile, finer particles passing through 106- and 55-µm sieves were enriched in starch and proteins. Although lignin appeared in all fractions, its presence in the finer samples resulted solely from trace amounts of palea and lemma. Beyond its role in analysis and separation, milling serves as a vital step for improving component extraction from spent grain. One study demonstrated that fine milling significantly enhanced the solubility of carbohydrates prior to enzymatic hydrolysis. This improvement was attributed both to the increase in surface area from particle size reduction and the mechanical release of water-soluble sugars [41,50].

Brewers’ spent grain is predominantly composed of the outer structural layers of the barley grain—specifically the husk, pericarp, and seed coat—which are rich in lignocellulosic materials such as cellulose, non-cellulosic polysaccharides (primarily hemicelluloses like arabinoxylans), and lignin [13]. These components make up the bulk of the biomass on a dry weight basis. In addition to these major components, brewers’ spent grain also contains moderate amounts of protein and lipids. The husk in particular is a significant contributor of silica, largely in the form of biomineralized phytoliths unique to barley. These silica bodies appear as highly charged, reflective points in scanning electron micrographs [13,51,52], and are thought to represent up to 25% of barley’s total mineral content [14,35,51,52].

While the composition of brewers’ spent grain can vary, it consistently reflects its origin in both grain and processing factors. Variables such as the barley cultivar, harvest timing, malting conditions, mash formulation, and even lautering parameters all influence the final makeup of spent grain [14,21,32,53,54]. Despite this, brewers’ spent grain is reliably classified as a lignocellulosic biomass, generally comprising around 70% dietary fiber (primarily cellulose and hemicellulose) and 20% protein by dry weight. Microscopic and compositional analyses reveal that spent grain retains numerous fibrous tissues from barley’s outer layers. These are largely composed of arabinoxylan, lignin (a polyphenolic structural polymer), and cellulose (a linear glucose homopolymer). In one study of oven-dried brewers’ spent grain, the material was found to contain approximately 24.2% protein, 3.9% lipid, and 3.4% ash [14,54]. These values reflect the effective removal of starch during mashing, which concentrates the fiber and protein in the spent fraction [49].

Among the hemicelluloses, arabinoxylans are the most common in brewers’ spent grain. Cereal grains typically contain 5–15% of their dry weight as cell wall material, mostly composed of non-starch polysaccharides like hemicelluloses [24,34,55,56]. In barley’s starchy endosperm, arabinoxylans make up approximately 20% of the cell wall polysaccharides, whereas in wheat this can be as high as 72% [24,57]. For comparison, commercial wheat bran—which mainly consists of the pericarp, testa, and aleurone layers, with some endosperm contamination—has been found to contain 38% glucuronoarabinoxylan, 16% cellulose, 6.6% lignin, and 25% protein, along with aleurone-derived (1,3;1,4)-glucans [58,59,60]. These values provide insight into the structural complexity and nutritional potential of brewers’ spent grain as a fiber-rich by-product.

2.2. Brewers Spent Grain: Composition, Stability Challenges

Brewers’ spent grain (BSG) is the solid by-product primarily generated during the mashing and lautering stages of the brewing process. Mashing involves mixing milled malt and other grains—collectively known as the grain bill—with water at controlled temperatures to extract soluble carbohydrates, enzymes, amino acids, and other key components into a fermentable liquid called wort. This wort is subsequently separated from the solid grain residue through lautering and then fermented, typically with yeast and, in some styles, additional microbes such as lactic acid bacteria or other eubacteria [3,11,31].

The resulting BSG is a moist, fibrous material high in polysaccharides, proteins, and—critically—water. This moisture-rich environment makes it especially susceptible to microbial contamination and rapid spoilage [7,8]. One frequently overlooked factor contributing to its perishability is its relatively high lipid content—specifically free fatty acids and triglycerides—due to its origin from barley’s bran-rich outer tissues. These lipids are prone to enzymatic, microbial, and oxidative degradation, leading to rancidity if the material is not promptly stabilized after collection.

Given its biochemical composition, long-term storage of BSG without proper treatment poses logistical and quality challenges. Efficient preservation—such as drying, refrigeration, or chemical stabilization—is essential for its successful reuse or industrial upcycling. Additionally, the chemical profile of BSG can vary depending on multiple factors: barley cultivar, harvest timing, malt kilning temperature, mash composition, and specific brewing conditions. For instance, ale malts are often kilned at higher temperatures than lager malts, which are typically derived from barley with higher protein content [3,8,61]. Regional agricultural practices also influence grain properties; barley grown in the Americas, for example, tends to exhibit slightly higher protein content than its European counterparts [3,40,44].

In some breweries, BSG is co-mingled with other production residues such as surplus yeast from fermentation and trub, a protein-rich sediment resulting from wort boiling and hop additions. These additions may further impact the microbial load, nutrient content, and shelf stability of the spent grain stream [8,42].

2.3. Unlocking the Biochemical Potential of Brewers’ Spent Grains

Brewers’ spent grain (BSG), often regarded as a waste byproduct, contains a wealth of underutilized phytonutrients—including phenolic compounds like ferulic acid, various lignans, non-cellulosic carbohydrates such as arabinoxylans, lipids, and proteins. Analytical techniques like microscopy and chemical profiling have revealed that BSG is rich in valuable components, notably feruloylated arabinoxylans and proteins [15,34]. Certain lignans—primarily syringaresinol and secoisolariciresinol—have been detected after enzymatic treatment of cell wall residues [41,50]. Ferulic acid, concentrated especially in the aleurone layer (up to 75% of total ferulic acid in barley), also resides in smaller amounts in the starchy endosperm [24,62]. Its antioxidant, antimicrobial, and anti-inflammatory effects are of growing interest in both health and industrial sectors [24].

Currently employed as a preservative in the food industry due to its ability to inhibit lipid peroxidation, ferulic acid also plays a pivotal role in the chemistry of arabinoxylans, influencing gel formation for applications like wound dressings [24,63]. Additionally, it serves as a natural precursor to vanillin through microbial fermentation [48]. In BSG, hydroxycinnamic acids like ferulic and p-coumaric acid are often ester-linked to arabinofuranosyl residues, a key factor influencing the structural and functional properties of arabinoxylans [64,65]. The extent and pattern of these esterifications—along with diferulic acid-mediated interpolymer crosslinks—directly affect arabinoxylan solubility and matrix behavior [66].

Diferulic acid dimerization can result in “lignan-like” (8,8’) or “neolignan-like” linkages, with the type and prevalence varying among grains and processing conditions. For instance, 5,5’ linkages are dominant in barley bran, whereas 8-O-4’ linkages have appeared more prominently in some BSG samples [67,68]. Comparative studies show millet and maize contain the highest levels of these dimers, with barley, rye, and wheat exhibiting moderate amounts, and oats and rice showing significantly fewer [69]. While arabinoxylans have been well characterized in barley and malt, there remains a notable gap in the structural and biochemical understanding of those left in spent grain after mashing, marking an area rich with research potential [24,57,70]. Figure 2 shows the primary constituents of different sources of plants.

2.4. Yeast, Lipids, and Beer Chemistry: Insights from Brewers Spent Grain

Although lipids are essential to yeast metabolism and brewing physiology—and of increasing interest in waste upcycling and valorization efforts—they make up a relatively small part of brewers’ spent grain (BSG), accounting for only 3.9–13.3% of its dry weight [13,30,50]. One way to isolate and recover these lipids is through enzymatic degradation of proteins and carbohydrates. A two-stage enzymatic process has been shown to solubilize up to 66% of BSG, revealing a lipid content of 11%, mostly triglycerides with significant amounts of free fatty acids [50]. This solubilization was primarily driven by the alkaline protease step. Key fatty acids identified include linoleic, oleic, and palmitic acids. Another study identified the main lipophilic compounds in BSG as trilinolein, various dilinoleins and monolinolein, palmitic, oleic, and linoleic acids, an alkane (n-heptacosane), and an alkylresorcinol (1, 3-dihydroxy-5-n-heneicosylbenzene) [71]. Generally, triglycerides in BSG outnumber free fatty acids by a factor of two to five [50,71,72]. Adding BSG-derived lipids to wort significantly affected yeast metabolism under both aerobic and anaerobic conditions, increasing fuel alcohols while decreasing esters and medium-chain fatty acids [46,73]. Yeast absorbed sitosterol and unsaturated fatty acids from the BSG lipids, altering the profiles of fatty acids and sterols produced. Although triglycerides were inactive in this process, the combined effect of free unsaturated fatty acids, sitosterol, and phospholipids drove the metabolic changes. Interestingly, phospholipids could be replaced with Triton X-100 without impacting the outcome. Ultimately, the modulation of fermentation products was attributed to the action of unsaturated fatty acids [12,42,46,74,75]. This mechanism differs from the typical causes of fusel alcohol production in beer—such as elevated fermentation temperatures exceeding 27 °C with Saccharomyces cerevisiae [3,42]—but remains highly relevant for both brewing practices and the sustainable valorization of BSG and spent yeast. Figure 3 shows the primary lipid constituents of BSG.

Table 2. Selected lipid-related and nonpolar compounds reported by GC-MS from brewers’ spent grain material in Poerschmann and Górecki, 2018 [72].

Compound	Molecular Formula	Molecular Mass, Parent ion (Da)	Solvent Concentration (µg/g)	Saponifiable Extract Concentration (µg/g)
Fatty Acids
Arachidic acid	C20H40O2	312	65	330
Behenic acid	C22H44O2	340	90	400
Dimorphecolic acid	C18H32O3	296	115	91
2-Hydroxyarachidic acid	C20H40O3	328	65	180
Lignoceric acid	C20H48O2	368	105	440
Linoleic acid	C18H32O2	280	2450	12,200
Linolenic acid	C18H30O2	278	330	2900
Myristic acid	C14H28O2	228	310	1350
Oleic acid	C18H34O2	282	970	5150
Palmitic acid	C16H32O2	256	2850	20,300
Pentadecylic acid	C15H30O2	242	105	450
Phloionic acid	C18H34O6	346	90	310
Stearic acid	C18H36O2	284	455	2100
Monoacyl Glycerols and Diacyl Glycerols
1,3-Dipalmitoyl glycerol	C35H68O5	568	370	—
1-Linoleoyl-3-palmitoyl-rac-glycerol	C37H68O5	592	150	—
1-Monopalmitoyl glycerol	C19H38O4	330	1390	—
2-Monopalmitoyl glycerol	C19H38O4	330	110	—
1-Monolinoleoyl glycerol	C21H38O4	354	240	—
1-Monooleoyl glycerol	C21H40O4	356	710	—
1-Monostearoyl glycerol	C21H42O4	358	85	—
1-Palmitoyl-3-linoleoyl-rac-glycerol	C37H68O5	592	850	—
Sterols and Tocopherols
Δ5-Avenasterol	C29H48O	412	35	120
Campesterol	C28H48O	400	95	330
β-Sitosterol	C29H50O	414	205	710
α-Tocotrienol	C29H44O2	424	28	—
β-Tocotrienol	C28H42O2	410	12	—
Alkylresorcinol Derivatives
5-(2,3-Dihydroxypropyl)-2-methoxy-benzene-1,3-diol	C10H14O5	214	105	—
5-(2-Hydroxyethyl)-2-methoxy-benzene-1,3-diol	C9H12O4	184	160	—
5-(2,3,4-Trihydroxy-n-butyl)-2-methoxy-benzene-1,3-diol	C11H16O6	244	270	—

shows the lipid-related non-polar compounds identified by GC-MS [72].

Ideally, for scientific studies or industrial post-processing and upcycling, the origin of the brewers’ spent grain is clarified with all attention possible to the minutiae of its production, given that the assumption by default is that this waste stream is derived directly from the remains of mashing and lautering processes, before boiling and the addition of yeast to the wort. Suppose brewers’ spent grain can almost always be characterized as a direct by-product during the creation of wort. In that case, as long as microbial contamination is minimized, its overall chemical and physical properties are consistent, especially given the massive amount of material available. A combination of attention to the anatomy of barley grain and the additional variables that contribute to the chemistry of brewers’ spent grain will help develop and troubleshoot future research processes to isolate and generate specific desired bioproducts. Moreover, judicious use of appropriate microscopic and physical processing techniques, complementing extraction, chromatography, and analytical chemistry methods, can improve the workflow for isolating valuable components from this feedstock. Enzymatic treatment is another effective option that can be utilized to assist in the purification process. Altogether, brewers’ spent grain represents a copious and diverse source of valuable starting materials at an industrial scale, with potential applications beyond animal feeds and dietary supplements.

3. Drying and Processing Brewers’ Spent Grain and Other Brewery Wastes to Produce Isolates and Other Value-Added Products

3.1. Drying Challengesof BSG

Brewers’ spent grain (BSG), a byproduct of the brewing industry, contains more than 50% water by mass, making drying a critical first step for its preservation and further processing. Various drying techniques—such as rotary drum drying, annular drying, autoclaving, lyophilization, and superheated steam drying—are essential to stabilize BSG and enable downstream valorization. Once dried, BSG can undergo a range of extraction and processing methods aimed at isolating valuable components like proteins, fibers, and bioactive compounds. These include supercritical carbon dioxide extraction, autohydrolysis, alkaline and dilute acid hydrolysis, solvent extraction, ultrasound- and microwave-assisted extraction, and enzymatic hydrolysis. Notably, enzymatic treatments can be coupled with microbial fermentation, either through intentional inoculation with specific strains or via spontaneously developing consortia, to produce energy-rich biofuels or other co-products, such as microbial lipids and carbohydrates. Such valorization routes not only enhance the economic viability of BSG but also mitigate environmental concerns by diverting organic waste from disposal streams [17,39].

Due to its high moisture content—exceeding 60% by mass—the drying process of brewers’ spent grain (BSG) is critical, as it significantly impacts both its chemical composition and subsequent upcycling potential. Thermogravimetric analysis has been employed to investigate drying kinetics under isothermal conditions (60–90 °C), offering insight into optimal dehydration strategies [30]. While rotary drum drying remains widespread, alternative methods like refrigeration and autoclaving have been explored as less energy-intensive options. Nevertheless, despite improved stability compared to room temperature storage, refrigeration at 4 °C and autoclaving have been shown to degrade starch, arabinoxylans, and polyphenolic compounds. In contrast, −20 °C storage best preserves BSG’s carbohydrate and phenolic fractions [68]. Superheated steam drying is a particularly promising alternative, as it minimizes starch gelatinization and monosaccharide release, while providing shelf stability at ambient conditions for up to six months [12,18,19,68]. The role of microstructure is vital in these processes, as it influences both material properties and subsequent processing behavior. Studies using X-ray micro-computed tomography revealed that superheated steam drying of distillers’ spent grain pellets increased expansion and open porosity by 90–133%, reducing drying time by 81% relative to hot air drying [76]. Similar structural behavior has been observed in compressed BSG, where porosity and permeability decrease linearly with increased compaction [77]. Thus, while various drying methods are viable, superheated steam drying stands out for its efficiency and preservation potential, and frozen storage at −20 °C remains a key strategy for maintaining biochemical integrity before further processing.

3.2. Microbial Activities

Research utilizing different incubation conditions with cellulase and hemicellulase enzymes as an attempt to hydrolyze and solubilize most of the carbohydrates remaining in brewers’ spent grain has led to seemingly counterintuitive and exciting results, with low yields demonstrating the diversity of carbohydrate chemistries present. While monosaccharides represented 13–14% of the dry weight of the solubilized material, this corresponds solely to 26–28% of the total carbohydrates present and 30–34% of the original arabinoxylan content [78]. The study concluded that the low hydrolysis level indicated the ineffective activity of the cellulase-hemicellulase formulae applied to the spent grain [78]. Although, where and when relevant, feruloyl esterase activity in each enzyme cocktail utilized in that study also led to the release of free ferulic acid as well as arabinoxylan-derived oligosaccharides and monosaccharides, the unhydrolyzed fraction always contained over 40% of carbohydrates after enzymatic treatment [78]. A small but potentially significant amount of residual starch is known to be present in brewers’ spent grain, but, in that case, it did not account for more than a tiny fraction of the total dry mass content (>2%) [15,78,79]. β-glucans are derived from the remaining cell wall material of the small proportion of cell wall material of the starchy endosperm, whereas arabinoxylans predominate in the aleurone layer [78]. Proteins were overall not solubilized or altered by any of the enzymatic treatments, and in addition to the protein content and refractory carbohydrates, the insoluble residuum was enriched in lignin and lipids [78]. The efficient valorization of brewers’ spent grain at the industrial level requires separations and chemical reactions to proceed at that scale [80,81]. One study attempted a novel fractionation of spent grain to produce arabinose and xylose as well as arabinoxylan oligomers, on the one hand, and bioethanol from the fermentation of glucose on the other [80]. Two-step acidic and enzymatic hydrolysis step regimes were investigated, leading to successive high arabinose and arabinoxylan oligomer-rich as well as xylose-rich acidic hydrolysates, with the subsequent enzymatic hydrolysate yielding a glucose-enriched supernatant usable for bioethanol production via fermentation with commercial baker’s yeast [80]. The composition and structural features of arabinoxylans have been studied in the past few decades and are now well known [24,57,82,83], although fewer studies exist that are specifically pertinent to brewers’ spent grain. Further fractionation of arabinoxylan can yield both xylose, an intermediate for the production of xylitol, and a variety of xylan oligosaccharides, both of which are already important products relevant to human diet and oral/gastrointestinal health [24,55,56,79,84,85]: xylitol has already been employed for food applications, including chewing gum and toothpaste, and may be utilized as a noncaloric sweetener for people with diabetes mellitus, whereas, moreover, xylooligosaccharides (also known as xylan oligosaccharides) are prebiotic compounds, bearing the potential to promote the growth of beneficial bifidobacteria.

Brewing residues include brewery wastewater and brewers’ spent grain (as well as excess yeast), among other categories, with these two being of primary concern for brewery operation and sustainability, and a major line of investigation for valorizing these waste streams is through anaerobic digestion or other processes to produce biofuels: biogas, including biomethane, bioethanol, and biodiesel. Brewing wastewater treatment has been thoroughly addressed and well investigated, with granular biomass technology being the predominant technological approach [1,20,44,65,86,87,88]. When evaluating brewery waste both to optimize its valorization and to prevent its release into the environment as a pollutant source, it becomes essential to monitor parameters such as pH; temperature; total suspended solids (TSS); chemical oxygen demand (COD); and biological oxygen demand (BOD), as well as to collect samples from multiple stages of the process on-site, including from the brewhouse, the fermentation sector, the barrel and bottling systems, and the pipe wastewater effluent. One such study found that the highest organic pollutant loads originate from the fermentation sector of the brewery, with samples examined bearing high values of BOD (500–1500 mg/L), COD (650–2500 mg/L), and TSS (70–450 mg/L) [22]. Wastewater from the brewhouse as well as the barrel and bottling systems contains significant quantities of organic matter, with a variable pH dependent upon the cleaning chemicals utilized (3.4–9.8) [22]. Waste minimization proceeded in this case through the clean-in-place (CIP) method, which involved neutralization and tank treatment for wastewater before discharge from the brewery [22]. These methods reduce organic wastewater content by ~40% [22]. The parameters and optimization of processes relating to waste yeast anaerobic digestion have also been investigated and tested successfully at an industrial scale [20,44,88,89], although valorization would be just as or more liable to serve nutraceutical and similar purposes given the amount (15 kg/10 hL)/underrealized potential of yeast waste derived from breweries in the food and beverage industry [45]. Exhausted brewery yeast, like spent grain, has, through the present day, primarily been offloaded as animal feed, if anything.

3.3. Overall Challenges and Limitations

The disposal and valorization of spent grain thus have remained, by comparison, a vastly underutilized waste stream for anaerobic digestion in particular [20]. Anaerobic digestion of brewers’ spent grain, consisting mainly of cellulose, hemicellulose, and lignin, is difficult, possibly partly because certain polyphenolics present can inhibit the degradation of these natural polymers, let alone proteins (and lipids) [20]. To deal with this reality of a potentially inefficient anaerobic digestion process for spent grain, a two-stage system was established: anaerobic digestion occurred in a solid-state anaerobic digestion reactor where microbiological hydrolysis and acidogenesis proceeded, and in a granular biomass reactor where mostly methanogenesis occurred [20]. The overall process was between 75.9 and 83.0% efficient in terms of total solids degradation, with the average specific biogas production being 414 ± 32 L/kg of added total solids, of which 224 ± 34 L/kg pertained to biomethane production [20]. This anaerobic process was stable over 198 days and exhibited healthy eubacterial and archaeal communities contributing throughout. The efficiency and production demonstrate that brewery spent grain can be successfully anaerobically digested and used for biogas production, a potentially implementable, economical option for reducing net energy usage and minimizing environmental impact [20]. Biodiesel production from brewers’ spent grain has also been reported, employing an acid-catalyzed in situ transesterification process at different catalyst concentrations, methanol-to-feedstock ratios, reaction times, and temperatures [30]. Promisingly, it seems that rather than focusing on a single energy-rich end-product, biodiesel, bioethanol, and biogas can be generated from an integrative, adjustable process depending upon need and upon the pre-processing of the spent grain [90]. The extraction efficiency of lipids (free fatty acids and triglycerides, etc.) from spent grain approached ~70%, utilizing a hexane-based solid-liquid extraction process [90]. Ethanol yield from spent grain material reached 45%, given the acid pre-treatment step, enzymatic hydrolysis with Cellic CTec2, and subsequent fermentation with Saccharomyces cerevisiae [90]. Raw spent grain, defatted spent grain, and spent stillage/digestate presented values equal to 379 ± 19, 235 ± 21, and 168 ± 39 L biogas generated/kg material, respectively [90]. Given these data and additional modeling, brewers’ spent grain could ensure energy production in the range of 4.5–7.0 million MJ/y if Europe’s spent grain alone were fully valorized in this way, yielding a potentially remarkable contribution to an overall energy strategy at the regional/global scale [90]. In another study, spent grain was pretreated by two distinct methods, namely, a microwave-assisted alkaline process and organosol, the latter a pulping technique using a heated organic solvent to solubilize lignin and hemicellulose from a substrate, treatments that were then evaluated for glucose and xylose production during enzymatic saccharification trials [26]. The hydrolysate from the organosolv-pretreated brewers’ spent grain (the more efficient, productive treatment in this study) was then employed as a substrate for the cultivation of Rhodosporidium toruloides, aiming to produce lipids from this yeast as a raw material for generating biodiesel [26]. The microbe generated up to 18.44 ± 0.96 g/L of cell dry weight and 10.41 ± 0.34 g/L lipids while cultivated on spent grain hydrolysate (C/N ratio of 500) [26,34]. The dry cell weight, lipid concentration, and lipid content were higher compared to the results obtained from the yeast’s growth on synthetic media containing glucose, xylose, or a mixture of glucose and xylose, and its lipid profile was similar to vegetable oils, promising for any biodiesel production that might result from these yeast lipid precursors [26]. In line with several studies evaluating anaerobic digestion as a possibility to upcycle brewery wastes but investigated at a smaller scale and in terms of chemical oxygen demand (COD), in another work, the specific methane production for both brewers’ spent grain and spent yeast was determined by way of biomethanation experiments carried out in 5 L fed-batch stirred reactors at several distinct substrate/inoculum ratios. The data ranged from 0.255 L CH₄ g⁻¹ COD for exhausted brewery yeast to 0.284 L CH₄ g⁻¹ COD for brewers’ spent grain, which certainly concords with the figures from other, industrial-scale studies [20,90].

Surfactants and emulsifying agents are other categories of useful value-added materials that have been investigated and derived from brewers’ spent grain. Often these are carbohydrate- or peptide/protein-based, although specific lipids and lipid fractions could also serve this purpose. A protein-enriched material derived from brewers’ spent grain was found to remain stable at 140 °C above pH 6.0 even after >300 min heating, and the barley proteins responsible for the emulsifying properties observed are known to exhibit more excellent foaming stability at alkaline pH [1,27,91]. Biosurfactants are produced naturally by microorganisms and are therefore biodegradable, often making ideal substitutes for energy-intensive and highly polluting chemical surfactants in many applications [92]. Large-scale production of biosurfactants is limited due to cost, but this could be reduced significantly through process optimization and by upcycling wastes as starting materials [92]. In one case, the optimization of biosurfactant production by the Bacillus subtilis N3-1P strain using brewery waste as the lone carbon source for the process was pursued by investigating five independent conditions in the context of this research: agitation speed, carbon concentration, nitrogen concentration, starting pH, and temperature. Surface tension and emulsification index were used to measure biosurfactant production, and in addition, experimental verification of modeled optima for this process was pursued [92]. Surfactin, a cyclic lipopeptide produced by many B. subtilis strains with excellent surfactant and emulsifying properties, has great potential as a next-generation antibiofilm agent in confronting antimicrobial resistance of extant and emerging pathogens [93]. As its high manufacturing cost is the major bottleneck currently, using widely, regularly available, and cheap feedstocks such as brewers’ spent grain to produce surfactin would represent an essential step toward more viable industrial-scale production [93]. One study showed that surfactin was made from Bacillus subtilis using brewers’ spent grain as the carbon source, with the strain producing 210.11 mg/L after 28 h [93]. Antimicrobial bioactivity was evident for all tested strains, with complete inhibition of growth for Pseudomonas aeruginosa, at 500 mg/mL; P. aeruginosa reached a growth log reduction of 3.91, Staphylococcus aureus and Staphylococcus epidermidis demonstrated between 1 and 2 log reductions in growth [93]. In the antibiofilm assays against P. aeruginosa, the co-incubation, anti-adhesive, and disruption showed significant bioactivity, where the inhibition observed was at its greatest with the co-incubation assay (79.80%) [93]. Overall, this study provides evidence that surfactin produced from brewers’ spent grain can be a promising biostatic agent, given its antimicrobial and antibiofilm abilities against pathogens [93]. Although surfactin is the most well-known component contributing to the surfactant and emulsifying properties of B. subtilis fermentations of brewers’ spent grain, levans, and/or other exopolysaccharides should also be considered and analyzed. These sticky materials excreted by the microbes are thought to contribute to the ropiness of bread and bread dough upon contamination, a defect of the crumb [48,94]. Interestingly, one of the main eubacterial taxa known to produce levans, Zymomonas mobilis [95], also carries out alcoholic fermentation or contaminates other similar processes as an ethanol-tolerant microorganism, given that it is a primary fermenter in contemporary palm wines [96], evidenced in ancient pulque production in Mexico [73], and sometimes found as the source of faults such as cider sickness and framboise (French for the word raspberry) in beers and ciders [97]. The biology of Z. mobilis leaves open the attractive possibility of using brewers’ spent grain as a carbon source for the simultaneous production of emulsifying/surfactant exopolysaccharides and bioethanol.

4. Monitoring and Studying Grain Precursor Materials and Brewers’ Spent Grain

4.1. Non-Destructive Phenotyping of Barley Husk Adhesion via Raman Spectroscopy

Strong husk-caryopsis adhesion is a defining attribute of malting barley cultivars and is notably rare among cereals. This trait is primarily governed by the caryopsis surface lipid profile and, over generations, has been enhanced through the selection of cultivars with sufficiently thin husks that facilitate efficient malting [59,98,99]. However, thinner husks also increase the risk of grain skinning—mechanical damage during harvest and processing that compromises husk integrity before malting—thereby diminishing both malting efficiency and final product quality [99]. To probe the biochemical basis of adhesion quality, refs. [98,100,101] employed Raman spectroscopy to characterize the lipid extracts of barley caryopses across cultivars with divergent husk adhesion properties. Multivariate analysis via principal component regression indicated that elevated concentrations of fatty acids, sterols, and triterpenes were associated with improved husk adhesion and reduced grain skinning. These data suggest a functional link between lipid class distribution and both mechanical resilience and adhesion strength. Although grain skinning is often used as a surrogate measure for husk adhesion [98,99,100,102], such assumptions should be cautiously evaluated. Skinning may arise not only from adhesion failure but also from intrinsic material weaknesses in the husk or caryopsis—a mechanism known as substrate failure [99]. Differentiating these causes is essential for accurately interpreting varietal traits.

Raman spectroscopy has shown potential for predicting husk adhesion quality in barley through cultivar-specific differences in the surface lipid composition of the caryopsis [101]. Principal component regression of spectroscopic data demonstrated the technique’s capacity to discriminate between high- and low-adhesion cultivars based on their associated lipidic signatures [101]. In earlier studies, total internal reflectance Raman spectroscopy was employed to directly analyze barley leaf surface waxes; its shallow penetration depth reduced interference from cell wall autofluorescence. However, to mitigate such interference in the current context, lipid extraction from the caryopses was required [59,101,103]. Looking forward, the development of Raman-based methodologies that enable direct, non-destructive measurement on intact caryopses may enhance the efficiency of phenotypic screening and accelerate breeding strategies for improved husk adhesion traits [101].

4.2. Lipid Complexity and Emerging High-Resolution Technologies

Starch granules within the endosperm of various cereals possess a distinctive lipid profile, notably containing significant quantities of monoacyl lipids. In cereals such as barley, rye, triticale, and wheat, these are predominantly lysophospholipids [104,105]. However, in other cereal taxa, free fatty acids comprise a larger proportion of the starch lipid fraction [104,105]. In barley specifically, saturated free fatty acids are a notable component of these starch-associated lipids [74]. Additionally, starch granules may adsorb surface lipids—primarily free fatty acids—from the surrounding endosperm lipid matrix [104,105]. While the caryopsis surface and embryo tissues tend to be relatively lipid- and protein-rich, the starchy endosperm compartment contains lipids both as inclusion complexes within starch granules and as distinct oil droplets. These droplets—termed oleosomes, oil bodies, or spherosomes—are stabilized, in part, by oleosin proteins [106]. Evidence from genomic, transcriptomic, and morphological analyses suggests these organelles also occur in the aleurone layer and embryo of barley and other cereal grains. In wheat, integrated lipidomic, transcriptomic, and bioimaging data have elucidated triacylglycerol (TAG) biosynthesis and accumulation in the developing starchy endosperm, with TAG content increasing markedly from 14 to 34 days post-anthesis—from 50 to 115 mg/100 g DW and from 35 to 175 mg/100 g DW in two experimental systems [106]. The principal fatty acids were palmitic (C16:0), palmitoleic (C16:1), stearic (C18:0), oleic (C18:1), linoleic (C18:2), and linolenic acids (C18:3), with unsaturated fatty acids constituting approximately 75–80% of the total lipid fraction during development. Linoleic acid (C18:2), in particular, was the most abundant and increased progressively throughout grain maturation [106]. Transcriptome analyses confirmed the Kennedy pathway as the primary route for TAG synthesis, involving elevated diacylglycerol acyltransferase activity. Confocal microscopy localized TAG-rich oil droplets to starchy endosperm cells, especially beneath the sub-aleurone layer, and revealed transcripts encoding 16-kDa oleosins, implicating their role in droplet stabilization [106].

The development of barley seeds involves rapidly differentiating tissues such as the pericarp, nucellus, nucellar projection, and endosperm, with maternal influences and hormonal signals like abscisic acid playing key roles, particularly through programmed cell death [75]. Integrative, spatiotemporal models utilizing microscopy, MRI, and emerging non-destructive techniques such as MALDI-based mass spectrometry are essential to synthesize disparate multi-omics datasets, including transcriptomics, proteomics, and metabolomics [75]. Tools like droplet probe LC-MS enable mm-scale resolution analysis of complex plant matrices, including barley grains and brewers’ spent grain [107,108], while SEM-EDS and X-ray micro-CT imaging have proven useful in characterizing structural and compositional features [77,109]. Portable FT-IR and Raman spectroscopy have also been applied effectively to analyze biological materials, indicating promise for in-field, noninvasive assessments of plant tissues [98,110,111]. Moreover, lipid imaging via MRI contributes to high-resolution, quantitative visualization of tissue-specific oil accumulation, supporting broader systems biology approaches to grain development and valorization [112].

5. Conclusions and Future Directions

Brewers’ spent grain (BSG), traditionally regarded as a waste byproduct of the brewing industry, has emerged as a valuable resource within the framework of the circular economy. Its repurposing not only mitigates environmental pollution but also supports sustainable development across multiple sectors. The compositional richness of BSG—including carbohydrates, lipids, proteins, and bioactive compounds—enables its transformation into a variety of products ranging from food and nutraceutical applications to uses in the pharmaceutical, biomedical, energy, and fuel industries. The integration of advanced analytical and processing technologies is pivotal for the systematic upcycling of BSG beyond conventional applications such as animal feed. A strategic focus on method development and reproducibility in processing, separation, and analysis is essential for the consistent recovery of high-value materials. This is particularly true when the interest in primary raw materials such as barley and malt is matched by a corresponding emphasis on BSG and its derivative products.

Recent advances in spatially resolved and noninvasive technologies—combined with insights gleaned from previous research and industrial practices—suggest a promising trajectory for the utilization of BSG. With continued innovation and cross-sector collaboration, the future of brewery waste valorization holds considerable potential for economic, environmental, and technological impact. However, future research should focus on a multidisciplinary approach for the complete utilization of BSG, such as biopolymers and carbon-based nanomaterials.