Brewing By-Products: Source, Nature, and Handling in the Dawn of a Circular Economy Age

Abstract

The brewing industry generates vast amounts of by-products of biotic and abiotic nature that require proper handling to reduce their environmental footprint annually. Simultaneously, and in alignment with the current circular economy dynamics, there is a growing trend towards the valorization of such by-products, through upcycling and/or repurposing. Biotic by-products are a low-cost source of valuable compounds, such as proteins, carbohydrates, lipids and phenolic compounds, which, with adequate recovery methods, can be used in various industries, e.g., agro-food and pharma, among others, where their bioactive and physical-chemical properties can be harnessed effectively. Abiotic by-products are increasingly valorized through pathways that prioritize material recovery and functional reuse. This work aims to address the most relevant by-products from brewing by providing a broad perspective that abridges their sources alongside the manufacturing chain, the composition of the different by-products, and current and foreseen handling and valorization strategies.

1. Introduction

Brewing is one of the oldest crafts known to mankind, with evidence dating back to at least 6000 BC, and possibly as early as 7000 BC according to archeological discoveries [1,2,3]. With its origin typically associated with Sumer, it is likely that beer emerged as a beverage in different locations [3]. Beer provided a relatively secure source of nutrition, which rapidly increased its relevance, prompted innovation in production, methods, and technology, and gained widespread popularity, so that currently the annual production reaches approximately 2 × 10⁹ hL [1,3,4]. The global beer market size was valued at USD 839.3 billion 7.4 × 10⁷ k€ in 2024 and is expected to expand at a compound annual growth rate (CAGR) of 6.8% from 2025 to 2030 [5]. The global beer market is undergoing a significant transformation, characterized by a rising consumer preference for premium, craft, and non- and low-alcoholic beer segments. This trend is more pronounced in developed regions, e.g., North America and Europe, where increasing disposable income and a demand for higher quality, differentiated products are influencing purchasing behavior. Craft breweries have gained traction through their emphasis on small-batch production and innovative flavor profiles, which are notably appealing to younger consumers. Emerging markets are also contributing to this shift, with a growing interest in premium and imported beers [5,6,7,8,9].

Brewing involves several steps, alongside which diverse by-products are generated, as illustrated in Figure 1.

Table 1. Key stages in beer production [10,11,12].

Stage	Comments	References
Malting	Barley (mostly) grains are soaked in water, germinated and dried, so that the sprouted grain acquires the intended color and flavor. Hydrolases, e.g., amylolytic and proteolytic enzymes that will be used to obtain fermentable sugars and peptides/amino acids, are produced.	[13]
Mashing	Malt is mixed with hot water to activate enzymes (amylases, glucanases, proteases) and facilitate the dissolution and breakdown of malt components, producing a wort rich in fermentable sugars and essential nutrients. Maximizing amylase activity is critical, as wort typically contains high concentrations of fermentable sugars (~90–100 g/L) compared to amino acids (~1–2 g/L), as advised for proper yeast metabolism.	[14,15,16]
Boiling	Hops are added and wort is boiled, resulting in relevant chemical and physical changes. Boiling serves to extract and convert hop compounds for bitterness, sterilize the wort, inactivate enzymes, coagulate proteins, and enhance flavor, aroma, and color through Maillard reactions. Water evaporation concentrates the wort, while unwanted volatile compounds, e.g., dimethyl sulfide, are removed. Prior to fermentation, wort undergoes clarification and cooling.	[10,17]
Fermentation	Yeasts primarily convert sugars into ethanol and carbon dioxide. Besides alcohol production, fermentation generates a wide range of secondary metabolites, e.g., isoamyl acetate (banana aroma), ethyl acetate (solvent-like), and ethyl hexanoate (pineapple), that shape the flavor and aroma of beer. The specific profile of these compounds depends on the yeast strain used.	[3,10]
Bottling/Packaging	After fermentation/maturation, beer is filtered (to remove the remaining yeast), carbonated (to adjust the final dissolved carbon dioxide level), pasteurized (to eliminate harmful bacteria) and packed (in bottles, cans, or kegs for distribution and consumption).	[11,12]

provides a summary of the transformation occurring at key stages of the brewing process.

Brewer’s spent grain (BSG) is by far the most abundant by-product of breweries, corresponding to about 85% of total by-products [18,19]. Approximately 20 to 22 kg of wet BSG is generated per hectoliter of beer [20,21], which corresponds to about 4.4 to 6.2 kg dry BSG per hectoliter [22,23]. BSG has an estimated market value ranging from €35 to 50 per ton [20]. Brewer’s spent yeast (BSY), also known as residual yeast or surplus yeast, is another major by-product of brewing. Thus, around 1.5 to 2.6 kg of BSY is generated per hectoliter of beer [24,25]. Brewer’s spent hop and trub, consisting essentially of hot trub and dry hopping residues, generates about 1.4 kg per hectoliter of beer [25]. Kieselguhr, also known as diatomaceous earth or diatomite, is widely used in brewing as a filter aid for beer filtration. Approximately 1.7 kg of wet kieselguhr is generated per hectoliter of beer [26]. Breweries also produce significant fluid by-products, including approximately 9.3 kg of carbon dioxide and 0.3 to 1.0 m³ of wastewater per hectoliter of beer [12,23,27]. Currently, these by-products are mostly used for low-value applications or simply discarded [24,25,28].

Thus, although a significant source of employment and revenue, the brewing industry faces challenges related to the improper disposal of by-products. These problems have major environmental impacts [12,29], some of which stand out (

Table 2. Significant environmental impacts of improper disposal of brewing by-products.

Environmental Impact	Comments	References
Water pollution	Brewery wastewater often depicts high concentrations of organic matter, nutrients, and various chemicals. Unless adequately treated prior to discharge, this effluent significantly increases the biochemical oxygen demand (BOD) in receiving water bodies, leading to oxygen depletion and eutrophication. These processes can result in harmful algal blooms, oxygen deficits, and subsequent negative impacts on aquatic ecosystems.	[30,31,32,33]
Soil contamination	Inadequate disposal of solid by-products may lead to soil nutrient overload or imbalances. Runoff from such sites may introduce excessive nutrients and disturb local fauna and flora.	[30,31]
Greenhouse Gas Emissions	Besides the direct emission of CO2 from alcoholic fermentation, inadequate landfilling of by-products such as BSG trub and spent hops leads to anaerobic decomposition and ultimately the generation of methane, which has higher global warming potential than carbon dioxide.	[23,34,35]

):

However, the by-products from brewing exhibit key characteristics, such as abundance, low cost, high organic and nutrient content, biodegradability, and potential for process integration, that make them ideal candidates for valorization within a circular bioeconomy framework. The reuse of by-products as raw materials contributes to resource efficiency by lowering energy consumption, reducing greenhouse gas emissions, and minimizing waste generation, pollution, and reliance on landfills [36,37]. Additionally, their conversion into value-added products such as biofuels, bioplastics, food and feed ingredients, or chemical precursors, further supports sustainable and circular production practices and helps mitigate the environmental impact of brewing [26,28]. Circular bioeconomy integrates the principles of the circular economy, namely the reduction, reuse, recycling and recovery of resources, with the focus of bioeconomy on the sustainable use of renewable biological materials. This convergence aims to close resource loops and optimize the value extracted from biomass throughout its lifecycle [38,39]. Accordingly, circular bioeconomy offers a strategic framework for the brewing industry by promoting a shift from waste disposal to resource valorization. This approach enhances resource efficiency, reduces environmental impact, and supports economic gains through new value chains. Given their consistent by-product streams, existing infrastructure, and increasing sustainability pressures, large-scale breweries (macrobreweries) are sound candidates to implement central bioeconomy principles [28,39,40]. Still, in recent years, the craft beer sector has grown considerably, driven by small, local breweries that prioritize quality, unique ingredients, and a strong association with local culture and social identity [40,41,42]. Unlike large industrial breweries, which operate at scales that allow them to efficiently manage and treat both solid and liquid waste internally, microbreweries face significant challenges in handling their by-products and environmental impacts. Microbreweries operate at a much smaller scale than microbreweries; therefore, they lack the infrastructure and resources of the latter and have limited capacity to influence upstream processes. Therefore, they must rely on optimizing their own internal practices to find cost-effective and sustainable solutions for managing brewing residues. This represents a major hurdle, as the increasing popularity of craft beer coincides with a need for these small producers to address their environmental responsibilities without the resources of macrobreweries [40,43].

2. Overview of Brewing By-Products and Valorization Strategies

Brewing by-products are largely organic, e.g., BSG, BSY, spent hops/trub and wastewater organics, although inorganic waste is also generated, e.g., kieselguhr and other filter aids, broken glass, plastic labels, mineral salts, and trace metals in sludge [12,25]. Given their specific composition, various valuable compounds can be sourced from these by-products (Figure 2). However, biowaste streams composition varies depending on factors such as barley variety, brewing methods, and processing conditions, which somehow condition the strategies used for their valorization [25,28,44].

2.1. Brewer’s Spent Grain (BSG)

BSG is primarily composed of the seed coat, pericarp, and husk of the original cereal grain, with varying amounts of residual starchy endosperm and aleurone cell walls, depending on mashing efficiency. Hop residues may be present, depending on the brewing process [19,21]. BSG is typically heterogeneous across different breweries due to various factors, e.g., cereal variety, geographical location, harvesting time, hop type, malting and mashing methods, and use of adjuncts. Still, it tends to be relatively homogenous within a single brewery. Chemically, BSG is a lignocellulosic material rich in fiber, namely cellulose (12–25% w/w); non-cellulosic polysaccharides, which is mostly hemicellulose (20–25% w/w) and (1–3,1–4)-β--glucan (<1% w/w); lignin (12–28% w/w); protein (19–30% w/w); lipids (10% w/w); and ashes (2–5% w/w). Minerals such as silicon, phosphorus, calcium and magnesium can be found [21,25,45]. BSG also contains small amounts of free amino acids and phenolic acids [46], and of starch [45,47,48].

Cellulose is composed of glucose residues linked by β-1,4 glycosidic bonds [21,45,49], whereas hemicellulose is essentially composed of arabinoxylan [21,25]. The latter consists of a xylose backbone, with xylose residues linked by β-1,4 glycosidic bonds, to which arabinose residues are linked by either α-1,2 or α-1,3-glucoside bond, at a xylose/arabinose ratio of 7/3. Moreover, ferulic acid can be esterified to some of the arabinose residues at the O-5 position of arabinose [21,50,51]. Proteins in BSG primarily consist of hordeins, a subtype of prolamins, which are rich in proline and glutamine residues, and glutelins, which are rich in glutamic acid/glutamine, with minor contributions from albumins and globulins [52,53,54]. Barley (or other cereals used for brewing) undergoes significant changes during malting and mashing. During malting, proteolytic enzymes hydrolyze more than 70% of the hordeins and glutelins, leading to a reduction in disulfide crosslinks and an increase in albumins and globulins. During mashing, the soluble proteins (albumins and globulins) continue to degrade, while the insoluble proteins (hordeins and glutelins) experience enhanced disulfide crosslinking and aggregation, eventually forming a gelled complex that settles at the bottom of the mash. The latter becomes the major protein component of BSG [52]. Lignin is a complex phenol polymer composed of phenylpropanoid units, termed monolignols, linked by both ether and carbon–carbon bonds, the three main types of units being guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H), all of which are hydroxycinnamyl alcohols [55,56]. Lignin is strongly associated with carbohydrates in the cell walls, forming lignin–carbohydrate complexes (LCC) through either covalent or non-covalent bonds. The former involves benzyl ether, benzyl, ferulate or diferulate esters, glycosidic or phenyl glycosidic, or hemiacetal or acetal linkages, whereas the latter features, e.g., hydrogen bonds and hydrophobic interactions [57,58]. The relative contents of monolignols (H, G, and S units) in lignin, which may vary across different tissues and plant species, can significantly affect how biomass can be broken down. The S/G ratio in cell walls reflects the performance of pretreatments on biomass and affects the degradability of lignin. A high S unit content can favor the removal of lignin, but the relationship between S/G ratio and biomass hydrolyzability is not always consistent, with both positive and negative correlations reported. The H unit content also plays a role, with high H unit content favoring the degradation of biomass, possibly due to its reduced molecular weight, reduced H unit–glucan bonding, and high activity. The H/G ratio has been shown to have a positive correlation with the glucose yield [55].

Thus, BSG is rich in dietary fiber, protein, and phenolic compounds, with potential to improve human health through improved nutrition when incorporated into a range of products such as bread, cookies, pasta, cereal bars, extruded snacks, and instant flours. Its application can improve the health profile of foods by enhancing antioxidant activity, lowering the glycemic index, and providing anti-inflammatory and antidiabetic properties [59,60], as illustrated in

Table 3. Illustrative examples of BSG incorporation in food products: main advantages and challenges.

Food Product	BSG Incorporation (%)	Key Advantages	Challenges	References
Cereal-based products, e.g., biscuits, bread, cookies, muffins, pasta)	Optimal level of BSG * bread and biscuits: <10 cookies: <25 muffins: <20 pasta: <12	High fiber, protein and antioxidants content, with potential overall health benefits, e.g., antidiabetic, anti-inflammatory, and antithrombotic features.	Variability of raw material composition compromises standardization; strong BSG flavor and aroma, overall acceptability.	[60]
Cereal bars	~12	Perceived as more natural; sustainability and nutrition information increase purchase intent; similar acceptable price range to control.	Lower sensory and hedonic ratings; relies on external info (e.g., sustainability) to enhance consumer interest; lower optimal price point.	[61]
Instant flours	10 to 20	Nutritionally comparable to commercial products; potential to reduce malnutrition; good protein source.	Poor sensory traits due to high fiber (20%); microbial instability; require further safety and quality research.	[62]
Snacks and breadsticks	20 to 40	BSG increases antioxidant capacity and fiber content and reduces glycemic index (GI).	Heat loss of nutrients; limited GI reduction in extrudates; formulation challenges at high BSG levels, e.g., unwanted polysaccharide–protein complexes.	[63]

* To achieve acceptable techno-functional and sensory properties [60].

. However, practical challenges remain, e.g., high levels of BSG inclusion can negatively affect sensory properties such as texture, color, and taste, which may limit consumer acceptance. Typically, inclusion levels up to 10–20% are optimal to achieve a balance between enhanced nutrition and acceptable sensory quality [21,60].

Given its compositional richness, BSG has thus been increasingly used in food formulations [60,64], although this only accounts for not more than ~10% of all BSG generated [25]. Currently, BSG is still vastly underutilized, as it is primarily used as animal feed [65], despite some noted limitations [66]. To a lesser extent, BSG is disposed of in landfills, used for biogas production, or applied as fertilizer or compost [25,53,67,68,69]. Still, BSG is not suitable for direct composting due to its high moisture content, low C/N ratio, and acidic pH (70 to 85%, 7.1 to 26.5, and 3.8 to 6.9, respectively), compared to common composting target ranges (60 to 65%, 20 to 30, and 5.5 to 7.5, respectively). Thus, incorporating lignocellulosic bulking agents and livestock manure has been suggested to improve composting conditions. Still, this approach comes with some drawbacks, such as the need for additional bulking materials, space, labor, and time, as well as the risk of leachate formation and ammonia volatilization if improperly managed [70].

Additionally, BSG has been evaluated for: microbial cultivation [71]; production of nutraceuticals [72]; biosynthesis of valuable biomolecules such as dextran [73], polyhydroxyalkanoates [74] and enzymes [75]; and as a carrier for microbial cells [76,77,78] and enzymes [79]. Moreover, BSG has been incorporated into ceramic brick formulations to enhance thermal insulation properties [80], assessed as a biosorbent for the removal of synthetic dyes, e.g., Acid Orange 7 [81], and evaluated as an alternative raw material for pulping and papermaking [82,83].

A major drawback of fresh BSG is its high moisture and nutrient content, which makes it highly susceptible to microbial contamination. This accelerates spoilage, reduces shelf life, and poses safety concerns. It hosts a vast diversity of microorganisms, which proliferate during storage and may produce metabolites, e.g., mycotoxins, that pose additional risks, limiting its safe use, namely in food and feed, without proper preservation [20]. Moreover, this high moisture content also increases the bulk weight, raising transportation and storage costs. While drying BSG to below 10% moisture can contribute to its thermal valorization or shelf-life extension, it introduces new challenges, as drying is highly energy-intensive and requires specialized equipment [84,85].

Other major brewery solid by-products (trub, spent hops and BSY) are also retrieved with high-moisture titers (70–85%), leading to the same handling, storage and energy use burdens as BSG [24,86].

In addition to whole-use applications, BSG components can be valorized upon extraction processes, although these can be challenging. Several approaches have been developed to enable the efficient recovery of a given class of compounds, e.g., proteins. Notwithstanding, some approaches rely on integrated extraction methods that enable the simultaneous recovery of more than one valuable compound [87,88]. Some representative examples are given in

Table 4. Some representative examples of recently suggested approaches for the deconstruction of BSG into easily usable raw materials and/or valuable products.

BSG Component	Deconstruction Method	Valorization Pathways
Cellulose and hemicellulose	Alkaline processing with hydrogen peroxide (AHP) effectively removed proteins and lignin from BSG, increasing the relative content of cellulose and hemicellulose. Higher AHP concentrations and longer treatment times enhanced removal efficiency, aiding in (hemi)cellulose recovery for further use [92].	Enzymatic or chemical hydrolysis of the polysaccharide to yield fermentable sugars towards the production of goods, e.g., ethanol, biodiesel, biopolymers, enzymes [88,93].
	Subcritical water extraction (SWE) of defatted BSG enabled temperature-controlled fractionation, yielding cellulose-rich residues, alongside phenolic- and protein-rich extracts. At 150 °C, cellulose recovery was highest, although yields (20–25%) and purity (42–71%) remained limited even after H2O2 bleaching. SWE offers a sustainable method for cellulose recovery while also retrieving bioactive phenolic co-products [94].	Through mechanical or chemical treatments, cellulose can be processed into nanocellulose, which finds applications in advanced biocomposites, packaging, and biomedical devices, due to its high strength and renewability [95].
		Hydrolysis of hemicellulose yields pentose sugars (e.g., xylose) for fermentation to ethanol, 1-butanol or xylitol, or dehydration to furfural (a precursor for bioplastics and platform chemicals) [96,97].
		Partial hydrolysis of hemicelluloses can yield oligosaccharides with prebiotic features that may enhance gut health when added to functional foods [98].
Lignin	Combining acidic natural deep eutectic solvents DES, namely choline chloride–lactic acid (ChCl-La), with microwave-assisted extraction enabled efficient lignin recovery from BSG. Lignin with 79% purity and strong antioxidant activity (IC50 * ≈ 0.022 mg/mL) was obtained under optimal conditions of irradiation (150 °C, 15 min). Lignin structural integrity was preserved, and low carbohydrate contamination was observed, suggesting a promising approach for high-quality lignin extraction [99].	Given its polyphenolic structure, lignin displays antioxidant activity which can be used by incorporating extracted lignin into polymeric composites and coatings [100].
	BSG was fractionated using ethanol organosolv pretreatment. Under optimal conditions (180 °C, 120 min of incubation and 50% ethanol concentration), lignin was recovered with 95% purity and 58% yield. Moreover, the process enabled separation of cellulose and hemicellulose [101].	Lignin can be used to obtain carbon-based materials with application in energy storage, novel catalysts and environmental remediation [102].
		Lignin can be used as feedstock for the chemical or biological production of valuable phenolic compounds [103] and other bioproducts [104], respectively.
Phenolic compounds	Pulsed Electric Field (PEF) was used as a pre-treatment to enhance ethanol/water (4:1, v/v) leaching of phenolic compounds from BSG. Optimized PEF conditions (2.5 kV/cm, 50 Hz, 14.5 s) increased total free and bound phenolic content by 2.7-fold and 1.7-fold, respectively, compared to untreated samples [105].	Phenolic compounds from BSG offer antioxidant and antimicrobial benefits, supporting their use in functional foods and nutraceuticals. Their incorporation is enhanced through techniques like encapsulation to improve stability, bioavailability, and effectiveness. Standardization of extracts is key to consistent application in health-promoting products [106,107,108]
	Optimized ultrasound-assisted extraction (UAE) using response surface methodology efficiently recovered phenolic compounds from BSG, achieving a 156% higher yield than conventional methods. Under optimal conditions (80 °C, 50 min, 65:35 ethanol/water) extracts rich in ferulic, vanillic, and p-coumaric acids with strong antioxidant activity were obtained [109].	Phenolic compounds in skincare and cosmetics offer antioxidant, antiaging, and protective benefits. They help treat skin issues like inflammation and pigmentation and support natural skin defenses. Although effective, they act slower than common synthetic-derived cosmetics and need more research for optimized use in humans [110].
	Pressurized liquid extraction (PLE) ** of propane pressed defatted BSG using water, ethanol, and their mixtures at varying temperatures and flow rates (10 MPa) achieved up to 20.1 (w/w) yield. Optimal conditions (120 °C, ethanol/H2O 0.5, 2 mL/min) favored recovery of total phenolic (2.130 g GAE ***/100 g), flavonoids (0.778 g CE ****/100 g), and antioxidant activity (9.944 mmol TE/100 g). Water and ethanol/H2O/H2O mixtures outperformed Soxhlet in extracting bioactives [111].
Polysaccharides	Hot water extraction of β-glucan from BSG yielded highest concentration and purity at 60 °C for 90 min, followed by ethanol precipitation. Extracted β-glucan displayed water-holding capacity (6.82 g/g) and outperformed oat β-glucan and gum arabic in viscosity and emulsion stability [112].	β-glucan can be used as a stabilizer and viscosity enhancer in various food products, e.g., in bakery, dairy, fruit juices and meat products, albeit its effectiveness is concentration dependent and must thus be tailored to each application. It can also be used as a fat replacer, e.g., in mayonnaises, with nutritional and sensory benefits [112,113], and play a prebiotic role [114]. Antioxidant activity depicted by β-glucan can potentially be used in (functional) foods, including upon partial depolymerization [115] and cosmetics [114,116], and incorporated in bioactive biofilms to be used as packaging materials in food and pharma [117,118].
Proteins	Proteins were extracted from BSG using PLE with water under near-subcritical conditions (<100 °C, ~1.0 × 107 Pa) and varying NaOH concentrations (0.01 to 0.1 M). Protein yields ranged from 21.3% to 65.3% (dry basis), with the highest yield (66%) from H2SO4-pretreated BSG. Peak protein purity (69.7%) was achieved at 40 °C, 60 min, 0.05 N NaOH. Mild alkaline extraction proved efficient for enriching protein with minimal chemical input [119].	Upon treatment with proteases, proteins retrieved from BSG depicted high oil-holding capacity, foaming formation capability, and foaming stability [120], as well as high solubility, water absorption, and emulsifying capacity [121], and dense, soft, and stable microstructure with consistent texture during food storage [122]. Overall, these techno-functional features suggest potential for improving food formulations [123,124].
	One-stage alkaline extraction of BSG yielded up to 87% protein at pH 11, 60 °C, and 1:17 g/mL solid/solvent ratio. Mild conditions (pH 8) produced protein concentrates with higher purity (>40%), better solubility, and emulsifying properties, while harsher conditions (pH 11–12, 80 °C) enhanced gelling behavior but lowered solubility and protein content (~30%). Temperature and pH of extraction pH strongly influenced both yield and functional properties [125].	BSG protein hydrolysates produced upon protease treatment have been shown to depict high antioxidant activity, alongside α-amylase and α-glucosidase inhibition [121,126]; the latter enables slow carbohydrate digestion, ultimately reducing postprandial blood glucose spikes [127]. These features highlight the potential of BSG proteins as sources of bioactive peptides that can be used to develop functional foods and nutraceuticals [128].
	Proteins were extracted from BSG using alkali (pH 12), ethanol (55% with 2-mercaptoethanol), and enzymatic methods. Alkaline extraction produced glutelin-rich, partially unfolded proteins with high water holding capacity (2.5–4.0 g/g) and gelation potential. Ethanol extraction yielded hordein-rich, aggregated proteins with gel-forming ability. Enzymatic extraction yielded soluble peptides (<10 kDa) with high emulsifying (83 m2/g) and antioxidant activity but no gelation. Protein structure, solubility, and functionality varied markedly with the extraction method, impacting future applications [129].
Ashes	Upon pyrolysis to produce bio-oil and biochar [130,131], ashes, typically rich in Ca, P, K, and S, among other minerals [132], can be recovered by sieving [133].	Ashes can be used as soil stabilizer [134] or fertilizer [135].
Phenolics and sugars	Supercritical CO2 (sc-CO2) extraction of BSG enables the recovery of phenolic-rich oil and enhanced enzymatic hydrolysis. High pressure and temperature improved phenolic yield and antioxidant capacity. Moreover, sc-CO2 extraction eased the assess of enzymes to polysaccharides and concomitantly led to a 20% increase in sugar release as compared to non-treated BSG [136].	Develop prebiotic dietary fiber blends, where the oligosaccharide fraction selectively stimulates beneficial gut bacteria (e.g., Bifidobacterium, Lactobacillus), while phenolic compounds convey radical-scavenging and anti-inflammatory properties, without compromising rheology or sensory profile [53,137]. Create active edible films and coatings that are mechanically robust, hinder oxygen diffusion, and release antioxidants over time, extending shelf life of packaged foods [138].
Proteins and bioactive (phenolic) compounds	PSE was used to extract proteins and bioactives from BSG. Under optimal conditions, namely 4.7% ethanol, 155 °C, and 10 min, PSE yielded 36% more protein than ultrasound-assisted alkaline extraction (UAE). A higher ethanol concentration (35%) and extended extraction time (17 min) maximized phenolic content. PSE extracts depicted a higher antioxidant capacity and angiotensin-converting enzyme and cholesterol esterase inhibition ability than UAE. The higher bioactivity of PSE extracts compared to UAE was attributed to a greater proportion of hydrophobic peptides [139].	Protein and polyphenol conjugates can be used as green emulsifiers and delivery vehicles for lipophilic or sensitive biomolecules. These complexes improve emulsion stability (low droplet coalescence), enhance oxidative stability (low peroxide formation) and enable the controlled release of encapsulated nutraceuticals in foods [140]. The protein fraction from BSG, together with bound phenolic compounds, yields an ingredient with a simultaneous increase in protein, fiber and antioxidant content. Its incorporation of food formulations, e.g., bread, cookies, snacks, may increase the content in protein and total phenolics, improving radical-scavenging capacity without adversely affecting dough handling or final texture [53].
Proteins and polysaccharides	Integrated process developed for sequential extraction of proteins (79–85%) and arabinoxylans (AX, 62–86%) from BSG using increasing concentrations of NaOH or KOH (0.1 to 4.0 M) at room temperature (24 h, 1:2 w/v BSG: alkali). Proteins were selectively precipitated by acidifying extracts to pH 3 with saturated citric acid (food-grade, recyclable); AX were recovered by further HCl acidification (pH < 2) and ethanol precipitation (70% v/v). The method avoids dialysis, reduces process time, and allows recovery of high-purity protein fractions and AX with minimized salt coprecipitation. Recycled solvents were used, stressing the environmental and economic advantages of the method [141].	Polysaccharides and proteins retrieved from BSG can be used to foster the cultivation of edible fungi, e.g., Neurospora intermedia, within a biorefinery concept [142,143].

* IC50 refers to the concentration of a compound resulting in a 50% reduction in DPPH (diphenyl-1-picrylhydrazyl) radical activity. ** Also known as accelerated solvent extraction (ACE) or pressurized solvent extraction (PSE). *** GAE: gallic acid equivalents. **** CE: catechin equivalents.

. Moreover, processing BSG through, e.g., extrusion, fermentation, or targeted enzymatic hydrolysis, consistently enhances the bioaccessibility of bound phenolics, antioxidants, and soluble fibers or peptides [89]; enhances antioxidant, prebiotic or antihypertensive activities [90]; and allows incorporation at substantial levels (up to 20–30% w/w) into breads, extruded snacks or meat analogs without compromising dough rheology, textural profiles or sensory acceptability [91].

2.2. Spent Hops and Trub

2.2.1. Spent Hops

Spent hops refers to the residual hop material left after wort boiling and from dry hopping [144]; the latter is a common practice in craft beer [41]. Spent hops are composed of fibrous plant material, residual bitter acids, phenolic compounds (namely xanthohumol), and essential oils [66,145]. Spent hops are often discarded in landfills [86] or repurposed for composting [144,146] or animal feed [66], although this option has been questioned given the bitterness and presence of 2-methyl-3-buten-2-ol [86,147]. However, given the presence of bitter compounds, essential oils, and other organic compounds, spent hops possess antimicrobial, antioxidant, and sedative properties. These properties make them more suitable for use in fertilizers, pharmaceuticals, cosmetics, and natural pest repellents among others [148,149,150]. Several extraction methods, e.g., involving apolar solvents, hydro-distillation and supercritical fluids, have been implemented, as detailed in a recent review [150]. Additionally, the reuse of spent hops from dry hopping was recently assessed as a sustainable and cost-effective source of bitterness in brewing Pilsener-style beer. Spent hops retain significant amounts of humulones (0.9–10.3% dry matter), offering up to 94% retention from the original hop pellets. Selected samples were used as the sole source of bitterness in test brews, achieving target bitterness and good sensory quality comparable to conventional hop pellet brews [151].

2.2.2. Trub

Trub is a term that abridges sediments precipitated out of the boiled wort (hot trub or hot break) [45,152], as well as sediments that precipitate when the wort is rapidly chilled or during cold storage (cold trub) [10,17]. Hot trub formation is mostly a protein denaturation and precipitation process influenced by thiol and disulfide bond dynamics that promote molecular aggregation and particle growth, leading to particles sized 30 to 80 μm in diameter that settle well [153,154]. Aggregation through protein–protein, protein–bitter acid, and protein–polyphenol interactions is driven by exposed hydrophobic sites [150,153]. Hot trub typically consists of carbohydrates, insoluble denatured proteins, lipids, minerals, phenols, tannins, and other insoluble hop materials [10,45,86,152]. The proportion of the components differ depending on the raw materials used and brewing process, but proteins are predominant (40 to 70%), alongside significant amounts of bitter compounds (7 to 32%), polyphenols (20 to 30%), carbohydrates (4 to 8%), fats and bitter acids such as adhumulone (1 to 2%), and ashes (around 5%) [86,152,153]. Moreover, hot trub is rich in bioactive compounds, namely phytochemicals, with antioxidant, antibacterial, antifungal, and antihypertensive properties, indicating promising potential for valorization in food and pharma [10,155,156,157]. The bitterness of trub, which can be associated with the presence of phenolic compounds such as catechins and epicatechins, but mostly to iso-α-acids that co-precipitate/bind into the protein/polyphenol aggregates of hot trub [158,159], has restricted its application in food and feed applications compared to other brewing residues like BSG [160]. Still, a yeast–trub mixture has been found acceptable for pigs’ feed and can be used in dried protein feed preparations, according to other feeding experiments [66,161]. Prior to its application in food products, namely as a protein source [157,160], the removal of bitter fractions is thus typically advised [160,162], e.g., through hot water [160] or ethanol/water [163] extractions [160,162].

Hot trub, alongside spent hops, has been used for composting [146]. Thus, Kopec and co-workers observed that both types of waste were microbiologically safe and not a contamination risk. However, hot trub showed plant growth inhibition, likely due to organic compounds, and its mineral content allows limited composting use. Spent hops had high nitrogen levels, requiring carbon-rich materials to balance composting. Overall, the authors suggested that effective and stable compost could be produced in 60 days with careful substrate selection. Wolny-Koładka and co-workers valorized hot trub through biodrying, using refuse-derived fuel as a bulking agent, to produce a solid fuel with high energy value and low moisture [164]. Tesio and co-workers used hot trub to produce carbon for battery cathodes through a simple pyrolysis method. The resulting material, combined with sulfur, served as an efficient and sustainable cathode, underscoring the potential of hot trub to contribute to enhancing the performance of next-generation energy storage systems [165].

Hot trub has also been tentatively used as a raw material for microbial fermentations [166,167], and proved effective to produce surfactin from Bacillus subtilis. Surfactin exhibited antimicrobial and antifouling abilities, showing its potential for use as a biocide [167]. Previously, the addition of hot trub to pitching wort had been shown to enhance yeast vitality, biomass yield, and fermentation performance of Saccharomyces cerevisiae, with effects increasing proportionally to the trub concentration. These benefits were attributed to various trub constituents, including lipids, zinc, and particulate matter [45,168]. Although unsaturated fatty acids are considered beneficial [169], zinc was also suggested to be the key contributor to improved fermentation [45,168]

Other illustrative examples of the strategies to process and upcycle hot trub are depicted in

Table 5. Some further representative examples of recently suggested approaches for the deconstruction of hot trub into easily usable raw materials and/or valuable products.

Hot Trub Component	Deconstruction Method	Valorization Pathway
Phenolic compounds	UAE with ethanol as extractor, upon optimization of key operational parameters [150,170]	Use of extracts as food-preservative or nutraceutical ingredients [66,171]
	A DES, composed of choline chloride and propylene glycol (1:2 mol/mol), was used to retrieve xanthohumol [172]	Use in drug design, given anti-inflammatory, antimicrobial, antioxidant, antihypertension and anticancer activities of spent hops major bioactives, e.g., α- and β-acids, essential-oil terpenes and phenolic compounds [155,157,173,174,175,176]
Proteins	Hydrothermal pre-treatment at 25 °C for 20 min followed by dying [157]	Production of biofilms for the development of active packaging materials [177].

. Further specific details on the isolation of specific molecules from hot trub can be found in a recent review [150].

Cold trub is also mostly composed of proteins (50%), high molecular mass carbohydrates (20–30%) and polyphenolic compounds (15–25%) [45,169], somehow akin to that of hot trub, in the form of particles sized around 0.5 µm [154]. Recently, cold trub has been shown to be a rich source of hydroxycinnamoylagmatine dimers [178]. These bioactive phenolic compounds are structurally diverse and offer significant potential for use in food, nutraceutical, and pharmaceutical applications [179].

Cold trub, combined with other brewery waste, has been incorporated into animal food products [180]. Moreover, given its composition, cold trub shows promise for application in food, extraction of functional ingredients, and microbial fermentation, thus positioning cold trub as a likely resource for sustainable use in food, nutraceutical, and biorefinery sectors.

2.3. Brewer’s Spent Yeast (BSY)

BSY, as other organic brewery waste, is rich in proteins (36 to 64%) and carbohydrates (31 to 42), with minor amounts of ashes (1.5 to 9.5%) and fiber (3 to 18%), e.g., β-glucans, mannans, polymeric hexosamines, and reducing sugar (~0.2%) [24,45,156,181,182,183,184]. BSY is also rich in amino acids, e.g., arginine, cystine, histidine, isoleucine, leucine, lysine, methionine, tyrosine, phenylalanine, threonine, tryptophan, and valine, thus including nine essential amino acids, in proportions that meet the requirements of both the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) [181,185]. Moreover, BSY is also a source of vitamins, namely of the B complex, e.g., thiamine (B₁), riboflavin (B₂), niacin (B₃), pantothenic acid (B₅), pyridoxine (B₆), biotin (B₇), folate (B₉), and cobalamine (B₁₂), and of phenolic compounds, e.g., gallic acid, ferulic acid, p-coumaric acid, and xanthohumol [66,88,186,187]. BSY presents some advantages as compared to yeast originating from other industries, namely the presence of residual hops, which, given their antimicrobial properties, may offer health benefits by reducing pathogenic risk and promoting gut health [188]. More specific details can be found in a recently published review [189].

Although BSY is generally recognized as safe (GRAS) for human consumption, its nucleic acid content (6 to 15%) limits its use as a protein source in human nutrition due to the risk of hyperuricemia associated with high uric acid levels. On the other hand, the relatively high titer of RNA in BSY can be advantageously used to produce nucleotides such as 5′-adenosine monophosphate (5′-AMP), 5′-guanosine monophosphate (5′-GMP), and 5′-inosine monophosphate (5′-IMP), which are known for flavor-enhancing properties [24,190]. SBY is thus primarily used as a cost-effective protein source in terrestrial animal feed [88,181], and at a minor scale, sold or donated to famers for use as fertilizer or compost, a practice more common to rural than to urban breweries [188,191,192]. The successful use of BSY in animal feed has suggested that this can be effectively replicated in aquafeed [188].

Some craft breweries co-dispose BSY and BSG, mainly for such agricultural use, with occasional application in anaerobic digestion [192]. Aligned with this, co-digestion of BSY and brewery wastewater to increase energy recovery in breweries has been tested as lab-scale with success, although long-term impacts on sludge development required further insight [193]. Disposal of BSY via sewage systems presents environmental risks due to high biological oxygen demand, suspended solids, and acidification of effluents. While landfill disposal after filtration is recommended, thermal and chemical inactivation methods remain resource intensive and may not fully mitigate pH-related impacts [188]. Besides these applications, BSY has also been used as substrate for microbial fermentations, namely targeting the production of yeast extract and γ-aminobutyric acid (GABA) [194], glutamic acid [195], succinic acid [196], bioethanol [197], and BSY with improved sensory quality and nutritional features for use as a food ingredient [198], among others, as recently reviewed elsewhere [189].

A key step in the valorization of BSY is cell disruption, which enables the efficient recovery of its bioactive compounds. Different cell disruption approaches have been implemented for processing BSY, as comprehensively reviewed recently [185]. Among them, mechanical/physical cell disruption techniques such as ultrasonication, high-pressure homogenization, and pulsed electric fields have proven effective for releasing intracellular proteins, enzymes, polyphenols, and antioxidants for use in the food and nutraceutical sectors. Enzymatic lysis and autolysis facilitate the production of antioxidant-rich protein hydrolysates and the extraction of valuable polysaccharides like β-glucans. Integrated biorefinery approaches have further advanced BSY utilization by enabling the co-production of single-cell proteins and fungal exopolysaccharides, thereby enhancing process efficiency and economic viability [24,186,189]. Some illustrative potential applications for compounds retrieved from BSY are given in Figure 3.

2.4. Other Waste

2.4.1. Kieselguhr

The conventional disposal methods for spent diatomite, such as landfilling or use as agricultural fertilizer, are associated with environmental concerns, including land resource depletion, CO₂ emissions, and potential leaching of nitrogenous substances [26,199]. Thus, alternative strategies for kieselguhr valorization have emerged. Accordingly, Gong and co-workers regenerated brewery spent kieselguhr using Lysinibacillus fusiformis. Besides achieving 51% protein degradation after 14 days, the regenerated kieselguhr displayed exhibited enhanced adsorption capacities for methylene blue (95.5%) and Cr(III) (71.7%), outperforming thermal regeneration [199]. Huaccalo Aguilar and co-workers combined geopolymers derived from spent kieselguhr with activated carbon for the treatment of winery wastewater [200]. These authors established that said preparation was effective for the treatment of for treating organic-rich agro-industrial wastewater. Previously, Ferraz and co-workers demonstrated that brewing spent kieselguhr could be safely and effectively incorporated into clay bricks without compromising mechanical properties. Optimal incorporation improved porosity and reduced density, with no observed ecotoxicity [201]. In the same trend, Halle evaluated the reuse of brewing spent kieselguhr for floor tile production. After alkaline pretreatment with NaOH and blending with sand, cement, water, and aggregate, tiles were molded and sun-dried for 14–28 days. While 100% kieselguhr substitution for cement proved ineffective, a 50% replacement yielded tiles with acceptable compression strength and water absorption, influenced primarily by porosity [202]. Further illustrative examples on the valorization of brewing spent kieselguhr can be found in a recent review [203].

2.4.2. Carbon Dioxide

Prior studies have shown that CO₂ can be recovered from fermentation, scrubbed, and compressed for in-process recycling and use in, e.g., beer carbonation, package-purge (O₂ displacement), and clean-in-place (CIP) spray gas, thus reducing costs and greenhouse gas emissions. Moreover, the CO₂ retrieved from fermenters is high quality, as it is free of industrial contaminants that may be present when CO₂ is purchased from ammonia and urea facilities. Recovered CO₂ can be further processed into dry ice and compressed or liquefied CO₂ for offsite applications. CO₂ recovery units are available as modular skid-mounted systems [26,204]. Other approaches are nevertheless being assessed for CO₂ valorization. Thus, Silkina and co-workers developed a scalable, low-maintenance system that uses microalgae, namely Limnospira maxima and Tetradesmus obliquus, to capture CO₂ emissions from breweries while producing valuable biomass. The harvested algae were rich in high-value components such as protein, phycocyanin, carbohydrates, and lipids, supporting both environmental and economic sustainability [205]. Notwithstanding, CO₂ retrieved from breweries could be used as raw material for the microbial and/or electrochemical production of value-added products such as short-chain fatty acids and green fuels, which are approaches that offer promising routes for sustainable CO₂ utilization [206,207,208].

2.4.3. Wastewater

Brewery wastewater contains high organic loads (chemical oxygen demand (COD): 1800 to 50,000 mg/L; biochemical oxygen demand (BOD): 1005 to 38,000 mg/L), suspended solids (550 to 3000 mg/L), total phosphorus (4 to 103 mg/L) and nitrogen (20 to 6000 mg/L) loads, and has a variable pH (3 to 12) due to cleaning cycles [209]. On-site treatment typically involves physical-chemical and biological methods that help meet discharge regulations and enable water reuse, mainly for cleaning, cooling, or irrigation [209]. Brewery wastewater treatment must balance environmental performance with economic feasibility, requiring systems that can handle fluctuating waste loads while keeping costs low. Aerobic processes, though effective, are energy-intensive and produce excess sludge. Therefore, anaerobic pretreatment is generally preferred for its energy efficiency and lower sludge production [210]. Moreover, anaerobic digestion enables sustainable biogas generation and energy production [27].

The most common anaerobic system is the up-flow anaerobic sludge blanket (UASB) reactor, known for its high treatment efficiency due to the formation of dense microbial granules. Versions like the expanded granular sludge bed (EGSB) reactor and the internal circulation (IC) reactor enhance performance through improved design and internal circulation [210,211,212]. To ensure treated water meets discharge standards, an aerobic polishing step is typically added. Sequencing batch reactors (SBRs) are adequate for this role due to their operational flexibility and ease of automation. Other aerobic options include jet loop, fluidized bed, and membrane bioreactors (MBRs) [209,210,211,212]. Still, alongside developments to improve classic treatments, several valorization routes have emerged, as summarized in

Table 6. Some illustrative examples of brewery wastewater valorization. The examples highlight the potential of the different approaches for sustainable, circular bioresource use.

Valorization Route	Comments	References
Dark fermentation	Anaerobic baffled reactors were used to produce biohydrogen and volatile fatty acids, FAs (e.g., acetate, butyrate, caproate).	[213]
Mixed cultures	The microalga Tribonema aequale was co-cultured with native bacteria to effectively remove major nutrients. The treated water met discharge standards, and the resulting biomass contained valuable compounds, e.g., chrysolaminarin, palmitoleic acid, and eicosapentaenoic acid, with potential commercial applications.	[214]
	Chain elongation to produce caproate without compromising waste management.	[215]
	Polyhydroxyalkanoates were successfully produced with similar yields from anaerobically treated and acidified streams, namely under acetate pulse feeding. Favors production of bioplastics.	[216]
Microalgae cultivation	Arthrospira platensis was effectively used for nutrient recovery, CO2 capture, and production of pigments, fatty acids, and biogas.	[217]
Nutrient recovery	Controlled struvite recovery using a crystallization reactor proved economically viable. Fertilizer shortages are addressed, and eutrophication is mitigated.	[218]

.

2.4.4. Wastewater Sludge

Brewery wastewater sludge (BWS) contains organic matter, phosphorus, nitrogen, and micronutrients beneficial for soil and plant growth. Thus, it has been used as a fertilizer. However, its large volume and high pathogenic load pose significant management challenges [219,220,221,222]. Two recent studies highlight how the positive and negative aspects must be balanced. Thus, Christian and co-workers observed that the amendment of soil samples from abandoned farmland with varying BWS concentrations resulted in increased microbial loads as compared to the control, which was considered suggestive of BWS on agricultural soil [223]. Alayu and Leta also performed field experiments in a maize field, comparing BWS-treated plots with a commercial fertilizer containing nitrogen, phosphorus and sulfur and control plots [224]. Results showed that BWS application significantly increased maize biomass (up to 37.2%) and grain yield (26.8%) over the control, and improved key soil nutrients, namely nitrogen, phosphorus, and potassium. However, BWS also altered soil pH and electrical conductivity and led to increased concentrations of lead (150%) and fecal coliforms (24.4%) compared to the control. The authors suggested that the short-term findings highlight the potential of BWS as a nutrient-rich soil amendment, but long-term monitoring is needed to address possible risks from heavy metals, pathogens, persistent organic pollutants, and antibiotic-resistant gene transfer.

To overcome the risk of high pathogen load when BWS is used as fertilizer, Demeke and co-workers performed the aerobic composting of BWS mixed with spent kieselguhr to produce biofertilizer. The composting process improved nutrient content and stabilized pathogens to levels well below Environmental Protection Agency (EPA) limits. Heavy metal concentrations remained within acceptable ranges, indicating that the resulting biofertilizer was safe and of suitable quality for agricultural use [225]. A different approach, to use BWS as a soil amendment to reduce cadmium availability and uptake by Brassica carinata grown in contaminated soil, was presented by Tsadik and co-workers [221]. These authors produced biochar by pyrolyzing BWS and applied it to cadmium-spiked soil. The treatment effectively immobilized cadmium, significantly reducing its uptake by the plants and improving their growth. Additionally, the biochar enhanced soil properties, making it a promising, sustainable option for remediating heavy metal contamination in agricultural soils.

BWS can also be used as feedstock for anaerobic digestion to recover energy as biogas. Agler and co-workers highlighted the importance of incorporating BWS digestion into brewery wastewater treatment to maximize methane recovery and reduce the load on subsequent treatment processes [226]. Edunjobi and co-workers highlighted the potential of BWS as an effective inoculum to enhance the anaerobic digestion of BSG [227]. The combination of BWS with poultry manure and FeCl₃ improved biogas and biomethane production through co-digestion, bioaugmentation, and biostimulation. In a different approach, Teshome and co-workers addressed the role of BWS in renewable fuel production through biomass briquetting [228]. When combined with BSG and molasses as a binder, BWS contributed to producing high-quality briquettes with calorific values comparable to sawdust-based alternatives, while meeting international fuel standards. These works further underscored the value of brewery sludge in optimizing waste-to-energy processes.

2.4.5. Packaging Waste

This section provides a brief overview of current and anticipated approaches to the valorization of packaging waste.

Glass and aluminum packaging are major environmental hotspots in beer production [229]. At the end of life, beer bottles are either returned to the manufacturer for refilling or collected, crushed into cullet, and remelted at glassworks to produce new bottles through open- or closed-loop recycling systems [230]. While returnable glass bottles reduce some impacts after reuse, they still perform worse than non-returnable bottles in areas like eutrophication and waste [229]. Moreover, both returnable and non-returnable bottles generate cullet or broken glass that require disposal [231]. Overall, considerable amounts of glass are still landfilled, contributing to a growing waste management challenge and placing increasing pressure on limited landfill space [232,233]. To tackle this, crushed bottle cullets can be embedded in cementitious composites (masonry blocks, tiles), improving mechanical strength and insulating properties [233,234,235].

Aluminum waste comes from defective or expired cans, which are collected and crushed for disposal [231]. Aluminum cans may be recycled by remelting and casting, but this method is energy-intensive and results in material loss. Solid-state recycling offers a more efficient alternative by consolidating shredded cans without melting, using processes like hot extrusion and forging, which produce high-quality material with better mechanical properties [236,237]. Still, the recycling ratio worldwide varies, and significant amounts of cans are landfilled, with negative environmental, health and economic impacts [238]. Zhang and co-workers developed a low-cost, small-scale aluminum can recycling system that operates using solar energy, a smart induction furnace for rapid melting, and dual gas filters for emission purification. Moreover, high-value products can be produced through direct casting and a life cycle assessment shows they outperform traditional methods in several environmental categories [238].

Cardboard, paper and labels are separated and sent to recycling facilities and repurposed, disposed of in landfills, or burnt in a boiler to supply energy to the factory, although the latter have associated environmental impacts [12,239]. On the other hand, cellulose nanofibers can be produced from wastepaper [240,241], with said nanofibers being used for fresh produce packaging [242].

Common brewery plastics include HDPE (high-density polyethylene) used in, e.g., kegs and casks, and PET (polyethylene terephthalate) used in, e.g., bottles and kegs, which contribute significantly to environmental pollution, as they are often discarded or incinerated [243,244]. Advances in decontamination and technology have made post-consumer recycled PET (rPET) safe for reuse in beverage packaging. FDA-approved for food contact use for over 30 years, rPET supports sustainability efforts through bottle-to-bottle recycling, as outlined by Benyathiar and co-workers [245]. Chemical methods have been developed that allow the depolymerization of PET and HDPE to regenerate the (macro)monomers, therefore establishing recycle routes for the polymers [245,246,247].

Overall, in response to concerns over single-use, fossil-fuel-based plastics, breweries are increasingly seeking sustainable packaging solutions, aiming to make all packaging reusable, recyclable, compostable, or biodegradable [248].

3. Case Studies on the Valorization of Brewing By-Products

The following case studies exemplify the potential for integrating circular resource flows and on-site renewable energy solutions within industrial brewing operations

Sierra Nevada Brewing Co. exemplifies a comprehensive, closed-loop approach to sustainability through a combination of on-site renewable energy generation, waste minimization, and resource recovery. Waste is managed through a 99.8% diversion rate, with spent grain and yeast repurposed as livestock feed, and fermentation-derived CO₂ captured and reused in brewing operations. Renewable energy is generated through one of the largest private solar arrays in the industry, supported by battery storage and biogas-powered microturbines fueled by methane from anaerobic wastewater treatment. These systems reduce dependence on external utilities while improving energy resilience. Additionally, energy efficiency strategies, including heat recovery and green building design, significantly lower operational demands. This integrated model demonstrates how environmental caretaking can align with economic performance in craft brewing [249,250].

Project Circle, developed by Heineken France, advances circularity in brewing by integrating resource recovery and renewable energy generation into the production process. The project uses fractionation technology to separate protein and fiber components from BSG. The protein-rich fraction, refined to concentrations exceeding 70%, is intended for use in food and feed applications. The fibrous component is dried and combusted on-site to generate process steam, thereby reducing the use of fossil fuels. While initial third-party evaluations at the pilot site foresee a reduction in thermal energy-related CO₂ emissions by approximately 50%, full value chain net-zero emissions by 2040 are envisaged. Project Circle exemplifies the potential for integrating circular resource flows and on-site renewable energy solutions within industrial brewing operations [251,252].

Anheuser-Busch InBev’s EverGrain initiative demonstrates large-scale valorization of BSG by converting it into high-protein and high-fiber ingredients for food and beverage applications. Products such as EverPro, a barley–rice protein isolate, and fiber- and protein-rich formulations (EverVita Prima and Fibra) exemplify circular reuse of brewing byproducts [253]

The CHEERS biorefinery pilot, integrated into the Mahou brewery in Spain, exemplifies a circular approach to brewery side stream valorization. Through this initiative, BSG is repurposed as feedstock for mealworm (Tenebrio molitor) cultivation, yielding protein flour suitable for high-protein food applications. Simultaneously, BSY, residual biomass, and brewery wastewater are processed via anaerobic digestion to produce biogas. This biogas, along with captured CO₂ from fermentation, feeds microbiological processes that generate five high-value bio-based products: single-cell protein and volatile fatty acids for pet food, ectoine for cosmetic formulations, caproic acid, and chlorine for disinfectant production. Together, these processes reduce waste, lower emissions, and create value-added products from traditional brewing byproducts [254].

In addition, EU-funded projects such as LIFE RESTART [255] and POLYMEER [256] are advancing the upcycling of BSG into sustainable bioplastics, addressing both environmental and economic challenges associated with this by-product. LIFE RESTART focuses on the recovery of 75% of BSG and the reuse of brewery wastewater to produce biodegradable, recyclable bioplastics, while establishing a semi-industrial production hub. Complementing this approach, POLYMEER aims to convert wet BSG into high-performance bioplastics through green, low-waste processes, developing tailored polymer materials for specific applications such as agricultural mulch films, automotive textiles, and industrial packaging.

4. Practical Considerations for Valorizing Brewing By-Products in a Scale-Sensitive Approach

The valorization of brewing byproducts presents a unique opportunity to combine environmental sustainability and economic resilience across the brewing sector. However, the feasibility and cost-effectiveness of implementing these pathways vary significantly depending on brewery size, resource availability, and technological capacity.

For small breweries, simple valorization strategies that require minimal investment and operational change are the most viable. Reusing spent grain and yeast as animal feed and incorporating spent grain into food products are well-established options with low technical risk and clear returns [21,257,258,259]. Composting trub and hop residues with carbon-rich materials is another accessible strategy with minimal equipment requirements [146,260]. While commercial CO₂ capture and reuse systems have traditionally been suited to large facilities, emerging compact units now make on-site CO₂ recovery increasingly feasible and economically viable for small breweries, offering both environmental and cost-saving benefits [34].

The repitching of yeast, reusing yeast across multiple fermentation cycles, can reduce raw material costs and improve product consistency when carefully managed. However, it also introduces several risks (contamination, genetic drift, off-flavor production, and inconsistent fermentation) if not supported by strict protocols, microscopy, and cell viability assessments [261]. These quality control needs may exceed the capacity of smaller operations, making repitching more viable for medium to large breweries or those with robust lab infrastructure.

For small-scale breweries, which often operate with limited capital and space, low-tech valorization strategies offer the most accessible entry points. Among these, the repurposing of BSG and BSY for animal feed remains the most common practice due to minimal processing needs and local demand [192,257]. Similarly, food enrichment with BSG, such as incorporating it into bread, snacks, or meat alternatives, can offer product diversification and value addition with low investment [21,259]. The composting of trub and other organic waste, when combined with carbon-rich materials such as straw or wood chips, can be carried out at the microbrewery scale with little more than land, time, and basic equipment [260].

In contrast, valorization methods such as the conversion of wastewater and BSG into bioplastics [262] require significant capital investment, specialized expertise, and regulatory compliance, currently making them impractical for most small-scale operations and instead more suitable for larger breweries or consortia of small brewers sharing infrastructure.

Such technologies can offer higher value returns per unit of waste, but only with economies of scale and centralized processing facilities, an approach increasingly explored through regional circular economy hubs or public–private partnerships. Within this scope, several European initiatives (e.g., LIFE RESTART [255] and POLYMEER [256]) focus on transforming brewing waste into industrial materials by leveraging cross-sector collaboration and shared technological platforms.

A comparative assessment of the feasibility of different brewing by-product valorization pathways across brewery scales is presented in Figure 4.

5. Preventive Strategies for By-Product Reduction in Brewing

The valorization of brewing by-products represents a key strategy for enhancing sustainability; nonetheless, a comprehensive approach should also consider upstream innovations aimed at minimizing waste generation at its source. These innovations can result from process intensification and technological changes [263,264,265,266]. Alternative fermentation strategies and clarification technologies are relevant within this scope [267,268]. Compared to batches, continuous fermentation of beer increases volumetric productivity and decreases process downtime and cleaning cycles, ultimately reducing water and energy inputs per product unit. Moreover, the product profile is more consistent, thereby leading to higher downstream processing efficiency, as well as reduced extract losses and waste disposal. Still, continuous fermentation of beer, whether using suspended flocculent yeast or immobilized on carriers, is hindered by yeast washout, contamination risks, lack of control over wort attenuation in carrier-free systems, high carrier costs, altered yeast metabolism, mass-transfer limitations, operational complexity, and dead-cell accumulation in carrier-loaded setups. Nonetheless, continuous fermentation of beer has been implemented on a commercial scale at Dominion Breweries in New Zealand [266,267,269,270].

The intensification of enzymatic activity during mashing, namely through the targeted use of thermostable or engineered amylolytic and proteolytic enzymes, significantly improves starch and protein breakdown, thus enhancing the conversion into fermentable sugars and bioavailable nitrogen. This enzymatic optimization reduces residual extract in spent grain, minimizing its volume and lowering the organic load entering subsequent process stages. The resulting improvements in wort composition, including lower viscosity, reduced β-glucan levels, better filtration, and increased clarity and consistency, support cleaner fermentation, reduce trub formation, and reduce the need for harsh processing conditions, thereby promoting overall process efficiency and decreasing the generation of residues [14,266,271]. Still, excessive enzyme use in brewing can weaken beer quality by reducing foam stability, body, and clarity, while also increasing process complexity, energy costs, and the risk of filtration problems [272,273].

High gravity brewing (HGB) involves fermenting worts with higher concentration of dissolved sugars than usual. This method increases brewhouse productivity and offers sustainability benefits, including reduced water, energy, labor, and waste costs. Moreover, under optimized conditions, HGB can yield beers with sensory profiles comparable to conventional brews, with no significant increase in off-flavor compounds and improved ethanol yields. Nonetheless, HGB poses challenges such as reduced foam stability and fermentation issues, particularly when high levels of unmalted adjuncts are used. Additionally, HGB requires larger and more advanced equipment for handling, heating, cooling, and dilution, leading to increased capital investment and higher energy costs [266,274,275,276].

Regarding clarification, the commonly used kieselguhr-based filtration poses occupational health concerns, environmental burdens and disposal limitations [277]. Alternative environmentally friendly strategies such as membrane-based microfiltration, under crossflow and dynamic modes of operation, have been evaluated [268,277]. Moreover, since microfiltration is size-selective, it can be used to remove yeasts, large macromolecules (e.g., proteins, β-glucans, and other large carbohydrates) and colloids responsible for haze formation, given proper membrane selection. Additionally, microfiltration is compatible with clean-in-place (CIP) protocols, scalable to the capacities required by small and medium-sized breweries and its integration enables significant reductions in solid waste and filter media consumption [278,279]. Early commercialization efforts focused on polymeric hollow fiber membranes, but these modules often delivered permeate fluxes well below those of kieselguhr-based filtration. More recently, tubular ceramic membranes have emerged to address the flux limitations and durability concerns of their polymeric predecessors [277,280].

Mechanical clarification using decanter and disk centrifuges enables the effective removal of solids, e.g., yeast and trub, without the introduction of exogenous filtration aids. This approach reduces solid waste volumes but also eases the recovery of valuable biomass fractions suitable for valorization pathways. The centrifuge is a compact, versatile tool with low oxygen pickup, which helps preserve the flavor and stability of beer, and can run continuously with minimal cleaning. However, it can damage yeast and proteins due to shear forces, potentially affecting flavor and foam. It also has high maintenance needs and generates significant noise during operation [281,282,283].

In a relatively recent work, Cimini and Moresi coupled an enzymatic pretreatment with centrifugal and membrane separations to clarify and stabilize rough beer, free from kieselguhr [284]. Unfiltered beer was first treated with a proteolytic and glucanolytic enzyme to take apart haze-forming complexes. A disk-stack centrifuge then removed most suspended solids, yielding a partially clarified brew that was further processed by crossflow microfiltration through a ceramic hollow fiber module operated with periodic CO₂ backflushing to limit fouling. This stage produced a clear permeate that ultimately led to a brilliant, chill-haze resistant and microbially safe beer, comparable to an industrial benchmark, hence justifying pilot plant studies to evaluate scalability.

Furthermore, body feed filtration is being optimized using alternative filtration materials with lower environmental impact, such as cellulose and natural zeolites, which have displayed adequate filtration performance and reduced toxicity, potentially offering viable substitutes for kieselguhr in existing filtration setups [285].

Further information on advancing technologies aiming to contribute to processing efficiency and environmentally friendly strategies in brewing can be found elsewhere [286].

6. Conclusions

Beer holds a significant share in the beverage market that is expected to grow further in the coming years. Naturally, this comes along with a growing output of by-products, generated at different stages of the production chain. If inadequately handled, these by-products contribute to organic loading, nutrients, and greenhouse gases, creating notable environmental issues and straining ecosystems. On the other hand, brewing by-products, ranging from biotic fraction rich in biomolecules, such as proteins, carbohydrates, lipids and phenolic compounds, to abiotic materials, such as CO₂ or kieselguhr, represent valuable resources whose recovery and reuse are increasingly promoted by economic, environmental and regulatory drivers. This approach aligns with the current movement towards the transition from linear production to circular economy, establishing paths for innovation, cost savings and new revenue sources.

Biotic by-products, namely BSG and BSY, have been mostly used as feed supplements, while spent hops and trub are used to a lesser extent. Alternatively, these by-products have been repurposed as compost or fertilizer or discarded in landfills. However, research has shown that incorporation of these biotic by-products in food can result in products with improved rheological and functional properties. They can also be used as substrates in fermentation processes to yield valuable biomolecules and biofuels. Targeted extraction processes can further yield biomolecules of interest for agro-food, pharmaceutical and other industrial sectors. Nevertheless, their high moisture content, susceptibility to microbial spoilage and compositional diversity, driven by differences in raw materials and brewing methods, pose logistical and processing challenges.

Abiotic by-products, namely CO_2, wastewater, sludge, kieselguhr, glass, aluminum cans, cardboard, and paper also present valorization potential. Current uses are relatively limited, as CO_2, is often vented or, on a minor scale, reused for beer carbonation; wastewater is sent to treatment plants; and sludge, kieselguhr and packaging waste are disposed of as fertilizer, via landfill or basic recycling. However, advances enable their use as a substrate for fermentation and production of valuable goods (wastewater and CO₂), incorporation in building materials (glass, kieselguhr), adsorption of toxic compounds (kieselguhr), a source of nutrients and biofertilizer (sludge), and raw materials for novel products for, e.g., packaging (plastics, aluminum, and cardboard). Additionally, emerging compact CO_2, recovery systems enable on-site reuse that is increasingly feasible for small-scale breweries.

Large breweries have already implemented integrated biorefineries, anaerobic digestion, continuous extraction lines, and modular CO₂ recovery systems, while small-scale operations often rely on animal feed partnerships, drying and milling for specialty ingredients, or small-batch yeast and microalgae processing. Upstream waste prevention, through approaches such as continuous fermentation, membrane filtration, reusable filter media, and mechanical separation, can further reduce waste generation, complementing downstream valorization. However, moving from pilot applications to widespread adoption will require scale-up readiness: technologies must maintain this performance with industrial throughputs, tolerate raw material variability, comply with regulatory and quality standards, and remain economically viable on both large and small scales. Standardized characterization protocols, robust life cycle and techno-economic assessments, and collaborative infrastructure models, particularly for smaller producers, will be essential to bridge the gap between promising research and full commercial deployment, positioning the brewing sector as an active driver of circular bioeconomy.