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Review

Research Trends on Valorisation of Agricultural Waste Discharged from Production of Distilled Beverages and Their Implications for a “Three-Level Valorisation System”

by
Kelly Stewart
1,2,
Nik Willoughby
1 and
Shiwen Zhuang
1,3,*
1
Institute of Biological Chemistry, Biophysics and Bioengineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
2
FlexBio Bioprocessing Scale-Up Centre, Industrial Biotechnology Innovation Centre, Heriot-Watt University, Edinburgh EH14 4AS, UK
3
International Centre for Brewing and Distilling, Heriot-Watt University, Edinburgh EH14 4AS, UK
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(16), 6847; https://doi.org/10.3390/su16166847
Submission received: 20 May 2024 / Revised: 7 August 2024 / Accepted: 8 August 2024 / Published: 9 August 2024
(This article belongs to the Topic Sustainable Recycling and Reuse of Industrial By-Products or Waste from Geo-Resource Exploitation)
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Abstract

:
The circular economy, driven by waste elimination, material circulation and nature regeneration, is crucial for business, people, and the environment. With the increasing demand for distilled beverages, managing agricultural waste like spent grains is paramount. While previous studies focused on individual beverages, investigating technologies across different types of beverages has been overlooked. This paper provides a systematic review of agricultural waste valorisation over the past five years, focusing on four representative distilled beverages: whisk(e)y, tequila, baijiu and shochu. Research efforts have primarily focused on bioenergy production from whisk(e)y and tequila waste, whereas extracting functional substances is common for baijiu and shochu. Through integrating different technologies, a “Three-level Valorisation System” was proposed to enhance the translation of agricultural waste into value-added products like proteins. This system is directly relevant to the distilled beverage industry globally and applicable to associated industries such as biofuel and food production.

1. Introduction

Distilled beverages are strong alcoholic drinks obtained from the distillation of fermented grains or plant/fruit-derived sugars. Examples of distilled beverages include whisky (or whiskey), rum, tequila, gin, baijiu, brandy, shochu and vodka. In 2022, Scotch whisky exports were worth GBP 6.2 billion, which accounted for 77% of Scottish food and drink exports and 25% of all UK food and drink exports [1]. The global market value for distilled beverages is expected to grow to GBP 150.2 billion in 2028 at a compound annual growth rate of 5.6%. The main driver of the trend is identified to be the rising population, leading to the growing demand for alcoholic beverages worldwide [2].
In parallel, the rise of agricultural waste discharged from beverage production (Figure 1) presents an urgent challenge for today’s society (Table 1). For instance, Scotch whisky distilleries yield almost 900,000 tons of distiller’s spent grain as agricultural waste based on the annual production of 400 million litres of alcohol [3]. These spent grains are traditionally used as animal feed and have been considered as a co-product of distilleries in recent years due to the discovery of its value; however, some concerns have been raised such as susceptible microbial contamination and intensive energy requirement of drying the materials [4]. Baijiu, the world’s most heavily consumed distilled beverage, is generating over 20 million tons of agricultural waste per annum [5,6], resulting from an annual production of 6–8 billion litres of Baijiu in recent years [7]. The Baijiu waste is typically disposed of by landfill and incineration [8], which could cause detrimental effects on environmental pollution, the fertility of farmland and health risks to human beings. Furthermore, the agricultural waste has caused a financial burden to craft distillers, who often rely on a third-party waste management service, resulting in significant operational costs (personal communication). Therefore, harnessing this waste and developing innovative approaches to utilise it is an urgent need.
The concept of “circular economy” was launched by the European Commission, to develop a sustainable, low-carbon, resource-efficient and competitive economy “where the value of products, materials and resources is maintained in the economy for as long as possible, and the generation of waste minimized” [16]. This is opposed to the traditional linear approach of “extract-produce-use-dump” which is unsustainable from an economic and environmental point of view [17]. The transformation to this new model would not only assist distilleries to fulfil their sustainability commitments but also boost economic growth and further cultivate a sustainable brand image for the business as an effective way of building customer trust. Indeed, consumers have recently started to focus on “sustainability” to minimise environmental footprints. Consequently, the valorisation of solid waste for value-added product recovery plays an important role in achieving a circular economy for the distilling industry and become an emerging field in sustainable research and development.
Previous research efforts have been put into the valorisation of the agricultural waste discharged from a chosen type of distilled beverage. For example, tequila waste has been studied for value-added products from a biorefinery perspective [18,19]; whisk(e)y waste has been researched for functional food ingredient [20] and enhanced biogas yield and quality by pretreatments [21]; Baijiu waste treatment has been investigated in an integrated approach to achieve minimum solid waste discharge and clearer production [22].
While the waste derived from different beverages shares common compositions such as proteins, fats, and fibres (Figure 2), there is a notable gap in the literature whereby different distilled beverages are accounted for and assessed concerning valorisation technologies and associated products. To bridge this gap, our study aimed to examine valorisation technologies associated with four representative distilled beverages: whisk(e)y, tequila, baijiu and shochu. Drawing from this examination, we proposed an integrated approach to unlock and maximise valorisation opportunities within the global distilling industry. We anticipated that our paper would not only strengthen valorisation methods for distiller’s agricultural waste but also identify the necessary support and resources to propel technology implementation towards sustainability in the sector.

2. Whisk(e)y

The production of whisk(e)y represents a significant economic resource and cultural asset worldwide. In particular, Scotch whisky and Irish whiskey are known globally as renowned geographical indications. The agricultural raw ingredients of Scotch whisky are barley and wheat. Malted barley is used by malt distillers whereas un-malted grains such as wheat and maize are employed by grain distillers [26]. For every litre of alcohol production (Figure 1), an average of 2.5 kg of agricultural waste (known as draff or spent grain) is generated [3]. The fraction is nutritionally rich in carbohydrates, proteins, and fats (Table 1) and reusing it to develop sustainable processes has become technically and economically appealing in the last five years (Table 2), where the valorisation of draff and pot ale was often combined.

2.1. Bioenergy Production from Combined Draff and Pot Ale

Over the last decade, distilleries have combined draff with pot ale as a feedstock to produce biogas such as biomethane via anaerobic digestion, by which the consortia of microorganisms converted the biodegradable materials to biogas [17,33]. Recently, a comprehensive assessment of biogas potential associated with anaerobic digestion was reported at nine operational distilleries in seven countries. The specific methane yield obtained was 79.9 m3 methane for every ton of draff and 12.6 m3 methane for every ton of pot ale, both on a wet weight basis [27]. In the meantime, the production of hydrogen by utilising these materials was also investigated through dark fermentation where bacteria convert sugars and proteins into carboxylic acids, hydrogen, and carbon dioxide in the absence of light [17,28,34]. These developments not only supported the development of a circular economy but also reduced the distillery’s carbon footprint, environmental impact, and dependency on fossil fuels.

2.2. Pretreatment to Optimise Renewable Energy Production from Draff and Pot Ale

The exploration and optimisation of these technologies are in the process of leading to a deeper understanding of the system and improved plant performance with pretreatment technologies. Specifically, a research study assessed and modelled four scenarios to generate renewable energy from pot ale and draff, including dark fermentation, two-stage anaerobic digestion, single-stage anaerobic digestion and integrated feedstock pretreatment and anaerobic digestion [17]. In a whisky plant producing 2 million litres of whisky per annum, the authors identified that the system consisting of the integrated hydrothermal pretreatment and anaerobic digestion was optimal, with the potential to cover 25% total thermal energy requirement and 446% total electricity requirement corresponding to a reduction of 61% CO2 emission. Preliminary economic analysis indicated the system had the lowest payback period of 3.94 years. In addition, an anaerobic digestion plant was designed by the integration of a novel pretreatment method, where the materials were pretreated with 0.6 M NaOH and high shear homogeniser for 24 h [21]. The study designed the facility for a whisky distillery that produces 2 million litres of whisky per annum, anticipating generating up to 750,000 m3 of methane per year. Compared to conventional anaerobic digestion plants, the novel facility could lead to a 20% increase in methane yield. The estimated capital cost was GBP 3.1 million with a payback period of 9.60 years compared to 15.13 years without the pretreatment process. The authors also anticipated over 1000 tons of CO2 reduction per year by the installation of the novel facility, corresponding to a 30% decrease and subsequent savings in carbon taxes. More recently, a group of researchers compared three anaerobic digestion systems to enhance the return of distillery waste and by-products into the production of quality products [30]. It included a system consisting of a single continuous stirred tank reactor, a system consisting of two reactors, and a system consisting of three leach bed reactors and one expanded granular sludge bed. The result showed similar biomethane yield across the three systems; however, the two-phase anaerobic digestion system could provide additional valorisation opportunities due to the production of volatile fatty acids such as butyric acid and caproic acid. The authors concluded that the choice of the system may vary depending on the alignment of the distillery’s objectives with the system’s benefits and limitations. Moreover, it was reported that acid pretreatment by 1% H2SO4 at 135 °C for 15 min did not lead to significant improvement of methane yield from the solid by-products but reduced the digestion time by 54.5% [28]. To better understand microbial consortia during anaerobic digestion, the microbial and archaeal biodiversity was studied [28,29]. It was revealed that Lithe dominant bacterial genera were Sedimentibacter, Clostridium, and Romboutsia and the archaeal genus were Methanoculleus, Methanosarcina, Methanomassiliicoccus and Methanobrevibacter, indicating the methane was produced through the hydrogenotrophic methanogenesis pathway [28].

2.3. Bioenergy Production from Draff Alone

Recent research investigated the electrochemical conversion of the draff alone into hydrogen via a novel two-stage electrolysis process, equipped with proton exchange membrane electrolysis cells and a Keggin-type structured catalyst known as phosphomolybdic acid. In particular, the author anticipated that a draff concentration of 40.9 g/L would give great potential for hydrogen production [31]. This route was attractive because the electrolysis of biomass required much lower voltages than a standard potential of water electrolysis. The study provides valuable information to further enhance hydrogen yield and efficiency.

2.4. Feed/Food

The draff and pot ale have also been identified as one of the most promising biomass streams to produce insect-based feed ingredients for Scottish Farmed salmon. Indeed, a recent study provided a techno-economic feasibility study on the utilisation of different biomass streams as insect-based salmon feed ingredients. The authors anticipated the production of 8500 tonnes of larvae meal and 3800 tonnes of larvae oil out of the draff and pot ale at the largest geographical hotspot in Scotland [32].
Moreover, the spent grains have been considered as a value-added food especially given its higher contents of protein, fibre, vitamins, and minerals while lower contents of starch and calories compared to other grain-based products. An updated review of the potential use of spent grain in the production of value-added food products was provided [20]. It included bread (10–15% spent grain flour addition), pasta (5–15% spent grain flour addition), cookies, shortbread, muffins, wafers, snacks, and yoghurt (up to 30% spent grain addition), as well as sausages, tarhana and fruit juice and smoothies (up to 10% spent grain addition).

3. Tequila

The tequila industry is a world-renowned agro-industry in Mexico, made from a blue agave plant (Agave tequilana). Agave bagasse is the main agricultural waste discharged after the extraction of the fermentable juice from agave heart (also known as “piña”) during the manufacturing of tequila (Figure 1). It accounts for approx. 40% of the total weight of the consumed agave plant and is a fibrous material that is rich in cellulose, hemicellulose, and lignin [24]. In recent years, research efforts have been focused on the utilisation of agave bagasse to obtain advanced materials and renewable bioenergy (Table 3).

3.1. Bioenergy—Pretreatment of Agave Bagasse for Increased Productivity

Agave bagasse is a promising feedstock for bioenergy production including hydrogen methane and ethanol. However, it is known that productivity is hindered by the presence of lignin and the heterogeneous structure of hemicellulose. To address this, numerous studies have been performed to develop the pretreatment technologies, including oxidative and enzymatic pretreatment [35], detoxification pretreatment [36], steam explosion pretreatment [37], ionic liquid-based pretreatment [39,41,44], hydrothermal pretreatment [40], and high-pressure treatment [42]. In particular, enhanced saccharification of agave bagasse was reported where pretreatment of oxidative delignification and enzymatic synergism of cellulase and hemicellulase were carried out [35]. The result indicated a 2-fold increase in total sugar yield and productivity by using the enzyme mix. Energy production using such enzymatic hydrolysates as substrates improved the overall yield of hydrogen and methane production by 1.5 and 3.6 times, respectively, when compared to the enzymatic hydrolysates of agave bagasse without pretreatment and hydrolysed with a single type of enzyme. Another study reported for the first time an enhanced biomethane production by using bioderived ionic liquid-based pretreatment (cholinium lysinate) on agave bagasse [39]. The authors optimised the pretreatment conditions resulting in an elevated sugar yield of 51.4 g total sugars per g bagasse. Consequently, a mass balance starting from 100 kg of bagasse was proposed indicating the potential of this biorefinery scheme.

3.2. Optimisation of Production Systems for Renewable Energy Production from Agave Bagasse

Researchers also investigated production systems to generate renewable energy from agave bagasse [45,46,47]. A two-stage continuous treatment system was designed to maximise hydrogen and methane production from agave bagasse hydrolysate [45]. The first stage was a continuous stirred tank bioreactor (CSTR), where homoacetogenesis was controlled with agitation speed and organic loading rate to produce biohydrogen. The effluent from the CSTR was used as a substrate in the second stage for methane production in an up-flow anaerobic sludge blanket (UASB). The system demonstrated the possibility of achieving 6 L H2/L/d, and 6.4 L CH4/L/d, equivalent to 1.34 kJ/g bagasse and 7.88 kJ/g bagasse, respectively, resulting in an increased energy conversion efficiency of 56% compared with single-stage hydrogen production (8.2%). The same group compared continuous methane production from an enzymatic agave bagasse hydrolysate using two different systems, CSTR and UASB. The result demonstrated better performance of the USAB than the CSTR, probably due to the high solid retention time and the microbial community composition [47]. Further, the maximum production of volatile fatty acid and methane was defined for the first time using a two-stage continuous model including an acidogenic reactor and a methanogenic reactor [46]. The author also successfully carried out an experimental set-up using instrumented and automated bioreactors with robust closed-loop responses against kinetics and parametric uncertainties.

3.3. Nanofibres and Green Composites

The potential of extracting nanofibers from the agave bagasse was reported using the materials obtained from a local tequila factory [24]. In this work, organic solvents were applied to remove lignin and extract cellulose from the materials. Subsequently, the cellulose was processed through 6 cycles of a microfluidiser. The obtained cellulose nanofibers (CNFs) displayed high temperature resistance of thermal decomposition, high tolerance to pressure, elasticity, and uniform length with an average diameter of 75 nm. The authors concluded that the product could be used in the elaboration of various materials in the food, biomedical and textile industries. Another group of researchers assessed the feasibility of the potential of agave bagasse fibres as a raw material source for natural fibres and green composites. The effect of four treatments including acetylation, silane, alkali, and enzymatic surface treatments on the fibre’s properties was evaluated. Consequently, the author showed that the fibre can be used as reinforcement fibres to manufacture polylactic acid-based green composites which bring several advantages such as low cost, low density, moderate toughness and biodegradability [48].

3.4. Chemical Production

Agave bagasse lignin was also converted to medium-chain-length polyhydroxyalkanoates (PHA) via fermentation of wild-type and engineered Pseudomonas putida strains, which enabled a PHA titre up to 0.97 g/L [49]. It was almost four times higher than a previous finding (<0.25 g/L) where p-coumaric acid was used as a substrate [50], and the first report about bioconversion of a lignin stream rich in valillin by P. putida. The results were confirmed by Nuclear Magnetic Resonance (NMR) and Gas Chromatography-Mass Spectrography (GC-MS) with great potential for this process to be integrated into agave bagasse bioprocessing. Further, a literature review highlighted rural development opportunities for the incorporation of the agave plant into the production of polyhydroxybutyrate (PHB), leading to the manufacture of green composites towards a circular economy [51]. The authors also identified biological (e.g., microbial strain development), technological (e.g., fermentation optimisation), economical (e.g., techno-economic analysis) and social challenges (e.g., protection of biopiracy) for the successful implementation of these technologies.

4. Baijiu

Baijiu is a traditional Chinese distilled beverage that dates back more than 5000 years. It is made by solid-state fermentation and distillation (Figure 1) using raw ingredients mostly sorghum, and other grains such as wheat, and rice depending on the specific catalogue. Jiuzao, containing various organic components such as lignin, cellulose, and hemicellulose (Figure 2A), is the agricultural residue after solid-state fermentation and distillation of Baijiu. The research trend in the valorisation of the waste discharged from Baijiu production is presented in Table 4. In particular, over the last five years utilising Jiuzhao to extract functional substances including peptides [52,53], glutelin [33,54,55,56] and xylooligosaccharides (XOS) [6,57] has been emerging, along with growing interests in human health benefits. Each compound will be reviewed separately.

4.1. Functional Peptides

Four functional peptides were obtained from Jiuzao protein hydrolysates by enzymatic digestion [52]. The antioxidant activities of the tetrapeptides were evaluated using in vitro assays and HepG2 cell models. The authors suggested the healthy benefits of Baijiu and other functional food could be increased by the addition of such functional substances. In addition, the same research group optimised the hydrolysis method by using different proteinases and by varying treatment conditions including temperature, time, pH, and proteinase/substrate ratio [53]. The optimised yield of the tetrapeptide was 158.24 mg/kg Jiuzao. In addition, the author reported the peptide could improve antioxidant capacity in vivo by activating the Nrf2/Keap1-p38/PI3K-MafK signalling pathway and also exhibited anti-inflammatory activity by inhibiting the secretion of inflammatory cytokines and the mediator via the activation of Nrf2. Moreover, a sensory evaluation demonstrated no significant difference by the addition of peptide into Baijiu, potentially paving the way for commercial application in Baijiu and other foods.

4.2. Glutelin

Furthermore, Jiuzao glutelin extract was extracted from strong flavour type Baijiu by using a pulse electric field assisted process, a gentle non-thermal processing technology [33]. In optimal conditions, the extracted content was improved by 14% compared to ultrasound auxiliary extraction. The resulting Jiuzao glutelin extract exhibited stable structure, water and oil holding capacities, as well as antioxidant and angiotensin-converting enzyme inhibitory activities in vitro. The study indicated the pulse electric field-assisted process could be a potential technique for extracting high-quality Jiuzao glutelin extract. Nevertheless, the stability of the proteins can still be affected by environmental stress conditions such as temperature and shear stress. To address this, this research group followed up with another technique to manufacture the Jiuzao glutelin extract by grafting the Jiuzao glutelin with pullulan, a water-soluble polysaccharide with high stability [54]. In this work, pullulan-Jiuzao glutelin conjugates (PJCs) were produced through a wet-heating Maillard reaction. An optimisation study was carried out by varying pullulan to Jiuzao glutelin ratios, and the best result was obtained when pullulan to Jiuzao glutelin ratios of 2:1 was applied. It displayed the most stable nanoemulsion property with the lowest particle size and highest zeta potential. Compared to native Jiuzao glutelin, it exhibited better antioxidant activity and low toxicity to selected cells. The work demonstrated the potential of pullulan-Jiuzao glutelin conjugates as nanostabilisers for a range of food and non-food applications. The authors also pointed out that the PJCs can be useful in high-protein beverages because of their dense packing structure and pre-denaturation. In addition, to further improve the stability of Jiuzao glutelin in food applications, the same research group also investigated the conjugation of Jiuzao glutelin with carboxymethyl chitosan [55] and polysaccharides including dextran, gum arabic, and pectin [56], respectively, leading to promising methods of improving functional properties of Jiuzao glutelin.

4.3. Xylooligosaccharides (XOS)

XOS was reported to have great prebiotic potential by selectively stimulating beneficial gut bacteria and inhibiting pathogens [60,61]. This substance was produced from Jiuzao through a two-stage process starting from Jiuzao, where autohydrolysis was integrated with enzymatic hydrolysis using a recombinant thermostable xylanase, XynAS, produced by Escherichia coli. Through optimisation studies of both steps, the maximum XOS yield of 30.4% was achieved, resulting in the major XOS products being xylobiose and xylotriose. Consequently, the work demonstrated the feasibility of producing XOS by autohydrolysis combined with xylanase hydrolysis using Jiuzao as a substrate [57]. Furthermore, the same research group reported a similar two-stage process to produce the XOS, where a modified thermostable xylanase (XynAR) was applied. The highest yield of 34.2% was obtained under optimised process conditions including the autohydrolysis pretreatment step (181.5 °C for 20 min with solid-liquid ratio of 1:13.6) and subsequent enzymatic hydrolysis step at 60 °C and pH 5 with 15 U/mL of XynAR for 2 h [6].

4.4. Biochar

In addition, many investigations have been carried out to develop activated biochar from Jiuzao [8,58,59]. Recently, it was reported a novel potassium-enriched biochar as a controlled-release potassium fertiliser, was prepared by potassium acetate-assisted pyrolysis of Jiuzao [8]. The fabricated biochar exhibited excellent potential for controlled release performance in the long term, likely dominated by a combination of dissolution, electrostatic attraction, adsorption, confinement effect, and chemical interaction. In addition, the author also demonstrated the beneficial effect of biochar on the growth of Komatsuna (a leaf vegetable) in a pot experiment. Consequently, the results indicated a great potential for biochar as a controlled-release potassium fertiliser. Similarly, Jiuzao was utilised as a raw material to prepare a modified biochar material with dual functions including sustained release of multiple nutrients and potential to remove chromium. It was achieved by the modification of Jiuzao through wet process phosphoric acid modification without washing and subsequent solid-phase pyrolysis [58]. Moreover, a low-cost activated biochar from Jiuzao was developed to remove turbidity in low-alcoholic Baijiu [59]. The biochar was prepared by the controlled pyrolysis of Jiuzao followed by steam activation, displaying a large specific surface area (320–480 m2/g) and pore volume (approx. 0.46 cm3/g). It exhibited a good adsorption selectivity to the components with long lipophilic chains such as ethyl palmitate which often lead to turbidity during the preparation process of low alcoholic Baijiu, while limited absorptions to the main flavour components in Baijiu were observed. This was explained by its excellent exterior hydrophily and interior lipophilicity.

4.5. Chemical Production

Jiuzao was also utilised to produce an ester of caproic acid (i.e., caproate), an emerging chemical platform with diverse applications [5]. In this work, shallow pit mud was used as an inoculum in anaerobic fermentation where lactic acid in the Jiuzao was identified as the main electronic donor. The dominant production microorganisms were Lactobacillus, the producer of lactic acid, and Caproiciproducens spp., which converted lactic acid to caproate. The author also provided useful information on the metabolic pathway of caproate production with the co-existence of ethanol and lactic acid. The study demonstrated another promising approach for the valorisation of Jiuzao.

5. Shochu

Shochu is a traditional Japanese distilled beverage made from grain, such as sweet potato, barley, rice, and wheat. Similar to Chinese baijiu, after fermentation and distillation the remaining agricultural residue, sometimes named shochu slop, is mostly regarded as industrial waste (Figure 1). Recent studies have been carried out on various Shochu distillery wastes, paving the way for using the waste as potential compounds for bacterial culture nutrients [62,63], electricity generation [14,64], functional feed [65,66], and advanced materials [67]. Research trends on shochu waste derived from sweet potato, barley rice and wheat were summarised in Table 5 and described below, respectively.

5.1. Bacterial Culture Medium

It was reported that sweet potato-based shochu slop was an excellent culture medium for Escherichia coli K-12 [62]. In this study, the authors first removed solid matter from the shochu slop and subsequently adjusted the pH of the supernatant to 7 followed by passing it through a membrane filter. The resulting liquid was used as a cultivating medium without dilution. The result indicated that the ability of shochu slop to support bacteria growth and enzyme production was much better compared to conventional media (LB Miller). In addition, the performance was comparable to another media type namely Terrific broth, which could grow E. coli to a high cell density or obtain a high yield of bacterial protein.
Kuma shochu, made from rice, is another brand with the protection of geographical indication. Researchers turned the Kuma shochu waste into culture nutrients for photosynthetic bacteria (PSB) [63]. The authors successfully isolated 4 PSB strains, showing the highest nucleotide sequence homologies to Rhodopseudomonas palustris, which grew well on the nutrient without extra additives. Field trials indicated significant increases in fresh weight in various vegetables by the treatment of Kuma PSB, including cabbage, Chinese cabbage, broccoli and Japanese radish. In addition, the authors also started manufacturing and distributing the Kuma PSB to local farmers, run by a university-student-run business venture.

5.2. Electricity Generation

Electricity generation from sweet potato-based shochu waste was demonstrated using cassette-electrode microbial fuel cells (CE-MFCs) [14]. The authors observed the maximum power density (1.2 W/m3) and COD removal efficiency (67.4 ± 1.8%) in the CE-MFCs with 10 g COD/L shochu waste. In addition, the microbial community analysis revealed an interesting microbial structure change with different ingredients. For instance, the genus Clostridium (75.4%) was predominant in the CE-MFCs with raw shochu waste (73 g COD/L), whereas Bacteroides (65.3%) and Clostridium (12.1%) were predominant in the CE-MFCs with diluted Shochu waste (10 g COD/L). The authors proposed future work to isolate and characterise the microorganisms for a detailed study of the electron transfer mechanisms.
Electricity generation was also reported in combination with the treatment of barley-based shochu waste through stacked microbial fuel cells, in which two pairs of electrodes were stacked in a single unit [64]. The study reported a maximum power density of 4.3 ± 0.2 W/m3 and a chemical oxygen demand removal efficiency of 36.7 ± 1.1%. Moreover, microbial community analysis suggested that Clostridiaceae, Acetobacteriaceae, and Enterobacteriaceae played important roles in organic waste decomposition and electricity generation in the SMFCs.
Finally, wheat-based shochu waste was processed by pre-carbonisation and then chemical activation to produce shochu waste-derived activated carbon (SWAC) [67]. Their potential to be used as the electrode active material of electric double-layer capacitors (EDLCs) was assessed by a series of studies. The authors concluded that the SWAC is highly promising as an active material for EDLC electrodes, potentially leading to better energy-power performance than commercial benchmark active carbons.

5.3. Functional Feed

Lactic acid fermentation was carried out on sweet potato-based shochu waste [65]. The resulting solid content was supplemented (1–10%) with salmon roe emulsion oil at various doses before applying in the larviculture of Japanese flounder (Paralichthys olivaceus). The authors evaluated its impact on the feeding response and performance of P. olivaceus larvae through a series of studies. The result indicated that the shochu waste could have potential as a live feed enrichment media for larval fish production, particularly with 5 and 10% enrichment dosage.
Another study investigated the effect of barley-based Shochu waste on pig stress and pork quality. The results indicated that the shochu slop was able to reduce the level of cortisol, a stress marker of pigs, and also increased oleic acids in sirloin and fillet, leading to improved pork meat quality [66].

6. Proposal of a “Three-Level Valorisation System”

The technologies and products outlined in Table 2, Table 3, Table 4 and Table 5 were traditionally segregated by the type of distilled beverage. Drawing from our review of waste valorisation technologies across four representative distilled beverages, we introduced a novel concept termed the “Three-Level Valorisation System” for agricultural waste treatment. This system aims to enhance the conversion of waste into value-added products and promotes resource sharing within communities, as illustrated in Figure 3.
In Level 1, craft distillers could consolidate their agricultural waste for pretreatment and temporary storage, potentially managed by a separate entity. Large distillers could opt for this central facility or utilise their waste treatment infrastructure such as for bioenergy production. The initial treatment step aims to preserve the chemical characteristics of the organic waste over an extended period. In Level 2, the centralised agricultural waste would undergo extraction of value-added substances like functional ingredients and advanced materials (e.g., proteins). The remaining materials could then be used for bioenergy or electricity generation to power distilleries, or for composting in agricultural applications. Any residual material from energy production could also be composted. Ultimately, primary products from Level 2 could undergo further processing in Level 3, managed by additional business partners to achieve enhanced value.
The proposed system is beneficial to businesses, people, and the environment, with the potential to enhance a “cascade approach” for processing spent grains from Chinese baijiu [22]. First of all, the extraction of protein before bioenergy production would be of particular benefit for two reasons: (1) these wastes are usually higher in nitrogen content than is ideal for digestion, resulting in excessive ammonia production and subsequent operational challenges [68]. Prior extraction of protein would minimise this problem and increase the Carbon/Nitrogen ratio of the material; (2) the historical use of such wastes is in animal feed. Removal of these protein sources from the food chain creates a protein shortfall, and a need to source such protein elsewhere. As an unintended consequence, sourcing such protein can come at a higher carbon cost than that is saved by the biogas being generated [69]. Secondly, this approach holds particular significance to craft distillers with limited in-house waste treatment capacities. By assessing the facilities for Level 2–3 valorisation, the small businesses could contribute to the United Nations’ sustainable development goals while enhancing their reputation through circular economy practices. Overall, our proposal is poised to make significant contributions towards sustainable manufacturing practices. It is anticipated that our framework will not only directly benefit the global distilled beverages industry but also hold promise for related sectors utilising agricultural materials for production such as biofuel and food industries. Indeed, this framework represents a valuable opportunity for collaboration between academic, governmental, and industrial clusters to develop local infrastructure supporting waste valorisation efforts in the field.
Despite the advantages, it is important to conduct a comprehensive techno-economic assessment to fully validate the efficiency of this strategy. Additionally, while biogas generation via anaerobic digestion has been scaled up successfully, it is essential to acknowledge the challenge that many promising studies on valuable compound extraction were conducted at the laboratory scale. In such settings, strict experimental conditions such as cleanliness, pressure and temperature, and high-grade chemical ingredients are often employed. While controlled conditions are necessary for method development, transitioning laboratory technologies to pilot or full-scale processes requires additional research, development and investment. Consequently, addressing scale-up challenges will be crucial for realising the full potential of these technologies in the proposed strategy.

7. Conclusions

With the rise in demand for distilled beverages, management of substantial amounts of post-production agricultural waste has become increasingly critical. Effectively managing this waste is essential for sustainability. This paper provides an overview of recent research trends in agricultural waste valorisation from four major distilled beverages: whisk(e)y, tequila, baijiu and shochu. Various technologies have been explored, resulting in the production of valuable products such as bioenergy, advanced materials, and functional feed/food ingredients. Although the production of four beverages is based in different regions of the world, we observed similarities in the methodology for processing post-production agricultural residues, targeting common compositions such as proteins, fats and fibres. Leveraging this knowledge, a “Three-Level Valorisation System” was proposed, integrating different technologies systematically and enhancing economic sustainability. In Level 1, craft distillers can centralise their agricultural waste for pretreatment and temporary storage, while large distillers can use either the central facility or their waste treatment systems. The initial treatment preserves the waste’s chemical properties. In Level 2, the waste is processed to extract valuable substances like proteins, with the remaining materials used for bioenergy, electricity generation, or composting. Residual material from energy production is also composted. In Level 3, primary products from the previous process can be further refined by business partners for added value. Future studies and investments will be required to implement this strategy. On one hand, a thorough techno-economic assessment is needed to fully validate this strategy’s efficiency. On the other hand, establishing infrastructure for scale-up development of these technologies is crucial for translating laboratory technologies into practical solutions.

Author Contributions

K.S.: draft preparation, reviewing and editing. N.W.: writing—reviewing and editing. S.Z.: conceptualisation and writing—original draft preparation, reviewing and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors gratefully acknowledge the support from Scotland’s Industrial Biotechnology Innovation Centre (IBioIC) and the Institute of Biological Chemistry, Biophysics and Bioengineering, Heriot-Watt University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Diagrams depicting the discharge of agricultural waste from the production of various distilled beverages [9,10,11,12]. Note that the start points of the big arrows indicate where the agricultural waste derives from in each process.
Figure 1. Diagrams depicting the discharge of agricultural waste from the production of various distilled beverages [9,10,11,12]. Note that the start points of the big arrows indicate where the agricultural waste derives from in each process.
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Figure 2. Compositions of fibres (A), crude protein and crude fat (B) in the agricultural waste discharged from the production of whisk(e)y [4,23], tequila [24], baijiu [6,22], and shochu [25]. Note that the composition of fibres in shochu and protein/fat contents in tequila are not available in the present literature hence they are not reported.
Figure 2. Compositions of fibres (A), crude protein and crude fat (B) in the agricultural waste discharged from the production of whisk(e)y [4,23], tequila [24], baijiu [6,22], and shochu [25]. Note that the composition of fibres in shochu and protein/fat contents in tequila are not available in the present literature hence they are not reported.
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Figure 3. Proposal of the “Three-Level Valorisation System”. Briefly, in Level 1, craft distillers could centralise their agricultural waste for pretreatment and temporary storage externally operated by a business partner, whilst large distillers have the option to use the central facility or use their waste treatment plant e.g., to produce bioenergy. In Level 2, the centralised solid waste could first be utilised to extract value-added substances such as functional ingredients and advanced materials (e.g., proteins). Subsequently, the remaining materials would be utilised in bioenergy/electricity production for energy supply returning to distilleries, or by composting for agricultural application. The leftover material from the energy production stream could also be composted. Ultimately, the primary products derived from Level 2 could achieve further added value by Level 3 processing, operated by other business partners. The benefits of the proposed system are discussed in the main text.
Figure 3. Proposal of the “Three-Level Valorisation System”. Briefly, in Level 1, craft distillers could centralise their agricultural waste for pretreatment and temporary storage externally operated by a business partner, whilst large distillers have the option to use the central facility or use their waste treatment plant e.g., to produce bioenergy. In Level 2, the centralised solid waste could first be utilised to extract value-added substances such as functional ingredients and advanced materials (e.g., proteins). Subsequently, the remaining materials would be utilised in bioenergy/electricity production for energy supply returning to distilleries, or by composting for agricultural application. The leftover material from the energy production stream could also be composted. Ultimately, the primary products derived from Level 2 could achieve further added value by Level 3 processing, operated by other business partners. The benefits of the proposed system are discussed in the main text.
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Table 1. Estimated agricultural waste discharged from the production of distilled beverages.
Table 1. Estimated agricultural waste discharged from the production of distilled beverages.
Spirit TypeEstimated Waste DischargeReferences
Discharge per Litre SpiritAnnual Discharge
Scotch whisky2.5 kg9 × 105 tons[3]
Tequila3.6 kg1 × 106 tons[13]
Baijiu3.0 kg2 × 107 tons[5,6]
Shochu2.0 kg8 × 105 tons[14,15]
Table 2. The literature investigating valorisation of agricultural waste discharged from Scotch whisky and Irish whiskey production in the past five years.
Table 2. The literature investigating valorisation of agricultural waste discharged from Scotch whisky and Irish whiskey production in the past five years.
Value-Added ProductsTechnologies/
Subjects		Key FindingsReferences
Bioenergy
(Biomethane)		Anaerobic digestionFour scenarios based on a whiskey plant size of 2 million L/a were assessed. The recommended scenario resulted in a biogas production containing 1.03 million m3 biomethane, corresponding to an energy yield of 10,300 MWh. It could cover 446% of the annual electricity demand and 25% of the heat demand of a traditional distillery.[17]
Bioenergy
(Biomethane)		Anaerobic digestionA comprehensive assessment of biogas potential was reported at nine operational distilleries. Large variation was observed from different distilleries depending on the feedstocks and the processes employed. The theoretical methane yield obtained ranged from 288 to 521 L CH4 per kg volatile solids.[27]
Bioenergy
(Biomethane)		Anaerobic digestionMethane production potential via anaerobic digestion was assessed, including specific methane yield, acid pretreatment and microbial analysis. The methane yield from the unprocessed by-products was 330 mL/g volatile solids from draff.[28]
Bioenergy
(Biomethane)		Anaerobic digestionA novel anaerobic digestion plant integrated with a pretreatment stage was designed for a whisky distillery that produces 2 million litres whiskey per annum. Compared with the conventional digesters, the proposed method achieved a 20% increase in methane yield.[21]
Bioenergy
(Biomethane)		Anaerobic digestionAcetoclastic methanogenesis and hydrogenotrophic methanogenesis are important pathways for biogas production under mesophilic and thermophilic anaerobic digestion plant.[29]
Bioenergy (biomethane) and volatile fatty acidsAnaerobic digestionThe three systems studied delivered similar methane yields which could supply up to 44% of the thermal energy demand of a distillery that produces 50 million litre of whiskey per annum. The two-phase anaerobic digestion system could provide additional valorisation opportunity due to the production of volatile fatty acid.[30]
Bioenergy
(Biohydrogen)		ElectrosynthesisDraff and pot ale were identified as possible substrate for hydrogen production via digestion and electrolysis.[31]
FeedFeasibility studyDraff and pot ale were identified as the most promising biomass to produce Insect-based salmon feed ingredients.[32]
Table 3. The literature investigating valorisation of agricultural waste discharged from tequila production in the past five years.
Table 3. The literature investigating valorisation of agricultural waste discharged from tequila production in the past five years.
Value-Added ProductsTechnologies/SubjectsKey FindingsReferences
Bioenergy
(Biomethane and biohydrogen)		Oxidative delignification and enzymatic pretreatmentIt resulted in yields 1.5 and 3.6 times (hydrogen and methane, respectively) superior to those obtained with hydrolysates of non-pretreated bagasse processed with a single enzyme.[35]
Bioenergy
(Biohydrogen)		Detoxification pretreatmentThe detoxified hydrolysate produced 33% more biohydrogen than the un-detoxified one at the optimal condition.[36]
Bioenergy
(Biomethane)		Steam explosion pretreatmentIt demonstrated steam explosion prior to anaerobic digestion could add on site output in energy recovery from agave bagasse.[37]
Bioenergy
(Biomethane)		Comparison of pretreatment methodBiological methane potential from most hydrolysates was the same whilst difference was observed in lag phase and the methane production rates.[38]
Bioenergy
(Biomethane)		Ionic liquid-based pretreatmentHigh methane generation was obtained using the hydrolysate from ionic liquid pretreated materials, 7.5 times higher when compared to that obtained without pretreatment.[39]
Bioenergy
(Biomethane)		Hydrothermal pretreatmentThe operating condition of 154 °C and 15 min achieve the maximum solubilisation of total carbohydrates.[40]
Bioenergy
(Biomethane)		Ionic liquid pretreatmentIonic liquid–water mixtures enhance pretreatment and anaerobic digestion of agave bagasse.[41]
Bioenergy
(Bioethanol)		High-pressure pretreatmentIt demonstrated a sustainable high pressure treatment of lignocellulosic residual biomass for ethanol production.[42]
Bioenergy
(Biomethane)		Condition optimisationData reveal a acid catalyst effect, batch optimisation and stability of the semi-continuous process.[43]
Bioenergy
(Bioethanol)		Protic ionic liquid pretreatmentPretreatment with a biocompatible low-cost protic ionic liquid was achieved, allowing one-pot ethanol production.[44]
Bioenergy
(Biomethane and biohydrogen)		Design of a two-stage systemThe continuous hydrogen and methane production system was able to achieve equivalent to approx. 9 kJ/g bagasse, significantly increased energy recovery efficiency compared to one-stage methane production system.[45]
Bioenergy (Biomethane) and volatile fatty acidsProcess developmentTwo stage continuous production was modelled and demonstrated, where maximum volatile fatty acid production in the first stage and maximum biomethane production in the second stage were defined for the first time.[46]
Bioenergy
(Biomethane)		Continuous methane production in CSTR and UASBThe UASB reactor and the use of Cellulase 50XL can be advantageous features for future industrial application of methane production.[47]
NanofibersOrganosolv treatment followed by microfluidiserThe product has high resistance to pressure, and elasticity, with a potential to be used in the elaboration of advanced materials.[24]
Green compositesFibre-surface treatmentsThe fibre can be used as reinforcement fibres to manufacture polylactic acid-based green composites.[48]
ChemicalBioconversion of lignin to PHAIncreased PHA titre was achieved by a series of biological, fermentation configuration and condition optimisation.[49]
Table 4. The literature investigating valorisation of agricultural waste discharged from baijiu production in the past five years.
Table 4. The literature investigating valorisation of agricultural waste discharged from baijiu production in the past five years.
Value-Added ProductsTechnologies/SubjectsKey FindingsReferences
Functional peptidesExtraction method developmentAntioxidant peptides was obtained from Jiuzao protein hydrolysates.[52]
Functional peptidesMethod optimisation and mechanism investigationThe optimised yield of the tetrapeptide was obtained. It could improve antioxidant capacity in vivo by activating the Nrf2/Keap1-p38/PI3K-MafK signalling pathway.[53]
GlutelinPulse electric field (PEF) assisted processPEF can be a potential technique to extract high-quality glutelin extract from Jiuzao.[33]
GlutelinGrafting of Jiuzao glutelin with pullulanPullulan to Jiuzao glutelin ratio was optimised. A resulting product (PJC-2) can be a potential nanostabiliser for a range of food and non-food applications.[54]
GlutelinUltrasound-stirring assisted Maillard reactionJiuzao glutelin conjugates was prepared and optimised between Jiuzao glutelin and carboxymethyl chitosan, leading to improved functional property of Jiuzao glutelin.[55]
GlutelinMaillard reactionJiuzao glutelin was grafted with dextran, gum arabic, and pectin via Maillard reaction, respectively, leading to improved stability.[56]
XylooligosaccharidesAutohydrolysis with a recombinant thermostable, xylanases (XynAS)Combining autohydrolysis with the xylanases (XynAS) to produce xylooligosaccharides was achieved with maximum yield of 30.4%.[6]
XylooligosaccharidesAutohydrolysis with a recombinant thermostable, xylanases, namely XynARA similar to above process was developed to produce xylooligosaccharides, by autohydrolysis coupled with enzymatic hydrolysis (XynAR), with a yield of 34.2%.[57]
BiocharPotassium assisted pyrolysisA novel potassium enriched biochar was developed as a controlled-release potassium fertiliser.[8]
BiocharWet-process phosphoric acid and subsequent solid-phase pyrolysisThe biochar materials displayed dual functions including sustained release of multiple nutrients and potential to remove chromium.[58]
BiocharControlled synthesis of biocharA low-cost activated biochar was prepared from Jiuzao, used for turbidity removal in low alcoholic baijiu.[59]
ChemicalAnaerobic fermentationJiuzao was used to produce ester of caproic acid using shallow pit mud as an inoculum in anaerobic fermentation.[5]
Table 5. The literature investigating valorisation of agricultural waste discharged from shochu production in the past five years.
Table 5. The literature investigating valorisation of agricultural waste discharged from shochu production in the past five years.
Value-Added ProductsTechnologies/SubjectsKey FindingsReferences
Bacterial culture mediumGrowing trialsSweet potato based Shochu waste was an excellent culture medium for Escherichia coli K-12.[62]
Bacterial culture mediumGrowing trialsRice based shochu waste could provide culture nutrient to selected photosynthetic bacteria. The resulting Kuma PSB promoted the growth of various vegetables.[63]
ElectricityCassette-electrode microbial fuel cellsElectricity generation from sweet potato based shochu waste was demonstrated.[14]
ElectricityStacked microbial fuel cellsBarley based shochu waste was utilised to generate electricity.[64]
ElectricityMaterial treatment and characterisationShochu waste-derived activated carbon is a promising ingredient as the electrode active material of electric double-layer capacitors.[67]
Functional feedFeeding trialsBarley based Shochu waste improved pork meat quality by reducing stress.[65]
Functional feedFeeding trialsSweet potato based Shochu waste could be used as a potential live feed enrichment media for larviculture of Japanese flounder.[66]
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Stewart, K.; Willoughby, N.; Zhuang, S. Research Trends on Valorisation of Agricultural Waste Discharged from Production of Distilled Beverages and Their Implications for a “Three-Level Valorisation System”. Sustainability 2024, 16, 6847. https://doi.org/10.3390/su16166847

AMA Style

Stewart K, Willoughby N, Zhuang S. Research Trends on Valorisation of Agricultural Waste Discharged from Production of Distilled Beverages and Their Implications for a “Three-Level Valorisation System”. Sustainability. 2024; 16(16):6847. https://doi.org/10.3390/su16166847

Chicago/Turabian Style

Stewart, Kelly, Nik Willoughby, and Shiwen Zhuang. 2024. "Research Trends on Valorisation of Agricultural Waste Discharged from Production of Distilled Beverages and Their Implications for a “Three-Level Valorisation System”" Sustainability 16, no. 16: 6847. https://doi.org/10.3390/su16166847

APA Style

Stewart, K., Willoughby, N., & Zhuang, S. (2024). Research Trends on Valorisation of Agricultural Waste Discharged from Production of Distilled Beverages and Their Implications for a “Three-Level Valorisation System”. Sustainability, 16(16), 6847. https://doi.org/10.3390/su16166847

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