Future applications for brewers’ spent grain

The brewery by-product Brewers’ Spent Grains (BSG) is composed of the insoluble cereal residue that is separated from the mash before fermentation. It is estimated that the annual production of BSG is approximately 30 million tonnes worldwide. BSG consists mainly of the insoluble covering layers of the barley malt, i.e. husk, testa and pericarp, as well as endosperm cell wall fractions and storage protein. The composition of BSG depends on the raw materials of the brewing process: barley variety, harvest year, malting and mashing conditions, as well as the type and quality of other cereals added to the brewing process.

BSG is typically recovered as a wet material containing less than 30 per cent (w/w) of dry matter. The main components of BSG are cellulose, hemicellulose, i.e. arabinoxylan, lignin, protein, lipids, and low molecular weight phenolic compounds (Table 1).

Potential applications

Currently, BSG is mainly used as cattle feed. This may indeed be the most attractive application in areas where cattle are available. However, due to the high water content, BSG is not microbiologically stable and cannot be stored for a long period. Therefore, finding new outputs for the by-product may be important in areas where no demand for cattle feed exists. There may also be net costs related to getting BSG removed. Consequently, a considerable amount of research has been conducted over the years with the aim of developing new uses for spent grains. A range of more or less exotic applications have been suggested. These include use as building material1, as raw material for paper production2, as adsorbent3 or as carrier material for beer fermentation4. The suitability of BSG as raw material for mushroom cultivation2, xylitol production5 or ethanol production has also been elucidated.

When BSG is used as a carbon source, for example, bioethanol production, a physical or chemical pre-treatment and enzymatic or acid-catalysed breakdown of carbohydrates into fermentable sugars is required. Possible pre-treatments include steam explosion6 and milling. The feasibility of the use of BSG as a carbon source for fermentation is restricted by the enzyme costs, low yield due to the incomplete break down of the matrix and long processing times needed for the enzyme hydrolysis stage (typically 24-72 hours). Furthermore, yeasts normally use only glucose to make ethanol, leaving pentose sugars arising from hemocellulose unused. Genetically engineered yeasts able to metabolise xylose after glucose are, however, also available7,8. The cost of the enzymatic hydrolysis stage is expected to decrease significantly in two to five years due to extensive research work carried out all around the world.

As BSG originates from a food process, it is natural that many researchers have looked into the food uses of BSG. For example, small amounts of spent grains have been added to baked food products9,10. It has also been shown that BSG could serve as raw material for ready-to-eat snacks11. Considering its high lignin and cellulose content, another logical application for BSG is energy production. The present article focuses on energy and food ingredients from spent grain.

Energy from brewers’ spent grains

There are basically two alternatives for energy production from BSG:

1. Drying and combustion to generate electricity and/or heat
2. Production of biogas by anaerobic micro-organisms (anaerobic digestion)

The possibilities and limitations of the two alternatives are briefly described below.

1. Combustion of BSG can serve two purposes: i) getting rid of the material or ii) energy production in the form of heat and/or electricity. For both purposes, either a small or medium-scale local boiler or alternatively co-firing in a larger boiler are options. In the latter case the spent grains can, for example, be sold as such or as dried, thus avoiding investing in capital intensive boilers at the brewery. The need for drying before combustion depends on the moisture content of the spent grain. As a rule of thumb, biomass of over 35-40 per cent d.w. can usually be combusted as such. If energy production is the goal, the energy balance has to be calculated case-by-case (need for energy for drying vs. generation of energy). Dewatering and combustion technology is widely available and some breweries are already applying this route. As an alternative, proteins can be separated first, and the remaining lignin fraction combusted after dewatering12.
2. In biogas production, a mixture of microbes breaks down the cellulose and other carbohydrates into sugars, which are then anaerobically converted to methane. The breakdown of cellulose is generally not very efficient in anaerobic conditions and some kind of physical or chemical pre-treatment is beneficial. Furthermore, the success of biogas production technology is dependent on the carbon/nitrogen ratio of the spent grain. The remaining biomass and lignin-rich residue can be dried and combusted, or alternatively composted. The rate-limiting step in both biogas and ethanol production is hydrolysis of carbohydrate polymers into fermentable sugars. The lignocellulose matrix is very resistant and matrix disruption requires energy and/or time. Currently, the disassembly of agrobiomass matrices by combined enzymatic and mechanical means is being investigated and more efficient process concepts can be expected in the future.

Food ingredients from spent grain

BSG is an interesting raw material for food ingredients. It has high fibre content, proteins and potential phenolic antioxidants. The proteins in BSG are rich in glutamine which has been reported to promote the recovery and preservation of intestinal mucosa and to prevent bacterial translocation from the gastrointestinal tract13. A protein-enriched fraction of BSG has proven useful in the treatment of the inflammatory bowel disease ulcerative colitis in humans14. Other mechanisms than those related to glutamine may also lie behind the positive effects of the protein-enriched BSG. The proteins in BSG can be almost completely solubilised by enzymatic hydrolysis releasing soluble peptides15. Removing protein material from BSG improves the energy value of the residue. Furthermore, the soluble peptides are themselves an interesting by-product. So far, the nutritional and functional properties of this fraction have not been characterised in-depth, but this will certainly be an interesting area for future research.

BSG is a potential source of soluble xylo-oligosaccharides, provided that the hemicelluloses are first hydrolysed into oligosaccharides. Xylo-oligosaccharides are indigestible by human enzymes in the small intestine, but are extensively fermented by desirable bacterial species, such as Bifidobacteria, in the large bowl and therefore have prebiotic activity16.

Spent grain is a good source of the phenolic antioxidants ferulic and p-coumaric acid, as well as dimeric forms of ferulic acid17. In BSG, ferulic acid is bound to arabinoxylan. The release of ferulic acid from BSG can be improved by the use of ferulic acid esterase together with xylanase18. Xylanases solubilise low-molecular weight feruloylated oligosaccharides, which are then further degraded by the esterase to produce ferulic acid. In fact, feruloylated, xylo-oligosaccharides may themselves be interesting food ingredients. They are water-soluble antioxidants, whereas free ferulic acid has limited solubility in water. An enzyme-aided process for fractionation of BSG to peptides and xylo-oligosaccharides has been developed in the REPRO project15,19-24.

A major part of the cell wall matrix as well as lignin remains in the solid residue after enzymatic processing. Lignin is known to be poorly fermented by colon microbiota. However, lignins are antioxidants and might have other types of beneficial effects in the GI tract. More research is, however, needed to investigate these impacts. The possible health-benefits of the lignin-enriched cell wall residues are best exploited when the whole, insoluble fraction is used as an ingredient. Such fractions can be incorporated into drinks by nano-scale wet grinding i.e. by milling in colloid mills or in agitated ball mills. The finely ground meal forms a dispersion with pleasant mouth-feel. However, one of the barriers with BSG is its bitter taste. Certain free phenolic compounds as well as peptides and fatty acids have an impact on flavour, mainly by increasing perceived bitterness. Masking or removing the bitter taste is one of the challenging but not impossible tasks of current and future research.

Future perspectives

With the increasing demand for sustainability in all operations, it is self evident that no side streams are wasted in the future. The worldwide effort to increase the proportion of renewable energy sources will probably increase the use of BSG as energy source. A future alternative to ethanol production may be the production of butanol, another potential road transport biofuel. Butanol has higher energy content, lower vapour pressure and higher flashpoint meaning reduced risk of fire in storage. Butanol can be produced from sugars by some bacteria (e.g. Clostridium acetobutylicum). Genetic engineering of microbes will probably be necessary to improve the feasibility of this approach.

BSG remains an attractive raw material for health-promoting food ingredients. The health-promoting food trend is not likely to die out as long as the developed countries are tackling the problems arising from an overweight and aging population. Therefore, we may see some interesting new applications for BSG fractions to emerge in the near-future.

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