Brewers’ spent grain (BSG) is the main by-product of the brewing industry. It is the insoluble cereal residue that is separated from the mash before fermentation. It represents an enormous 85% of the total by-product produced by this industry. One estimate back in 2009 from a team at the VTT in Finland estimated 30 million tonnes was being produced globally. Those amounts must be considerably larger now and it is a global resource. Where there is brewing, there is BSG. The VTT team has reported extensively on the potential use for BSG in a number of areas (Wilhelmson et al., 2009). BSG is a possible substrate for solid-state fermentation because it can support the growth of fungi. As a high volume and low cost resource, it is ripe for industrial exploitation but has so far eluded entrepreneurs developing it as a suitable material for food use.
It is valuable source of potential food material for human nutrition.
How Does BSG Come About?
In the brewing process, barley (Hordeum vulgare L.) is germinated to produce and activate hydrolytic enzymes in the malting process. These enzymes hydrolyze in the first instance barley starch as well as other stored food reserves including carbohydrates and proteins in the barley seed. Following mashing, the sugar-rich liquor called wort is fermented to beer. The material left behind is this brewers’ spent grain which is the barley malt residue remaining when the wort is separated from it. We know about 200g of wet BSG is produced per litre of beer. (Mussatto et al., 2006).
Some references call BSG brewers dried grain (BDG) (Chaudhary & Weber, 1990).
What is BSG?
It is primarily the cellular remains of the fermentation medium but also includes cellular components from other microorganisms although clearly not yeast. Generally, BSG is all the insoluble covering layers from the barley malt such as the husk, pericarp and testa, the endosperm cell wall fractions and barley storage protein. Hordein is the main protein in barley seed but albumins and a plethora of protein types will also be present.
It has a high protein (approx. 20-25%) and fibre content (70%) on a dry weight basis.
The main fibre fraction is lignocellulosic material containing 17% cellulose and a similar fraction amount of hemicellulose, 28% non-cellulosic polysaccharides, chiefly arabinoxylans which we will discuss later, and 28% lignin (Mussatto et al., 2006). Residual starch contributes up to 13% by dry weight and is what remains after malting. The content and type of material obtained depends on what type of grain was used in the brewing process. Different malts undoubtedly produce different levels of protein and fiber for example.
BSG is available in large quantities on a year round basis but has only been used on a large scale as animal feed.
The BSG has a strong resident microflora mainly of thermophilic aerobic bacteria (<107 g-1 fresh weight). Much more should be done to explore the microorganisms associated with this material.
The high fibre content has excited plenty of interest as has the proteins. Many of these proteins are rich in glutamine which makes it as a suitable nutrient for bacteria.
BSG not only offers plenty of insoluble fibre in the form of the celluloses and hemicelluloses, there is plenty of soluble xylo-oligosaccharide when the hemicelluloses are hydrolysed into oligosaccharides. The xylooligosaccharides are incapable of being digested in the human upper intestine. However, it is a possible prebiotic because this type of fibre is fermented by probiotic bacteria including the Bifidobacteria in the lower intestine (Okazaki et al., 1990).
Barley arabinoxylans are polysaccharides with a poly-(1,4)-β-D-xylopyranose backbone, which is substituted with arabinofuranosyl residues at O-2, O-3 or at both O-2 and O-3 positions (Kabel et al., 2002).
It should be borne in mind that different breweries produce differing degrees of BSG material so there will always be a quality and consistency issue contrary to what is often reported about this byproduct (Roberston et al., 2010).
The major portion of total lipids present in BSG are triacylglycerols, followed by fatty acids such as linoleic, palmitic, and oleic acids. Minor amounts of diacylglycerols and monoacylglycerols have also been reported (del Río et al., 2013, Fărcaş et al., 2015, Niemi et al., 2012).
Preparation of Brewers Spent Grains For Applications
Brewers Spent Grain (BSG) is usually collected from the brewer and then commercially dried by a variety of industrial methods to between 8 and 10% moisture. To obtain a powder, BSG is milled using impact mills. Size fractions are obtained using internal sieves and classifiers. Particle sizes range from 40 microns up to 600 microns depending on use. Most dietary fibres methods are suitable – the Prosky, Lee and McCleary methods can all be applied.
Applications
Some applications include its use in the cultivation of mushrooms and actinobacteria, as a source of the value-added products such as ferulic and p-coumaric acids, xylose and arabinose. It is a potential raw material for xylitol and arabitol production.
BSG (SBG) was used as the basic substrate for cultivation of Pleurotus ostreatus, while SBG alone and mixed with maguey tequila bagasse was best used for P. ostreatus and Pleurotus pulmonarius cultivation (Wang, 2001; Gregori et al., 2008). Rhizopus sp. have been tested to improve the nutritional value of this substrate (Ibarruri et al., 2019).
BSG in its own right has been used as an ingredient in bread (Prentice & D’Appo for cultivation of lonia, 1977; Dreese & Hoseney, 1982). The low cost of BSG potentially means that as an ingredient it would be of lower cost than the addition of beta-glucan. It can also decrease the energy content of the bread and raise the protein content if that fraction is also used. Up to 30% BSG can be added although caveats such as flavour might not be acceptable. The level of dietary fiber if that 30% level of BSG is used can certainly increase five fold in a loaf. All aspects of the bread are improved such as bread volume, texture and shelf-life.
In more recent times an enzymatically modified form of BSG added to bread dough has demonstrated that altering the structure of the arabinoxylans improved their solubility but not the quality of the bread it was added to (Steinmacher et al., 2012). That study also showed how adding enzymes to the dough also altered bread quality. perhaps the most important findings were that bread which was made with untreated BSG and the subsequent addition of enzymes such as hemicellulases produced the best bread quality because it was softer and had a larger specific volume.
When Prentice & D’Appalonia (1977) used BSG in their application they compensated for any changes in functionality by adding between 1 and 2% w/w soy lecithin per unit of flour as an emulsifier and surfactant. In that example they managed up to 40% in their blends with soft-wheat cookie flour without losing the attributes of the control flour. The sensory panel only liked the products from the modified dough system with a maximum BSG content of 15% because of the unpleasant flavour. Later on Prentice and others (1979) added spent grain with different heating histories to various cookie formulas with differing degrees of success.
It is especially useful when producing a sourdough in combination with various enzymes such as Maxlife 85 (ME), Lipapan Extra (LE), Pentopan Mono (PE), and Celluclast (CL). Using all four enzymes with BSG produced by far the best quality sourdough (Stojceska, 2011).
Snacks such as breadsticks will also benefit from incorporation especially extruded ones as will beverages. Extrusion technology lends itself very well to using dietary fiber from various sources (Stojceska, 2013). One research study noted that the phenolic content increased a number of times in extruded snacks if somewhere between 20 and 25g/100g of BSG was added to the final product (Reis & Abu-Ghannam, 2014). The breadsticks with BSG had more arabinoxylans, a lower glycaemic index and were certainly richer in antixodants and fibre.
BSG has been successfully added to cookies (Ozturk et al., 2002; Heredia-Sansoval et al., 2020). It has also been added egg pasta although beyond a certain point of addition, the quality of the fresh pasta being produced began to lose structural and taste impact. The addition was to some extent ameliorated by addition of egg white powder which improved texture (Cappa & Alamprese, 2017).
Recovery Of Protein And Fiber
Processes to recover both the protein and the arabinoxylans have been devised using sequential extraction. The use of increasing alkali from 0.1M to 4 M using NaOH and KOH has been tested (Viera et al., 2014). The research group found a ratio of 1:2 (w/v) of BSG to alkali solution worked effectively at ambient temperature. Acidification by dropping the whole mixture to pH3 using citric acid produce protein-rich fractions. The arabinoxylan was recovered by ethanol precipitation. The process yielded between 82 and 85% total protein and between 66 and 73% arabinoxylans. The remaining residue was cellulose rich with no nitrogen compounds supposedly present.
Membrane separation using ultrafiltration has been used to remove salts and water from aqueous suspensions of brewer’s spent grain. This is part of a process in the recovery of protein from BSG. Over 92% of the protein was retained with membranes having a molecular weight cut-off of either 5 kDa and 30 kDa. The protein contents were 20% and 15% respectively for these types of membrane. Clearly, the tighter membrane of the two retained 5% more of the protein. The process was more effective than rotary evaporation (Tang et al., 2009).
Minerals
BSG contains less than 0.5% dry weight of a range of minerals including calcium, cobalt, copper, iron, magnesium, manganese, phosphorous, potassium, selenium, sodium and sulphur (Wilhelmson et al., 2009).
Antioxidants and Phenolics
Brewer’s spent grain extracts produce varying levels of antixoxidants and phenolic acids. The two main phenolic acids are ferulic and p-coumaric acids but others include syringic acids and several isomeric ferulate dehydrodimers and at least one unnamed dehydrotrimer (Bartolomé et al., 2002; Moreira et al., 2013).
Many of these phenolics are bound to arabinoxylan such as ferulic acid. Further treatment of the arabinoxylan using ferulic acid esterase together with xylanase helps release much of this important phenolic acid (Faulds et al., 2004). The xylanases help to produce soluble forms of low molecular weight feruloylated oligosaccharides. These are then further degraded by the added esterases to release ferulic acid.
The feruloylated xylo-oligosaccharides could be offered as food ingredients in their own right. They appear to act as water-soluble antioxidants but freely released ferulic acid is only partially soluble. Considerable interest in developing an enzyme based process has led to the fractionation of BSG to produce peptides and xylo-oligosaccharides in the REPRO project (Treimo et al., 2008; 2009,Forssell et al., 2008; 2009).
The different levels depend on the type of malts used. Generally, malt can be divided into light and dark forms. Further differentiation follows; light malts can be split into pilsen, melano and carared for example whilst a dark malt might be chocolate or black. The level of kilning of the malt dictates the availability of the antioxidants in the BSG. The greater kilning the poorer the yield of antioxidants. Clearly, light malts like the Pilsen types give the best yields of both antioxidants and phenolic acids based on antioxidant assays such as the 2,2-diphenyl-1-picrylhydrazyl, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) and deoxyribose assays.
Incidentally, the protein content from lager malts is higher than those of ales.
Nutritional Benefits
The consumption of BSG and products containing BSG has been shown to prevent gastrointestinal disorders and cardiovascular diseases (Aman et al., 1994). Ingestion is also associated with increased faecal weight, accelerated transit time, increased cholesterol and fat excretion (Fastnaught, 2001).
Adding about 15 to 20% of a soluble protein extract from BSG suppressed the spread of sugar-snap cookies (Finley et al., 1976).
Fermentation medium
BSG is a popular substrate for liquid and solid-state fermentation. It is usually modified by hydrolysis to improve its nutritional value which can then be used further as a fermentation medium as well as used in foods. Solid-strate fermentation too also improves its nutritional value and has been suggested as the best approach to developing a high quality food supplement.
Generally, a glucose-rich hydrolysate from BSG is used for the production of lactic acid by Lactobacillus delbrueckii (Mussatto et al., 2008). Studies have demonstrated the ability of fungi using solid-state fermentation to increase the protein content of BSG (Xiros and Christakopoulos, 2012).
Rhizopus growth on BSG has enhanced its nutritional value by raising the amino acid, citric acid, vitamin and antioxidants content.
References
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