Valorisation of brewers’ spent grains for food applications

As the world population continues to grow and food demand is projected to increase, our global food security is facing an impending threat. At the same time, food losses and waste are increasing, which not only brings about negative repercussions to the environment, and limits the efficiency in which we use our resources, but also signifies economic loss. As such, it is pertinent to critically examine stages of the production chain where food losses can be cut and convert these wastes from a problem into a resource. Side streams from food processing usually contain underutilised fractions that have potential in other applications but they are often disposed in landfills or used as animal feed. Brewers’ spent grains (BSG), which is a major byproduct of the brewing industry, represent an inexpensive and valuable resource owing to its high protein and fibre content, alongside other nutrients such as phenolic compounds and lipids. With rising consumer interest in alternative sources of proteins, BSG fits well into the scene as a plant-based product derived from sustainable sources. The objective of this thesis is to obtain an understanding of the processing and fractionation methods that are possible for BSG to possess functional properties which are industrially relevant. This thesis starts with the conventional approach of extracting proteins from BSG that aims at comparing the influence of extraction methods on the characteristics of proteins and their properties. Given the heterogeneous composition of BSG, it is however more realistic to consider the use of functional fractions or mixtures rather than purified components. Being rich in nutrients and high in moisture content, BSG is prone to microbial degradation, but this also illustrates the possibility of using fermentation to upgrade its properties. Chapter 2 reports on a study of the effect of BSG protein extraction methods using alkali, ethanol and enzymes, on the composition, structure and function of the proteins. Fractions extracted with alkali, ethanol and enzymes contain more than 70% protein, with some carbohydrates and lipids present. Alkali-extracted proteins were more enriched in glutelins and were partially unfolded while the ethanol-extracted proteins comprised mostly of hordeins that were strongly aggregated. Enzymatic hydrolysis resulted in peptides smaller than 10 kDa with no distinct structural elements. The changes in composition and structure led to different functional properties, mainly related to solubility differences. Protein hydrolysates from enzymatic extraction were completely soluble in water and exhibited good emulsifying properties and high antioxidant activity. Conversely, alkali-extracted and ethanol-extracted proteins were poorly soluble in water but demonstrated good water and oil holding capacities that allowed heat-induced gelation from as low as 6% protein. These results indicate that the extraction method impacts the protein composition and structure which in turn affects 117 functionality. Therefore the intended application of the protein concentrate should drive the choice of the procedure for protein extraction. As different protein extraction methods give clear differences in functionality, the possibility of using non-purified BSG containing proteins and fibres to stabilise oil-in-water emulsions was investigated in Chapter 3. First, BSG was mildly treated by a colloid mill to reduce particle size. Colloid mill-treated BSG yielded samples with about 26-32% protein and 52-62% fibre, with sizes generally smaller than 10 µm. The soluble fraction in full BSG without centrifugation yielded a 35% reduction in the interfacial tension of oil-water interfaces with an interfacial elasticity that is characteristic of soluble proteins. Emulsions prepared with full BSG were reversibly flocculated whereas those with insoluble BSG showed particle bridging. Despite sufficient electrostatic repulsion between the emulsion droplets, flocculation still occurred due to the presence of large amounts of fibres. Even though the emulsion droplet sizes ranged between 25 to 40 µm, both emulsions remained stable against coalescence after 10 days. This was attributed to Pickering stabilisation by insoluble proteins bounded to fibres. The fibres created a network in the continuous phase that sterically hindered droplets from coalescence. These findings indicate that mild processing methods like colloid milling can be sufficient to functionalise BSG for emulsification purposes, and chemicals and harsh treatments to obtain pure components are not always necessary, especially for particle-stabilised emulsions. Considering that BSG is a nutrient-rich material, Chapter 4 approached the treatment of BSG from a different perspective by employing fungal fermentation to secrete enzymes that degrade the lignocellulosic BSG and enable the release of valuable compounds. In particular, protein extracts obtained from fermented BSG showed that up to 15% of the original protein was hydrolysed, and the amino acid profile revealed that proline and glutamic acid were prominently present, due to the extraction of prolamins in BSG. There was a good balance of hydrophobic and hydrophilic amino acids, which indicated the possibility of emulsification. The fermentation improved the protein solubility, alongside other functionalities such as emulsifying properties as well as oil holding capacities. Protein extract from fermented BSG also presented a higher antioxidant activity and non-cytotoxic effects. To utilise these promising characteristics, the fermented BSG protein extract was applied in a mayonnaise-type product to prove its ability as a plant-based emulsifier. Fermented hydrolysates had larger oil droplets and lower viscosity than egg-based mayonnaise, but could be used as a partial substitute for eggs. Chapter 5 concludes this thesis with a general discussion of the main findings. Processing methods that were applied in Chapters 2 and 4 were compared in terms of yield, functionality and the antioxidant activities of the extracts. The findings were put into a wider perspective 118 that examines how BSG can be processed for real-world applications, emphasising the need to shift our focus from high purity ingredients to functional fractions instead, especially for natural complex materials that are composed of more than one component. The concept of a circular economy was discussed, and an integrated approach of the total use of BSG in a low-cost and sustainable manner was proposed. The chapter ends with a few considerations concerning BSG valorisation and recommendations for future research.