Brewers' spent grains (BSG) are a major byproduct from the brewing industry that are currently discarded or used as animal feed. This study systematically explored the effects of BSG protein extraction methods using alkali, ethanol or enzymes on the protein's structure, composition and function. Proteins extracted at pH 12 were partially unfolded and glutelin-rich while those extracted by 55% ethanol containing 2-mercaptoethanol were highly aggregated and hordein-rich. Enzymatic-assisted extraction resulted in peptides below 10 kDa that had no distinct structural elements. Being completely water-soluble, these peptides gave good emulsifying properties (activity: 83 m2/g protein; stability: 35 min) and a high antioxidant activity. On the other hand, alkali-extracted and ethanol-extracted proteins were mostly insoluble but exhibited high water holding capacities (2.5–4.0 g/g) that enabled gelation. The results highlighted that selection of the extraction method is critical, as the composition, structure and function of proteins are modified, which affects its potential applications.

As a major side stream from the production of beer, brewers' spent grains are present in huge amounts. These residues are a promising source of alternative proteins, but before separating them, it is important to first consider an extraction method that does not compromise on its functionality. This study offered insights into the influence of extraction methods on the structure and function of the resulting protein concentrates as well as its future applications.