Residual barley proteins in brewers' spent grains: Quantitative composition and implications for food ingredient applications

Brewer's spent grain (BSG) is the major side-stream from beer production with millions of tons produced each year. The direct use of BSG as a food ingredient has been limited due to subpar protein techno-functionality following extraction. As such, BSG is primarily used for low-value purposes and can be considered an underutilized resource. BSG has a protein content of up to 30 %, making it a readily available and high potential feedstock for valorization into a functional food ingredient through approaches such as enzymatic hydrolysis. However, little attention has previously been given to the protein-level composition of BSG, which is essential for developing hydrolysis strategies to improve functionality in an efficient and targeted manner. Here, we present an in-depth quantitative characterization of the BSG proteome and investigate dynamic proteome changes during malting and mashing, the initial phases of beer production. We show dynamic and selective changes in the proteome across the different process steps, where 29 % of reproducibly identified proteins display differential abundance. BSG was found to have a significantly higher proportion of intracellular protein compared to both barley and malt as well as a nutritionally favorable amino acid composition. The major constituent of the BSG proteome was B3-Hordein, constituting more than 30 % (mol/mol) of the BSG protein. Moreover, a large proportion (> 45 % (mol/mol)) of the BSG protein are classified as potential allergens and antinutritional factors. This emphasizes the need for care during downstream processing of BSG to produce safe and functional food ingredients, while also providing protein-level insights for the development of targeted protein extraction and hydrolysis strategies. • An optimized sample preparation for proteomics of BSG has been developed. • 29 % of barley proteins are differentially abundant across malting and mashing. • B3-Hordein is the major protein in BSG with an abundance over 30 % (mol/mol). • BSG contains a high content of potential allergenic and antinutritional proteins. • A protein-level basis for targeted downstream processing of BSG is presented.