With an ever-increasing global population and dwindling natural resources, a shift towards more sustainable food systems is required. Important aspects to aid in this transition are the reduction of food waste, and a movement towards non-animal protein sources. Brewers spent yeast (BSY) is an abundant by-product of the brewing industry, which is generally regarded as waste, despite its high nutritional value. Previous work has shown that fermentation of BSY with Lactobacillus amylovorus FST 2.11 resulted in changes in composition, functionality, and improved palatability of the processed raw material (PBSY). In this study, in vitro protein digestibility, amino acid bioaccessability, and protein quality of PBSY was explored using the static INFOGEST in vitro model. In vitro protein digestibility of PBSY (73.0 %) was almost two-fold higher than that of CBSY (40.0 %), while PBSY also displayed significantly higher in vitro bioaccessability values for all essential amino acids, except for tryptophan. Investigation of protein quality using the digestible indispensable amino acid score (DIAAS) values and the FAO recommended amino acid scoring pattern for individuals >
3 years old showed that the protein quality for CBSY was low (DIAAS of 17.0 %), while PBSY was considered to be of "good" protein quality (DIAAS of 98.2 %). Investigation of the modulation potential of PBSY on the gut microbiome using an in vitro colon model system showed an increase in gut microbiome α-diversity indices and an abundance of beneficial Mediterranean diet-responsive taxa after 24 h. Overall, this study highlights the potential of BSY as raw material for the production of a high-quality food ingredient with potential prebiotic effects, aiding in the reduction food waste and supporting global food systems.