Food by-product valorisation in nutrients through spent brewer's yeast bioprocessing with Propionibacterium freudenreichii

This study investigated the potential of Propionibacterium freudenreichii to enhance the nutritional quality of spent brewer's yeast (SBY) by producing short-chain fatty acids (SCFAs) and vitamin B12 (B12). SBY was also evaluated as a growth medium for P. freudenreichii. Four SBY samples from different breweries were utilized and compared to microbiological yeast extract (YE). The tested SBY samples exhibited similar chemical compositions in terms of protein content, free amino acids (FAAs), SCFAs, carbohydrates, riboflavin, and cobalt, whereas YE was richer in most of these substances. However, P. freudenreichii growth was hindered by polyphenols in SBY, as evidenced by better growth in samples with lower polyphenol levels and the control sample (YE). SCFA synthesis was efficient in the SBY sample, resulting in 1.3 mg mL−1 of acetic acid and 1.70 mg mL−1 of propionic acid. Furthermore, P. freudenreichii fermentation increased essential free amino acids (FAAs) in all tested samples. SBY was found to be a suitable medium for supporting the production of the active form of B12, with a concentration of 25.2 ng mL−1. However, YE outperformed SBY as a medium for B12 synthesis, yielding a concentration of 214.3 ng mL−1 after fermentation. The results indicate that both SBY and YE can serve as one-component sustainable growth media, valorised with valuable compounds through P. freudenreichii fermentation. In conclusion, SBY offers a novel opportunity for the biotechnological synthesis of SCFAs, FAAs, and B12 through P. freudenreichii fermentation.