Metabolization of Free Oxidized Aromatic Amino Acids by Saccharomyces cerevisiae.

The aromatic amino acids tryptophan, phenylalanine, and tyrosine are targets for oxidation during food processing. We investigated whether S. cerevisiae can use nonproteinogenic aromatic amino acids as substrates for degradation via the Ehrlich pathway. The metabolic fate of seven amino acids (p-, o-, m-tyrosine, 3,4-dihydroxyphenylalanine (DOPA), 3-nitrotyrosine, 3-chlorotyrosine, and dityrosine) in the presence of S. cerevisiae was assessed. All investigated amino acids except dityrosine were metabolized by yeast. The amino acids 3-nitrotyrosine and o-tyrosine were removed from the medium as fast as p-tyrosine, and m-tyrosine, 3-chlorotyrosine, and DOPA more slowly. In summary, 11 metabolites were identified by high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS). DOPA, 3-nitrotyrosine, and p-tyrosine were metabolized predominantly to the Ehrlich alcohols, whereas o-tyrosine and m-tyrosine were metabolized predominantly to α-hydroxy acids. Our results indicate that nonproteinogenic aromatic amino acids can be taken up and transaminated by S. cerevisiae quite effectively but that decarboxylation and reduction to Ehrlich alcohols as the final metabolites is hampered by hydroxyl groups in the o- or m-positions of the phenyl ring. The data on amino acid metabolism were substantiated by the analysis of five commercial beer samples, which revealed the presence of hydroxytyrosol (ca. 0.01-0.1 mg/L) in beer for the first time.

Physical, chemical, and sensory properties of water kefir produced from Aronia melanocarpa juice and pomace.

Water kefir is widely consumed all over the world due to its potential health benefits. The aim of this current study was to compare non-fermented juice and fermented beverage of water kefir produced from Aronia melanocarpa juice and pomace in terms of chemical, physical and sensory quality as well as valorisation of pomace in the production of water kefir. When compared to water kefir made with aronia juice, less reduction in total phenolic content (TPC), total flavonoid content (TFC) and total anthocyanin content (TAC) was observed in samples made with aronia pomace during the fermentation process. Similarly, greater antioxidant activity was demonstrated in water kefir made with aronia pomace than juice. Based on sensory evaluation, no difference was found in overall acceptability, taste, aroma/odor, and turbidity of water kefir made with aronia pomace before and after fermentation. Results indicated that aronia pomace has potential in water kefir production.