Transformation of Free and Dipeptide-Bound Glycated Amino Acids by Two Strains of Saccharomyces cerevisiae.

The yeast Saccharomyces cerevisiae transforms branched-chain and aromatic amino acids into higher alcohols in the Ehrlich pathway. During microbiological culturing and industrial fermentations, this yeast is confronted with amino acids modified by reducing sugars in the Maillard reaction (glycation). In order to gain some preliminary insight into the physiological "handling" of glycated amino acids by yeasts, individual Maillard reaction products (MRPs: fructosyllysine, carboxymethyllysine, pyrraline, formyline, maltosine, methylglyoxal-derived hydroimidazolone) were administered to two strains of S. cerevisiae in a rich medium. Only formyline was converted into the corresponding α-hydroxy acid, to a small extent (10 %). Dipeptide-bound pyrraline and maltosine were removed from the medium with concomitant emergence of several metabolites. Pyrraline was mainly converted into the corresponding Ehrlich alcohol (20-60 %) and maltosine into the corresponding α-hydroxy acid (40-60 %). Five specific metabolites of glycated amino acids were synthesized and characterized. We show for the first time that S. cerevisiae can use glycated amino acids as a nitrogen source and transform them into new metabolites, provided that the substances can be transported across the cell membrane.

Reduction of 5-Hydroxymethylfurfural and 1,2-Dicarbonyl Compounds by Saccharomyces cerevisiae in Model Systems and Beer.

Glycation and caramelization reactions in malt lead to the formation of 1,2-dicarbonyl compounds, which come in contact with yeast during fermentation. In the present study, the metabolic fate of 5-hydroxymethylfurfural (HMF) and 1,2-dicarbonyl compounds (3-deoxyglucosone, 3-deoxygalactosone, 3-deoxypentosone, 3,4-dideoxyglucosone-3-ene) was assessed in the presence of Saccharomyces cerevisiae. HMF is degraded very fast by yeast with the formation of 2,5-bis(hydroxymethyl)furan (BHMF). By contrast, only 7-30% of 250 µM dicarbonyl compounds is degraded within 48 h. The respective deoxyketoses, 3-deoxyfructose (3-DF), 3-deoxytagatose, 3-deoxypentulose, and 3,4-dideoxyfructose, were identified as metabolites. While 17.8% of 3-deoxyglucosone was converted to 3-deoxyfructose, only about 0.1% of 3-deoxypentosone was converted to 3-deoxypentulose during 48 h. Starting with the parent dicarbonyl compounds, the synthesis of all deoxyketose metabolites was achieved by applying a metal-catalyzed reduction in the presence of molecular hydrogen. In a small set of commercial beer samples, BHMF and all deoxyketoses were qualitatively detected. 3-DF was quantitated in the four commercial beer samples at concentrations between 0.4 and 10.1 mg/L.