FORMATION OF ESTERS, ACIDS AND ALCOHOLS FROM α-KETO ACIDS BY BREWER'S YEAST

α-Keto acids are metabolized to a considerable extent by brewer's yeast in anaerobic fermentation of a synthetic medium using glucose as the carbon source. The fraction metabolized is mainly recovered as the corresponding fusel alcohol and, to a lesser extent, as the fatty acid with one carbon atom less than the α-keto acid; the molar ratio between alcohol and acid is about 20. In the concentration range 0–15 mM there is a linear relationship between transformed and added α-keto acid; at higher concentrations this relationship is replaced by a saturation curve, with maximum at about 50 mM for α-ketovaleric acid. The rate-limiting step is most probably the penetration into the cells. The main process (alcohol formation) is reductive, but the formation of the normal fusel alcohols is not affected by the addition of α-keto acids; the redox balance is maintained by a decrease in glycerol formation. Ethyl esters of fatty acids with one carbon atom less than the added α-keto acids are formed in amounts corresponding to the concentration of these fatty acids. There seems to be no formation of acyl-CoA directly from α-keto acids.