Behaviour of dehydrated baker's yeast during reduction reactions in a biphasic medium

The behaviour of dry baker's yeast (Saccharomyces cerevisiae type II, Sigma) used as biocatalyst without preliminary growth for the synthesis of 2-heptanol from 2-heptanone in a biphasic system is presented. Cells undergo intracellular trehalose consumption with a stoichiometric ethanol production during the first 15 h of the process. This metabolism is then replaced by acetate accumulation. These reactions are disconnected from the biocatalytic reaction and do not provide reduced cofactors. 2-Heptanone is metabolised by two pathways. The first leads to 2-heptanol (molar yield close to 55%, enantioselectivity higher than 99%, with a slight decrease at the end of the process) and the second corresponds to material incorporation into the biomass. This latter phenomenon is assumed to provide the biocatalyst with the reduced cofactors needed for the reduction process. Overall, the process yielded ca. 1.4 g/l 2-heptanol in 50 h reaction, which is close to that observed with fresh cells previously grown for 15 h.