Optimization and mechanism of diacetyl accumulation by Enterobacter aerogenes mutant UV-3

Abstract

A mutant designated as UV-3 was obtained from wild-type Enterobacter aerogenes 10293 through u.v. radiation. The activities of a-acetolactate decarboxylase (Ald), lactate dehydrogenase (Ldh) and diacetyl reductase (Dr) in UV-3 were strongly attenuated, with the lowest activities at pH 7.0-7.5, and temperature between 36 and 39C. Compared to the wild-type, the yield of diacetyl by UV-3 was increased 18.7-fold, up to 1.05 ± 0.01 g l -1 . Acetoin and ethanol productions were decreased by 48.4 and 71.4%, respectively, but acetate yield was increased by 34.6%. Optimum medium for diacetyl production by UV-3 contained 10% glucose, 0.5% peptone, 0.5% yeast extract powder, 0.01% (NH4)2SO4, 0.1% citric acid, 0.2% MnSO4 and 0.2% MgSO4, and this was determined by one-factor-at- a-time approach. Data from the five level central composite designs demonstrated that initial pH of 7.0, temperature of 37C and rotational speed of 180 rev/min were optimum processing parameters for diacetyl production. The maxi- mum yield of diacetyl could reach 1.35 g l -1 in a 5-l bioreactor. These results showed an enhancement of the non-enzymatic oxidative decarboxylation of a-acetolactate and a decrease in the activities of Ald, Ldh and Dr as a consequence of diacetyl accumulation in UV-3.