ABSTRACTThe highest quantity of acetoin is produced by enzyme reduction of diacetyl caused by Lc. diacetylactis and L. brevis at pH 7.21–7.08. This quantity considerably decreases with the decrease of pH down to 4.34–4.26. Diacetyl is being continuously produced by dehydrogenation of acetoin under the catalytic action of acetoindehydrogenase of lactic acid bacteria within pH range of 7.21 to 4.26. It has been established that acetoin is being produced out of acetaldehyde in the process of pre-ferment fermentation. The quantity of acetoin decreases with the decrease of acetaldehyde value. Acetaldehyde is also source of crotonaldehyde and glyoxal production in the process of pre-ferment fermentation.