It is the Lact, diacetilactis and Lact, cremoris that form the largest amount of diacetyl in the pre-ferment mixed with separately added cultures of lactic acid bacteria and not adding yeast of Sacch. cerevisiae while the L. plantarum and Lact, diacetilactis form the largest amount of acetoin. In that semi-manufacture Lact, lactis and L. bulgaricus form a considerable amount of acetaldehyde. Greater amounts of diacetyl, acetoin and acetaldehyde are found in sour with singly added lactococci unlike the leavens prepared with lactobacilli. The dough showed less content of acetoin and higher of diacetyl. The quantity of the acetaldehyde increases in the dough and to the highest amount in the dough fermented with leaven composed of lactococci in mixture with lactobacilli. Acetoin is formed in the bread crumb as a result of the chemical changes in the acetaldehyde in the process of baking. Part of the diacetyl in the bread crumb originates during the thermal oxidation of acetoin.