Threonine aldolase and alcohol dehydrogenase activities inLactobacillus bulgaricusandLactobacillus acidophilusand their contribution to flavour production in fermented milks

Summary Cell-free extracts of bothLactobacillus bulgaricusandL. acidophilusdemonstrated threonine aldolase activity, the end product of which was acetaldehyde, the major flavour compound of yoghurt.L. acidophilusalso possessed an alcohol dehydrogenase activity capable of reducing acetaldehyde so that little yoghurt flavour was present in milks fermentation with this organism. Addition of threonine to fortified milk before fermentation withL. acidophilusincreased acetaldehyde production and resulted in a well flavoured product similar to that of yoghurt made withL. bulgaricus. The contribution of these 2 enzymes to flavour production is discussed.