Eleven strains of homofermentative and heterofermentative lactic acid bacteria were screened for acetoin (A) and diacetyl (D) production from pyruvate and citrate in a peptone-yeast extract-glucose broth. The homofermenters, except Streptococcus faecalis subsp. liquefaciens, produced much more AD from pyruvate than from citrate; the opposite was true for the heterofermenters. Acetoin and diacetyl were produced from pyruvate as soon as growth was initiated. The production was exponential up to 24 h. Destruction of the accumulated AD coincided with entry into the stationary phase. Production of AD from citrate did not begin until 6 h of the logarithmic phase of growth. Formation of gas from citrate by Lactobacillus plantarum did not implicate greater ability to form AD from citrate than from pyruvate. Fifty μmoles ml−1 citrate caused about 50% inhibition of growth of Streptococcus lactis subsp. diacetylactis. All strains examined for ability to use pyruvate as a sole source of carbon were able to do so. Acetate (50 μmoles ml−1 generally stimulated AD formation from pyruvate. With the exception of a Pediococcus sp. and S. faecalis subsp. liquefaciens, acetaldehyde (100 μg ml−1) enhanced AD production but not growth. Concentrations higher than 100 μg ml−1 had different effects.

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