Whole-cell suspensions of Lactobacillus plantarum grown on lactose in the presence of citrate did not produce acetoin and diacetyl (AD) (D) from citrate in succinate buffer, pH 4.4 unless both a source of energy and nitrogen was present, but did from pyruvate. The total AD and the amount of D, produced by citrate-grown cells, from citrate were about two times the amounts formed from pyruvate, calculated on a molar basis. It appears, that AD are formed not only from pyruvate resulting from cleavage of citrate but also from acetyl-coenzyme A arising during a probable breakdown of citrate in a reversible reaction of citrate synthetase. Neither acetate nor acetaldehyde had any effect on the total AD or the amount of D produced from pyruvate by pyruvate-grown cells. The rates of AD production from pyruvate by whole-cell suspensions of pyruvate-grown L. plantarum and Streptococcus subsp. diacetylactis represented only 69.7 and 6.6%, respectively, of that produced by Lactobacillus casei. These were 0.075 μmoles/mg dry wt−1 ml−1 min−1 for L. casei, 0.053 for L. plantarum and 0.005 for S. lactis subsp. diacetylactis.

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