SUMMARY
The homofermentative lactic acid producing bacteria,
Lactobacillus lactis, Lactobacillus bulgaricus, Streptococcus
lactis, and Streptococcus thermophilus, were similar
with respect to their ability to produce keto acids
during the fermentation of milk. Pyruvic and a-ketoglutaric
acids were formed in relatively large concentrations
and small amounts of oxaloacetic, oxalosuccinic,
glyoxylic, a-acetolactic, and a-ketoisocaproic acids
were formed under certain conditions. An initial
utilization of pyruvic and a-ketoglutaric acids originally
present in the milk was followed by subsequent increases
in these acids and the appearance of a-acetolactic
acid during 24 hr of incubation. However, the
addition of pyruvic and a-ketoglutaric acids to milk had
no effect on the acid production of the various lactic
acid bacteria. Increasing the frequency of transfer of
the test cultures resulted in changes in the concentrations
and number of keto acids formed. Combinations
of species of lactic acid bacteria changed the concentrations
and types of keto acids found. The formation of
keto acids by lactic acid bacteria during cheese manufacture
was similar to that in sterile milk.