| Summary: | Single-strain cultures of Streptococcus cremoris, Streptococcus
lactis, Streptococcus diacetilactis, and Leuconostoc citrovorum produced
little or no acetone and no dimethyl sulfide when grown in milk
culture. These organisms had little or no ability to decarboxylate
antexogenous source of acetoacetic acid nor were they capable of
producing dimethyl sulfide from methyl methionine sulfonium chloride.
The dimethyl sulfide content of milk was increased by heating
which indicated that a heat labile dimethyl sulfide precursor was
present in milk. The precursor remained in the skimmilk fraction
and was dialyzable. The precursor was identified as a methyl methionine
sulfonium salt on the basis of its thin-layer chromatographic
mobility and the heat instability of the compound. Heating of samples
caused the disappearance of the precursor compound with a
subsequent increase in the content of homoserine and dimethyl sulfide.
Single strain cultures of Pseudomonas fragi, Pseudomonas fluorescens, Pseudomonas putrefaciens, and two marine Pseudomonas
species reduced acetaldehyde, propionaldehyde, and butyraldehyde
to the corresponding alcohols at 21°C. All species studied
reduced propionaldehyde at 6°C. P. fragi and the marine species
reducted butanone and/or acetone at both 6 and 21°C. Under aerobic
conditions a strain of P. fragi quantitatively reduced added
propionaldehyde to n-propanol.
The quantities of acetaldehyde and ethanol produced by single-strain
cultures of Lactobacillus brevis, Lactobacillus casei, Lactolactis,
and Lactobacillus plantarum differed significantly both between
species and between strains of a species on incubation at both
their optimum growth temperature and 8°C. Growth and production of
these Compounds were very slow at 8°C. All organisms studied were
capable of reducing acetaldehyde and propionaldehyde to the corresponding
alcohol. L. brevis strains alone reduced added butanone
to 2-butanol. A strain of L. brevis produced n-propanol as a normal
metabolite when grown in milk culture.
Single-strain cultures of L. casei and L. plantarum accumulated
diacetyl when grown in milk culture at both 8 and 30°C, but
strains of L. lactis and L. brevis did not. Diacetyl reductase activity
was demonstrated in single-strain cultures of L. casei, L. brevis,
and L. lactis. Diacetyl reductase could be induced in L. plantarum
by growth in the presence of citrate. Growth in milk medium supplemented with citrate resulted in a stimulation of diacetyl reductase activity with L. casei. === Graduation date: 1968
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