Sq. Liu et al., INFLUENCE OF REDUCED WATER ACTIVITY ON LACTOSE METABOLISM BY LACTOCOCCUS-LACTIS SUBSP CREMORIS AT DIFFERENT PH VALUES, Applied and environmental microbiology, 64(6), 1998, pp. 2111-2116
The influence of reduced water activity (a(w)) on lactose metabolism b
y Lactococcus lactis subsp, cremoris 2254 and 2272 was studied at diff
erent pH values. In control incubations (a(w), 0.99) with nongrowing c
ells in pa-controlled phosphate buffer, the levels of carbon recovered
as L-(+)-lactate were 92% at pH 6.1 and 5.3 and 78% at pH 4.5. Howeve
r, the levels of recovery decreased to similar to 50% at all pH values
tested when the a(w) was 0.88 (with glycerol as the humectant), When
growing cells in broth controlled at pH 6.3 were used, a reduction in
the a(w) from 0.99 to 0.96 resulted in a decrease in the level of lact
ose carbon recovered as L-(+)-lactate from 100 to 71%, Low levels of L
-(+)-lactate carbon recovery (<50%) were also observed with cells resu
spended in pa-uncontrolled reconstituted skim milk at a(w) values of 0
.99 and 0.87 and in young cheese curds. The missing lactose carbon cou
ld not be accounted for by acetate, ethanol, formate, acetaldehyde, or
pyruvate, Attempts were made to determine where the missing lactose c
arbon was diverted to under the stress conditions used. Some of the mi
ssing lactose carbon was recovered as galactose (0.1 to 2.5 mM) in cul
ture supernatants. Decreasing either the a(w) or the pH resulted in in
creased galactose accumulation by nongrowing cells; adjusting both env
ironmental factors together potentiated the effect. The sensitivities
of the two lactococcal strains tested were different; strain 2272 was
more prone to accumulate galactose under stress conditions. A methyl p
entose(s) and additional galactose were found in acid-hydrolyzed super
natants from cultures containing both growing and nongrowing cells, in
dicating that a saccharide(s) rich in these components was formed by l
actococci under low-a(w) and low-pH stress conditions.