Influence of Reduced Water Activity on Lactose Metabolism by Lactococcus lactis subsp. cremoris at Different pH Values

The influence of reduced water activity (a(w)) on lactose metabolism by Lactococcus lactis subsp. cremoris 2254 and 2272 was studied at different pH values. In control incubations (a(w), 0.99) with nongrowing cells in pH-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. However, the levels of recovery decreased 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 lactose carbon recovered as l-(+)-lactate from 100 to 71%. Low levels of l-(+)-lactate carbon recovery (<50%) were also observed with cells resuspended in pH-uncontrolled reconstituted skim milk at a(w) values of 0.99 and 0.87 and in young cheese curds. The missing lactose carbon could not be accounted for by acetate, ethanol, formate, acetaldehyde, or pyruvate. Attempts were made to determine where the missing lactose carbon was diverted to under the stress conditions used. Some of the missing lactose carbon was recovered as galactose (0.1 to 2.5 mM) in culture supernatants. Decreasing either the a(w) or the pH resulted in increased galactose accumulation by nongrowing cells; adjusting both environmental 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 pentose(s) and additional galactose were found in acid-hydrolyzed supernatants from cultures containing both growing and nongrowing cells, indicating that a saccharide(s) rich in these components was formed by lactococci under low-a(w) and low-pH stress conditions.