The lactose found in milk is degraded to lactic acid during fermentation; this causes
the pH to drop and the milk to thicken. As much as 30% of the milk lactose is broken
down during fermentation. β-galactosidase is the enzyme that hydrolyses lactose
into glucose and galactose. It is found in yogurt and kefir grains, but its activity is very
low in kefir.The bacteria in kefir are able to break down glucose to lactic acid. Both
D-(−)-lactic and L-(+)-lactic acid are produced from the milk lactose, and in kefir the
L+ form tends to predominate. However, the ratio of L to D depends on the source and
microbial composition of the grains. Applying high pressure (> 1000 MPa) appears
to increase the amount of D-(−)-lactic acid produced by some kefir bacteria