During fermentation bacteria and yeasts convert lactose in the milk to various degradation products depending on the species present. Lactobacilli and streptococci give rice to lactic acid and monosaccarides (especially galactose).
Bifidobacteria give rice to lactic acid, acetic acid and monosaccarides,
while yeasts, present only in some few fermented milk products, produce CO2 and ethanol.
Different bacterias may be used for fermentation, giving products of special flavour and aroma, and with several potential health beneficial metabolites. The bacteria
contain cell wall components that bind Toll-like receptors on dendritic cells (and also other leucocytes) found in the mucosa of the small intestine and colon, thus stimulating
the Th1 immune response. It has been shown that fermented milk stimulates the Th1 immune response, and down-regulates the Th2 immune response . The immune system may thus be strengthened against cancer, virus infections and allergy . Bacterial DNA has also
a similar effect, binding to Toll-like receptor-9. Some bacteria can also improve the intestinal microbial balance, and the fermented milk may have positive health
effects both in the digestive channel and in metabolism. During the fermentation of milk, lactic acid and other organic acids are produced and these increase the absorp tion of iron. If fermented milk is consumed at mealtimes, these acids are likely to have a positive effect on the absorption of iron from other foods. Lactic acid is
also a poorer substrate for growth of pathogenic bacteria than glucose and lactose .