Lactose is almost uniquely found in milk. Few bacteria can break down lactose into usable forms of energy. Lactic acid bacteria, also known as Probiotic bacteria, use the enzyme lactase to break down the disaccharide lactose into the more usable monosaccharides glucose and galactose. The bacteria produce lactic acid as a by-product of lactose digestion. The low pH preserves the milk as other microbes cannot survive in acidic conditions. In addition, casein proteins come together in semi-solid curds. The flavor and texture favorably change in a process referred to as milk fermentation.
The history of milk fermentation dates back at least 2,000 years. In the early years, people believed that the fermentation was a spontaneous process. Later, the process was managed by the inoculation of fresh milk with fermented milk. By the late 19th century, bacteria had been identified as a causative agent, though the process was not entirely understood. By the 1900s, starter cultures of unknown mixed bacteria became commercially available. By the 1930s, pure single-strain cultures had evolved for the specific production of sour creams, yogurts, and cheese.
Fermenting lactic acid bacteria include species in the genera Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, and Pediococcus. Some of these species also colonize the mouth, intestine, and vagina of mammals as normal flora.
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