ABSTRACT
Having a very old product history, fermented milks originated on the basis of the generally applicable rule that raw milk will spoil after storage because of microbial action. At moderate temperatures, lactic acid bacteria commonly are predominant, and the milk becomes spontaneously sour. When the sour milk has been used, and fresh milk is put in the same vessel without rigorous cleaning of that vessel, the fresh milk is inoculated with the remaining bacterial flora. The milk now sours more quickly, generally due to a smaller number of bacterial species and strains. If this process is repeated under fairly constant conditions (especially in regard to temperature), natural selection leads to an almost pure lactic acid fermentation, although some other bacteria may remain present. The process can be improved by rigorously cleaning the vessel, heat-treating the milk to kill undesirable microbes, and inoculating the milk with a little bit of the sour milk from the previous batch; this then acts as a starter for the fermentation. The fermented milk thus obtained has a longer keeping quality and, often, a pleasant flavor. It is also much safer to the consumer because pathogenic bacteria have been killed, and contamination with pathogens afterwards can almost never lead to growth of these organisms. The lactic acid bacteria alter the conditions of milk in such a way that most undesirable organisms cannot grow or will even die. These conditions include a low pH (4.6 to 4.0), a low redox potential, and growth inhibition by undissociated acids (e.g., lactic acid) and other metabolites such as H
O
and compounds with an antibiotic activity. As a result of variations in conditions, a great number of fermented milk
types have developed. Variables include species of milch animal, heat treatment of the milk, percentage fat in the milk, concentration of the milk, fermentation temperature, and inoculation percentage. According to these conditions, various species of lactic acid bacteria become predominant, producing various flavor components. Most types of fermented milk contain two to four species of bacteria. In some products, yeasts or molds participate in the fermentation.
