Fermented milks are very old products. If raw milk is kept, it spoils by microbial action. At moderate temperatures, lactic acid bacteria generally are predominant, and the milk becomes "spontaneously" sour. When the sour milk is 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, and generally due to a smaller number of bacterial species and strains. If this process is repeated under fairly constant conditions (especially 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 afterward can almost never lead to growth of these organisms. Moreover, individuals suffering from lactase deficiency (see Section 20.1) can tolerate the product quite well.