With the development of microbiological and nutritional sciences in the late 19th century came the technology necessary to produce cultured dairy products on an industrial or commercial basis. Fermented milks had been made since early times, when warm raw milk from cows, sheep, goats, camels, or horses was naturally preserved by common strains of Streptococcus and Lactobacillus bacteria. These harmless lactic acid producers were effective in suppressing spoilage and pathogenic organisms, making it possible to preserve fresh milk for several days or weeks without refrigeration. Cultured products eventually became ethnic favourites and were introduced around the world as people migrated.
Central to the production of cultured milk is the initial fermentation process, which involves the partial conversion of lactose (milk sugar) to lactic acid. Lactose conversion is accomplished by lactic-acid-producing Streptococcus and Lactobacillus bacteria. At temperatures of approximately 32 C (90 F), these bacteria reproduce very rapidly, perhaps doubling their population every 20 minutes. Many minute by-products that result from their metabolic processes assist in further ripening and flavouring of the cultured product. Subsequent or secondary fermentations can result in the production of other compounds, such as diacetyl (a flavour compound found in buttermilk) and alcohol (from yeasts in kefir), as well as butyric acid (which causes bitter or rancid flavours).
Cultured buttermilk, sour cream, and yogurt are among the most common fermented dairy products in the Western world. Other, lesser-known products include kefir, koumiss, acidophilus milk, and new yogurts containing Bifidobacteria. Cultured dairy foods provide numerous potential health benefits to the human diet. These foods are excellent sources of calcium and protein. In addition, they may help to establish and maintain beneficial intestinal bacterial flora and reduce lactose intolerance.