The history of fermented milk products is long and quite diverse culturally. Although the exact origins are difcult to ascertain, they likely date back to more than 10,000 yr ago. According to Persian tradition, Abraham owed his fecundity and longevity to the regular ingestion of yogurt. In the early 1500s, King Francis I of France was reportedly cured of a debilitating illness after eating yogurt made from goats’ milk.1 Scientic interest in the health benets of yogurt was initiated by Élie Metchnikoff in the early 1900s. Metchnikoff proposed that the lactic acid microbes of fermentation must be antagonistic to the putrefying microbes of the gut, and once introduced into the intestine, they would prevent the breeding of the noxious microbes that required an alkaline environment. His hypothesis was stimulated by the fact that populations such as those living in the Balkans regularly ate yogurt and were noted for their longevity. He experimented on himself and reported that his health, which was generally poor, improved with regular ingestion of sour milk prepared with cultures of the Bulgarian lactic bacillus. Metchnikoff’s enthusiasm about yogurt became more publicized, and doctors began recommending yogurt or sour milk as a hygienic food. Metchnikoff credited his relatively long life in part to the lactic bacilli in his diet, and hypothesized “When people have learnt how to cultivate a suitable ora in the intestines of children as soon as they are weaned from the breast, the normal life may extend to twice my 70 yr.”2 (See Chapter 1 for more details on the history of yogurt.)

Over the past several decades the consumption of fermented milk products, especially yogurt, has greatly increased. The most dramatic increase occurred during the 1980s and 1990s, which is certainly in part due to increased knowledge regarding the health benets of yogurt and other fermented milk products.3 Moreover, the addition of fruit and sweeteners to yogurt has made it more widely palatable. However, it is likely the increasing knowledge regarding the health benets of fermented foods, especially live-culture yogurt, has driven the recent growth in consumption. The following sections of this chapter will discuss the fermentation process, the compositional changes of milk following fermentation, and health effects of consumption of fermented milk in both animals and humans.