Well sweet, but not for everyone tolerable: the lactose
Whether breast milk - or milk from a cow, goat, sheep or mare: Milk sugar (lactose) is always responsible for the sweet taste of the white juice. In addition, lactose is a digestible carbohydrate that provides usable energy.
But worldwide, most people cannot digest this valuable sugar after infancy. Cause for the lactose intolerance is the lack or absence of the enzyme lactase, which in the small intestine has the function of a "lactose breaker". If the cleavage does not occur, it can digest lactose and leads to stomach pain, flatulence, cramps and diarrhoea. Almost all Africans and Asians live with this missing enzyme, in Europe about 3-10% of the population.
Lactase deficiency is the "normal" thing after infancy. As a kind of genetic defect, it has preserved the enzyme lactase in evolution of all the populations that kept dairy cattle and imported milk as food. This is also the case with us where lactase deficiency is therefore regarded as a disease.
But why does nature produce this elaborate, only in milk occurring luxury sugar, which is only digestible for the infant or the suckling young animal? To provide the newborn babies in need of help with their food source, because as long as the older siblings or adults cannot tolerate lactose, no "jealousy about food" can arise on the coveted milk.
Lactose is used in many biscuits and baked it must declare goods for sweetening on the list of ingredients.
Lactose is converted into lactic acid in well acidified dairy products and cheese.