Milk
A liquid secreted by mammals, rich in calcium, protein and lactose, sold with different heat treatments that determine how long it keeps and how it tastes.
Cow’s milk provides about 120 mg of calcium per 100 ml, in a well-absorbed form, along with high-quality protein (caseins and whey proteins), lactose, B vitamins and, in whole milk, fat-soluble vitamins.
What sets shop-bought milks apart is mainly their heat treatment. Raw milk undergoes none and keeps for a few days. Pasteurised milk is heated to about 72 °C for a few seconds: it keeps a flavour close to raw milk and lasts a few days in the fridge. UHT milk, briefly brought to around 140 °C, keeps for several months without refrigeration.
The nutritional gap between these milks is small. This is often exaggerated: the losses mainly affect fragile vitamins present in small amounts, such as vitamin C, of which milk is not a source anyway. Protein, calcium and minerals are unaffected. The real difference lies in taste and shelf life.
Lactose is the main factor in tolerance: the ability to digest it declines in a majority of adults worldwide, with lactase persistence into adulthood being a genetic trait found mostly in northern Europe. Lactose-free milks are ordinary milks in which the enzyme has already done its work, hence their sweeter taste.