Lactose or Milk Sugar is the main carbohydrate or sugar of milk. Like sucrose, lactose is a disaccharide which forms hard crystals, but is less sweet than natural sugars and rarely used as a pure sugar. Lactose is present in the whey of coagulated milk and is the food that bacteria digest to make lactic acid and sour milk.
Lactose comprises is a double sugar of glucose and galactose. Hence, in digestion, lactose is broken down to these simple sugars using the enzyme, lactase. It is the lack of this enzyme, which causes lactose intolerance. Lactase is available in infants to enable them to digest milk, however in most peoples, lactase stops being produced in adulthood. For those peoples who do not produce lactase, they often process milk to break down the lactase through their cooking - for example, in India, milk is transformed into curd or yoghurt, where the milk can be digested without having any lactase. Likewise, in yogurt and cheese, much of the lactose is metabolised by lactic acid bacteria to lactic acid, which is more digestible.
For those with lactose intolerance, milk intolerance or wishing to cut milk out of their diet, there are many plant-based alternatives now available.