Lactose
The natural sugar in milk, most of which runs off in the whey or is turned into lactic acid during cheesemaking.
Also: milk sugar
Lactose is the sugar found in all mammals' milk. In cheesemaking most of it leaves the curd in the whey, and what remains is eaten by lactic acid bacteria, which turn it into lactic acid in the first days and weeks. As a result, long-aged hard cheeses such as Parmigiano Reggiano or aged cheddar contain almost none, while fresh cheeses such as ricotta, which is made from whey, and paneer keep more. Whey cheeses such as Norwegian brunost are made by boiling down whey until its lactose caramelises, which gives them their colour and sweetness. Khoa, which is boiled-down whole milk, keeps all of its lactose, which is why it tastes sweet.