quality:correction
Occurrence and isolation

Lactose comprises about 2–8% of milk by weight. Several million tons are produced annually as a by-product of the dairy industry.

Whey or milk plasma is the liquid remaining after milk is curdled and strained, for example in the production of cheese. Whey is made up of 6.5% solids, of which 4.8% is lactose, which is purified by crystallisation. Industrially, lactose is produced from whey permeate – whey filtrated for all major proteins. The protein fraction is used in infant nutrition and sports nutrition while the permeate can be evaporated to 60–65% solids and crystallized while cooling. Lactose can also be isolated by dilution of whey with ethanol.

Dairy products such as yogurt and cheese contain very little lactose. This is because the bacteria used to make these products breaks down lactose through the use of β-Galactosidases.