Lactose is the major carbohydrate present in the milk of most mammals. The amount varies from species to species. Cow and goat milks contain approximately 4.5% lactose; human milk contains approximately 7%.

Lactose is produced commercially from cow's milk that has had the casein coagulated (the first step in the manufacture of cheese) by adjustment of the pH of the milk to the isoelectric pH of casein (4.5-4.7) and heating or by use of rennin. The resulting sweet whey, the solids of which are approximately 70% lactose, is subjected to ultrafiltration to remove remaining proteins (7). Then minerals are removed by ion exchange, and the solution is concentrated to 50 to 65% solids so that the lactose can be crystallized or precipitated. The recovered lactose is redissolved, decolorized with carbon, and recrystallized as a-lactose monohydrate (see the article Carbohydrates: classification, chemistry, labeling), the most commonly prepared form of lactose and the one that crystallizes from supersaturated solutions at temperatures below 93.5°C (16). For every kilogram (2.2 lb) of cheese produced, approximately 9 kg (19 lb) of whey is recovered (3). Because whey contains about 4.7% lactose, about 450 g (1 lb) is potentially available as a by-product of the manufacture of each kilogram of cheese. However, little commercial use is made of lactose.

Milk also contains 0.3 to 0.6% lactose-containing oligosaccharides, many of which are important as energy sources for growth of a specific variant of Lactobacillus bi-fidus, which as a result is the predominant microorganism of the intestinal flora of breast-fed infants.

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