<P> Lactose monohydrate crystals have a characteristic tomahawk shape that can be observed with a light microscope . </P> <P> Several million tons are produced annually as a by - product of the dairy industry . Whey is made up of 6.5% solids of which 4.8% is lactose that may be purified by crystallisation . Whey or milk plasma is the liquid remaining after milk is curdled and strained, for example in the production of cheese . Lactose makes up about 2--8% of milk by weight . Industrially, lactose is produced from whey permeate--that is whey filtrated for all major proteins . The protein fraction is used in infant nutrition and sport nutrition while the permeate can be evaporated to 60--65% solids and crystallized while cooling . Lactose can also be precipitated from whey using ethanol . Since it is insoluble in ethanol, lactose precipitates, in about 65% yield . </P> <P> Infant mammals nurse on their mothers to drink milk, which is rich in lactose . The intestinal villi secrete the enzyme called lactase (β - D - galactosidase) to digest it . This enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can be absorbed . Since lactose occurs mostly in milk, in most mammals, the production of lactase gradually decreases with maturity due to a lack of continuing consumption . </P> <P> Many people with ancestry in Europe, West Asia, South Asia, the Sahel belt in West Africa, East Africa and a few other parts of Central Africa maintain lactase production into adulthood . In many of these areas, milk from mammals such as cattle, goats, and sheep is used as a large source of food . Hence, it was in these regions that genes for lifelong lactase production first evolved . The genes of adult lactose tolerance have evolved independently in various ethnic groups . By descent, more than 70% of western Europeans can drink milk as adults, compared with less than 30% of people from areas of Africa, eastern and south - eastern Asia and Oceania . In people who are lactose intolerant, lactose is not broken down and provides food for gas - producing gut flora, which can lead to diarrhea, bloating, flatulence, and other gastrointestinal symptoms . </P>

What are the products of the breakdown of lactose