Acid bacteria lactose digestion

Lactose is readily fermented by lactic acid bacteria, especially Lactococcus spp. and Lactobacillus spp., to lactic acid, and by some species of yeast, e.g. Kluyveromyces spp., to ethanol (Figure 2.27). Lactic acid may be used as a food acidulant, as a component in the manufacture of plastics, or converted to ammonium lactate as a source of nitrogen for animal nutrition. It can be converted to propionic acid, which has many food applications, by Propionibacterium spp. Potable ethanol is being produced commercially from lactose in whey or UF permeate. The ethanol may also be used for industrial purposes or as a fuel but is probably not cost-competitive with ethanol produced by fermentation of sucrose or chemically. The ethanol may also be oxidized to acetic acid. The mother liquor remaining from the production of lactic acid or ethanol may be subjected to anaerobic digestion with the production of methane (CH4) for use as a fuel several such plants are in commercial use. 

Martini, M.C., Lerebours, E.C., Lin, W., and Harlander, S.K. Strains and species of lactic acid bacteria in fermented milks (yoghurts) effect on in vivo lactose digestion. Am. J. Clin. Nutr., 54,1041,1991. 

Lactose is hydrolyzed to glucose and galactose in the intestine by the enzyme lactase. People who are lactose-intolerant lack this enzyme. In these people, lactose advances in the digestive tract to the large intestine, where it is fermented by intestinal bacteria to produce large amounts of carbon dioxide and organic acids. Today, people with lactose intolerance can purchase milk in which the lactose has already been hydrolyzed, or can buy lactase supplements. 

Thousands of diseases related to deficient or defective enzymes occur, many of which are rare. For example, in phenylketonuria (which has an incidence of 1 in 10,000 births in whites and Asians), the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine, is deficient. Phenylalanine accumulates, and tyrosine becomes an essential amino acid that is required in the diet. Mental retardation is a result of metabolic derangement. A more common problem is lactase deficiency, which occurs in 69% to 90% of American Indians, blacks, and Asians, and in 10% of whites. Lactose is not digested normally and accumulates in the gut where it is metabolized by bacteria. Bloating, abdominal cramps, and watery diarrhea result. 

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