n. (Chem.) A white, crystalline sugar, C6H12O6, isomeric with dextrose, obtained by the decomposition of milk sugar, and also from certain gums. When oxidized it forms mucic acid. Called also lactose (though it is not lactose proper). [ 1913 Webster ]

n. 1. (Physiol. Chem.) The main sugar present in milk, called also sugar of milk or milk sugar. When isolated pure it is obtained crystalline; it is separable from the whey by evaporation and crystallization. It is a disaccharide with the formula C12H22O11, being chemically 4-(β-D-galactosido)-D-glucose. It has a slightly sweet taste, is dextrorotary, and is much less soluble in water than either cane sugar or glucose. Formerly called lactin. When hydrolyzed it yields glucose and galactose. In cells it may be hydrolyzed by the enzyme β-galactosidase. [ 1913 Webster +PJC ]

2. (Chem.) See Galactose. [ 1913 Webster ]

n. A physiological condition in which lactose cannot be hydrolyzed in the intestine, leading to uncomfortable gaseousness, cramps, or diarrhea after eating a lactose-containing food such as milk or ice cream. The symptoms may be prevented by ingesting a preparation of beta-galactosidase before or together with dairy products. [ PJC ]