Lactose is composed of glucose and galactose, two simpler sugars used as energy directly by human body. Galactose and glucose are linked by a β-L-4 glycosidic bond. Its systematic name is β-0-D-galactopyranosyl-(1–4)- α-D-glucopyranose (α-lactose) or β-0-D-galactopyranosyl-(1–4)-β-D-glucopyranose (β-lactose).
It is normally broken down or digested by an enzyme in human body called lactase and ends up in the bloodstream as glucose (a simple sugar). Lactase splits lactose into glucose and galactose.
Lactose is readily fermented by the lactic acid fermenting bacteria producing lactic acid and has significance in milk and milk products.
Lactose in solid form can either be in a crystalline state or in an amorphous state. Crystalline lactose can exist in a number of distinct forms. Most well-known are α-lactose monohydrate and β-lactose. β- form is more soluble than the α form in water.
Lactose is more soluble in molar solutions of calcium chloride or of calcium nitrate than in pure water. Increase in the concentration of salt brings further increase in the solubility of lactose.
Neither α-lactose hydrate nor β-lactose is hygroscopic. However, anhydrous α-lactose is strongly hygroscopic and can absorb moisture from the air, forming the hydrate that occupies more volume than the anhydrous form.
Lactose is not digested until it reaches the small intestine, where the hydrolytic enzyme lactase is located. Lactase (β-galactosidase), which is located in brush border epithelial cells of the small intestine, catalyzes the hydrolysis of lactose into its constituent monosaccharides. Only monosaccharides among the carbohydrates are absorbed from the intestines.
For people who are lactose intolerant, the lactose doesn’t get broken down. Instead, it goes on to the colon, where it mixes with normal bacteria and ferments. It can cause things like gas, bloating and diarrhea.
Lactose may play a role in the absorption of calcium and other minerals such as copper and zinc, especially during infancy. Moreover, if it is not digested in the small intestine, lactose may be used by the intestinal microbiota (the microorganism population that lives in the digestive tract) as a nutrient (prebiotic).
Properties of lactose
