NUTRIENTS IN MILK

Lactose

It is the principal and typical carbohydrate of milk, known as milk sugar. Glucose, galactose and other sugars such as oligosaccharides are also present in traces. Lactose exists in true solution in milk. It is a disaccharide, composed of two molecules of monosacharides; glucose, and galactose. Lactose is readily fermented by the lactic acid fermenting bacteria producing lactic acid and has significance in milk and milk products. 

It exists in two isomeric forms, designated as a and ß forms of which the ß form is more soluble than the alpha form in water. Lactose content of cow milk is 4.9 percent. The lactose content of milk is inversely proportional to the ash content of the milk. Udder infection promotes an increased level of chloride in the milk and depresses the secretion of lactose.

The metabolism of lactose in the intestine is considerably slower and hence it passes through the intestine slowly where it promotes the growth of lactic acid fermenting bacteria. The acidic condition produced due to fermentation of lactose to lactic acid increases the solubility of lactose and improving its absorption by forming soluble complex with calcium.

Slow absorption of lactose from the intestine coupled with low pH induces laxative effect by increasing peristalsis. Even on a higher lactose diet, the lactose concentration does not shoot up. Hence, diabetic persons can safely consume milk or milk products rich in lactose with out any adverse impact. 

Proteins

Milk protein is a rich source of essential amino acids. Whey proteins are proteins that are passed along with the whey portion after the coagulation of milk. They contain 51% essential amino acids when compared to 45% in casein. The sulphur containing amino acids, which are considered essential and important, are found in higher concentration in whey protein than in casein. Usually the quality of egg proteins are regarded as very high. But the net protein utilization, biological value and the protein efficiency ratio of milk protein come neck in neck with the quality of egg protein. Lactalbumin, a whey protein , whose biological value, net protein utilization and protein efficiency ratio is considered superior when compared to the major milk protein, casein. Normally double the quantum of vegetable protein is required to meet the daily requirement of essential amino acids when compared to that obtained from the milk proteins.

Vegetable milk proteins which are naturally deficient in some essential amino acids such as lysine and threonine can be enriched by supplementation with milk protein, especially the cereal based vegetable products. Whey proteins can be used to supplement the ordinary food which increases the net protein utilization of the entire food consumed. With regard to digestibility pattern of different fractions, the whey proteins are considered to have better digestibility when compared to casein. The higher biological value of milk proteins naturally makes it as an ideal candidate for persons suffering from renal impairment. They also find a place in the fitness diets.

The buffering capacity of milk proteins is very high and hence useful in the treatment of inflammation of mucous lining of the stomach and stomach ulcers. They are also used in the treatment of hyperuricaemia. Milk contains a number of protein components, which differ in composition. They are the principal nitrogenous constituents of milk, made up of several amino acids bound together by a peptide bond. In milk they are partly in true solution and mostly as colloidal suspension. The main proteins of milk are caseins, alpha lactalbumin and beta lactoglobulins. 

Milk derived peptides have functional properties. Several peptides with sleep inducing or opium like property have been extracted from milk proteins and they are beta casomorphins from beta casein, exorphin from alpha s1 casein, beta lactostensin from lactogobulin and serorphin from serum albumin. 

Casein

May be defined as the major protein, which is precipitated at pH 4.6 and is exclusive to milk. It is present in spherical bodies as micelles, which vary in size with negative surface charge. The caseins of milk may be sub-divided into five main classes, as1, as2, ß, gamma and k-caseins. In milk, casein is present in combination with calcium in the form of calcium caseinate or more precisely calcium hyhdrogen caseinate.

Whey proteins

Whey proteins are those in the whey fraction, after the precipitation of casein at pH 4.6. These are the alpha-lactalbumin, beta-lactoglobulin, immunoglobulins, lactoferrin, transferrin, proteose-peptone fractions etc. Most of these are globular proteins subject to heat denaturation. a-lactalbumin exists in variants A and B forms and is susceptible to denaturation by unfolding of the tertiary structure. ß-lactoglobulin is identical to blood globulin and insoluble in water and is responsible for the transfer of antibodies. Normal milk contains 0.1%, whereas colostrum contains 6 %.  

Source : From Jensen, R. G. Ed., Handbook of Milk Composition, Academic Press, New York, p. 465, 1995.

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