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 monosaccharides; 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 α and β forms of which the β 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.
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.
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, αs1, αs2, β, gamma and k-caseins. In milk, casein is present in combination with calcium in the form of calcium caseinate or more precisely calcium hydrogen caseinate.
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. α-lactalbumin exists in variants A and B forms and is susceptible to denaturation by unfolding of the tertiary structure. β-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 %.
Milk fat The digestibility of milk is comparatively higher than other oils and fats. This can be attributed to existence of fat globules in aqueous phase forming an emulsion. This facilitates its easy absorption through the intestinal tract when compared to other fats which have to be emulsified with bile salts, enzymes from pancreas and fat splitting lipases. Endowed with short and medium chain fatty acids, milk fat can be easily absorbed when compared to long chain fatty acids because of the ability of the lipases to split the ester bonds in the former. Supplementation of milk fat in the diet increases the energy density. When compared to human milk, the cow milk is low in essential fatty acids such as linoleic and linolenic acids. Short and medium chain fatty acids with 4-12 carbon atoms, which occur at comparatively higher concentration in milk fat, reported to have antibacterial and fungistatic activity. Milk fat plays another important role in preventing tooth decay by forming a protective coat over the surface of enamel. It is composed of triglycerides of fatty acids. A fatty acid molecule is composed of hydrocarbon chain and carboxyl group. Triglycerides are of two types, simple and complex. In simple, all the three fatty acids are of same nature, whereas complex triglycerides on hydrolysis give glycerol and different fatty acids. The milk fat exists in the form of small globules of sizes ranging from 2 to 10 microns with different glycerides of low melting points in suspension. Milk fat varies in amount and composition, according to the breed, species, feed and lactation time, of which, feed being a major factor. Fat is distributed in globules as triglycerides (98-99 %), fat globule membrane in combination with phospholipids and lipoprotein (0.2 to 1.0 %) and also as free fatty acids, cholesterol and phospholipids in the serum.
Phospholipids These contain phosphorus in their molecules in addition to the fatty acids and glycerol; they also contain a nitrogenous base. Principally milk phospholipids are the Lecithin, Cephalin and Sphingomyelin. Though fat-soluble, they are hydrophilic and imbibe large quantity of water and swell. They are used as antioxidants for fat rich dairy products. Vitamins Various vitamins present in milk are as follows. Fat Soluble Vitamins - include vitamins A, D, E and K. Water-soluble vitamins are the, B1 (thiamine), B2 (riboflavin), B6 (pyridoxine), Biotin, Niacin (nicotinic acid), Pantothenic Acid, Para-amino benzoic acid, Inositol, Choline, Folic acid, B12, and Ascorbic acid. Fat rich milk products contain large quantities of fat soluble vitamins, whereas whole milk, skim milk, buttermilk and whey are a good source of water soluble vitamins.
Minerals The portion left after ashing of milk at 150ºC is known as the 'ash of milk' and is composed of various inorganic constituents. Ash forms about 0.75 % of milk and plays a very important role in milk and is basic in character. The minerals in milk consist principally of the chlorides, citrates and bicarbonates of calcium, magnesium, potassium and sodium.
Nutrients in Milk
Article Source: http://EzineArticles.com
By Thenmozhi Kathirvelu
