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Milk occupies a special position among foods in being an animal food that has a vegetarian connotation. Breast milk is the sole food for humans during the first part of their lives, and even animal milk carries many nutrients that the infant needs for growth and development. For children, adolescent, elderly people pregnant and nursing mothers, milk plays an important role in meeting the requirements of many essential nutrients (Table), and hence milk is considered as a protective food. Milk helps to balance human diet by supplementing good quality protein, calcium and vitamins particularly, vitamin A, riboflavin, niacin and folic acid. In addition milk contains several bio-protective molecules that ensure health security to humans. Further, milk is an excellent medium for fortification with probiotics, health promoting bacteria that colonize intestine and confer health benefits to the host.
Nutritional quality of milk proteins
Milk proteins are rich in essential amino acids. Whey proteins that constitute 20% of milk proteins especially are of high nutritional quality, containing 51% essential amino acids, compared to 45% in casein. The sulphur amino acids are higher in whey proteins then in casein. The Protein Efficiency Ratio (3.1), Biological Value (91) and Net Protein Utilization (82) of milk protein are very close to that of egg proteins (3.8, 100 and 94, respectively). Lactalbumin is superior to casein having Biological Value (BV), Net Protein Utilization (NPU) and Protein Efficiency Ratio (PER) 100, 92 and 3.6, respectively. The corresponding values for casein are 77, 76 and 2.5, respectively. Only 14.5g of lactalbumin or 28.5g milk proteins is sufficient to meet the daily requirement of essential amino acids for adult humans.
Supplementary value and digestibility of milk proteins
Because milk proteins contain a surplus of certain essential amino acids (lysine and threonine), they can raise the BV of vegetable proteins. Milk proteins can be added to cereal based products to increase their lysine and threonine content. A breakfast consisting of milk, egg and bread has an excellent NPU value, and provides balanced amounts of minerals and vitamins. Whey proteins have even better supplementary value. A mixture of whey protein, wheat, rice or maize has PER value even greater than that of whey proteins. Whey proteins can raise the BV of soy proteins because of their high concentration of sulphur amino acids. Whey
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obtained as a by-product in cheese manufacture has thus a great potential for incorporation in cereal based products.
The digestibility of milk proteins is rated higher (96%) then that of plant proteins (74-78%). Because of their high BV, the milk proteins are useful in the diet of patients suffering from liver and gall bladder diseases, hyperlipidaemia and diabetes. Patients with impaired kidney functions rely on protein with high BV for relieving strain on the excretory function of the kidney. The milk proteins are also used in slimming diets.
Nutritional benefits of milk fat
Compared to other fats and oils, milk fat is easily digestible. The digestibility of milk fat is 99%, while that of natural palm oil is 91%. The excellent digestibility of milk fat is due to dispersion of fat globules in the aqueous phase of milk forming an emulsion. They are absorbed directly unlike other dietary fats that have to be emulsified by bile, pancreatic enzymes and intestinal lipases before they can pass through intestinal well. Also, milk fat is rich in short and medium-chain fatty acids, which are more easily absorbed than long chain fatty acids. The ester bonds involving short-chain fatty acids are more easily cleaved by lipases. The easy digestibility of milk fat makes it a valuable dietary constituent in diseases of stomach, intestine, liver, gall bladder, kidney and disorders of fat digestion.
In infant and child nutrition, milk fat is of immense benefit. It helps them in meeting their energy requirements by increasing energy density of the diet. A sufficient fat supply is essential for thriving babies, a rosy and smooth skin and also resistance to bacterial infections. Mother's milk contains lipase which plays an important role in digestion of fat in infants, particularly so since the glands responsible for digestion of fat are not fully developed in infants.
No Justification for replacing milk fat with vegetable oils
Milk fat has a low content of essential fatty acids (EFA), linoleic and linolenic acid. The EFA requirement is only 3% of total calories, two-third of which is met from invisible fat present in dietary cereals, pulses and vegetables. Therefore there is no justification to replace milk fat with another fat having higher linoleic acid content. Mother's milk has relatively higher EFA content than cow or buffalo's milk. The babies fed purely on
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