What are the sources of vegetable and animal protein?

This group comprises food stuffs which contain mostly proteins, but in widely varying amounts. Animal foods, such as meat, fish and eggs are rich sources of protein. Among the vegetable foods, pulses and huts are the richest sources of protein with amounts often exceeding those present in animal foods. Soya bean is unique in this respect in that it contains over 40% of protein.

Vegetable Proteins

Pulses are a term which includes dals and grams. These are really important in the Indian diet as sources of proteins because flesh foods are consumed only in small amounts.

Pulse proteins, however, are of relatively low biological value because of the deficiency of an essential amino acid, methionine. Red gram is also deficient in tryptophan. However, pulse proteins are rich in lysine and they are, therefore, of good supplementary value to cereal diets. The lysine deficit in cereals is made good by the lysine present in pulses and thus the overall biological value of the cereal­ pulse diet is better.

In the amounts consumed pulses cannot be considered rich sources of minerals but they are rich in vitamins of B-Group espe­cially thiamine and folic acid. Dried peas do not contain Vitamin C in significant amounts, but when they are sprouted they become rich sources of Vitamin C. This is particularly true of sprouted green gram dal and sprouted Bengal gram.

Nuts and oil seeds, are rich in proteins, contain fats and are good sources of calories. Most of the oil seeds produced in India is used for extraction of edible oils, but the cake left behind is richer in protein than the original seed. Till recently oilseed cakes were not being used as human food to any significant extent. Now oil cake can be used in various ways as food for human beings and ‘Protein Isolates’ are being produced from them.

Dried groundnuts are leguminous and are widely used in this country. Besides protein, groundnut is a rich source of B-complex vitamins especially thiamine and nicotinic acid. However, groundnut protein is poor in the amino-acid, methionine.

Animal Protein

Milk serves as the sole food of the young during the most critical period of life. Although the milk of various mammals is used for food, cow’s milk is by far the most common and will be discussed here. Milk products include cheese, khoya, rabri, curd.

There is no adequate substitute for milk. No food has wider acceptability or offers a greater variety of uses. Milk is a complex substance in which over 100 separate components have been identi­fied. It is fluid in spite of the fact that it contains more solids than many solid foods. Fresh cow’s milk contains 85% water and 13% solids.

The essential amino acids present in milk proteins are supplied in almost ideal proportions for maximum tissue synthesis. Casein accounts for 4/5th of the protein in milk.

Milk supplements cereal proteins in an excellent manner, for supplies the amino acids, lysine and tryptophan which are limited in the cereals. The biological value of proteins in white wheat flour is only 5% when used alone, but this is raised to 75% when milk is used with the wheat flour. The fat of milk is highly emulsified and easily digested.

Milk-sugar or lactose is a carbohydrate occurring only in milk. This sugar is particularly adapted to making milk an ideal food for the young, because it is much less sweet, less soluble and more stable than sucrose and other sugars. It gives to milk a bland flavour. Lactose favours the growth of lactic-acid-producing bacteria which are believed to retard or prevent the growth of putrefying bacteria. Lactose probably favors the absorption of calcium and phosphorus and the synthesis of some B-complex vitamins in the small intestine.

Milk supplies several mineral elements in abundance. Milk products provide a practical basis for meeting the recommended allowances for calcium. Phosphorus occurs in correct proportions with calcium to support optimum skeletal growth. Milk contains appreciable amounts of sodium, potassium and magnesium, but it furnishes very little iron. An infant’s diet must be supplemented with iron rich foods at an early age to prevent anaemia.

Milk is an outstanding dietary source of riboflavin and also supplies fair amounts of Vitamin A, thiamine, Vitamin B₆ and Vitamin B_12. It is low in niacin, but is an excellent source of tryptophan, which functions as a precursor of niacin. Milk lacks in Vitamin C.

Curd. Whole milk is made into curd by the addition of curd as a ‘starter’. Curd contains lactic acid bacteria which cause fermentation and set the milk. All the good nutrients present in the milk remain in the curd. It is, however, more easily digested than milk.

Cheese. The composition of cheese depends upon the kind of milk used-whole or skimmed-and the amount of water present. The proteins in cheese contain all the essential amino acids and are, therefore, of high biological value. Only a trace of the lactose present in milk remains in the cheese. Depending upon the method of preparation varying amounts of calcium, thiamine and riboflavin are lost.

Eggs may be used in many ways in breakfast, lunch or dinner dishes. They are excellent sources of protein. In addition to the protein of the highest quality, they contain a good deal of fat, a high content of Vitamins A and B; iron, phosphorus and calcium in fair amounts. Only Vitamin C is absent. The proteins contained in eggs are so well proportioned in their amino acid composition that egg protein is considered to be a perfect protein. Because of its high biological value and easy digestibility, egg protein is used in nutrition work as a reference protein for comparing the quality of protein in other foods. Eggs of different species of birds are said to be similar in nutritive value. Raw egg white contains a protein known as avidin which renders the vitamin biotin unavailable to the body. Duck’s egg white contains in addition, a substance which inhibits the action of trypsin on protein. Heating egg, as for instance, in the preparation of boiled egg, destroys both avidin and trypsin inhibitor. Cooking makes egg-protein easily available to the body.

Fish, Meat, Poultry and other Flesh Foods

Regardless of the species, the amino acid composition in the flesh foods is relatively constant and of such balance and quality that meats, fish and poultry rank only slightly below eggs and milk in their ability to effect tissue synthesis.

The meat group is the principal source of cholesterol in the diet. This lipid is not uniformly present in all flesh foods; and brain and shellfish are the outstanding sources.

The mineral element of special importance in the meat group is iron. Meats are also rich in phosphorus, sulphur, potassium; moder­ately high in sodium and poor in their calcium content. Fish, however, is a good source of calcium, especially the small varieties which can be consumed whole. Salt-water fish contain an appreciable amount of iodine.

All flesh foods like meat and fish are good sources of B-complex vitamins. Pork, liver and other organ meats are excellent for their thiamine content; poultry is rich in niacin. Vitamin B_12, which is contained only in foods of animal origin and not in plant foods, is supplied well by organ meats and muscle meats. Flesh foods are generally not good sources of Vitamin A, but liver is an outstanding source of Vitamin A. Other organs such as kidney contain some Vitamin A.