Biological value of protein

BIOLOGICAL VALUE OF PROTEIN

Biological value of protein is the percentage of a protein nitrogen that is absorbed and available for use by the body for growth and maintenance.

Proteins are functionally divided into complete, partially complete and incomplete proteins. A complete protein contains all essential amino acids in relatively the same amounts as human beings require to promote and maintain normal growth. (eg) Protein derived from animal foods. A partially complete protein contains sufficient amounts of amino acids to maintain life but fail to promote growth. (eg) Gliadin in wheat. Incomplete proteins are incapable of replacing or building new tissue and cannot support life or growth. (eg) Protein in Wheat germ.

The quality of a protein is determined by the kind and proportion of amino acid it contains. Proteins that contain all essential amino acids in proportions capable of promoting growth are described as complete protein, good quality protein, or proteins of high biological value.

A good quality protein is digested and utilized well. Egg protein is a complete protein and is considered as a reference protein with the highest biological value. The quality of other proteins is determined based on their comparison with egg protein as in figure.

The eight essential amino acids (EAA) must be present in a protein in specific ratios. Egg protein has all eight in the correct proportions used most efficiently and completely by the body.

Wheat germ is an incomplete protein because it is deficient in tryptophan (incomplete circle). As a result of this deficiency only less of the total protein can be used (as represented by the inner dotted circle).

The protein of animal foods like milk, meat, and fish generally compare well with egg in the essential amino acid composition and are categorized as good quality proteins.

Plant proteins are of poor quality, since the essential amino acid composition is not well balanced.

The amino acid, which is not present in sufficient amount in food protein, is called the limiting amino acid of that food. For (eg) Lysine in cereal protein, Tryptophan in Wheat germ.

Biological value of food proteins.

Food Stuff Biological Value

I        Animal Protein    

          Egg    96

          Milk  90

          Meat  74

          Fish   80

II       Vegetable Protein

          Cereals       

          Rice   80

          Wheat         66

          Maize          50

III      Pulses                  

          Bengal gram         74

          Red gram    72

IV      Oil Seeds   

          Ground nut 55

          Gingelly      62

The limitation in cereals can be overcome by a judicious combination with pulses, which are rich in lysine.

The resulting mixture of cereals and pulse will have an amino acid pattern better than either of the constituents. Thus a combination of cereal and pulse has a supplementary effect. For (eg) recipes like, Pongal, Idli, dhokla are based on cereal pulse combination.

Thus the habitual diets in India based on cereal and pulse have indeed a rational basis. The biological value of some important food proteins is given in table-18A.