Fundamentals of Food Technology

Functions, properties, sources and requirements of proteins

Functions of proteins

The important functions of dietary proteins are
i) To replace the daily loss of body proteins
ii) To provide amino acids for the formation of tissue proteins during growth
iii) To provide amino acids necessary for the formation of enzymes, blood proteins and certain hormones of protein nature and
iv) To provide amino acids for growth of fetus in pregnancy and for the production of milk proteins during lactation.

Properties
Amphoteric nature
Like amino acids, proteins are amphoteric, that is, they act as both acids and bases. Since proteins have electric charges, they migrate in an electric field, the direction of migration depending on the net charge on the molecule. For each protein, there is a pH at which the positive and negative charges will be equal and protein will not move in an electric field. This pH is known as the isoelectric point of the protein.
Solubility
Each protein has a definite and characteristic solubility in a solution of known salt concentration and pH. Albumins are soluble in water. Globulins are soluble in neutral sodium chloride solutions but are almost insoluble in water.
Colloidal nature of protein solutions
Proteins have large molecular weights. Protein solutions are colloids. They do not pass through semi permeable membranes. This property of proteins is of great physiological importance

Sources
Best sources: Meat, fish, poultry, eggs, milk and milk products
Good sources: cereals and legumes
Requirements - RDA
Children g/day
1-3 years - 16.7
4-6 years - 20.1
7-9 years - 29.5

Boys
10-12 years - 39.9
13-15 years - 54.3
16-18 years - 61.5

Girls
10-12 years - 40.4
13-15 years - 51.9
16-18 years - 55.5

Adult man - 60
Adult woman - 55.5
Pregnant woman - 82.2
Lactation - 77.9