Nutritional quality of proteins can be assessed by the following methods:
Chemical score The chemical scoring is one of the easiest methods if appropriate instruments for measuring amino acid content of a protein are available. The chemical score for a protein is arrived at by determining the most limiting amino acid for that protein by comparing it with standard protein like hen’s egg.
Information on the most limiting amino acid is generally sufficient to arrive at the chemical score.
Biological value Biological value of a protein is determined from the nitrogen (N) balance of an individual or an animal after feeding the test protein. It is defined as
After feeding a protein-free diet for a period of three days, the test protein is fed to the animal or human for seven days. Urinary and faecal nitrogen are measured in 24 hrs. collection, at the end of each period for consecutive three days. The biological value can be arrived at by the following equation.
Where I, F and U are dietary, faecal and urinary nitrogen on the test diet and Fp and Up are faecal and urinary nitrogen on a protein – free diet.
Protein utilization Net utilization of dietary protein , which is a product of digestibility coefficient and biological value divided by 100.
Thus it accounts even for digestive losses of nitrogen. NPU is generally determined at or below the nitrogen intake required for maintenance. In humans it is calculated from nitrogen balance studies. In rats the actual N retained is determined by estimating the carcass nitrogen content after feeding the test proteins.
Protein efficiency ratio: The protein efficiency ratio is calculated by