Protein

Protein Quality

Scientists have developed several methods for evaluating the quality of food proteins. These methods aim to identify high quality-proteins, that is, proteins which contain all the essential amino acids in relatively the same proportion as human beings require.

This is a simple and relatively inexpensive way to evaluate a food protein's quality. The amino acid composition is determined and compared with a reference protein. Scientists can identify the limiting amino acid which is the one which falls shortest compared to the reference. If the test food protein's limiting amino acid is 70% of the amount of the amount found in the reference protein, it receives a chemical score of 70. Limitations of this method include the inability to predict the digestibility of a protein, which may effect its quality.

The biological value of a protein measures its efficiency in supporting the body's requirements. Scientists will feed a given food protein to experimental animals as the sole source of protein in their diets and measure their protein retention and loss. The more protein retained, the higher the protein quality.

Biological value is expressed as a percentage of the absorbed nitrogen that is retained.

Examples

Like, biological value, net protein utilisation (NPU) measures nitrogen retention. However, instead of measuring retention of absorbed nitrogen, the NPU measures the retention of food nitrogen.

The protein efficiency ratio (PER) measures the weight gain of a growing animal and compares it to the animal's protein intake. The PER is expressed as:

PER = Weight gain (g)__
Protein Intake (g)

This method has advantages because of its easy and economical. However this is a time-consuming method and the amino acids needs of rats are not the same as humans.

The protein-digestibility-corrected amino acid score (PDCAAS) method compares the amino acid content of protein with human amino acid requirements and corrects for digestibility. The protein's amino acid profile is determined as in the amino acid scoring method and then it is compared against the amino acid requirements of preschool-aged children. This comparison reveals the most limiting amino acid. The rationale behind using the requirements of this age group is that if a protein will effectively support a young child's growth and development, then it will meet or exceed the requirements of older children and adults. Therefore, the PDCAAS method evaluated dietary protein quality of for all ages except infants (The PER method is used to evaluate proteins for infants).

Examples

Note: 1.0 is the maximum PDCAAS a food protein can receive.