Early measurements of protein quality were based on a 1919 study, where the protein quality was calculated using a method known as PER (Protein Efficiency Ratio). The basic flaw of PER was that it was calculated based on the amino acid requirements of young growing rats, and not young growing humans. This result overestimated the quality of animal protein and underestimated the quality of plant protein for human nutrition. Why was this so? Very simply because rats are much hairier than humans, and therefore need higher levels of the sulphur containing amino acids – methionine and cysteine, which are required for producing fur. As a result, the sulphur containing amino acids were considered the “limiting” amino acids in soy proteins, and therefore the quality of soy protein for human nutrition was wrongly assumed to be inferior to animal protein.
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Since 1993, the Food and Agricultural Organisation of the United Nations/World Health Organisation adopted the PDCAAS as the preferred best method to determine protein quality for humans.
Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a method of evaluating the protein quality based on both the amino acid requirements of humans and their ability to digest it.
Using the PDCAAS method, the protein quality rankings are determined by comparing the amino acid profile of the specific food protein against a standard amino acid profile, with the highest possible score being a 1.0. This score means that, after digestion of the protein, it provides per unit of protein 100% or more of the essential amino acids required (there are 9 essential amino acids that the human body needs to have in the food eaten, as the human body cannot synthesise them itself).
The four proteins that score the maximum PDCAAS of 1.0 are soy protein, casein (milk protein), whey (another milk protein) and egg white. Beef has a score of 0.92, peanuts 0.52 and gluten (from wheat) a score of 0.25.
PDCAAS does have some limitations, but to date it is still considered to be the best available method for calculating protein quality for humans.
Of course, humans often consume more than one protein source at meal times. In these instances it therefore becomes necessary to look at the “total pool” of amino acids present in order to determine the PDCAAS.