This is an excerpt from Sport Nutrition 3rd Edition by Asker Jeukendrup & Michael Gleeson.
The quality of a protein relates to the degree to which that protein contributes to daily requirements. Various methods have been proposed to measure the protein quality of a food. The most recent method is the protein digestibility corrected amino acid score (PDCAAS). PDCAAS is a method of evaluating protein quality based on the amino acid requirements of humans. Using the PDCAAS, protein-quality rankings are determined by comparing the amino acid profile of a specific food protein against a standard amino acid profile. The maximum score of 1.0 means that after digestion of the protein, it provides, per unit of protein, 100% or more of the indispensable amino acids required. Although this classification was adopted by the U.S. Food and Drug Administration (FDA), the Food and Agricultural Organization of the United Nations, and the World Health Organization in 1993 as “the preferred best” method to determine protein quality, it has received much criticism. People rarely eat a single source of protein; therefore, having the information for individual sources of protein does not give information about the protein quality of the overall diet unless all protein-containing food sources are considered to calculate an average score. In addition, the fact that common protein sources, which have different amino acid profiles, receive identical scores of 1.0 limits its usefulness as a comparative tool. But in the absence of a better method, the PDCAAS is frequently used.
A PDCAAS value of 1.0 is the highest and 0.0 is the lowest. The following are the ratings of some common foods: