Protein quality

Protein quality is the digestibility and quantity of essential amino acids for providing the proteins in correct ratios for human consumption. There are various methods that rank the quality of different types of protein, some of which are outdated and no longer in use, or not considered as useful as they once were thought to be. The current protein ranking standard in use is the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which was recommended by Food and Agriculture Organization (FAO), and became the industry standard in 1993, and FAO has recently recommended the newer Digestible Indispensable Amino Acid Score (DIAAS) to supersede PDCAAS, due to the limitations of PDCAAS. The dairy industry is in favor of this, because while PDCAAS truncates all protein types that exceed the essential amino acid (EAA) requirements to 1.0, DIAAS allows a higher than 1.0 ranking, and as such, while for example both soy protein isolate and whey isolate are ranked 1.0 according to PDCAAS, in the DIAAS system, whey has a higher score than soy.

The main limitations of PDCAAS is that it doesn't take into account anti-nutrient factors like phytic acid and trypsin inhibitors, which limit the absorption of protein among other nutrients. For this reason, DIAAS is promoted as the superior method and preferable over the PDCAAS. Other older methods like BV, PER, NPU and nitrogen balance may not reveal much about the amino acid profile and digestibility of the protein source in question, but can still be considered useful in that they determine other aspects of protein quality not taken into account by PDCAAS and DIAAS.

Below follows a table that compares various proteins based on their rankings. Some of these results may differ and vary significantly depending on if it is soybeans or soy protein isolate, and so on. For example, while soybeans have a PDCAAS score of 0.91, many soy protein isolates (though not all) typically get a PDCAAS score of 1.0, and as such, is a complete protein. Likewise, the amino acid profile may differ from crop to crop depending on the soil, and between different breeds of soy. Generally speaking, however, soybeans rarely outperform whey protein isolate in PDCAAS rankings.

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