The validity of the protein digestibility-corrected amino acid score (PDCAAS) method in predicting the quality of fourteen protein products was compared with the commonly used protein quality methods, protein efficiency ratio (PER) and net protein ratio (NPR). A rat growth and balance study was conducted to determine protein digestibility and quality of the animal and vegetable protein products by the PER and NPR methods. Amino acid compositions of the products were also determined, and PDCAAS were calculated using a rat and a human pattern of amino acid requirements. Compared to the biological methods, the scoring method overestimated protein quality of mustard flour [PDCAAS of 84-92% vs. relative PER (RPER) or relative NPR (RNPR) of 0], raw black beans (PDCAAS of 45-72% vs. RPER or RNPR of 0), alkaline-treated lactalbumin and soybean protein isolate (PDCAAS of 44-67% vs. RPER or RNPR of 0) and heated skim milk (PDCAAS of 29-31% vs. RPER and RNPR of 0-5%). The scoring method also overestimated the protein quality of zein (true protein digestibility of 63%) supplemented with Lys, Met, Thr and Trp (PDCAAS of 63-71% vs. RPER and RNPR of 3-44%). These data demonstrate that the PDCAAS method is inappropriate for predicting protein quality of those protein sources which may contain naturally occurring growth-depressing factors or antinutritional factors formed during alkaline and/or heat processing.

Anti-nutritional factors and technological processes may modify the nutritional quality of plant proteins. Heated rapeseed meal, soybean, and lupine proteins dried by various processes were used to compare in vivo methods of nutritional quality measurement such as protein efficiency ratio and true digestibility (TD) to in vitro methods such as pH-stat giving degree of hydrolysis (DH) and digestion cell giving nitrogen digestibility (ND). Combined methods taking into account amino acid score were as follows: PDCAAS (protein digestibility-corrected amino acid score) derived from TD, NDCAAS (nitrogen digestibility-corrected amino acid score), and DHCAAS (degree of hydrolysis-corrected amino acid score). Correlations (p < 0.001) were 0.81 (TD vs DH) and 0.88 (TD vs ND). PDCAAS was significantly (p < 0.001) correlated with DHCAAS or NDCAAS (r = 0.92 and 0.98, respectively). Time-consuming and expensive TD determination could be supplanted by both in vitro methods; DHCAAS and NDCAAS could replace PDCAAS.