THE PROTEIN DIGESTIBILITY-CORRECTED AMINO-ACID SCORE METHOD OVERESTIMATES QUALITY OF PROTEINS CONTAINING ANTINUTRITIONAL FACTORS AND OF POORLY DIGESTIBLE PROTEINS SUPPLEMENTED WITH LIMITING AMINO-ACIDS IN RATS

The validity of the protein digestibility-corrected amino acid score ( PDGAAS) method in predicting the quality of fourteen protein products was compared with the commonly used protein quality methods, protein e fficiency ratio (RER) and net protein ratio (NPR). A rat growth and ba lance study was conducted to determine protein digestibility and quali ty of the animal and vegetable protein products by the PER and NPR met hods. Amino acid compositions of the products were also determined, an d 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. r elative PER (RPER) or relative NPR (RNPR) of 0], raw black beans (PDCA AS 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 h eated skim milk (PDCAAS of 29-31% vs. RPER and RNPR of 0-5%). The scor ing method also overestimated the protein quality of zein (true protei n digestibility of 63%) supplemented with Lys, Met, Thr and Trp (PDCAA S of 63-71% vs. RPER and RNPR of 3-44%). These data demonstrate that t he PDCAAS method is inappropriate for predicting protein quality of th ose protein sources which may contain naturally occurring growth-depre ssing factors or antinutritional factors formed during alkaline and/or heat processing.