Protein quality assessment of soy products

Three chick assays were conducted to compare the protein quality of dehulled soybean meal (SBM; 49.0% CP), soy protein concentrate (SPC; 63.9% CP), functional soy protein isolate (FSPI; 82.4% CP), and edible soy protein isolate (ESPI; 85.0% CP). Weight gain and protein efficiency ratio (PER) were highest for SBM, similar between chicks fed SPC and FSPI, and lowest for chicks fed ESPI. Supplementation with methionine and threonine increased weight gain and PER values of all protein sources and made PER values for FSPI and ESPI similar to those for SBM and SPC. Upon addition of methionine alone, weight gain and PER of all protein sources were increased substantially, and no difference was observed between SPC and ESPI, although weight gain and PER for FSPI was superior to both SPC and ESPI. Upon addition of both methionine and threonine, PER values for FSPI and ESPI were equivalent and were superior to those for SPC. These results indicate that the inferior protein quality of unsupplemented SPC, FSPI, and ESPI relative to SBM results from a decrease in the effective sulfur amino acid and threonine content of the processed soy products.