Protease inhibitor activities and isoflavone content in commercial soymilks and soy-based infant formulas sold in Ottawa, Canada

Abstract

The content of isoflavones and residual activities of soybean trypsin (SBTI) and chymotrypsin (SBCI) inhibitors in the major soymilks and soy-based infant formulas sold in Ottawa, Canada were examined. The residual SBTI activities (% whole soybean) measured as the ability to inhibit the release of p-nitroaniline from N-benzoyl-dl-arginine-p-nitroanilide by trypsin were from 6.4% to 55.6% in soymilks (8 brands), 4.2% to 8.8% in liquid formulas (4 brands) and 1.9% to 5.1% in powdered formulas (8 brands). SBCI activities (% whole soybean) measured as the ability to inhibit the degradation of casein by chymotrypsin in soymilks were from 3.5% to 33.7%. Total isoflavones measured as aglycones were from 26.4±0.18 to 55.3±3.48μg/g wet weight in liquid formulas or 137.5±3.0 to 229.1±2.3μg/g dry weight in powdered formulas and 49.8±0.5 to 279.9±3.4μg/g wet weight in soymilks. Compared to soymilks, the residual SBTI activities and total isoflavones in infant formulas were lower. The residual SBTI activities in 6 out of the 8 soymilks measured were higher than the level (10%) considered being safe.