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

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 ± 3.48 μg/g wet weight in liquid formulas or 137.5 ± 3.0 to 229.1 ± 2.3 μg/g dry weight in powdered formulas and 49.8 ± 0.5 to 279.9 ± 3.4 μg/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.

► Isoflavone contents in soymilks and soy infant formulas measured are quite variable.
► Trypsin inhibitor activities in most soymilks measured are higher than expected.
► Best practices for soymilk should be developed for better quality products.