Cereal Chem 52:85 - 92.  |  VIEW ARTICLE
Biological Threshold Levels of Soybean Trypsin Inhibitors by Rat Bioassay.

J. J. Rackis, J. E. McGhee, and A. N. Booth. Copyright 1975 by the American Association of Cereal Chemists, Inc. 

In conventional processing of soy flour by moist heat (toasting) an apparent destruction of at least 95% of the trypsin inhibitor (TI) is observed when assayed by various procedures. A more accurate and reproducible method, particularly suitable for determining TI activity of heat-processed samples, has been used in this study. Toasted soy flour has much greater TI activity when assayed by the modified procedure because it is now possible to measure the activity of insoluble TI. The destruction of TI activity in defatted soy flakes by live steam at 100 C. was determined by the modified procedure. Samples containing graded levels of TI activity were fed to rats. As TI activity decreased, weight gain, protein efficiency ratio, and nitrogen digestibility rapidly increased and pancreatic hypertrophy decreased. The adverse effects of raw meal as measured by these four biological parameters disappeared at different levels of TI activity in the diet. No pancreatic hypertrophy occurred in rats fed soy flour in which only 55 to 69% of TI activity had been destroyed. Maximum body weight, protein efficiency values, and nitrogen digestibility were obtained with rats fed soy samples in which 79 to 87% of the inhibitors were inactivated. Additional heat treatment did not further improve nutritive value. Our data do not support claims that residual TI in properly processed soy flour has antinutritional properties.