Effect of Mild Chemical and Enzymatic Treatments of Soybean Meal and Soybean Trypsin Inhibitors on Their Nutritive and Biochemical Properties

Abstract

Raw soybean meal (RSBM) was extracted with HCl (pH 4.2) yielding an insoluble residue (SBM_4.2) with a higher protein content. Although SBM_4.2 retained about 50% of the original RSBM trypsin-inhibiting activity and almost no hemagglutinating activity, it was only slightly superior to RSBM when added to diets of chicks and rats. Fraction F_4.2, the acetone precipitated and vacuum-dried extract (pH 4.2) of RSBM, when added to a properly heated soybean meal diet, impaired growth only to the extent of one-third of the growth-impairment caused by SBM_4.2 although F_4.2 contains about 50% of trypsin-inhibiting and most of the hemagglutinating activities of RSBM. The stability of the trypsin-inhibiting and hemagglutinating activities of RSBM, SBM_4.2 and three crude soybean trypsin inhibitors towards HCl (pH 1.6) and peptic digestion were examined in an attempt to elucidate the fate of the two detrimental activities prior to pancreatic digestion. Analyses of residual trypsin-inhibiting and hemagglutinating activities showed that hemagglutinating activity was completely destroyed even when incubated with HCl alone, whereas trypsin-inhibiting activity was destroyed to a varying extent, depending upon the material examined. The trypsin-inhibiting activity of SBM_4.2 was found relatively more resistant towards HCl and pepsin, thus leading to the conclusion that its higher than expected growth-depressing activity might be due partly to the presence of a comparatively more resistant trypsin-inhibiting activity. Growth experiments with chicks maintained with diets including RSBM, SBM_4.2 and properly heated soybean meal supplemented by various methionine levels showed no interaction between the various soybean meals and the level of methionine added.