Preparation of unheated soy protein isolates with low trypsin inhibitor content

The nutritional quality of soy protein products is affected by the processing conditions employed in their manufacture. Heat treatment during processing serves to inactivate the inhibitors of trypsin and chymotrypsin, enzymes which play a key role in the digestion of protein in animals. In the processing of defatted soy flour to prepare a soy protein isolate, generally no heat treatment is used. Instead, a high percentage of the trypsin inhibitors (TI) is physically removed in the whey fraction. However, depending on their mode of preparation, soy isolates may contain trypsin inhibitor activity as high as 40% of that found in raw soybeans. Using "salting in" techniques we found that a higher percentage (97.7%) of the TI was solubilized and removed with the whey fraction when the protein curd was precipitated from 0.1 N NaCl solution at pH 5.4. Using membrane techniques for the separation of TI from non-TI-protein, best results were obtained with 0.1 N. MgCl2 where 79% of the TI was removed in the permeate.