Immunohistological visualization of the effect of soybean processing on Kunitz trypsin inhibitor

Anti-nutritional factors such as trypsin inhibitors represent a major hindrance in the usage of soybean-based products as feed for non-ruminant feedstock. Here we studied the effect of four different methods of soybean processing to generate soybean meal (oil-extraction with a solvent, toasting, mechanical extraction, and fermentation) on the Kunitz trypsin inhibitor and soy protein. We visualized both proteins in situ using indirect immunofluorescence on resin sections with specific rabbit-raised polyclonal antibodies and laser-scanning confocal microscopy We characterized their cellular localization in raw soybeans and show that Kunitz trypsin inhibitor and soy protein occupy different domains related to protein globoids. Semi-quantification of the signal intensities showed an only moderate reduction in detectable Kunitz trypsin inhibitor epitopes after any type of processing. On the other hand, a considerable loss of trypsin inhibitor activity was shown with a standardized assay. The effect of toasting can be seen as significantly increased green fluorescence possibly due to the production of advanced glycosylation end products. This work introduces antibody-based in situ visualization as a new promising way to study anti-nutritional factors in soybean-derived products. •It is possible to study the in situ distribution of Kunitz trypsin inhibitor and soy protein in raw and processed soybean using specific antibodies.•Kunitz trypsin inhibitor localizes to the rim of protein bodies.•Any type of common processing of soybean reduces but does not completely remove Kunitz trypsin inhibitor antigen.