Effect of soybean processing on cell wall porosity and protein digestibility

Apart from the presence of antinutritional factors, digestibility of soybean proteins is limited in intact cells by cell wall permeability to proteolitic enzymes. Food processing may modulate cell wall permeability and hence the accessibility of protease enzymes to intracellular proteins. In this study, soybeans were processed in various ways, e.g. cooking applied alone or with either germination or fermentation processes, and the modification in cell wall permeability was investigated using confocal microscopy to visualize the penetration of FITC-dextran probes into isolated cells/cell clusters. Diffusion of fluorescently labelled trypsin into cells and cell clusters was also monitored. Microscopy observations showed that fermentation and germination as well as proteolitic enzymes increase the permeability of boiled soybean cotyledon cells. The diffusion of trypsin into all the isolated cells was observed at an early stage of simulated in vitro digestion, whereas diffusion into cell clusters was delayed due to a bigger size and limited permeability of cell clusters. A modest, although significant, increase in protein digestibility was observed when boiling was combined with fermentation or germination likely due to pre-digestion of storage proteins and inactivation of trypsin inhibitors. This study highlights the positive role of fermentation and germination in improving protein digestibility in soybeans but overall suggests that cell wall permeability to trypsin plays a minor role in the extent of protein digestion of intact soybean cells.