Applying nanotechnology to okara for developing soy protein gel-based foods

Okara, the residue of the production of tofu and soybean milk, is rich in dietary fibers (DFs) and phytochemical components such as soy isoflavones and soyasaponins. Despite its nutritive value, okara is scarcely used as a food source as the DFs in okara are mostly insoluble and have adverse effects on food texture. Thus, improving the physicochemical properties of okara for various applications in foods is of great importance. Here, okara were atomized using a wet-type grinder (WG) that is commonly used to produce nanocelluloses (NCs). We treated 2 wt% okara with the WG at different passages (1−5 times). The particle size distribution (PSD) and viscosity of WG-treated okara decreased and increased, respectively, with passages up to 3 times. In addition, we treated okara with the WG at different concentrations (1−3 wt%). The viscosity of WG-treated okara increased with increasing concentrations. The WG-treated okara also dispersed in water homogeneously at more than 2 wt% after 24 h, whereas untreated and 1% WG-treated okara did not. The soybean protein isolate (SPI) gels were not formed without sodium chloride (NaCl). The breaking stress and strain increased with NaCl concentrations up to 0.2% and then decreased. The addition of 1% okara treated at different passages by the WG increased the breaking stress and strain of SPI gels with increasing of passages. The breaking stress and strain of SPI gels were also increased with increasing WG-treated okara concentrations up to 1%. These results indicate the possibilities that the NC technologies can improve the physicochemical properties of okara and are useful to develop protein gel-based foods.