Impact of the incorporation of soybean processing by-products (okara) on the physicochemical properties of carrageenan-based soybean-tofu

Okara (solid soybean processing by-product) can improve the properties of tofu gel. However, there are few studies on the effect of okara on tofu gel in the presence of a structurally-reinforcing hydrocolloid. Therefore, this study investigated the effect of okara on the properties of κ-carrageenan-reinforced tofu. Tofu formed by two types of coagulants (glucono-δ-lactone-GDL acidification and CaCl 2 electrostatic flocculation) were studied in that aspect. The water holding capacity (WHC) of GDL tofu was lowest when 30 % okara was added, while CaCl 2 tofu showed a WHC maximum at 10 % okara. When 50 % okara was added, the hardness of GDL tofu and CaCl 2 tofu decreased from 565 g to 409 g–182 g and 221 g, respectively. The crystallinity of GDL tofu decreased from 14.99 % to 10.91 % after adding 50 % okara. Fourier transform infrared spectroscopy indicated for okara-added tofu a stronger absorption peak at 1050 cm −1 and a shift in the amide I band, suggesting that okara affected the secondary structure of tofu proteins. Disulfide bonds and hydrogen bonds were dominant in the tofu gel, accounting for 78.04 ± 0.92 % in the tofu with 30 % okara. These results can provide theoretical guidance for the valorization of okara in tofu-type products.