Abstract

This study’s aim was to compare the effects of various processing, including non-fermentation and fermentation technologies on polyphenols and antioxidant properties in traditional soy foods by using the same Prosoy soybean cultivar. Soybean was processed into non-fermented products (soymilk by two technologies, and tofu (by two technologies), and fermented products (natto aged for 2, 4, and 6 days; tempeh aged for 2, 4 and 6 days;  and soy-yogurt aged for 2, 4, 6, 8 days). Total phenolic content (TPC), total flavonoids content (TFC), condensed tannins content (CTC), individual phenolic acids, isoflavones, DPPH, ferric-reducing antioxidant power (FRAP), and oxygen radical absorbance capacity (ORAC) were measured. Results showed natto had the highest TPC among all products.  Cooked soymilk(s) had the highest CTC (1.1-0.9 mg CAE/g) among all products. In general, fermented products showed higher (p <0.05) DPPH and FRAP antioxidant activities compared to non-fermented soy foods.  ORAC values of natto and cooked soymilk ranked higher (p<0.05) than all products. Aging did not affect the total phenolic acid content in natto, tempeh or yogurt. Total isoflavone content of soymilk, yogurt, filled tofu (2027-2248 µg/g) was higher (p < 0.05) than natto and tempeh (1552.2-2013.7 µg/g). Significant changes occurred in the phenolic and antioxidant properties of soybean products and the extent of changes was dependent upon processing methods.  Fermented products and non-fermented soy foods have unique compositions of isoflavones, phenolic acid compounds and antioxidant activities.  The study contributes to new knowledge for soy consumers to choose different types of products for enhancing health, and for the processing industry to optimize products for the consumers.