Pre-treatment of Soy Okara Using Multi-enzyme Complex on Sugar Extraction and Its Effect on Chemical Composition, Morphological Structure, and Antioxidant Capacity

Okara is a highly perishable food processing by-product from soymilk and tofu manufacture. The present study aimed to optimize the pre-treatment of okara on the extraction of reducing sugars by applying a multi-enzyme complex, Viscozyme, containing a variety of carbohydrases to hydrolyze the okara cell walls. The chemical composition and the morphological structure of enzymatic-treated okara (ETO) were scrutinized. Enzymatic hydrolysis pre-treatment was carried out under different enzyme concentrations (5–8%), temperatures (50–60 °C) and pH values (3–4) according to a central composite rotatable design, in which the ranges were chosen based on preliminary study. The optimal conditions for maximum reducing sugars extraction (4.78 mg/mL) were 8.0% (w/w) of enzyme, 50 °C and pH 3.0 for 4 h hydrolysis time, representing a 158.33 times increment, compared to the sample without enzymatic pre-treatment. The chromatogram from high performance liquid chromatography (HPLC) revealed that the carbohydrates such as glucose, fructose, raffinose and stachyose increased after hydrolysis. Surprisingly, arabinose which was not detected in raw okara (RO) was present in ETO. Scanning electron microscopy (SEM) confirmed the highly structured matrix of okara was unfolded and broken by the action of enzyme pre-treatment while energy-dispersive X-ray spectroscopy (EDS) imparted the changes of elemental components in ETO. The ETO showed higher antioxidant capacity than the control by DPPH, ABTS and FRAP tests. Enzymatic hydrolysis proved to be an efficient way to enhance the physicochemical properties of okara for possible uses, making this residue into a newly added value product.