3. Results and discussion
3.1. Chemical composition of raw and pretreated okara
The composition of okara on a dry weight basis was 2.5% moisture, 45.5% carbohydrate, 27.4% protein, 13.6% fiber (lignin and pectin), 9.5% fat/oil, and 4.0% ash (values are a mean of three replicates). The carbohydrate in raw okara consisted mainly of glucose (17.2%), galactose (12.1%), arabinose (5.8%) and xylose (4.0%) (Table 1) (values are a mean of three replicates). The protein was considered valuable byproduct. Mielenz et al. [17] demonstrated the soybean hull as biomass to bioethanol production and proposed the remained protein would be suitable for bovine livestock. However, we mainly focused on suitable in-house enzyme production, carbohydrate enzymatic hydrolysis and ethanol fermentation in this study. After an autoclave pretreatment, the raw carbohydrate exhibited little degradation. Glucose and galactose were affected the least by the pretreatment with losses of only 0.2 and 0.1%, respectively, because the autoclave pretreatment was conducted under mild conditions, without the addition of any chemicals or the use of mechanical force. This process may have allowed the okara to swell, which improved the subsequent enzymatic hydrolysis by increasing the okara surface area and making it more accessible to hydrolytic enzymes [23].