3.4. Water solubility index (WSI)
The results of water solubility index (WSI) for different experimental conditions are showed in Table 2. Thewater solubility index (WSI) of extrudates ranged between 2.7 g/100 g (protein content50 g/100 g, moisture content 40 g/100 g (wb), barrel temperature160 C) and 6.8 g/100 g (protein content 60 g/100 g, moisture content 45 g/100 g (wb), barrel temperature140 C). The response surface plot of water solubility index were likewise prepared but not shown. Under lower barrel temperature (140 C), the WSI continued to decrease as the feed moisture and protein content decreased. With higher barrel temperature (180 C) theWSI increased with a decrease in the feed moisture and with an
increase in the protein content. When the barrel temperature is in the middle (160 C), WSI showed the lowest value in the middle part. ANOVA analysis demonstrated that the model was significant (p < 0.05). Again barrel temperature and protein content significantly affected the WSI through a linear model. All three factors were significant for WSI with quadratic model, and the interaction between barrel temperature and moisture content was also significant (p < 0.05). The regression equation, showing empirical relationship between water solubility index (YWSI) and the processing variables are shown in Table 2.