Effect of Independent Variables on the Responses
The significance of each term was determined using the F ratio and P value as shown in Table 3. The significant and magnitude of the coefficients indicated the effect of the variables on the response and the negative coefficient means a decrease in response when the level of variable increased while a positive coefficient signifies an increase in the response [14]. Furthermore, a significant interaction indicates that the level of one interactive variable may increase while the other may decrease for a constant value of the response [15]. Tables 3 and 4 showed that all linear terms, two quadratic terms (fermentation time and temperature and all interactive terms had significant (P < 0.05) effect on the pH and TTA. In Figure 1, the 3D response surface plot of pH and fermentation time displayed a linear effect on the response where pH decreases with increase in fermentation time. This was evident in the regression equation where fermentation time had a negative coefficient which indicated a decrease in the response. The 3D response surface plots of TTA during fermentation are shown in Figure 2. The TTA increased to an optimum level when the fermentation time and mass of root individually increased to 105 h and 3kg respectively, followed by a decline with further increase in each of these variables (Figure 1). The decrease in TTA as fermentation increased above 30oC could be due to the fact that the optimum fermentation time have exceeded for maximum TTA. It could be that microorganism was not active enough to produce more acid. This is in accordance within the range reported by Oyewole and Ogundele [11] for fufu (0.08 – 0.22)% lactic acid.