Optimization of Ethanol Production Process from Cassava Starch by Surface Response
Fig. 5. Fermentation process with Saccharomyces cerevisiae, a) Contour plot showing the highest yield region, b) Response surface showing a maximum region to conversion ethanol.
alcohol increased proportionally as with the concentration of glucose in the range previously analyzed. However an exces- sive concentration of glucose can stop or prevent fermentation because an osmosis process would begin . The optimal values of ethanol concentration were obtained with CTFS of 100 g/L and an Ay of 3 g producing ethanol with 38.83 % Alc/vol, 57 % higher than those obtained using sorghum and potato [22, 23, 24].
To study the influence of concentration of the total fer- mentable sugar (CTFS) and the amount of yeast (AY) on ethanol content (EC) we used the NCSS-2004 software, a second-order Taylor-series model was used. Equation 6, shows the math- ematical model obtained, Ay was a variable no significant, with an adjustment of 91.21 %. This established that the opti- mal value of AY (2.5 g) was sufficient to achieve an optimal investment of glucose into ethanol.
Kinetic model of acid hydrolysis of cassava starch proposed together with the experimental design approach allowed deter- mining the optimal conditions at hydrolysis and fermenta- tion steps, from second-order polynomial, contours plot and response surface fits. Using the optimal conditions obtained (under hydrolysis and fermentation), in semi-continuous pro- cess, substantially increased the final concentration of ethanol, reducing the total process time.
Kinetic model of the acid hydrolysis
EC = -28.27 + 1.3385CTFS
Optimized semi-continuous process for production of ethanol
A starch solution with a concentration of 170 g/L at pH of 0.8 was prepared, using sulfuric acid 20 % (w/w). The solution was brought to the boil at a temperature of 98 ºC with reflux at atmospheric pressure and a agitation speed of 400 rpm to perform the conversion of cassava starch to total fermentable
From the optimal conditions obtained (both hydrolysis and fermentation), acid hydrolysis was realized obtaining a final conversion of 89.84 % the TFS initially considered, which compared with the value predicted by the kinetic model of equation 4 which shows a deviation of 14 % from the experi- mental value (Figure 6), which validates the proposed kinetic model fit. Subsequently it was carried out the fermentation yielding ethanol concentration of 49.76 % which equates to an increase above 60 % higher than in other investigations [2, 9, 25, 26] which considered different conditions (bacterium, type of enzymes, yeast species, process design), demonstrating the optimization of the process.
Fig. 6. Representation of kinetic model for the degradation of starch at different initial concentrations.