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# J. Mex. Chem. Soc. 2010, 54(4)

Leticia López Zamora et al.

200

- 0 C_{TFS }= 10.879 + 0.426Cs_{i }+ 0.121A_{s }- 2.02E 4 C s i 2 - 3 . 7 8 E - 0 5 A s 2 - 4 . 6 6 E - 0 4 C s i A s

(3)

## Cs (g/L)

150

100

Experimental Data Kinetic Model Regression Model

The methodology of the response surface (RSM) was used in order to determine the optimal working region. The contour graph (Figure 4a), shows that the highest yield was obtained in the region from 177 to 233 g/L, and from 510 to 760 rpm. Figure 4b clearly shows a saddle point whereas the central experimentation conditions, while the conditions of higher concentration and agitation present the maximum conversion of starch to glucose.

50

0

TFS optimal production from cassava starch through acid was obtained for a starch concentration of 190 g/L of starch and an agitation of 600 rpm with sulfuric acid (90.5 % conver- sion). The results obtained in this work were higher than those reported in [20] who employ 1.5 h more in the process, reach- ing a conversion of 4.3 % lower.

0

1

2

3

4

5

## Time (h)

# Optimization of the fermentation process

Fig. 2. Behavioral comparison of experimental data against kinetic (equation 1) and the regression models (shown on equation 2).

### 20 0

15 0

[150 g/L] [170 g/L] [190 g/L]

Statistical software NCSS-2004 was employed to analyze the results, considering the ethanol content (E_{C}) in % Alc/vol as the response variable. The trend of optimization was identified by initially adjusting (equation 4) and secondly (equation 5), obtaining an adjustment of 82.56 % and 91.43 % respectively, which guarantees the reliability of the correlations.

## Cs (g/L)

10 0

# E_{C }= 11.3514 + 0.6599A_{y }+ 0.2685C_{TFS }

(4)

50

+ 13.0177A 2 y - 0.205A_{y}C E C = - 4 . 7 2 9 9 - 6 2 . 7 8 8 9 A y + 2 . 6 9 1 5 C T F S TFS - 0.0418C 2 TFS

(5)

0

0

1

2

3

4

5

6

### T im e (h)

Fig. 3. Kinetic behavior of starch degradation at different initial con- centrations.

A study was conducted in the optimal region from the contour plot (Figure 5). It was possible to observe an opti- mal region by considering a mass of yeast Saccharomyces cerevisiae between 2.78 and 3.34 g and a glucose concentra- tion between 94.5 and 122.5 g/L. Figure 5b shows that the %

(a)

(b)

Fig. 4. Behavior of acid hydrolysis process using the 22 experimental designs with 3 central points a) graphical contours showing the highest yield region, b) Surface Response showing a saddle point in the center and maximum conversion at the dotted region.