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Optimization of Ethanol Production Process from Cassava Starch by Surface Response - page 5 / 6

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

sugar (TFS). Once filtered the hydrolysate was neutralized with NaOH, centrifuging the solution to remove the salts gen- erated in the neutralization process.

It employed a HPLC chromatograph Dionex ICS-3000 for determination of TFS. They were injected 2.5 mL of this one using a mobile phase of 200 mM NaOH with a flow of 0.25 mL/min at room temperature with a CarboPacTM PA1 column (2 x 200 mm) using an ED40 electrochemical detector with the AOAC method 996.4 [27]. External standard was used to validate the data.

The model and kinetic data were important in the design, development and operation of carbohydrate conversion pro- cesses [13]. The proposed model was based on an irreversible first order homogeneous reaction for hydrolysis with H2SO4, where the average constant was a function of the concentration of starch (CS) and the time (t). The saccharification reaction is obtained by equation 7.

CS

k1

CTFS (7)

Getting the rate of equation for species A from the equa- tion 7 is:

d[CS ] dt

r C S

k1[CS ]

(8)

Getting the integrated solution of the equation 8 it was obtained an exponential drop of starch and its corresponding exponential increase in TFS

[CS] = [Csi] e-kt

(9)

Design of the Experiment

Acid hydrolysis and fermentation process were developed using a 22 experimental designs with three central points. This statistical technique allowed evaluating the influence among the most important factors as well as the significant interac- tion, using a small number of trials.

Seven tests with three central points were made, Figure 7, shows the design of the experiment. Each experiment was made randomly so that errors are independently distributed. Specific values to acid hydrolysis and fermentation process are shown on Table 4.

Y

(-1,1)

(0,0)

(1,1)

X

Acid hydrolysis X = concentration of cassava starch Cs (g/L) Y = Agitation speed, AS (rpm) Fermentation process X = Conversion of total fermentable sugar CTFS (g/L) Y = Amount of yeast, Ay(g)

(-1,-1)

(1,-1)

Fig. 7. Factorial Design 22 with 3 central points used both in acid hydrolysis and fermentation process.

Leticia López Zamora et al.

Table 4. Factorial design data used in acid hydrolysis and fermenta- tion process.

0

0

170

400

80

2.5

0

0

170

400

80

2.5

0

0

170

400

80

2.5

-1

-1

150

200

60

2.0

1

-1

190

200

100

2.0

-1

1

150

600

60

3.0

1

1

190

600

100

3.0

Acid hydrolysis

Fermentation process

Cs (g/L)

AS (rpm)

y CTFS (g/L) A (g)

XY

Optimization of the acid hydrolysis process

To achieve the hydrolysis process it was employed a 22 experi- mental design with three central points for considering two independent variables: 1) concentration of cassava starch (CS) and 2) agitation speed (AS), being the response variable the percent conversion of starch to total fermentable sugar (CTFS).

The starch was hydrolyzed for 4.5 h, according to kinetic analysis of the process, to 98 ºC to a pH 0.8 using H2SO4 to 30 % (w/w). Three levels of concentrations of starch (150, 170 and 190 (g/L)) and three agitation speeds (200, 400 and 600 rpm) were considered. The TFS was quantified using 996.4 AOAC method´s.

The experiments were randomly conducted, the order is shown under Table 5.

Optimization of fermentation process

Completed the saccharification process, the syrup was filtered, adjusting to pH 5 with NaOH 5N and considering the concen- tration of TFS at three different levels (100, 80, 60 g/L). The f e r m e n t a t i o n m e d i a w a s e n r i c h e d w i t h n i t r o g e n ( ( N H 4 ) 2 S O 4 phosphate (KH2PO4) and magnesium (MgSO4) at concentra- tions of 0.96 g/L, 0.02 g/L and 0.5 g/L, respectively. A total volume of 800 mL was considered. Subsequently, the medium was sterilized by autoclave (121 ºC and 1.2 kg/cm2 pressure) during 15 min. 200 mL from the fermentation media were taken adding the mass of yeast Saccharomyces cerevisiae to employ (2, 2.5 and 3 g) activated by aeration for 15 min at a ) ,

Experiment

Cs (g/L)

AS (rpm)

1

170

400

2

150

200

3

170

400

4

190

600

5

170

400

6

150

600

7

190

200

Table 5. Experimentation

sequence for

acid hydrolysis process.

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