Monday, July 27, 2009
P16
Use of response surface methodology in lactic acid production: Focus on medium supplementation, temperature and pH control
Two response surface methodologies involving central composite designs were successfully applied to evaluate the effect of cheese whey, corn steep liquor, ammonium sulfate, temperature and pH control on D(-)lactic acid fermentation by Lactobacillus sp. LMI8 isolated from cassava flour wastewater. In the first central composite design, corn steep liquor and ammonium sulfate were investigated as low cost nitrogen sources in combination with other components to substitute yeast extract for economical production. The experiments were performed in 250-mL Erlenmeyer flasks containing 50 mL of mineral medium at 200 rpm and 37 ± 1°C for 48 h of fermentation and at an initial pH of 6.7. The best results were obtained with 55 g/L of lactose, 15 g/L of corn steep liquor and 5.625 g/L of ammonium sulfate. At the maximal point, the D(-)lactic acid concentration reached 18.68 g/L. After defining the optimal nutritional conditions for D(-)lactic acid production, a second central composite design was performed to determine the extent to which temperature and pH influence lactic acid synthesis with the aim of improving the fermentation process. The statistical analysis revealed that the isolated terms of these two variables had significant effects. A second-order polynomial regression model determined that the maximal D(-)lactic acid production of 52.37g/L would be obtained when the optimal temperature and pH were 39.64 ºC and 5.9, respectively. Comparing the D(-)lactic acid production in shake-flask fermentation, there was an increase of 180% after 30 h of processing, with a conversion efficiency of about 95% of the initial reducing sugars.
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