Abstract
The energy demand in Indonesia increases due to a significant growth in population, yet fossil fuel storage as the main non-renewable energy source has been significantly depleted. There are a lot of researches on renewable energy; one of the most prominent is the development of bioethanol as a result of fermentation of the sugar. One of the most abundantly available sugar sources in Indonesia is the palmyra sap. Palmyra sap from palmyra tree (Borassus flabelliffer) is a seasonal and low priced drinking juice in tropical countries such as Indonesia and it is highly available in a coastal area of Indonesia. Currently in Indonesia, ethanol is mainly produced from molasses or cassava which can hamper the needs of crop plants. Palmyra sap can be used as a cheap alternative of ethanol feed stock and in the same time can enhance its economic value. Conventional fermentation is usually carried out with the help of Baker’s yeast (Saccharomyces cerevisiae), but recent study shows that Gram-negative bacteria Zymomonas mobilis could be a promising alternative of fermentative microorganism. This research aims to determine and compare physical parameters needed in the fermentation of palmyra sap by Saccharomyces cerevisiae and Zymomonas mobilis to obtain optimum concentration of ethanol. Using a batch fermentation process in a 1.8 L bioreactor with 1 L working volume, these microorganisms were cultivated in sterilised palmyra sap and the different physical parameters applied are pH and inoculum concentration. Response surface methodology was generated based on the result of 13 runs generated by central composite design (CCD). Response surface methodology was applied to determine the optimum conditions for the maximum yield of ethanol with the variation of temperature and pH. The highest yield of ethanol concentration using Saccharomyces cerevisiae was obtained at pH 5.05 with inoculumconcentration of 3,973,760 cell.mL-1/gL-1glucose. The model showed that the value of R2 (0.9201) was highand from ANOVA analysis the model is said to be significant. Highest concentration of ethanol obtained by fermentation is 85.68 g/L, with yield of 0.49 and error compared to predicted value is 9.05 %. The highest yieldof ethanol concentration using Zymomonas mobilis was obtained at pH 5.57 with inoculum concentration of 2,660,000 cell.mL-1/g.L-1glucose. The model showed that the value of R2 (0.9290) was high but analysis of ANOVA shows that the model is not significant. Highest concentration of ethanol obtained by fermentation is12.75 g/L, with yield of 0.49 and error compared to predicted value is 0.65 %. The study concluded that fermentation with Saccharomyces cerevisiae gives better model of optimisation but Zymomonas mobilis gives better result of ethanol production.
