Optimization of Anaerobic Fermentation conditions for Bioethanol production from Banana Peels using yeast in a still reactor

Abstract

Limited scientific information on optimal biofuels production conditions to both the small and medium scale enterprises has led to non-economical and inefficient processes hence uncompetitive low-grade biofuels. Fermentation process behavior prediction and optimization is very crucial especially while using relatively low fermentable sugars substrates in simple and sustainable bioreactors such as the still batch anaerobic systems. In the current work, banana peels derived from three comparative cultivars were dried, ground into a fine powder to pass through a 1 mm screen, and then hydrolyzed using 60% concentrated H 2 SO 4 at 50 o C. Bioethanol was produced by anaerobic fermentation of these hydrolysates using Saccharomyces cerevisiae. Sterilized Erlenmeyer flasks fitted with non-return air valves were used as laboratory scale still reactors. Fermentation systems were subjected to various conditions based on half factorial Central Composite Rotatable Design (CCRD). Total Reducing Sugars (TRS) concentrations and bioethanol yields analyses were done by the Dubois and Gas Chromatography methods respectively. Various mathematical models estimating the behavior of these simple fermentation systems were developed, analysed, and statistically revised for future predictions and enhancement of bioethanol yield. Dried banana peels powder derived from the three banana cultivars under study presented average TRS contents of 15.80% w/w and 36.21% w/w before and after concentrated acidic hydrolysis respectively. Ash rangedbetween 5.67% and 7.85% w/w in various banana cultivars used.Optimum bioethanol yields of 13.93 ml/L, 12.59 ml/L, and 13.09 ml/L from Sialamule, Uganda Green, and Ngombe respectively were obtained at 180 g/L Substrate Concentration, 35 o C Fermentation Temperature, 5.5 Initial medium pH, 2 g/L Yeast Concentration, and 120 hours Incubation Period for all the three banana cultivars under study. These corresponded to TRS degradation of 31.57 g/L, 29.88 g/L, and 30.30 g/L in Sialamule, Uganda Green, and Ngombe respectively. Lowest bioethanol yields of 1.51 ml/L, 1.33 ml/L, and 1.44 ml/L from Sialamule, Uganda Green, and Ngombe respectively were obtained at 84.86 g/L Substrate Concentration, 35 o C Fermentation Temperature, 5.5 Initial medium pH, 2 g/L Yeast Concentration, and 120 hours Incubation Period for all the three banana cultivars under study. These similarly corresponded to TRS degradations of 3.28 g/L, 2.85 g/L, and 2.85 g/L in Sialamule, Uganda Green, and Ngombe respectively. From the Analysis of Variance (ANOVA) and Correlation Coefficients (R 2 ), there were strong indications that the set of mathematical models predicting bioethanol yields from the three different cultivars were the same and could be used alternatively. The student test proved a significant interaction between both the substrate concentration and incubation temperature across all the mathematical models developed. Other statistical features such as the R 2 and f- test of various regression models developed also showed that they were significant in estimating both bioethanol yields and TRS degradations associated with this simple fermentation process.Concentrated acidic hydrolysis raised TRS concentrations in these peels powder by more than twice the free TRS.Bioethanol yields closely coincided with TRS degradation in various experimental runs and showed that optimal fermentation conditions is important to achieve higher yields.This study recommends quantitative measurement and elimination of various inhibitors contained in banana peels hydrolysates thus enhancing bioethanol yields. Further, economical and cost analysis and implementation of a pilot plant for bioethanol production from banana peels in the country and dissemination of results from this to small and medium scale bioethanol manufacturers to help them better their yields in such fermentation processes.