Optimizing bio-ethanol production from sweet sorghum stalk juice using Saccharomyces cerevisiae, finger millet malt and sorghum malt by taguchi method

Abstract

Bio-ethanol is a viable alternative source of energy because it is renewable and environmentally friendly. However, the cost of its production remains prohibitive due to the high cost of feedstock, more so, food insecurity is caused if food crops are used. Studies on the use of sweet sorghum stalk juice for bio-ethanol production are ongoing due to its adaptability to different climatic and environmental conditions coupled with its high ability to accumulate high concentration of fermentable sugars within its stalk. Most of these fermentations are carried out using Saccharomyces cerevisiae as the main yeast which is obtained industrially. However, due to high cost of producing industrial yeast for bio-ethanol production, there is need to establish an alternative source of yeast that would be of low cost but of good quality and efficient in bio-ethanol production. The use of fossil fuels causes environmental pollution, adverse human health effects and high cost of production therefore there is an urgent need to produce bio-ethanol which is a cleaner alternative source of fuel. The objectives of this study were to find: the best sweet sorghum variety with the highest °Brix, optimum bio-ethanol production temperature, pH, yeast to substrate ratio and reaction time, compare the effectiveness of Saccharomyces cerevisiae, finger millet malt and sorghum malt as sources of enzyme for bio-ethanol production and finally characterize the bio-ethanol produced in terms of calorific value, pH, density and flame test. Five sweet sorghum varieties namely: IESV 92001 DL (V1), NTJ (V2), 15233 IESV (V3), 92008 DJ (V4) and IESV 92028 DL (V5) were planted at Jaramogi Oginga Odinga University of Science and Technology experimental farm. °Brix content of their stalk juice was determined using a digital refractometer (Model MA871, Milwaukee Co. Ltd., Romania) at the 11th to 16th week after sowing. The highest °Brix for all the genotypes, indicated by ANOVA, was registered at the 15th week where V1 had the highest °Brix of 22.07 (P≤𝟎.𝟎𝟓). It was then harvested for bio-ethanol production. Fermentation factors were optimized using L16 (44) Taguchi approach. The optimal conditions were temperature of 30 ℃, 36 hours, pH 5 and yeast to substrate ratio of 5 g/L using Saccharomyces cerevisiae while optimal conditions of pH 5, temperature of 35 ℃, 48 hours and yeast to substrate ratio of 5 g/L on using sorghum malt finally with finger millet malt the optimal conditions were yeast to substrate ratio of 5 g/L, pH 5, 48 hours and temperature of 30 ℃. Kinetics of the fermentation reaction for Vmax and Km were 0.35 g/L/h and 12.56 g/L respectively using finger millet malt, while a Vmax and Km of 0.34 g/L/h and 14.09 g/L obtained for sorghum malt and a Vmax and Km of 0.69 g/L/h and 13.96 g/L with Saccharomyces cerevisiae. Fermentation of sweet sorghum stalk juice with the 3 sources of enzyme followed Michaelis Menten model. Both the optimized and kinetic parameters were within reported literature values and therefore results show that finger millet malt has a greater potential, as a substitute yeast source in application in bio-ethanol production industries. In terms of characterization of the bio-ethanol produced the calorific value, pH, density and flame test were found to be 8740±29 kcal/kg, 6.3±0.2, 0.895±0.076 g/cm3 and a blue flame produced respectively. The bio-ethanol produced burnt with a hot blue flame hence a viable alternative fuel for domestic cooking. This information is important to policy makers in designing ways that can help implement it as a clean source of cooking fuel and at the same time create employment to the people living in the rural areas.