Ethanol production from sweet sorghum / Mutepe R.D.
Mutepe, Rendani Daphney
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The use of fossil fuels contributes to global warming and there is a consequent need to resort to clean and renewable fuels. The major concerns with using agricultural crops for the production of energy are food and water security. Crops that do not threaten food security and that can be cultivated with a relatively low amount of water and produce high yields of fermentable sugars are therefore needed. Sweet sorghum is a fastgrowing crop that can be harvested twice a year and that can produce both food (grain) and energy (sugar juice from stems). Sweet sorghum bagasse can also be utilised for ethanol production. The aim of this study was to determine the sugar content of different sweet sorghum cultivars at different harvest times, and determine the cultivar that will produce the highest ethanol yield at optimized fermentation conditions. Sweet sorghum bagasse was also pretretated, enzymatic hydrolysed and fermented and the best pretreatment method and ethanol yield was determined. In this study, sweet sorghum juice, which mostly consists of readily fermentable sugars (glucose, sucrose and fructose), as well as the bagasse obtained after juice extraction, were converted to bio–ethanol. Sweet sorghum juice was fermented to ethanol using Saccharomyces cereviciae without any prior pretreatment. The effect of pH (4–6), yeast concentration (1–5g.L–1), initial sugar concentration (110–440g.L–1) and the addition of a nitrogen source (urea, ammonium sulphate, yeast extract and peptone) on the ethanol yield was investigated. The pretreatment of bagasse using sulphuric acid (3wt %), and calcium hydroxide (0.2g/g bagasse), followed by enzymatic hydrolysis using Celluclast 1.5L (0.25g/g bagasse), Novozyme 188 (0.24g/g bagasse) and Tween 80(1.25g.L–1) were adapted from Mabentsela (2010). Fermentation was done using Saccharomyces cerevisiae, but it was unable to ferment the xylose sugar. The results show that the USA 1 cultivar contains the highest sugar content at 3 months. An ethanol and glycerol yield of 0.48g.g–1 and 0.05g.g–1 was observed respectively at a pH of 4.5, a yeast concentration of 3wt%, initial sugar concentration of 440g.L–1 and when ammonium sulphate was added to the fermentation broth as nitrogen source. The glycerol yield formed as a by–product during fermentation and at a maximum ethanol yield was 0.05g.g–1. The glucose yield obtained from sulphuric acid, base and ultrasonic wave pretreatment was 0.79g.g–1, 0.62g.g–1 and 0.62g.g–1 respectively. The glucose yield obtained after each type of pretreatment was higher than that obtained for unpretreated bagasse, which was 0.55g.g–1. Base pretreatment, ultrasonic wave pretreatment and unpretreated bagasse also contained fructose at the end of enzymatic hydrolysis. Base, sulphuric acid pretreatment disrupted the crystal structure of cellulose and increased the available surface, and therefore cellulose was easily accessible for enzymatic hydrolysis. Ultrasonic wave pretreatment showed potential in increasing the surface area for enzymatic hydrolysis but further investigations need to be done. From bagasse fermentation, 0.45g.g–1 – 0.39g.g–1 of ethanol per g of available fermentable sugar was obtained.
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