Optimization of chemical saccharification of sweet sorghum straw and bagasse for ethanol fermentation

Abstract

Sweet sorghum straw and sugarcane bagasse are lignocellulosic materials that are promoted as an alternative feedstock for ethanol production because it is available and inexpensive. Due to its composition of cellulose and hemicellulose, that could be hydrolyzed into fermentable sugars. Conversion of these potential feedstocks requires a pretreatment step to alter the microscopic size and structure of the lignocellulose. This research was studied in order to find the optimum conditions on hydrolysis of sweet sorghum straw and sugarcane bagasse using dilute-acid or dilute-alkaline pretreatment. The biomass was mixed with dilute sulfuric acid (0-3%v/v) or dilute sodium hydroxide (0-4%w/v) with solid loading of 10% w/v and then pretreatment at hight temperatures (120-190°C) for 10-30 min of pretreated times. The maximum yield of glucose and xylose from sorghum straw was 0.234 g glucose/g dry substrate and 0.208 g xylose/g dry substrate, respectively, at the pretreatment condition : 120ºC, 3%H₂SO₄ for 10 min. In this case, a total of 50.04% of glucan and 76.41% of xylan were converted to glucose and xylose, respectively. In the case of bagasse, pretreatment at 170°C, 3%H₂SO₄ for 20 min gave the maximum yield of glucose of 0.367 g glucose/g dry substrate and a total of 78.52% of glucan was converted to glucose. Pretreatment at 120°C, 3%H₂SO₄ for 20 min, gave the maximum yield of xylose of 0.226 g xylose/g dry substrate and a total of 83.05% of xylan was converted to xylose. After chemical pretreatment, the hydrolyzates of sorghum straw and bagasse were detoxified and concentrated by overliming and evaporation method, respectively. The hydrolyzates containing 20 g/l and 50 g/l of reducing sugars were fermented with separate hydrolysis and fermentation (SHF) process using Saccharomyces cerevisiae and Pichia stipitis at 30ºC, pH 5.5 and agitation rate of 150 rpm. The fermentation of sorghum straw hydrolyzate, containing 20 g/l of reducing sugars by P. stipitis gave the highest ethanol concentration of 10.17 g/l at 46 hr. Whereas S. cerevisiae gave the ethanol concentration of 6.38 g/l at 12 hr. In case of fermentation bagasse hydrolyzate by P. stipitis and S. cerevisiae gave ethanol concentration of 3.73 g/l at 32 hr and 1.78 g/l at 9 hr, respectively.