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bioweb.sungrant.org » Technical » Biofuels » Technologies » Ethanol Production » Ethanol from Sucrose

Ethanol from Sucrose	Printer Friendly

Ethanol is produced by fermenting plant carbohydrates with yeast. Plant carbohydrates are grouped as soluble sugars (such as sucrose from sugarcane), storage carbohydrates (such as starch from grains and tubers), and structural carbohydrates which make up the plant cell wall (such as cellulose, hemicellulose, and pectin). Sucrose, commonly called table sugar, is composed of two simple sugars (fructose and glucose). It is the main sugar extracted from sugar cane and sugar beets, and is highly soluble in water. Industrial strains of yeast such as Saccharomyces can be used to hydrolyze sucrose into fructose and glucose, and ferment the sugars into ethanol. Ethanol production from sucrose predominates in tropical regions with sugar cane production, such as Brazil, which produced 4.2 billion gallons (15 billion liters) of fuel ethanol in 2004 (Barros, 2005). In the U.S., a typical sugarcane stalk weighs about 3 pounds and contains a little more than 0.3 pounds of sugar. Sugar is produced by first squeezing the juice out of the stems. The raw juice is clarified, impurities and solids removed, and thickened, followed by a series of crystallization steps to produce sugar crystals, which are removed. The remaining syrup is molasses. Approximately 3 gallons of molasses are produced for every 100 pounds of raw sugar produced (Shapouri, 2006). Fermentation of sucrose from sugar cane can be conducted using the juice directly obtained from squeezing the sugar cane stalks, or from the molasses. Most ethanol plants in Brazil produce ethanol from molasses, typically molasses A, which has had the sugar removed by a single crystallization step. The ethanol concentration in the fermented molasses is about 9% v/v (7% w/w). As sugarcane molasses is naturally low in free nitrogen, urea is typically added as a nitrogen source to insure proper yeast performance. Depending upon the quality of the molasses, other nutrients such as phosphorous, biotin, pantothenic acid, and inositol may also be added (Piggot, 2003). From each gallon of sugar cane molasses, 0.41 gallons of ethanol can be produced. If the raw sugar and molasses in sugar cane is used to produce ethanol, 19.6 gallons of ethanol can be produced per ton of harvested sugar cane (Shapouri, 2006). The fermentation and ethanol recovery processes for sugarcane juice or molasses use technologies similar to those used to produce ethanol from corn, thus distillation and molecular sieves are used to purify the ethanol following fermentation of the sucrose (Rosillocalle, 1986). The stillage left after the distillation of ethanol is used as a fertilizer for sugarcane fields. The fibrous plant material that remains after the juice has been squeezed out of the sugarcane stalk is called bagasse. Approximately 0.3 lb of wet bagasse (about 50% moisture) is produced per 1 lb of wet sugarcane (Shapouri, 2006). Bagasse is rich in cellulose, hemicellulose, and lignin (Okano, 2006; Pandey, 2000; Silva, 2005), but has no food value. In most processing facilities, bagasse is burned to produce the heat and steam used to evaporate the water from the sugar in the crystallization process, and to distill the ethanol. Bagasse is also used to produce the electricity used by the plant with excess electricity sold to the power grid (Bhatt, 2001; Goldemberg, 2004). However, bagasse could be used as a cellulosic biomass resource to produce ethanol, other biofuels, and bioproducts.

References
Barros, S. (2005). Brazil Sugar Semi Annual (No. GAIN Report BR5020): USDA Foreign Agricultural Service. Bhatt, M. S. and Rajkumar, N. (2001). Mapping of combined heat and power systems in cane sugar industry. Applied Thermal Engineering, 21 (17), 1707-1719. Goldemberg, J.; Coelho, S. T.; Nastari, P. M.; and Lucon, O. (2004). Ethanol learning curve - the Brazilian experience. Biomass & Bioenergy, 26 (3), 301-304. Okano, K.; Iida, Y.; Samsuri, M.; Prasetya, B.; Usagawa, T.; and Watanabe, T. (2006). Comparison of in vitro digestibility and chemical composition among sugarcane bagasses treated by four white-rot fungi. Animal Science Journal, 77 (3), 308-313. Pandey, A.; Soccol, C. R.; Nigam, P.; and Soccol, V. T. (2000). Biotechnological potential of agro-industrial residues. I: sugarcane bagasse. Bioresource Technology, 74 (1), 69-80. Piggot, R. (2003). Treatment and fermentation of molasses when making rum-type spirits. In K. A. Jacques, T. P. Lyons & D. R. Kelsall (Eds.), The Alcohol Textbook, 4th Edition. Bath, England: Nottingham University Press. Rosillocalle, F. (1986). The Brazilian Ethanol-Chemistry Industry (a Review). Biomass, 11(1), 19-38. Shapouri, H. and Salassi, M. (2006, July) The economic feasibility of ethanol production from sugar in the United States. U.S. Department of Agriculture, Office of the Chief Economist. Silva, S. S.; Matos, Z. R.; and Carvalho, W. (2005). Effects of sulfuric acid loading and residence time on the composition of sugarcane bagasse hydrolysate and its use as a source of xylose for xylitol bioproduction. Biotechnology Progress, 21 (5), 1449-1452.