Ethanol is produced by fermenting plant carbohydrates with yeast. Sucrose, commonly called table sugar, is a soluble sugar carbohydrate 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 break down 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.
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.
Fermentation of sucrose from sugar cane can be conducted using the sugar cane juice directly obtained from squeezing the 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% by volume, or 7% by weight.
As sugarcane molasses is naturally low in free nitrogen, urea is typically added as a nitrogen source to ensure proper yeast performance. Depending upon the quality of the molasses, other nutrients such as phosphorous, biotin, pantothenic acid, and inositol may also be added. From each gallon of sugar cane molasses, 0.41 gallons of ethanol can be produced. If both 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.
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. 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. 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. However alternatively, bagasse could be used as a cellulosic biomass resource, to produce ethanol, other biofuels, and bioproducts.