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It is also possible to generate ethanol out of cellulosic materials.That, however, requires a pretreatment that splits the cellulose into glucose molecules and other sugars that subsequently can be fermented.Although there are various ways ethanol fuel can be produced, the most common way is via fermentation.The basic steps for large-scale production of ethanol are: microbial (yeast) fermentation of sugars, distillation, dehydration (requirements vary, see Ethanol fuel mixtures, below), and denaturing (optional).Rather than grow algae and then harvest and ferment it, the algae grow in sunlight and produce ethanol directly, which is removed without killing the algae. Currently, the first generation processes for the production of ethanol from corn use only a small part of the corn plant: the corn kernels are taken from the corn plant and only the starch, which represents about 50% of the dry kernel mass, is transformed into ethanol.Two types of second generation processes are under development.Prior to fermentation, some crops require saccharification or hydrolysis of carbohydrates such as cellulose and starch into sugars.Saccharification of cellulose is called cellulolysis (see cellulosic ethanol). Ethanol is produced by microbial fermentation of the sugar.
It is most often used as a motor fuel, mainly as a biofuel additive for gasoline.
This process, known as distillation, separates the ethanol, but its purity is limited to 95–96% due to the formation of a low-boiling water-ethanol azeotrope with maximum (95.6% m/m (96.5% v/v) ethanol and 4.4% m/m (3.5% v/v) water).
This mixture is called hydrous ethanol and can be used as a fuel alone, but unlike anhydrous ethanol, hydrous ethanol is not miscible in all ratios with gasoline, so the water fraction is typically removed in further treatment to burn in combination with gasoline in gasoline engines.
Microbial fermentation currently only works directly with sugars.
Two major components of plants, starch and cellulose, are both made of sugars—and can, in principle, be converted to sugars for fermentation.