Processes for producing ethanol, or ethyl alcohol, from materials that contain sugar, from materials that contain starch, and from materials that contain cellulose.
Production of ethanol from materials that contain simple sugars is the easiest; because simple sugars can be fermented by the addition of yeast.
Production of ethanol from materials that contain starches is more difficult and costly; because the starches must be converted to simple sugars before the yeast can ferment the material and produce ethanol.
The breaking down of starches into sugars includes mechanically reducing the starchy material to the smallest practical size by milling or grinding, making a slurry of the starchy material and water, and heating the slurry with a first enzyme to a temperature that is high enough to break the cell walls of the starch. This step is called enzymatic hydrolysis and it produces dextrins from the starches.
The dextrins are then reduced to simple sugars, or glucose, by the use of a second enzyme. This second step is called saccharification.
Breaking down cellulose into material that is fermentable is difficult and costly. Commonly this process includes the use of acid and heat; and the process is called acid hydrolysis.
Materials containing sugar, which are suitable for the production of ethanol, include: sugar cane, sugar beets, sweet sorghum, American artichokes, fodder bets, and fruit crops such as grapes, apricots, peaches, and pears.
Materials containing starch, which are suitable for the production of ethanol, include grains such as corn, barley, wheat, and sorghum, and tubers such as potatoes and sweet potatoes.
Materials that contain cellulose include stalks and leaves of sugar and starch crops, and forage crops. Forage crops, such as forage sorghum and Sudan grass, in early stages of growth, include a larger proportion of their carbohydrates in starch and a smaller proportion of their carbohydrates in cellulose. In addition there is very little lignin in these two forage crops in the early stages of growth; so conversion to sugars is more efficient.
The conversion of starch to glucose normally requires that the temperature of the slurry be raised to ninety-three degrees Celcius (two hundred degrees Farenheit) and that the slurry be held at this temperature for two and one-half hours. Then for the saccharification step, the temperature must be dropped to between sixty degrees and fifty degrees Celcius (between one hundred forty and one hundred twenty-two degrees Farenheit).
The heating, temperature maintaining, and temperature reducing steps that are required to convert starch to sugar adds to the energy requirements of the distillation process; so that, in some cases, the production of ethanol requires more energy, in coal and electricity, than the energy of the ethanol that is produced.