Students learn to
describe conditions under which fermentation of sugars is promoted
Ethanol is made largely from corn and from sugar cane, though it is also produced from many other sources, including waste matter from cheese production. The main process followed is fermentation, the conversion of sugars to ethanol by microscopic yeasts in the absence of oxygen. There are many other fermentation reactions, all of which allow micro-organisms to obtain energy from chemicals in their environment when oxygen is unavailable - originally, the term was applied to any reaction that produced effervescence, or bubbles. Among the products of the biological reactions we now call fermentations (all of which produce carbon dioxide) are acetone, butanol, glycerol, and hydrogen gas.
The physical process in which ethanol is produced from corn is outlined below - for a more detailed picture, see the Alternative Fuels Data Centre.
- Grind the corn up and mix it with water and with alpha-amylase, the enzyme that breaks down the bonds between glucose molecules in starch to generate maltose. This reaction is carried out at high temperatures to discourage bacteria from settling down for a good feed themselves.
- Glucoamylase is then added - this enzyme can attack all kinds of glucose-glucose bonds, but is most effective in splitting the maltose dimer to give glucose.
- Yeast is added to the mash to convert glucose to carbon dioxide and ethanol, and it passes through a series of fermenters from which air is excluded at about 60 °C. This gives a mixture containing about 10% alcohol after about 24 hours.
- Finally this mash is distilled to give 96% alcohol, dried using molecular sieves to remove the excess water, and sent out to for use mixed with a small amount of nasty organic chemical to make it unfit for human consumption.