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Fermentation is the digestion of some matter by bacteria or other small organism (such as yeast), especially the anaerobic breakdown of sugar into alcohol and biogas.

Fermentation is popularly taken to refer to the fermentation of sugar in solution to alcohol using yeast, but other fermentation processes include the making of yoghurt. The science of fermentation is known as zymology.

In the process, the organism metabolises one or more substances to produce the energy and chemicals it needs to live and re-produce. This process of chemical reactions will produce some form of by-product. In the case of yeast used for brewing, these by-products are carbon dioxide, and ethanol.

The 1811 Household Cyclopedia adds:

'Alcoholic Beverages'

May be divided into fermented drinks including beer and wines, and distilled drinks or spirits which are obtained from the former by distillation. Spirits usually contain about fifty per cent. of alcohol, beer and wines from one to twenty per cent. The alcohol in all cases results from the breaking up of the sugar in the fermenting liquid.


Ordinary sugar, or cane sugar, uncrystallizable, or fruit sugar; and grape sugar, or glucose, are the three most important varieties. Fruit sugar exists in all the sub-acid fruits as grapes, currants, apples, peaches, etc. When these are dried, it changes to grape sugar forming the whitish grains which are seen on the outside of prunes, raisins, etc. Grape sugar is found to a limited extent in fruits associated with fruit sugar. Cane sugar is readily changed by the action of acids or ferments into fruit sugar, and the latter into grape sugar, but the process cannot be reversed. Grape sugar is the only fermentable variety, the others becoming changed into it before fermentation.

'Transformation of Starch, etc.'

Under the influence of acids, or diastare, a principle existing in germinating grains, starch is changed first into gum (dextrine) and afterwards into grape sugar. Hence one of our most important sources of alcohol is to be found in the starch of barley, corn, wheat, potatoes, etc. Wood may be converted into grape sugar by the action of strong sulphuric acid which is afterwards neutralized. An attempt to produce alcohol in this way on a commercial scale was made in France, but was not successful.


A solution of pure sugar will remain unchanged for an indefinite period of time. To induce fermentation, a portion of some nitrogenous body, itself undergoing decomposition, must be added. Such ferments are albumen (white of egg), fibrin (fibre of flesh), casein (basis of cheese), gluten (the pasty matter of flour). Yeast consists of vegetable egg-shaped cells, which is increased during its action as a ferment.

'Circumstances influencing Fermentation'

In order that fermentation shall begin we require, besides the contact of the ferment, the presence of air. The most easily decomposed articles of food may be preserved for an indefinite period by hermetically sealing them in jars, after drawing out the air. When once begun, however, fermentation will go on, if the air be excluded. Temperature is important. The most favorable temperature is between 68 and 77 Fahr. At a low temperature fermentation is exceedingly slow. Bavarian or lager beer is brewed between 32 and 46 1/2 Fahr. A boiling heat instantly stops fermentation, by killing the ferment.

To check fermentation we may remove the yeast by filtration. Hops, oil of mustard, sulphurous acid (from burning sulphur), the sulphites, sulphuric acid, check the process by killing the ferment.

Too much sugar is unfavorable to fermentation, the best strength for the syrup is ten parts of water to one of sugar.

'Changes during Fermentation, etc.'

The grape-sugar breaks up into carbonic acid which escapes as gas, alcohol and water which remain. In malting the grain is allowed to germinate, during which process the starch of the grain is changed into gum and sugar: the rootlets make their appearance at one end and the stalk or acrospire at the other. The germination is then checked by heating in a kiln; if allowed to proceed a certain portion of the sugar would be converted into woody matter, and lost.

In brewing the sacharine matter is extracted from the malt during the mashing. Yeast is added to cause fermentation; an infusion of hops afterwards, to add to the flavor and to check fermentation. In wine making there is sufficient albuminous matter in the grape to cause fermentation without the use of yeast.

Distillation separates the alcohol in great part from the water. Alcohol boils at 179 Fahr., and water at 212. It is not possible, however, to separate entirely alcohol and water by distillation.

'Acetic Fermentation'

Weak fermented liquors will become sour on exposure to the air. This is owing to the conversion of their alcohol into acetic acid (see Vinegar). This change is due to the absorption of the oxygen of the air and is much promoted by the presence of a peculiar plant, the mother of vinegar. It is sometimes called the acetous fermentation.

'Viscous Fermentation'

By the action of yeast on beet-sugar a peculiar fermentation is set up; but little alcohol is formed. The same gives ropiness to wines and beer. It is checked by vegetable astringents.