A process of the nature of that resulting from the operation of leaven on dough or on saccharine liquids.
The features superficially recognizable in the process in these instances are an effervescence or internal commotion, with evolution of heat, in the substance operated on, and a resulting alteration of its properties. Before the rise of modern chemistry, the term was applied to all chemical changes exhibiting these characters; in Alchemy, it was the name of an internal change supposed to be produced in metals by a 'ferment', operating after the manner of leaven. In modern science the name is restricted to a definite class of chemical changes peculiar to organic compounds, and produced in them by the stimulus of a 'ferment'; the various kinds of fermentation are distinguished by qualifying adjs., as acetous, alcoholic, butyric, lactic, putrefactive, etc. In popular language the term is no longer applied to other kinds of change than those which it denotes in scientific use, but it usually conveys the notion of a sensible effervescence or 'working', which is not involved in the chemical sense.