Its function is to convert the starch which occurs in barley into sugar. These unorganised ferments act most rapidly at about 75° C. (167° F.).[31] We may now return to the work of Pasteur and the question of _organised ferments_. Let us preface further remark with an axiom with which Professor Frankland sums up the vitalistic theory of fermentation, which was supported by the researches of Pasteur: "_No fermentation without organisms, in every fermentation a particular organism_." From these words we gather that there is no one particular organism or vegetable cell to be designated the micro-organism of fermentation, but that there are a number of fermentations each started by some specific form of agent. It is true that the chemical changes induced by organised ferments depend on the life processes of micro-organisms which feed upon the sugar or other substance in solution, and excrete the product of the fermentation. Fermentation nearly always consists of a process of breaking down of complex bodies, like sugar, into simpler ones, like alcohol and carbonic acid. Of such fermentation we may mention at least five: the _alcoholic_, by which alcohol is produced; the _acetous_, by which wine absorbs oxygen from the air and becomes vinegar; the _lactic_, which sours milk; the _butyric_, which out of various sugars and organic acids produces butyric acid; and _ammoniacal_, which is the putrefactive breaking down of compounds of nitrogen into ammonia. We have already referred at some length to this process when considering denitrifying organisms in the soil. There are four chief conditions common to all these five kinds of organised fermentation. They are as follows:-- 1. The presence of the special living agent or organism of the particular fermentation under consideration. This, as Pasteur pointed out, differs in each case. 2. A sufficiency of pabulum (nutriment) and moisture to favour the growth of the micro-organism. 3. A temperature at or about blood-heat (35-38° C., 98.5° F.). 4. The absence from the solution or substance of any obnoxious or inimical substances which would destroy or retard the action of the living organism and agent. Many of the products of fermentation are themselves antiseptics, as in the case of alcohol; hence alcoholic fermentation always arrests itself at a certain point.