CONDITIONS OF FERMENTATION 95 



barley, into sugar. These unorganised ferments act most rapidly at a 

 high temperature.* 



We may preface our consideration of the organised ferments by 

 an axiom by 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 it is to be inferred 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 fermenta- 

 tions 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 always consists of a process of breaking down of 

 complex bodies, like sugar, into simpler ones, like alcohol and 

 carbonic acid. Of such fermentations 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 the ammoniacal, which 

 is the putrefactive breaking down of compounds of nitrogen into 

 ammonia. We shall have occasion to refer 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 follow : — 



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, 985° 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 organisrn and agent. Many of the products of fermenta- 

 tion are themselves antiseptics, as in the case of alcohol; hence 

 alcoholic fermentation always arrests itself at a certain point. 



The causal micro-organisms of particular fermentations are of 

 various kinds, belonging, according to botanical classification, to 



* The unorganised ferments are frequently otherwise classified than as above, 

 according to function. The chief are these : — amylolytic, those which change starch 

 and glycogen (amyloses) into sugars, e.g., ptyalin, diastase, amylopsin (organisms 

 of the subtilis group and the micrococcus of mastitis are said to produce amylolytic 

 ferments); proteolytic, those which change proteids into proteoses and peptones, 

 e.g., trypsin, pepsin; inversive, those which change maltose, sucrose, and lactose 

 into glucose, e.g. , invertin (various species of bacteria produce inversive ferments) ; 

 coagulative, those which change soluble proteids into insoluble, e.g., rennet ; steato- 

 lytic, those which split up fats into fatty acids and glycerine, e.g., steapsin. 



