BACTERIA AND FERMENTATION II5 



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,).' 



We may now return to the work of Pasteur and the ques- 

 tion 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 organ- 

 isms, in every fermentation a particular organism, ' ' From 

 these words we gather that there is no one particular organ- 

 ism 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 de- 

 pend 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 car- 

 bonic 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. 



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

 not according to the locality, but according to the function. The chief are 

 these : — amylolyiic, those which change starch and glycogen (amyloses) into 

 sugars, e.g., ptyalin, diastase, amylopsin ; proteolytic, those which change 

 proteids into proteosis and peptones, e. g., trypsin, pepsin ; inversive, those 

 which change maltose, sucrose, and lactose into glucose, e.g., invertin ; coagu- 

 iative, those which change soluble proteids into insoluble, e. g., rennet- 

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



