on Milk and Milk- Products. 
375 
fermentative actions caused by enzymes are the conversion of 
starch into sugar by diastase, the dissolution of albuminoids by 
pepsine, and the coagulation of milk by rennet. 
The second group comprises those fermentations which are 
caused by orfjanizcd ferments, and which at present are almost 
exclusively acknowledged as and called "true" fermentations. 
To this group belong the various kinds of fermentations which 
will be treated of in the following pages. 
Although water containing organized ferments may appear 
quite limpid, such ferments are nevertheless not soluble. They 
are bodies with distinct outlines, of a well-defined form, in 
fact, " cells," to which, on account of their microscopic size, the 
name of " micro-organisms " has been given. Micro-organisms 
are the active agents in the decomposition of sugar into 
alcohol and carbonic acid, the transformation of alcohol into 
acetic acid, and the splitting up of milk-sugar. 
It is well known that the lowest forms of animal and vege- 
table life are of so simple and similar a character that it 
frequently becomes very difficult to decide to which kingdom 
they belong. The difficulty is increased by the fact that vege- 
table cells not infrequently exhibit a considerable power of 
locomotion, which may easily lead to the assumption that they 
are animals. This is the case, for instance, with several forms 
of micro-organisms occurrins: in milk. Although until a short 
time ago much difference of opinion existed on this point, 
the micro-organisms which cause fermentative decompositions 
are at present generally recognized as belonging to the vege- 
table kingdom. Indeed they differ in no way in their structure 
from the young cells of living plants of higher orders, and 
exhibit in a great many respects a striking similarity to certain 
kinds of fungi and algse. 
Micro-organisms consist of single cells, or several united, 
possessing a cellular membrane, in which is enclosed a viscid 
nitrogenous substance, protoplasm ; there are further present 
minute particles of fat, as well as mineral salts. When brought 
under suitable conditions, these cells multiply in the following 
manner : — A parting membrane is formed, making two cells out 
of one. These two cells grow, and both soon reach the size of 
the mother-cell, when new divisions take place. The newly- 
formed cells either separate entirely, or they are notched around 
the point where the divisions took place, still remaining united, 
and forming beads or chains : or else no indentation takes place, 
and threads of cells are produced. The chains or threads may 
be single rows, or have ramifications. 
Sooner or later there occurs a stage in the life of these 
organisms at which their power to produce new cells by 
