10 Dairy Bacteriology. 



Products of growth. All bacteria in their development 

 form certain more or less characteristic by-products. With 

 most dairy bacteria, these products are formed from the 

 decomposition of the medium in which the bacteria may 

 happen to live. Such changes are known, collectively, as 

 fermentations, and are characterised by the production of 

 a large amount of by-products, as a result of the develop- 

 ment of a relatively small amount of cell-life. The souring 

 of milk, the formation of butyric acid, the making of vin- 

 egar from cider, are all examples of fermentative changes. 



With many bacteria, especially those that affect proteid 

 matter, foul-smelling gases are formed. These are known 

 as putrefactive changes. All organic matter, under the 

 action of various organisms, sooner or later undergoes 

 decay, and in different stages of these processes, acids, al- 

 kalies, gases and numerous other products are formed. 

 Many of these changes in organic matter occur only 

 when such material is brought in direct contact with the 

 living bacterial cell. 



In other instances, soluble, non-vital ferments known as 

 enzyms are produced by the living cell, which are able to 

 act on organic matter, in a medium free from live cells, or 

 under conditions where the activity of the cell is wholly 

 suspended. These enzyms are not confined to bacteria 

 but are found throughout the animal and plant world, 

 especially in those processes that are concerned in diges- 

 tion. Among the better known of these non-vital fer- 

 ments are rennet, the milk-curdling enzym; diastase or 

 ptyalin of the saliva, the starch-converting enzym; pepsin 

 and trypsiu, the digestive ferments of the animal body. 



Enzyms of these types ar3 frequently found among the 

 bacteria and yeasts and it is by virtue of this characteristic 



