12 DAIRY BACTERIOLOGY 



animal tissues consist for the most part of proteins, the term putre- 

 faction has become synonymous with the bacterial decomposition 

 of proteins. Formerly, when nothing was understood of the real 

 nature of the different fermentations, they were generally looked 

 on as chemical changes which apparently originated spontaneously, 

 and which might improve a natural product. In cases where the 

 natural product was spoilt instead of being improved the process 

 was known as putrefaction. As alcohol fermentation is the most 

 widely-known example, it is often supposed that the evolution 

 of gas is characteristic of fermentations in general ; but in its 

 modern sense the term fermentation includes a number of 

 processes, such as the lactic acid and acetic acid fermentations, 

 in which no gas is produced. 



ENZYMES 



The manifold chemical changes due to microorganisms are 

 carried out through the agency of certain special substances, 

 known as enzymes. These are bodies of unknown composition, 

 possibly more complex than the proteins, which may be separated 

 more or less easily from the protoplasm, and which are able in 

 small amounts to induce certain chemical changes in relatively 

 large amounts of material. Enzyme action may be extracellular 

 or intracellular, according as the enzyme acts outside (exoenzyme) 

 or inside (endoenzyme) the cell in which it has been formed. To 

 the former class belong the digestive enzymes of animals ; as the 

 function of these is to prepare the food for assimilation, they 

 must necessarily act outside the assimilating cells. Many micro- 

 organisms form enzymes which are analogous to these. On the 

 other hand, the typical fermentation enzymes decompose or 

 oxidise the assimilated food material inside the cell, and must 

 therefore act inside the cell, so that the energy liberated in the 

 process may be directly available to the cell. 



Distinction was formerly made between unorganised ferments, 

 i.e., enzymes such as are contained in the digestive juices, and 

 organised ferments, by which were understood the microorganisms. 

 It was not until 1897, when Buchner succeeded in separating from 

 yeast the enzyme which brings about alcoholic fermentation 

 (zymase), that it became clear that all fermentations are ultimately 

 due to enzyme action. 



The activity of enzymes increases with the temperature, but 

 above certain limits the enzymes, like the proteins, become de- 

 natured and lose their special properties. The optimum for most 

 enzymes lies between 35 and 65 C. In aqueous solution they 

 are generally rendered completely inactive at temperatures 



