168 AGRICULTURAL AND INDUSTRIAL BACTEEIOLOGY 



brought about in carbohydrates. The term putrefaction is 

 sometimes applied to decomposition, particularly of protein 

 materials, occurring under more or less anaerobic condi- 

 tions, usually with the development of malodorous sub- 

 stances. Decay is similar decomposition occurring usually 

 under aerobic conditions without noteworthy development 

 of malodorous compounds. 



Practically all fermentation, putrefaction, and decay in 

 nature occur as the result of the activity of microorganisms, 

 that is, of the bacteria, the yeasts and the molds. 



The Origin of the Products of Fermentation. The prod- 

 ucts developed during the process of fermentation may be 

 grouped under four headings, using the origin as a basis for 

 the classification. 



1. The Analytic Action of Extracellular Enzymes. The 

 digestive changes brought about by extracellular enzymes 

 break down compounds on the exterior of the cell into 

 simpler compounds. These are usually formed greatly in 

 excess of the immediate needs of the microorganisms. Such, 

 for example, is the transformation of starch into maltose by 

 certain molds, or the digestion of casein or gelatin by bac- 

 teria. 



2. The Analytic Action of Intracellular Enzymes. Many 

 of the intracellular enzymes are active in bringing about 

 changes in those food substances which diffuse into the cell. 

 Such, for example, is the origin of alcohol and carbon di- 

 oxide in yeasts, and of lactic acid in the lactic acid bac- 

 terium. It may be noted furthermore that the activity of 

 the analytic enzymes contained within a cell do not cease 

 necessarily with the death of that cell. Such enzymes as 

 continue to act after the death of the cell and bring about 

 more or less decomposition of the cell substance itself are 

 termed autolytic. In an old culture in broth in which most 

 of the organisms have died, the cells will be found more or 

 less autolyzed; and chemical test will reveal substances 



