128 BACTERIA, YEASTS, AND MOLDS 



and since some of them are vapors they may give it a 

 strong odor. Consequently the food material which is 

 being consumed by bacteria soon begins to have a very 

 strong taste and odor, due to some of these decomposition 

 products. The character of the food changes very greatly, 

 and after the bacteria have had an opportunity of feeding 

 upon the material for a comparatively few hours no resem- 

 blance to the original food mass remains, in appearance, 

 taste, or smell. 



This whole phenomenon is spoken of as putrefaction, or 

 decay. There is, however, a slight difference between 

 these two terms. By putrefaction we commonly mean a 

 change in food masses by which a series of very unpleas- 

 ant odors and tastes make their appearance in the putre- 

 fying mass. By the term "decay" we properly mean a 

 more complete destruction of the food material, in which 

 the unpleasant odors and flavors finally disappear, leaving 

 behind a comparatively odorless material which represents 

 the final debris remaining from the total destruction of 

 the food masses. Putrefaction, with its high flavors and 

 odors, is an incomplete process ; decay, a more complete 

 process of destruction. Putrefaction is produced in gen- 

 eral by bacteria when they do not have an abundance of 

 air or oxygen, whereas decay occurs when the amount of 

 air or oxygen present is abundant. In other words, when 

 a bit of food is being consumed by bacteria without suffi- 

 cient oxygen it putrefies and becomes offensive in taste 

 and smell ; when, however, the oxygen is abundant, the pro- 

 cess of putrefaction goes on to a more complete destruc- 

 tion, ending finally in what is known as decay, by which 

 the material is converted into inoffensive substances. 



