DECAY 231 



animals and plants back to the simpler substances from which 

 green plants manufacture their food and build up protoplasm. 



Food may be kept indefinitely when under conditions that 

 hinder the growth of bacteria, as in cold storage. The exclusion 

 of all bacteria from hermetically sealed tinned foods, in which 

 all germs have been previously killed by heat, is the chief prin- 

 ciple in the success of the canning industry. The agreeable 

 flavors of high-grade butter and certain cheeses, as well as the 

 gamy taste of meat, are largely due to bacteria, and really indi- 

 cate the first stages in the process of decay, although usually not 

 at all harmful or distasteful. Not infrequently, however, incip- 

 ient putrefaction forms certain organic poisons, called ptomaines, 

 in nitrogenous foods, and these may give rise to distressing 

 symptoms, or even prove fatal to the consumer. 



253. Fermentation. Decay rnay take place in two very dif- 

 ferent classes of substances : (1) the carbohydrates, such as cel- 

 lulose, starch, sugar, etc., and (2) the proteids or nitrogenous 

 substances that make up protoplasm, flesh, and many food prod- 

 ucts. The breaking down of the carbohydrates is called fermen- 

 tation, and many other fungi besides the bacteria are concerned 

 with the process. Yeasts, for example, are the most important 

 organisms in the fermentation of sugar, and the decay of cell 

 walls in timber is chiefly due to some of the higher fungi. 



The best-known types of fermentation are the alcoholic and 

 the acid. Alcoholic fermentation involves the change of sugars 

 to alcohols, accompanied by the formation of large quantities of 

 carbon dioxide, and will be considered more especially in the 

 account of the yeasts. Acid fermentation is the transformation 

 of sugars and alcohols into organic acids, and bacteria play the 

 most important part in this process. Thus the change of cider 

 to vinegar is one of sugars and alcohols into acetic acid, and 

 the souring of milk is the formation of lactic acid from milk 

 sugar. Both processes are caused by bacteria. There are a 

 number of stages in the processes of fermentation. For exam- 

 ple, cellulose is first changed into some kind of sugar, and this 



