TYPES OF FERMENTATION AND DECAY. 25 



sition by bacteria are thus of the nature of excretions (e.g., 

 ptomaines). In other cases the new chemical bodies are apparently 

 produced entirely outside the body of the bacteria, and are not 

 in any sense excreted products. In such cases the microorganisms 

 excrete substances which act upon the outside substances, producing 

 chemical changes in them. The substances thus produced are not 

 excretions, but by-products. 



TYPES OF FERMENTATION AND DECAY. 



The various processes known as fermentation, decay, putre- 

 faction, etc., are all closely related, and, while an attempt has been 

 made to distinguish between them, no real logical distinctions can 

 be made. They are all progressive chemical changes taking place 

 under the influence of organic substances which are present in small 

 quantity in the fermenting mass. They are fundamentally of the 

 nature of chemical decompositions by means of which organic 

 substances are broken down and new substances are formed. 

 They may best be understood by the consideration of three examples 

 illustrating three different types: 



Alcoholic Fermentation. When yeast is added to a sugar 

 solution, it grows rapidly and soon changes the sugar into carbon 

 dioxide and alcohol. The sugar is probably taken into the body of 

 the yeast and decomposed into these two products, which are then 

 liberated from the yeast, the gas appearing as bubbles and the alcohol 

 remaining in the solution. The change is sometimes expressed by 

 the chemical equation, C 6 H I2 O 6 = 2C 2 H 6 O +2CO 2 . But this 

 equation represents only the end-products and by no means correctly 

 expresses the changes that occur. This fermentation occurs in 

 malt to make beer, in apple juice to make cider, and is the basis 

 of alcoholic industries in general. It also occurs in the raising of 

 bread. 



The Amyolytic Fermentation. If a little saliva is mixed 

 with starch and water, there begins at once a conversion of the 

 starch into sugar, and it may continue until all the starch is thus 

 changed. This fermentation is also expressed by a chemical 



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