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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