42 APPLIED PHYSIOLOGY 



for an indefinite time. Fruit when boiled and at once 

 sealed in air-tight cans will keep unchanged for a long 

 time. If there is a great deal of sugar present no germs 

 at all will grow, and the solution will keep indefinitely. 

 This is why fruit can be preserved in open jars if a great 

 deal of sugar is used. 



50. Fermentation. Changing sugar to alcohol or vine- 

 gar is an example of fermentation. A substance which 

 can change the composition of other bodies without losing 

 its own identity or characteristics is a ferment. A very 

 small amount of a ferment can change a very large amount 

 of another substance. 



A very small amount of yeast will cause an indefinite 

 amount of sugar to become changed to alcohol or vinegar. 

 At the same time the yeast may not grow weaker, but 

 on the contrary may become stronger than at first. In the 

 same way a small amount of " mother " will change a 

 large amount of weak alcohol to vinegar, and itself will 

 greatly increase in amount. 



51. Kinds of ferments. Nature uses many ferments in her 

 actions. Some are living beings and some are lifeless substances. The 

 chief part of the digestion of food is done by lifeless ferments. Fer- 

 mentation is commonly spoken of as a process of decay, but the 

 common process of decay or rot is in itself only a special kind of fer- 

 mentation. Ordinary decay is caused by a living being like the 

 vinegar germ. By its growth and multiplication it softens and liquefies 

 the albumin of animal and vegetable matter. This process is called 

 putrefaction (see p. 24). Some of the matter passes off as foul smell- 

 ing gases, while the liquid part soaks into the soil. Putrefaction is 

 nature's way of giving dead bodies back to the soil and air so that 

 plants can build them into useful forms again. 



Yeast germs are found everywhere, but they are often grown in wet 

 meal or flour. The mass is then dried in cakes and sold as yeast. 

 When a small piece is added to sugar and water it starts alcoholic fer- 

 mentation at once. Alcoholic fermentation only is usually meant when 



