CHAPTEE XII. 



BACTERIA AND THE PRESERVATION OF 

 FOOD-PRODUCTS. 



1. INTRODUCTION. 



IN dealing with this subject, the main fact to be borne in mind is that 

 all organic material, whether animal or vegetable, is subject to the 

 attacks of microorganisms. The dead animal or vegetable body 

 becomes food-material for the living animal or plant. Now this food- 

 material is strictly limited in amount and cannot be wasted ; so re- 

 constitution must be continually taking place. To the myriads of 

 small organisms which are everywhere present, this important function 

 has been relegated. If the trunk of a fallen tree be left exposed, after 

 a number of years, it will have disappeared. Many millions of 

 organisms, of many different kinds, will have fed on it. Those that 

 came first will effect certain changes and then disappear, their place 

 being taken by a different set of organisms. These again will leave 

 their mark on the tree, and be succeeded by still other organisms, 

 and so the endless series of changes goes on, till the tree finally 

 disappears. The tree will become less and less, because gases and 

 liquids will be formed as a result of these activities, and their loss 

 naturally reduces the volume of the tree. These changes happen to 

 all other organic substances, for if they ceased, owing to the strictly 

 limited amount of the food-supply, all life would soon come to an 

 end. When, therefore, an article of food turns "bad," it simply 

 means that another step in the cycle of changes has been taken. 

 Nature's laws are very drastic, and the order to "move on" cannot 

 be evaded, though in a few cases, which we are about to consider, 

 the order can be suspended for a short time. 



Everything that we class as food-products is of an organic nature, 



