CHAPTER XVI 
THE PRESERVATION OF FOOD PRODUCTS 
THE SPOILING OF FOOD 
Practically all the materials raised on the farm as food for 
man or animals may serve also as food for microdrganisms. If 
any of the various fungi attack foods, their presence is shortly 
made manifest by signs of decomposition. The food, we say, 
begins to spoil. Any one of our food products that contains suffi- 
client water is sure to be attacked sooner or later by some of the 
various types of microérganisms, especially by the three classes we 
have recognized. 
By Bacteria.—These are particularly adapted for feeding upon 
proteid matter, but they are not fond of sugar. Hence we find 
them especially concerned in the spoiling of proteid foods. Meats, 
milk, eggs, wheat, and other cereals, all contain unusually large 
quantities of proteids and are liable to putrefaction, whenever 
they are moist enough; and their putrefaction will practically 
always be found to be due to bacteria. In all attempts to preserve 
these substances it is to be remembered that we are dealing with 
bacteria, some of which are liable to form spores and for that rea- 
son are difficult to destroy. Pure sugar solutions, on the other 
hand, will not undergo a bacterial fermentation, although impure 
sugars may do So. 
By Yeasts.—Although sugars are not attacked by bacteria, 
they are the favorite food of yeasts, which destroy them by setting 
up an alcoholic fermentation. Jf the sugar is in considerable 
abundance, it serves as a partial protection against bacterial 
growth, although it favors yeast activities. From this it follows 
that fruit juices in particular are subject to yeast fermentation, but 
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