248 THE PRESERVATION OF FOOD PRODUCTS. 
killing bacteria spores has caused great losses of canned corn, peas, 
and beans, as well as tomatoes. It must be recognized that, for 
successful canning, every spore must be killed; for if a single one 
be left alive in the middle of the can, the product is sure to spoil 
(Fig. 50). The slowness with which heat will pass to the middle of 
the can was not recognized until many losses had resulted from 
insufficient heating. But the failures proved instructive, and after 
bacteriologists studied the different problems presented by the 
attempts to can food stuffs, the questions were answered one by 
one, and successful rules were devised for the canning of any food 
products subject to such methods. To-day failures are rare and 
are all attributable to carelessness in the process. So thoroughly 
has this subject been mastered that to-day any food that can stand 
heat may be perfectly preserved by boiling and canning. Of course 
the flavors are commonly changed by the process, and few of them 
appear like the fresh material. But usually the flavors are less 
changed than by any other method of preservation, and canned 
goods are vastly superior to the dried foods with which our grand- 
fathers were forced to be content in winter. Canning is especially 
adapted to foods containing a good deal of water, and hence is 
of especial use among foods that cannot be well preserved by drying. 
Fruits are well adapted to canning, but ill adapted to drying, and 
are ruined by salting. 
BACTERIA IN EGGS. 
The presence of bacteria in eggs results in trouble experienced 
by every farmer, and one which it seems impossible to avoid. 
It might be supposed that eggs, when freshly laid, would be free 
from bacteria and hence not liable to decay. But this is certainly 
not the case. Bacteria are known to enter the oviduct and contami- 
nate the mass of the egg even before its shell is deposited. Hence 
when the egg is laid it will commonly contain bacteria in greater or 
less numbers. These bacteria can obtain plenty of oxygen from 
the air that enters through the porous shell, and are thus able to 
grow readily within the egg, where they soon cause its decay. A 
