264 THE PRESERVATION OF FOOD PRODUCTS 
PRESERVATION BY CANNING 
One of the most important methods of preserving perishable 
food products was invented a century ago, long before the signifi- 
cance of bacteria was known and long before the meaning of 
the process was understood. It was invented by Appert, a 
Paris confectioner in 1810, and consisted first in boiling the 
material to be preserved and subsequently sealing it hermetically. 
It was at first supposed that the significance of the sealing was 
to prevent the access of air, but it is now known that its purpose 
is simply to prevent the entrance of bacteria; for if these can be 
kept out, the presence of air does not interfere with the pre- 
servation. It is interesting to note that this method of preserva- 
tion of food products was invented and put to a wide practical 
use while scientists were disputing and laboriously experimenting 
over the problem of spontaneous generation. The experiments 
by which scientists tried to settle this question consisted in 
exactly the same devices as just mentioned, viz., the heating 
of varlous organic materials to a high temperature in order to 
kill all living organisms and then, after hermetically sealing, 
watching to see if life developed in the sterilizing mass. While 
the scientists were disputing as to the results, the method of 
canning was put into practical use, and every can of preserved 
fruit was evidence against spontaneous generation. 
The general method adopted in canning is well known. Micro- 
drganisms are so abundant everywhere that every bit of food is 
certain to be infested with them. The first step taken must be 
the killing of all the organisms that may be adhering to the food 
to be preserved. This is done by heat, the material being com- 
monly placed first in the receptacles in which it is to be finally 
sealed. The amount of heat necessary varies much with the 
nature of the material. If it is of a proteid nature, like meat, 
beans, peas, or corn, it is likely to contain spore-forming bacteria, 
and will require high and prolonged heating. This is especially 
