210 
The N.Z. Journal of Science and Technology. [Nov. 
the can so that no fresh invasion by bacteria or other micro-organisms 
can occur. This is accomplished by heating the cans and their contents 
to a temperature that kills not only the bacteria themselves, but also the 
most resistant spores. 
The complete sealing of the can previous to heating is inadvisable, for 
the steam-pressure inside the can might lead to the formation of leaks 
that would encourage subsequent infection ; hence a small hole is punched 
in the cover, through which the air and steam escape during the heating 
process. After the sterilization the hole is closed with a drop of solder, 
and if the work is effectively performed the contents of the can will remain 
unchanged for an indefinite period. 
The temperature necessary for sterilization varies throughout wide 
limits according to the nature of the product; for the destruction of 
bacteria it has been shown that a higher temperature is necessary for the 
spore form than for the vegetative. Again, it is found that the higher 
the acidity (within the range of acidity of foods) the lower is the temperature 
required for sterilizing the food. In general, the spore-forming bacteria 
do not thrive as well in the more strongly acid foods as in foods of low 
acidity. This applies especially to fruits and certain vegetables. Meat, 
probably owing to the difficulty with which it conducts heat to the centre 
of the can, requires a slightly higher temperature or longer time of sterili¬ 
zation than the majority of other foods. 
When it is considered how great the variety is of foods that may be 
canned, and their contact with air and water, there is no limit to the num¬ 
ber and variety of micro-organisms which may be present. Most types of 
micro-organisms, however, are readily killed in the processing of canned 
foods, but in some rare cases bacteria appear to be present which with¬ 
stand the heat of sterilization, and if the cans are thoroughly cooled after 
the processing, these remain quiescent until the can is stored in a warm 
place. Such organisms are unusual, but have been known to cause spoilage. 
When the closing of the cans is not perfect, bacteria sometimes find 
their way into the can, either in the water used in the cooling-tank or 
sprays or subsequently from the air. When a can becomes infected in 
this way there would appear to be no limit to the variety of bacteria 
which may be expected. Practically, however, types of organisms will 
appear which thrive in the constituents characteristic of the food : for 
example, if sugars are abundant, yeasts and other sugar ferments are 
usually present ; if proteins are prominent and sugars are practically 
absent, as in meats, then proteolytic organisms will prevail. 
Spoilage in canned foods is usually, perhaps always, due to one of two 
causes—defective sealing of the can, or under-sterilization. Spoilage 
betrays itself either by the swelling of the can or by an abnormal taste 
or appearance of the food. Usually it occurs rapidly, and is evident a 
day or two after the food is packed. When it is remembered that the 
rate of penetration of heat through the contents of a can depends largely 
upon the physical nature of the food, it is evident that slight changes in 
the character or packing of the food will greatly influence the time of 
heating necessary for sterilization. Then, again, the bacteria capable of 
destroying canned goods are not only of different species, but—what is 
of more importance to the canners—the spores of different species are 
capable of withstanding different amounts of heating. As a result of this 
it is possible to operate successfully for a number of seasons at one 
temperature, and suddenly find trouble when a more resistant species gets 
into the cans. 
