If practised properly, canning is a safe and useful
method of preservation. Food is placed in a can and the
top of the can is sealed. Heat sterilisation during the
canning process destroys the enzymes and micro-
organisms that would otherwise spoil foods. The
atmosphere inside the can remains aseptic and food
spoilage cannot subsequently take place. As different
enzymes and micro-organisms are likely to be present
in different foods the amount of heating (both the
temperature and time) needed to safely process the
food will vary according to the type of food being
canned. In addition, the temperature and time will vary
according to the size and shape of the can being used.
If the food is not heated sufficiently there is a risk that
micro-organisms will survive and grow inside the can.
Figure 1: Canning. Photo: Neil Noble /
In some foods (especially vegetables, meat, fish, milk
and other 'low acid' foods) a particular type of bacteria called Clostridium botulinum can grow
and cause severe food poisoning (see Technical Brief on Food Poisoning).
Conversely, if the food is over-heated the quality is reduced and it can become colourless,
tasteless or burned and have a soft mushy texture which customers will find unpleasant. It is
therefore essential that the correct heating conditions are carefully established and
maintained for every batch of food that is canned. This requires the skills of a qualified food
technologist or microbiologist. When foods are heated in a sealed can, there must be an
equal pressure outside the can as inside (otherwise the can will explode). This is achieved by
heating the cans in a strong metal vessel called a retort (similar to a domestic pressure cooker
but much more robust), using high-pressure steam. When the hot cans are cooled using
water, the pressure in the retort is kept the same as that inside the can using compressed air.
The retort must therefore withstand pressures of up to 60psi and be fitted with a safety valve
and other pipework for water, steam and compressed air. All the equipment described above
requires a controller and will need regular maintenance by a skilled technician.
Assuming that cans are available, they are usually considerably more expensive than other
types of packaging materials. The inside of the cans should be lacquered to prevent foods
reacting with the metal during storage. Different types of lacquer are needed for different
foods (fruit products, vegetables, meat and fish each require a different type). In addition a
'seamer' is needed to correctly fit the can lid and regular checks are needed to make sure that
the seams are properly formed. This needs training and experience and specialized
equipment (a seam micrometer).
Finally, because of the potential dangers from food poisoning with some types of food, it is
necessary for a trained microbiologist to routinely examine samples of canned food that have
been subjected to accelerated storage conditions. This requires a supply of microbiological
media and equipment.
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