CANNING TERMS
Acid Foods
Fruits, acidified tomatoes, pickles, relishes, chutneys, jams, jellies and preserves are acid foods – those
foods with enough natural acid or enough additional acid (lemon juice or vinegar) to result in a pH of 4.6 or
lower. Acid foods may be safely canned using a hot water bath method.
Botulism
A deadly form of food poisoning caused by toxins produced by the growth of botulinus spores. For more on
botulism and how to prevent it, see page 15.
Buckling
This term applies to cans in which one or both ends have been permanently distorted by excess pressure in
the can. Slack filling (more liquid than solid food), sealing at too low a temperature, or too sudden of a
release of pressure after cooking can all cause buckling.
Raw-Pack Method
Filling hot jars with raw, unheated food prior to processing.
Hot-Pack Method
Filling hot jars with hot precooked food before processing.
Hot Water Bath
The canning method used to process acid foods.
Headspace
The distance between the food in the jar/can and the cover of the jar/can. Leave at least ½-inch headspace to
most canned foods. Failure to leave headspace may cause bulging of the cans due to over filling.
Low-Acid Foods
Foods with little natural acid which have a pH above 4.6. This includes meats, poultry, seafood, milk,
vegetables and some varieties of tomatoes. Low-acid foods must be processed in a pressure canner.
Swell
Swell is the commercial term given to cans whose ends have been distended or swelled by the gas which has
been generated by spoilage of the food in the can. Any can with ends swelled out should be thrown away as
it is not fit for use, unless the can was filled too full or was not sealed while hot. See “Headspace.”
Vacuum
Vacuum as it applies to cans refers to a lower pressure within the container than on the outside. This higher
pressure on the outside causes the ends of the cans to be held in a concave position. Vacuum is obtained by
sealing the cans while they are hot. The condensation of the vapor in the headspace and the contraction of
the gases and contents of the can upon cooling produce a partial vacuum in the can. Proper vacuum may be
obtained by thorough pre-heating or exhausting just previous to closing. A can with a vacuum – that is, with
the ends properly pulled in – is considered to contain good food. If the products start to spoil, gas is
generated which releases the vacuum and causes the ends of the can to bulge.
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