Canning is one of the most popular preservation methods for food.
However, it can be much more labor-intensive than freezing or drying
foods, and you will need special equipment to process the food correctly.
The benefits to canning, in spite of the extra trouble, are that many foods
taste better when they are canned rather than frozen. Some produce, such as
plums, apples, and carrots, develop a richer, complex taste in the canning
process. In addition, your freezer usually has a small amount of room for
storage, whereas canned foods can be piled up in a pantry, basement, or
closet for years. Finally, there are some foods, such as pickles, that cannot
complete the fermenting process in a freezer.
A Glossary of Canning Terms
- Acid foods Foods that contain enough acid to result in a pH of 4.6 or lower.
This includes all fruits except figs; most tomatoes; fermented and pickled
vegetables; relishes; and jams, jellies, and marmalades. Acid foods may be
processed in boiling water.
- Altitude The vertical elevation of a location above sea level.
- Ascorbic acid The chemical name for vitamin C. Lemon juice contains
large quantities of ascorbic acid and is commonly used to prevent browning
of peeled, light-colored fruits and vegetables.
- Bacteria A large group of one-celled microorganisms widely distributed in
nature. See microorganism.
- Boiling-water canner A large standard-sized lidded kettle with jar rack, designed for heat-processing 7 quarts or 8 to 9 pints in boiling water.
- Botulism An illness caused by eating toxin produced by growth of
Clostridium botulinum bacteria in moist, low-acid food, containing less than
2 percent oxygen, and stored between 40 degrees and 120 degrees F. Proper
heat processing destroys this bacterium in canned food. Freezer
temperatures inhibit its growth in frozen food. Low moisture controls its
growth in dried food. High oxygen controls its growth in fresh foods.
- Canning A method of preserving food in air-tight vacuum-sealed
containers and heat processing sufficiently to enable storing the food at
normal home temperatures.
- Canning salt is Also called pickling salt. It is regular table salt without the
anticaking or iodine additives.
- Citric acid A form of acid that can be added to canned foods. It increases
the acidity of low-acid foods and may improve the flavor and color.
- Cold pack Canning procedure in which jars are filled with raw food. “Raw
pack” is the preferred term for describing this practice. “Cold pack” is often
used incorrectly to refer to foods that are open-kettle canned or jars that are
heat-processed in boiling water.
- Enzymes Proteins in food which accelerate many flavor, color, texture, and
nutritional changes, especially when food is cut, sliced, crushed, bruised,
and exposed to air. Proper blanching or hot-packing practices destroy
enzymes and improve food quality.
- Exhausting Removal of air from within and around food and from jars and
canners. Blanching exhausts air from live food tissues. Exhausting or
venting of pressure canners is necessary to prevent a risk of botulism in
low-acid canned foods.
- Fermentation Changes in food caused by intentional growth of bacteria,
yeast, or mold. Native bacteria ferment natural sugars to lactic acid, a major
flavoring and preservative in sauerkraut and in naturally fermented dills.
Alcohol, vinegar, and some dairy products are also fermented foods.
Headspace The unfilled space above food or liquid in jars. Allows for food expansion as jars are heated, and for forming vacuums as jars cool.
- Heat processing Treatment of jars with sufficient heat to enable storing
food at normal home temperatures.
- Hermetic seal An absolutely airtight container seal that prevents reentry of
air or microorganisms into packaged foods.
- Hot pack Heating of raw food in boiling water or steam and filling it hot
- Low-acid foods Foods that contain very little acid and have a pH above
4.6. The acidity in these foods is insufficient to prevent the growth of the
bacterium Clostridium botulinum. Vegetables, some tomatoes, figs, all
meats, fish, seafoods, and some dairy foods are low acid. To control all
risks of botulism, jars of these foods must be (1) heat processed in a
pressure canner, or (2) acidified to a pH of 4.6 or lower before processing in
- Micro-organisms Independent organisms of microscopic size, including
bacteria, yeast, and mold. When alive in a suitable environment, they grow
rapidly and may divide or reproduce every 10 to 30 minutes. Therefore,
they reach high populations very quickly. Undesirable microorganisms
cause disease and food spoilage. Microorganisms are sometimes
intentionally added to ferment foods, make antibiotics, and for other
- Mold A fungus-type microorganism whose growth on food is usually
visible and colorful. Molds may grow on many foods, including acid foods
like jams and jellies and canned fruits. Recommended heat processing and
sealing practices prevent their growth on these foods.
- Mycotoxins Toxins produced by the growth of some molds on foods.
- Open-kettle canning A non-recommended canning method. Food is
supposedly adequately heat processed in a covered kettle, and then filled
hot and sealed in sterile jars. Foods canned this way have low vacuums or
too much air, which permits rapid loss of quality in foods. Moreover, these
foods often spoil because they become recontaminated while the jars are
- Pasteurization Heating of a specific food enough to destroy the most heat-resistant pathogenic or disease-causing microorganism known to be
associated with that food.
- pH A measure of acidity or alkalinity. Values range from 0 to 14. A food is
neutral when its pH is 7.0, lower values are increasingly more acidic, and
higher values are increasingly more alkaline.
- Pickling The practice of adding enough vinegar or lemon juice to a lowacid food to lower its pH to 4.6 or lower. Properly pickled foods may be
safely heat processed in boiling water.
- Pressure Canner A specifically designed metal kettle with a lockable lid
used for heat processing low-acid food. These canners have jar racks, one
or more safety devices, systems for exhausting air, and a way to measure or
control pressure. Canners with 16- to 23- quart capacity are common. The
minimum volume of canner that can be used is one that will contain 4 quart
jars. Use of pressure saucepans with smaller capacities is not recommended.
Raw pack The practice of filling jars with raw, unheated food.
Acceptable for canning low-acid foods, but it allows more rapid quality losses in acid foods heat processed in boiling water.
Spice bag A closeable fabric bag used to extract spice flavors in pickling solution.
Style of pack Form of canned food, such as whole, sliced, piece, juice, or sauce. The term may also be used to reveal whether food is filled raw or hot into jars.
Vacuum The state of negative pressure. Reflects how thoroughly air is removed from within a jar of processed food — the higher the vacuum, the less air left in the jar.
Yeasts A group of microorganisms which reproduce by budding. They are used in fermenting some foods and in leavening breads.
The goal of proper canning is to remove oxygen, destroy enzymes, and kill harmful microorganisms like mold, bacteria, and yeast.
Proper canning will
also produce jars with a strong vacuum seal that will prevent liquid from leaking out or microorganisms from getting into the food.
Properly canned foods can last for several years.
Proper canning practices include:
• Selecting fresh, undamaged foods
• Carefully inspecting and washing fruits and vegetables
• Peeling fresh foods, if necessary
• Using the hot packing method where appropriate
• Adding acids (lemon juice or vinegar) to foods that need acid-packing
• Following recipes and directions precisely
• Using clean jars and lids that seal properly
• Using the right processing time when canning jars in a
boiling-water or pressure canner Collectively, these practices remove oxygen; destroy enzymes; prevent the growth of undesirable bacteria, yeasts, and molds; and help form a high vacuum in jars.
Good vacuums form tight seals, which keep liquid in and
air and microorganisms out.
Most health-related problems arise when people do not follow the canning directions properly.
Today, canning experts agree that old methods of canning and outdated cookbooks give unhealthy or inaccurate directions for food safety. However, the Center for Home Food Preservation, working with the University of Georgia, interviewed home canners and found that they often used unsafe directions or instructions from friends or relatives.
Different methods are now considered best for different types of foods.
The USDA recommends water bath or pressure-canning methods when preserving high-acid products such as pickles, fruits, and tomatoes.
In the past, people canned these products with open-kettle canning, but experts no longer considered this a safe canning method.
Oven and microwave procedures are also considered unsafe.