Guide Principles of Home Canning
Why Can Foods?
Canning can be a safe and economical way to preserve quality food at home. Disregarding the value of your labor, canning homegrown food may save you half the cost of buying commercially canned food. Canning favorite and special products to be enjoyed by family and friends is a
fulfilling experience and a source of pride for many people.
Many vegetables begin losing some of their vitamins when harvested. Nearly half the vitaminsmay be lost within a few days unless the fresh produce is cooled or preserved. Within 1 to 2 weeks, even refrigerated produce loses half or more of some of its vitamins. The heating process
during canning destroys from one-third to one-half of vitamins A and C, thiamin, and riboflavin. Once canned, additional losses of these sensitive vitamins are from 5 to 20 percent each year.
The amounts of other vitamins, however, are only slightly lower in canned compared with fresh
food. If vegetables are handled properly and canned promptly after harvest, they can be more
nutritious than fresh produce sold in local stores.
The advantages of home canning are lost when you start with poor quality fresh foods; when jars
fail to seal properly; when food spoils; and when flavors, texture, color, and nutrients deteriorate
during prolonged storage.
The information and guides that follow explain many of these problems and recommend ways to
How Canning Preserves Foods
The high percentage of water in most fresh foods makes them very perishable. They spoil or lose their quality for several reasons:
•growth of undesirable microorganisms-bacteria, molds, and yeasts,
•activity of food
•reactions with oxygen,
Microorganisms live and multiply quickly on the surfaces of fresh food and on the inside of bruised, insect-damaged, and diseased food. Oxygen and enzymes are present throughout fresh food tissues. Proper canning practices include:
•carefully selecting and washing fresh food,
•peeling some fresh foods,
•hot packing many foods,
•adding acids (lemon juice or vinegar) to some foods,
•using acceptable jars and self-sealing lids,
•processing jars in a boiling-water or pressure canner for the correct period of time.
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.
Ensuring Safe Canned Foods
Growth of the bacterium Clostridium botulinum in canned food may cause botulism-a deadly
form of food poisoning. These bacteria exist either as spores or as vegetative cells. The spores, which are comparable to plant seeds, can survive harmlessly in soil and water for many
years. When ideal conditions exist for growth, the spores produce vegetative cells which multiply
rapidly and may produce a deadly toxin within 3 to 4 days of growth in an environment consisting of:
•a moist, low-acid food
•a temperature between 40 and 120 F
•less than 2 percent oxygen
Botulinum spores are on most fresh food surfaces. Because they grow only in the absence of air, they are harmless on fresh foods.
Most bacteria, yeasts, and molds are difficult to remove from food surfaces. Washing fresh food
reduces their numbers only slightly. Peeling root crops, underground stem crops, and tomatoes
reduces their numbers greatly. Blanching also helps, but the vital controls are the method of
canning and making sure the recommended research-based process times, found in these
guides, are used.
The processing times in these guides ensure destruction of the largest expected number of
heat-resistant microorganisms in home-canned foods. Properly sterilized canned food will be free
of spoilage if lids seal and jars are stored below 95 F. Storing jars at 40 to 70 F enhances
retention of quality.
Food Acidity and Processing Methods
Whether food should be processed in a pressure canner or boiling-water canner to control
botulinum bacteria depends on the acidity of the food. Acidity may be natural, as in most fruits, or
added, as in pickled food. Low-acid canned foods are not acidic enough to prevent the growth of
these bacteria. Acid foods contain enough acid to block their growth, or destroy them more
rapidly when heated. The term "pH" is a measure of acidity; the lower its value, the more acid the
food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.
Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk,
and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also
have pH values above 4.6 unless their recipes include enough lemon juice, citric acid, or vinegar
to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles,
sauerkraut, jams, jellies, marmalades, and fruit butters.
Although tomatoes usually are considered an acid food, some are now known to have pH values
slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be
canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice
or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in
a boiling-water canner.
Botulinum spores are very hard to destroy at boiling-water temperatures; the higher the canner
temperature, the more easily they are destroyed. Therefore, all low-acid foods should be
sterilized at temperatures of 240 to 250 F, attainable with pressure canners operated at 10 to
15 PSIG. PSIG means pounds per square inch of pressure as measured by gauge. The more 1-5
familiar "PSI" designation is used hereafter in this publication. At temperatures of 240 to 250 F,
the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The
exact time depends on the kind of food being canned, the way it is packed into jars, and the size
of jars. The time needed to safely process low-acid foods in a boiling-water canner ranges from 7
to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes.