Organisms that cause food spoilage -- molds, yeasts and bacteria -- are present everywhere in the air, soil and water. Enzymes that may cause undesirable changes in flavor, color and texture are present in raw vegetables.
When vegetables are canned, they are heated hot enough and long enough to destroy spoilage organisms. This heating (or processing) also stops the action of enzymes.
Pressure canning is the only safe method of canning all vegetables (except tomatoes). Jars of food are placed in a pressure canner, which is heated to a temperature of at least 240º F. This temperature can be reached only in a pressure canner.
The Clostridium botulinum mircoorganism is the main reason pressure canning is necessary. Though the bacterial cells are killed at boiling temperatures, the spores they form can withstand these temperatures. The spores grow well in low acid foods in the absence of air, such as in canned low acid foods (vegetables and meats). When the spores begin to grow, they produce the deadly botulinum toxins (poisons). This growth can occur without any noticeable signs of spoilage in the sealed jar.
Luckily, these spores can be destroyed by canning the food at a temperature of at least 240º F. This temperature is above the 212º F boiling point of water, so it can only be reached in a pressure canner at the appropriate pressure (10 pounds at sea level). Because most people do not can at sea level, use the pressure given with the directions for canning vegetables. The pressures are different for dial and weighted gauge canners, because the weighted gauge canners have extra weight built in. Also the pressures are different for different altitudes.
A pressure canner is necessary for home canning. It must have a rack in the bottom, a tight-fitting cover, exhaust vent (or petcock), safety valve and an accurate pressure gauge. The pressure gauge may be a weighted gauge that fits over the vent or a dial gauge on which a needle indicates the pressure inside the canner. Newer canners may also have a extra cover lock as an added safety feature. Canning jars should be checked closely for cracks or chips. Use jars specifically designed for home canning. Commercial food jars such as mayonnaise or coffee jars break easily, especially in pressure canners, and may not seal. Use only one half-pint, pint and quart sizes.
Before using the jars, wash them in hot soapy water and rinse well. Keep the jars hot until they are filled and placed in the canner. This will help prevent jar breakage.
Two-piece metal canning lids need to be prepared for use. The lids can be used only once, but the screw bands can be reused as long as they are in good condition. Read the manufacturer's instructions for treating the lids. Some need to be covered with hot water, while others need to be boiled for a minute for more. Do not reuse lids from commercially canned foods for home canning.
Preparing the Vegetables
Select only fresh, young, tender vegetables for canning. The sooner you can get them from the garden to the jar, the better. If you buy vegetables to can, try to get them from a nearby garden or orchard.
For ease of packing and even cooking, sort the vegetables for size and ripeness. Wash all vegetables thoroughly whether or not they will be pared. Dirt contains some of the bacteria hardest to kill. Do not let vegetables soak; they may lose flavor and nutrients. Handle them gently to avoid bruising.
Filling the Jars
Vegetables may be packed raw into jars or pre-heated and packed hot. Read the directions for each vegetable to determine which method may be used. If given a choice, the hot pack produces a higher quality product. Remember, have the jars hot to prevent breakage as they are filled.
* To raw pack -- Put raw vegetables into jars and cover with boiling water. Most raw vegetables (except for starchy ones) should be packed firmly into the jars. Starchy vegetables such as corn, peas and lima beans should be packed loosely, because they expand during processing.
* To hot pack -- Heat vegetables in water or steam before packing. Then cover with the boiling cooking liquid. Pack the hot food loosely.
For either pack, use enough liquid to fill around the solid food in the jar and to cover the food. See directions for each vegetable for the correct amount of head space to leave between the top of the food and the top of the jar. This head space is important for obtaining a good seal.
Salt may be added to each jar, if desired. The salt is only for seasoning and does not help to preserve the food. If salt is used, canning salt is recommended to prevent the liquid from turning cloudy. Usually half teaspoon salt per pint is adequate.
Closing the Jars
To remove any trapped air bubbles, slide a non-metallic spatula between the food and the sides of the jar. Add more liquid if necessary to obtain the proper head space. Wipe the jar rim with a clean damp cloth to remove any food particles.
Place the treated lid on the jar. Screw the screw band fingertip tight.
Read the manufacturer's instructions on the use of your pressure canner. The following are general instructions:
* Have the 2 to 3 inches of water in your canner hot but not boiling if you are canning raw pack foods. For hot pack foods, the water can be hot or simmering.
* Place the jars of food on the rack in the canner so steam can flow around each jar.
* Fasten the canner lid so no steam can escape except through the vent
* Turn heat to high and watch until steam begins to escape from the vent. Let the steam escape steadily for 10 minutes.
* Close the vent, using a weight, valve or screw, depending on the type of canner. If you have a weighted gauge canner that has a weight of varying pressures, be sure you are using the correct pressure.
* For a dial gauge canner, let the pressure rise quickly to 8 pounds of pressure. Adjust the burner temperature down slightly and let the pressure continue to rise to the correct pressure. (If the burner were left on high, the pressure would be hard to regulate when the correct pressure is reached.) Start counting the processing time as soon as the pressure is reached. For weighted gauge canners, let the canner heat quickly at first and then reduce the heat slightly until the weight begins to rock gently or "jiggle" two or three times per minute, depending upon the type of canner you have. Start counting the processing time as soon as the weight does either of these.
* Keep the pressure constant by regulating the heat under the canner. Do not lower the pressure by opening the vent or lifting the weight. Keep drafts from blowing on the canner.
* When processing time is completed, carefully remove the canner from the heat. If the canner is too heavy, simply turn off the heat.
* Let the pressure in the canner drop to zero. This will take 30 to 45 minutes in a 16-quart canner and nearly an hour for a 22-quart canner. Do not rush cooling by setting the canner in water or by running cold water over the canner. Never lift the weight or open the vent to hasten the reduction of pressure.
* When the gauge on a dial gauge canner registers zero or when a gentle nudge to the weight on a weighted gauge canner does not produce steam or resistance, wait two minutes, and then open the vent or remove the weight. Wait two more minutes and then open the canner.
* Unfasten the lid, and tilt the far side up, so the steam escapes away from you. Do not leave the canner unopened, or the food inside could begin to spoil.
* Carefully remove the jars from the canner. To prevent the jars from breaking on contact with a cold surface, place the hot jars on a rack, dry towels, boards or newspaper, right side up.
* Allow the jars to cool untouched, away from drafts. Do not be alarmed at popping sounds as the jars cool and seal.