Food preservation: basics for canning vegetables

Preserving food yourself provides an abundant supply of a variety of foods when fresh products are not readily available. It also allows you to enjoy the bounty of fresh summer produce all year.

Proper canning of fresh vegetables is an excellent way to preserve vegetables for later use. Vegetables that are not canned using current research-based canning methods can pose serious health risks.

Understanding Clostridium botulinum

Pressure canning is the only safe method for home canning vegetables. Vegetables are low­acid foods and must be canned in a pressure canner at the appropriate pressure to guarantee their safety. Deadly foodborne illness can occur when low­acid vegetables are not properly preserved. Clostridium botulinum bacteria causes botulism food poisoning in low­acid foods such as vegetables. In vegetables and meat, the bacteria produce spores that can only be destroyed by temperatures that reach 240 degrees Fahrenheit for the correct amount of time. This can only be accomplished by using a pressure canner.

Clostridium botulinum is harmless until it finds itself in a moist, low­acid, oxygen­free environment or in a partial vacuum. These are the conditions found inside a jar of canned vegetables. Under these conditions, the bacteria can grow and produce toxins dangerous to people and animals.

Do not process low­acid vegetables using a boiling water bath, because botulinum spores can survive.

Caution: All home-canned vegetables should be canned according to the procedures in this fact sheet. Low­ acid and tomato foods not canned according to the recommendations in this publication or according to other United States Department of Agriculture (USDA) endorsed recommendations present a risk of botulism. Even a taste of food containing this toxin can be fatal. Even if there are no signs of spoilage, any deviation from USDA recommended canning practices requires food to be boiled for 10 minutes at altitudes below 1,000 feet to destroy the toxin. For altitudes above 1,000 feet, add one additional minute per 1,000 feet of additional elevation. This process should not be used with food known to be significantly under-processed.

Please be aware that all possible defects and hazards with non-recommended methods may not be overcome by the boiling process described above.

Equipment

Pressure canner

The pressure canner is a large pot with a tight­fitting lid, a clean exhaust vent (or petcock) and safety valve, and an accurate dial pressure gauge or a weighted gauge. The dial pressure-canning gauge needs to be checked for accuracy every year to guarantee proper pressure processing. Check with your local Ohio State University Extension office for information on this service.

Every pressure canner should have a metal rack in the bottom to keep jars elevated which helps prevent breakage.

USDA does not have recommended processing instructions for canning in a small pressure cooker. Small pressure cookers that accommodate less than four, quart jars in a standing position should not be used for canning. Processing times listed in this fact sheet are not adequate for pressure cookers.

Preparing the produce

For best results, select only fresh, young, tender vegetables. If unable to can immediately after harvest or purchase, place the produce in the refrigerator to keep it fresh. Rinse all produce thoroughly.

Soaking vegetables may cause a loss of flavor and nutrients. Handle produce gently to avoid bruising.

Small equipment

Use kitchen utensils that make the canning process more efficient and less frustrating:

• funnels to fit jar openings
• spatulas
• a bubble freer
• a jar lifter for easy removal of jars from canners
• knives
• cutting boards
• a timer or a clock
• clean cloths and towels
• hot pads

Filling jars

Raw-pack method

When vegetables are raw packed, they are cleaned but not heated. Then, they are added to the jar, and the jar is filled with boiling water. Fill the jar with freshly prepared, unheated vegetables, making certain to pack the jar tightly because the vegetables will shrink during the canning process. Corn, lima beans, peas, and potatoes should be packed loosely because they expand during processing. Add boiling water to the jars, leaving the headspace recommended in Table 2 between the jar rim and liquid.

Hot-pack method

Hot-packed vegetables are boiled for a period of time, then the hot food and hot liquid are placed in jars. Hot-packed vegetables should be packed fairly loosely in the jar. Make certain to leave the headspace recommended in Table 2 between the jar rim and the liquid.

Salt is for flavor only and is optional in home-canned vegetables. It does not serve as a preservative. If using salt, the recommended amount is ½ teaspoon per pint and 1 teaspoon per quart for most vegetables.

Closing jars

Remove trapped air bubbles before closing jars. Insert a long, nonmetallic spatula or bubble freer along the inside edge of the jar to allow any excess air to escape. Move the spatula up and down along the inside wall in several places around the jar. Add more boiling liquid if needed. Wipe the jar rim with a clean, damp towel to remove any food particles.

Place a prepared lid on the jar and adjust the metal screw band until fingertip tight. Avoid overtightening.

Using a pressure canner

For more detailed information, refer to HYG-5338-Canning Basics.

If available, use the manufacturer's directions to operate the canner. If a pressure canner is more than 10-years-old, directions may not include steps for fully venting the canner. Following the steps below will ensure proper venting for adequate processing.

Put 2 to 3 inches of hot water in the canner. Place filled, closed jars on the rack using a jar lifter. Fasten canner lid securely and leave vents and petcocks open. Heat at the highest setting until steam flows freely from the vents. Maintain the high heat and exhaust steam for 10 minutes. Then, close the petcock or place the weight on the vent port.

Begin timing the process when the dial gauge or weighted gauge indicates the recommended pressure has been reached. Regulate heat under the canner to maintain a steady pressure at, or slightly above, the correct gauge pressure. When the timed process is completed, turn off the heat, and then remove canner from the heat source if possible. Allow the canner to depressurize. Do not force the cooling of the canner; this could result in liquid boiling out of the jars and seal failure.

When the pressure is at zero, open the petcock, or remove the weight. Allow canner to set for 10 minutes before unfastening and removing the lid. As you remove the lid, direct the remaining trapped steam away from you. Use a jar lifter to remove the jars from the canner, being careful not to tilt the jars. Place jars on a rack, dry towel, or newspapers away from drafts. Do not retighten screw bands; this could break the seals. Also, do not push down on the center of the lid until the jar is completely cooled. Let jars cool 12 to 24 hours before testing seals.

Checking seals and storing

Listen for the familiar "ping" as the jars cool. Look for a slight depression in the lid's center. When completely cooled, test for proper seal by pushing down on the center of the lid (lid should not pop up and down). Allow the canned foods to cool on the countertop before removing the screw bands. Jars should not be stored with the screw bands on. After removing the screw bands, wipe jars with a clean, damp cloth to remove any residue or stickiness. Add labels with date, batch, food product, and any other special information. Store jars in a cool, dry, dark place.

Signs of spoilage

If a jar does not seem completely normal before or after opening, do not use. This includes leaking jars, bulging lids, and jars that spurt when opened. If the food looks spoiled, foams, or has an off odor, discard it. Spoiled food should be discarded in a place where humans or animals will not eat it.

Treat all jars and cans of spoiled, low­acid foods, including tomatoes, as though they contain botulinum toxin and handle in one of two ways:

1. If the suspect glass jars are still sealed, place them in a heavy garbage bag. Close the bag and place it out for trash collection or take it to a landfill.
2. If the suspect glass jars are unsealed, open, or leaking, detoxify (destroy the bacteria) before disposal:

• Carefully place the containers and lids on their sides in an 8­quart or larger pan.
• Wash your hands thoroughly.
• Carefully add water to the pan until it is at least one inch above the containers.
• Avoid splashing the water.
• Place a lid on the pan and heat the water to boiling.
• Boil 30 minutes to ensure that you have destroyed all toxins.
• Cool and discard the lids and food in the trash or take to the landfill.
• Sanitize all counters, containers, and equipment that may have touched the food or containers—don't forget the can opener, your clothing, and your hands.

Place any sponges or washcloths used in the cleanup in a plastic bag and discard them.

Additional resources 

• Download a PDF of this fact sheet.

Note: This fact sheet uses language, recipes, procedures, and data from reliable sources such as the ones listed below. For the most part the original text has been preserved because safety around food preservation is especially important. For safe and healthy preserved foods, be sure to carefully follow directions.

Learn more about the basics for canning vegetables:

• National Center for Home Food Preservation
• USDA Complete Guide to Home Canning

Adapted from information compiled by: Jacqueline LaMuth, Extension Agent, Home Economics, and Marcia Jess, Extension Agent, Family and Consumer Sciences, both retired.

Revised by Christine Kendle, Educator and Area Leader, Food, Agriculture, and Environmental Sciences; Ohio State University Extension; and Katharine Shumaker, Educator, Family and Consumer Sciences, Ohio State University Extension.