Food Preservation: Basics for Canning Vegetables

Preserving food yourself means having an abundant supply of a variety of foods when the fresh products are not readily available. It also allows you to save the bounty of fresh produce of summer to enjoy 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 low-acid foods and must be canned in a pressure canner at the appropriate pressure to guarantee their safety. Deadly foodborne illness can occur when low-acid vegetables are not properly preserved. Clostridium botulinum is the bacterium that causes botulism food poisoning in low-acid foods such as vegetables. In vegetables and meat, the bacteria produce spores that can only be destroyed by temperatures that reach 240°F for the correct amount of time in a pressure canner.

The Clostridium botulinum bacterium is harmless until it finds itself in a moist, low-acid, oxygen-free environment or in a partial vacuum. These are the conditions found inside a jar of canned vegetables.

Under these conditions, the bacterium can grow and produce toxins dangerous to people and animals.

Do not process low-acid vegetables using a boiling water bath, because botulinum spores can survive that method.

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 USDA-endorsed recommendations present a risk of botulism. If it is possible that any deviation from the USDA-endorsed methods occurred, to prevent the risk of botulism, low-acid and tomato foods should be boiled in a saucepan before consuming even if you detect no signs of spoilage. At altitudes below 1,000 feet, boil foods for 10 minutes. Add an additional minute of boiling time for each additional 1,000 feet of elevation. However, this is not intended to serve as a recommendation for consuming foods known to be significantly under processed according to current standards and recommended methods. It is not a guarantee that all possible defects and hazards with nonrecommended methods can be overcome by this boiling process. The recommendation is to can only low-acid and tomato foods according to the procedures in this fact sheet.

Equipment

Pressure Canner
The pressure canner is a heavy pot that has a tight-fitting lid, a clean exhaust vent (or petcock) and safety valve, and an accurate pressure gauge or a weighted gauge. The pressure canning gauge needs to be checked for accuracy every year to guarantee proper pressure processing. Check with your local OSU Extension office for information on this service.

Every pressure canner should come with a metal rack in the bottom to help keep jars elevated and prevent breakage.

USDA does not have recommended processing instructions for canning in a small pressure cooker. Small pressure cookers that hold less than 4-quart jars should not be used for canning. The research for USDA pressure processing for vegetable and meat products was conducted in pressure canners that are most similar to today's 16-quart or larger pressure canners. Processing times listed in this fact sheet are not adequate for smaller cookers.

Preparing the Produce
For best results, select only fresh, young, tender vegetables, and can them within three hours of harvesting. If canning takes place later, place the produce in the refrigerator to keep it fresh. Rinse all produce thoroughly. Soaking vegetables will cause loss of flavor and nutrients. Handle gently to avoid bruising.

Small Equipment
The following kitchen utensils are not mandatory but 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; and hot pads.

Filling Jars

Hot Pack Method
Vegetables that are hot packed have come to a boil for a period of time and have then had hot food and hot liquid placed in jars. Hot packed vegetables should be packed fairly loosely in the jar because the cooking has already caused shrinkage to take place. A food funnel that fits the top of the jar is helpful when canning small vegetables. Make certain to leave headspace between the jar rim and the liquid according to the table at the end of this fact sheet.

Raw Pack Method
When vegetables are raw packed, they are cleaned but not heated. Then, they are simply 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. Add boiling water to the jars, leaving recommended headspace between the jar rim and liquid. Corn, lima beans, potatoes and peas should be packed loosely because they expand during processing.

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 paper towel to remove any food particles.

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

Using a Pressure Canner

If available, use the manufacturer's directions for canner operation. Otherwise, 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 that the recommended pressure has been reached. When the timed process is completed, turn off heat and remove canner from the heat source. 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 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 to not tilt the jars. Then, place jars on rack, dry towel or newspapers. Do not retighten ringbands; 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 the slight depression in the lid's center. When completely cooled, test for proper seal by pushing down on center of lid; lid should not pop up and down. Allow the canned foods to cool on the countertop before removing the ringbands. Jars should not be stored with the ringbands on. After removing the ringbands, 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 during preparation, discard it.

Boil all spoiled, low-acid canned foods for 30 minutes before disposing of them. This destroys any toxins present and prevents their spread. Sanitize all containers and equipment that might have touched the food. Discard any sponges that might have been used in the cleanup. Place them in a plastic bag and discard in the trash.

References
Andress, E., and J. Harrison. So Easy to Preserve. 6th ed. Athens: University of Georgia Cooperative Extension Service, 2014.
United States Department of Agriculture. Complete Guide to Home Canning, Agriculture Information Bulletin No. 539. 2009.

Adapted from information compiled by: Jacqueline LaMuth, Extension Agent, Home Economics, and Marcia Jess, Extension Agent, Family and Consumer Sciences, both retired. (Originally published in 2015.)