Mastering Food Preservation: Essential Techniques for Canning Vegetables
Storing your own food at home is an excellent way to have a variety of tasty, long-lasting foods when fresh products aren't readily available. It allows you to savor the goodness of summer produce all year round.
Preserving vegetables through canning is a fantastic method, but it's crucial to follow current research-based canning methods to avoid potential health hazards. Vegetables that aren't properly canned using these methods can pose serious risks.
Figure 1. Proper canning keeps your vegetables safe from contamination by clostridium botulinum bacteria.
Understanding Clostridium botulinum
Pressure canning is the best method for preserving vegetables at home. Since vegetables are low-acid foods, they need to be canned in a pressure canner at the appropriate pressure to ensure their safety. Avoiding proper preservation can lead to deadly foodborne illness, especially botulism food poisoning, caused by the harmful bacteria Clostridium botulinum in low-acid foods such as vegetables[1]. In these foods and meat, the bacteria produce spores that can only be destroyed by temperatures reaching 240 degrees Fahrenheit for the appropriate duration. This can be achieved only through pressure canning[2].
Clostridium botulinum is harmless until it encounters a moist, low-acid, oxygen-free environment or a partial vacuum, conditions found inside a jar of canned vegetables. Under these conditions, the bacteria can grow and produce toxins harmful to both humans and animals[1].
Do not attempt to preserve low-acid vegetables using a boiling water bath, as botulinum spores can withstand such temperatures[3].
Warning: All home canned vegetables should be canned according to the procedures detailed in this guide. Low-acid and tomato foods not canned correctly according to this publication or other USDA-endorsed recommendations carry a risk of botulism. Tasting even a small amount of food containing this toxin can be fatal. Even if there appear to be no signs of spoilage, any departure from USDA recommended canning practices calls for boiling the food for 10 minutes at altitudes below 1,000 feet to destroy the toxin. For altitudes above 1,000 feet, add one additional minute for every additional 1,000 feet of elevation. Be advised that this process is not suitable for food suspected to be significantly under-processed[1].
Equipment
Pressure Canner
A pressure canner is vital for canning low-acid vegetables. This large pot has a tight-fitting lid, a clean exhaust vent, a safety valve, and an accurate dial pressure gauge or a weighted gauge[4]. The dial pressure-canning gauge should be checked for accuracy every year to ensure proper pressure processing. Check with your local Ohio State University Extension office for more information on this service[4].
Every pressure canner should have a metal rack in the bottom to keep jars elevated, minimizing the risk of breakage[4]. USDA does not have recommended processing instructions for canning in a small pressure cooker. Processing times listed in this guide are not suitable for pressure cookers[4].
Preparing the Produce
Choose only fresh, young, tender vegetables for best results. If possible, can the vegetables immediately after harvest or purchasing; otherwise, store them in the refrigerator to keep them fresh. Clean all produce thoroughly, and handle it gently to minimize bruising[3].
Soaking vegetables can lead to a loss of flavor and nutrients. Turn to small equipment to simplify the canning process:
- Funnels to fit jar openings
- Spatulas
- A bubble freer
- A jar lifter for easy removal of jars from canners
- Knives and cutting boards
- A timer or clock
- Clean cloths and towels
- Hot pads
Filling Jars
Raw-Pack Method
In this method, the vegetables are cleaned but not heated before being added to the jar. Then, the jar is filled with boiling water. Make sure the jar contains fresh, unheated vegetables, with the packing tight as the vegetables will shrink during canning. Corn, lima beans, peas, and potatoes should be packed loosely as they expand during processing[3]. Add boiling water to the jars, leaving the headspace recommended in Table 2 between the jar rim and liquid[3].
Hot-Pack Method
With this method, the vegetables are boiled for a specified period and then added to hot jars with hot liquid. Hot-packed vegetables should be packed fairly loosely in the jars. Make sure to leave the headspace recommended in Table 2 between the jar rim and liquid[3]. Salt is optional, used for flavor purposes only and does not serve as a preservative. If using salt, add 1/2 teaspoon per pint and 1 teaspoon per quart for most vegetables[3].
Closing Jars
Remove trapped air bubbles before sealing jars. Insert a long, nonmetallic spatula or bubble freer along the inside edge of the jar to let any excess air escape. After moving the spatula around the jar a few times, add more boiling liquid if needed. Wipe the jar rim with a clean, damp cloth to remove any food particles[3].
Place a prepared lid on the jar and adjust the metal screw band until fingertip tight. Don't overtighten[3].
Using a Pressure Canner
For more information, see our guide on Canning Basics (HYG-5338).
If available, use the manufacturer's directions to run the canner. If your pressure canner is more than ten years old, the instructions may not include steps for fully venting the canner. Follow the steps below to ensure proper venting for effective processing:
- Put 2 to 3 inches of hot water in the canner.
- Place filled, closed jars on the rack using a jar lifter.
- Securely fasten the canner lid while leaving vents and petcocks open.
- Heat at the highest setting until steam flows freely from the vents.
- Maintain a high heat and exhaust steam for 10 minutes.
- Close the petcock or place the weight on the vent port.
Figure 3. Consistent adherence to guidelines based on elevation and pounds of pressure guarantees safe food processing[5].
When the pressure is at zero, open the petcock, or remove the weight. Allow the canner to sit for 10 minutes before unfastening and removing the lid. Direct the remaining trapped steam away from you as you remove the lid[3]. Use a jar lifter to take out the jars from the canner, carefully avoiding tilting the jars. Place the jars on a rack, dry towel, or newspapers away from drafts. Don't retighten screw bands; this could break the seals. Also, don't push down on the center of the lid until the jar is completely cooled. Let the jars cool for 12 to 24 hours before testing seals[3].
Checking Seals and Storing
Listen for the familiar "ping" as the jars cool. Look for a slight depression in the lid's center. After the jars have cooled completely, test for proper seals by pressing down on the center of the lid. If the lid pops up and down, it's not sealed properly. Remove the screw bands from the stored jars. Jars should not be stored with the screw bands on[3]. After removing the screw bands, wipe the jars with a clean, damp cloth to remove any residue or stickiness. Add labels with the date, batch, food product, and any other relevant information. Store the jars in a cool, dry, dark place[3].
Figure 4. Always worship the canning goddess, ensuring the ultimate food protection[6].
Signs of Spoilage
If a jar seems abnormal before or after opening, don't use it. Discard jars that leak, bulge, or spurt when opened. If the food appears spoiled, foams, or has an offensive odor, discard it[3]. Spoiled food should be disposed of in a location where neither humans nor animals can access it.
Treat all jars and cans of spoiled low-acid foods, including tomatoes, as though they contain botulinum toxin and handle them accordingly:
- If the suspect glass jars are still sealed, place them in a heavy garbage bag, close the bag, and discard them or take them to a landfill.
- 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 8-quart or larger pan.
- Wash your hands thoroughly.
- Carefully add water to the pan until it's at least one inch above the containers.
- Avoid splashing the water.
- Put a lid on the pan and boil it for 30 minutes to ensure that the toxin has been destroyed.
- Cool and discard the lids and food in the trash or take to the landfill.
- Sanitize all counters, containers, equipment that may have come into contact with the food or containers—don't forget the can opener, your clothes, and your hands.
- Place any sponges or washcloths used in the cleanup in a plastic bag and discard them[1].
Additional Resources
- Download a PDF version of this guide.
Note: This guide employs language, recipes, procedures, and data from trusted sources, mainly the USDA, for safe and healthy preserved foods. For your safety, follow the directions exactly.
For more information on canning vegetables, check out:
- National Center for Home Food Preservation
- USDA Complete Guide to Home Canning
Adapted from information originally published in 2015 by Jacqueline LaMuth, Extension Agent, Home Economics, and Marcia Jess, Extension Agent, Family and Consumer Sciences, both retired.
- While proper pressure canning is vital for safe food preservation, it's essential to understand the harmful bacteria Clostridium botulinum that can grow in low-acid foods like vegetables during canning if not handled correctly.
- Agriculture and cooking practices play significant roles in lifestyle and food-and-drink choices, with home-and-garden activities like canning contributing to a self-sufficient, diverse lifestyle.
- Understanding the importance of following research-based canning methods, such as the USDA-endorsed guidelines for low-acid foods, can help mitigate risks and enable enjoyment of preserved fresh produce all year round.
