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Bulletin #4078, Let’s Preserve: Food Canning Basics

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Let’s Preserve

Food Canning Basics

vegetables and jars of canned food; photo by Edwin RemsbergRevised by Jason Bolton, Beth Calder, and Kathy Savoie, University of Maine Cooperative Extension and Department of Food Science and Human Nutrition.
Adapted from Complete Guide to Home Canning, USDA bulletin No. 539.

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Canning preserves food primarily by using heat to destroy the bacteria that cause spoilage. Heat processing forces air out of the jar causing a vacuum to occur. When the jar cools, a seal forms. The processing times and temperatures noted in Cooperative Extension and other approved publications have been set using scientific research. For safe, high-quality home-canned food, it’s important that you follow these directions carefully. Altering these directions in any way can result in improperly canned food, which can be dangerous to consume.

How Canning Preserves Food

Fresh foods spoil for a variety of reasons. Microorganisms such as bacteria, molds, and yeasts can cause spoilage. In addition, enzymes naturally found in many foods can cause spoilage. Microorganisms live and multiply quickly on the surfaces of fresh food and inside bruised, damaged food.

Proper canning techniques will stop the growth and activity of microorganisms and can prevent spoilage and quality loss. Use these techniques to ensure safe food canning practices:

  • Carefully select and wash fresh food. DO NOT use produce from diseased plants or those that have been frost killed.
    • Be sure to use clean potable water to wash fresh fruits and vegetables.
  • Prepare canned foods according to Cooperative Extension publications or fact sheets, reputable canning books or other approved canning authority recommendations.
    • Approved recipe sources include, but are not limited to: Ball canning books such as the Ball Blue Book Guide to Preserving, So Easy to Preserve, the USDA Complete Guide to Home Canning, and UMaine Cooperative Extension’s Let’s Preserve canning series. Please contact your local county office for updated UMaine Cooperative Extension publications or visit extensionpubs.umext.maine.edu. Another canning Internet resource is the National Center for Home Food Preservation.
  • Use recommended jars with dome lids and screwbands.
    • Two-piece dome lids with metal screw bands are recommended.
    • Lids should be used only once.
    • Canning jars with wire bails and rubber seals are no longer recommended.
    • One-piece zinc, porcelain-lined caps, and plastic one-piece lids are also no longer recommended. They do not form a proper seal.
    • Paraffin wax is no longer recommended.
    • Glass canning jars may be used several times as long as they are free of chips and are cleaned thoroughly. The rim should be inspected carefully for defects.
  • Process jars in a boiling-water bath or pressure canner according to the instructions for the correct period of time.

For Safety’s Sake

Pressure canning is the only canning method recommended for foods that are naturally low in acid, which means the pH of the food is above 4.6, such as meat, poultry, seafood, and vegetables. Clostridium botulinum is a spore-forming bacteria that can cause a foodborne illness called botulism from eating improperly canned foods. The botulinum toxin produced by this bacteria is potent, and according to the Centers for Disease Control and Prevention, botulism is one of the deadliest toxins. Just one small taste of contaminated food with this toxin can cause paralysis or it could be lethal. This bacteria is destroyed in low acid foods when they are processed at the correct time and temperature in pressure canners only. Pressure canners reach a higher temperature than a boiling-water bath.

Canning low acid foods in a boiling-water bath canner is absolutely unsafe because the C. botulinum spores can survive this process. For example, hot-water bath canning green beans is not recommended and is an unsafe practice because of the botulism risk.

Using acceptable canning recommendations from reliable recipes and sources are the only way to be sure home canned foods are safe to eat. Use the following precautions:

  • Process low acid foods in a pressure canner with an accurate gauge.
    • If using a pressure canner with a dial gauge, it should be checked for accuracy yearly.
    • Call your local UMaine Cooperative Extension county office for free accuracy check.
  • Make sure the recipe process time and pressure match the size of the jar, style of pack, and type of food being canned.
  • Be sure the jar lid is firmly sealed after processing and check that the seal is concave (curved inward).
  • Check that nothing has leaked from the jar.
  • Check for unusual or “off” odors. Please be sure to dispose of canned foods with off-colors and odors.

Use Safe Equipment and Canning Methods

  • Never open-kettle can or process jars of foods in conventional ovens, microwave ovens or dishwashers. These practices do not prevent spoilage.
  • Sterilize glass jars before canning (see sterilization section below).
  • Steam canners are not recommended because safe processing times have not been adequately researched. Using boiling-water canner processing times with steam canners may result in spoilage or foodborne illness. Canning powders are not recommended to be used as preservatives and do not replace the need for proper heat processing.
  • Do not use a pressure cooker to can foods. They do not use a calibrated weight gauge and are not made for canning purposes.

Sterilization of Empty Jars

  • High acid foods, such as fruit-based jams and jellies that are heated for less than 10 minutes, should only be filled into jars that have been properly sterilized. To sterilize jars, first place empty jars right side up on a rack in a boiling-water canner. Fill the canner with warm water, filling it 1 inch above the tops of the jars and bring water to a boil. Once the water comes to a rolling boil, start the timer and continue boiling for 10 minutes. If at altitudes of 1,000 feet or more, add 1 minute for every additional 1,000 feet in elevation. Carefully remove and drain the jars and set them on a cooling rack upside down. Always fill canning jars while they are still hot. Be sure to wipe off any spills from the jar rims. Place the lid level onto the jar and finger-tighten with screw bands.
  • Glass jars to be used for low acid, pressure canned food do not have to be sterilized prior to filling.
  • Sterilizing jars in a microwave, dishwasher or oven are not approved methods and do not adequately sterilize jars.

Food Acidity Affects Processing Methods

Whether you should process food in a pressure canner or boiling-water canner to control botulinum bacteria all depends on the amount of acid in the food. The term “pH” is a measure of acidity or alkalinity in the food (similar to measuring pH in pool water), and the pH scale ranges from 0-14. A pH less than 7 is acidic and a pH greater than 7 is basic. Foods that fall under a pH of 4.6 are considered to be high acid foods. High acid foods include most fruits, especially berry fruits.

In pickling, an acid is added to decrease the pH by adding lemon juice, citric acid or commercial vinegar. High acid foods contain enough acidity to prevent the growth of botulinum producing bacteria without having to add acid. Low acid foods do not contain enough acid to prevent the growth of botulinum bacteria. These foods require processing at temperatures of 240°F to 250°F. These high temperatures are attainable only with pressure canners operated at 10 to 15 PSI (PSI means pounds per square inch of pressure). The exact processing time depends on the type of food being canned, the way it is packed into jars, and the size of jars. Low acid foods include red meats, seafood, poultry, milk, all fresh vegetables, and some tomatoes. When you mix low acid and high acid foods, assume the mixture is low acid and therefore must be acidified or pressure canned.

Although tomatoes used to be considered a high acid food, some varieties are now known to have pH values slightly above 4.6. Tomatoes may still be water-bath canned, but lemon juice or citric acid must be added. For specific instructions on canning tomatoes, follow the guidelines from an approved canning resource such as the Let’s Preserve fact sheet on tomatoes.

Spot Spoilage by Careful Examination

It is not recommended to taste foods that show any signs of spoilage, and never taste food from a jar with an unsealed or rusty lid. Some types of spoilage are easier to detect in jars stored without screw bands. When bacteria and yeast grow, they produce a gas that swells lids and breaks jar seals. Examine lids for tightness and vacuum. Lids with concave (curved inward) centers have maintained proper seals.

Next, hold the jar at eye level and while rotating the jar, look for streaks of dried food that has dripped down the exterior. Also, check for rising air bubbles and unnatural color in the food.

While opening the jar, be sure to smell for possible off-odors. Also watch for signs of bubbling or cotton-like mold growth (white, blue, black or green) on the food surface and underside of the lid.

If You Suspect Spoilage, Handle with Care

Treat all jars of spoiled low acid foods, including tomatoes, as if they contained the botulinum toxin, and handle them in one of two ways:

  1. If the canned foods are still sealed, place them in a heavy garbage bag. Close the bag and place it in a regular trash container or dispose of it at a nearby landfill.
  2. If canned foods are unsealed, open or leaking, follow the detoxification process below.

Detoxification process: Put on disposable rubber gloves. Carefully place the containers and lids in a large pot (eight-quart or larger). Carefully add water to cover at least 1 inch above the jars and lids in the pot. 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 bacteria and possible botulinum toxin. Cool and discard the containers, lids, and food in the trash.

Thoroughly scrub (with soap and water and then use a bleach sanitizer) for all counters, containers, and equipment that may have touched the food or containers; don’t forget to clean the can opener, your clothing, and hands. Place any gloves, sponges or washcloths used for cleaning in a plastic bag and discard in the trash.

Safe Canning

Just to summarize, be sure to use recommended sources for canning recipes, follow recipe instructions, use the recommended canning methods, lids, and equipment. Canning is a great way to preserve your harvest and enjoy your produce into the winter months, but it is important to follow proper canning practices to ensure safe food products for you and your family.

Other Reliable Sources


Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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