Food Canning Basics
Revised 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.
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:
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:
Use Safe Equipment and Canning Methods
Sterilization of Empty 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:
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.
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.
© 2004, 2011
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Image Description: vegetables and jars of canned food; photo by Edwin Remsberg