How are Canning Times Determined?
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What's On This Page |
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Canning does two things: Foods are heated for specified times to kill the ever-present molds, yeasts, and bacteria that cause food spoilage. Heating also destroys food enzymes that continue the ripening process, eventually resulting in spoilage. Spoiled food can make you sick, the degree or severity of illness ranges from mild to death. Canning means heating foods at a specified temperature for a specified time according to the pH value of the food, the size of the jar, the use of a pressure canner (NOT a pressure cooker) and the altitude where you live. These are not things that you can determine at home, you have`to use correct methods and a modern and tested canning recipe! Some foods with high acid content are safely processed with the boiling-water-bath method (212°), other foods, especially low acid foods, require the higher temperatures reached in a pressure canner at 10 pounds pressure (240°). |
What's All The Fuss About? |
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Every year I get emails from people who say they can their
garden produce in a water bath canner, or they always use their grandmother old-time
recipes and no one ever got botulism. And thank goodness! Canning
times and techniques have changed with modern, scientific
improvements in food safety. Our Grandmothers used information
that was considered safe in their day, but as the
varieties of produce changed so has the canning guidelines.
For example, the heirloom seeds used by home canners
in the past changed, and modern varieties of tomatoes
now yield low acid crops. Tomatoes need to have acidifiers
added, such as lemon juice,
vinegar, or citric acid to all canned tomato products. We enjoy the benefits from current
scientific research and modern technology and
there is no excuse for risking the health of our family by using incorrect
or outdated canning techniques. The next batch may be the one with the deadly toxin
- you can't tell.
Botulism bacteria are present in soil and water and are found naturally on the surface of fresh foods. Washing removes some of them, so does peeling, shelling, and other preparation steps. However, the final "killer" of botulism bacteria is heating food using the proper method and time. Botulism bacteria grow in the absence of air, and if the botulism bacteria haven't been destroyed by proper heating, a sealed jar is a perfect place for them to grow and produce the deadly toxin. |
Why Do Some Foods Take Longer To Process? |
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Why can some be processed at boiling-water temperatures while others must be heated under pressure to higher temperatures? The temperature at which a food is canned is determined by the amount of acid in the food. In canning, foods are divided into two groups: High Acid High acid foods contain more natural acids. Many fruits are high acid foods and the presence of these natural acids helps prevent growth of some spoilage microorganisms. If the food product has a high enough acid level, boiling-water temperatures are high enough to destroy spoilage organisms. This is a prevention method for the deadly Clostridium botulinum bacteria. Low Acid Low acid foods, such as vegetables and meat products, contain very little natural acid. They must be processed at higher than boiling-water temperatures to destroy any Clostridium botulinum bacteria. Water boils at 212 degrees F, at sea level, and at a lower temperature at higher elevations. Turning up the temperature under the pot or letting the water boil for a long time does not raise the temperature of the water above its boiling point. To make water boil at a higher temperature, it has to be put under pressure, such as in a pressure canner. When a food is processed at 10 pounds pressure, the water boils when it gets to 240°, rather than at 212°. This is high enough to kill the bacteria that causes botulism poisoning. Pressure canners must be used to process vegetables, meats, poultry, seafood and soups. Mixed foods that contain both high and low acid foods, are not recommended, but if done, must be processed using the time for the vegetable with the longest pressure canning processing time. |
Acidity Determines Processing Times |
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Whether you should process food in a pressure canner or boiling-water canner to control botulinum bacteria depends on the amount of acid in the food. The term "pH" is a measure of acidity. The lower the pH, the more acid the food. Acid foods include pickles, most fruits and jams and jellies made from fruit. (In pickling, the acid level is increased by adding lemon juice, citric acid or vinegar.) Acid foods contain enough acidity either to stop the growth of botulinum bacteria or destroy the bacteria more rapidly when heated. Low-acid foods don't contain enough acid to prevent the growth of botulinum bacteria. Process these foods at temperatures of 240 degrees F to 250 degrees F. To reach these high temperatures, you must use a pressure canner operated at 10 to 15 pounds per square inch of pressure (PSI). The exact time depends on the kind 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 acid foods, assume that the mixture remains low-acid. Although tomatoes used to be considered an acid food, some are now known to have pH values slightly above 4.6, which means they are low-acid. To safely can them as acid foods in a boiling-water canner, you must add lemon juice or citric acid. |
Adjust For Altitude To Ensure Safety |
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It's important that you know your altitude.. Don't use process times recommended for canning food at sea level if you live at altitudes above 1,000 feet. Water boils at lower temperatures as altitude increases. Lower boiling temperatures are less effective for killing bacteria. You must increase either the process time or canner pressure to make up for lower boiling temperatures. Because altitude affects pressure and the boiling point of liquid, adjustments must be made when canning foods at altitudes of 1,000 feet above sea level or higher. When using the boiling water bath method, processing time must be increased. Add 5 minutes to processing time for altitudes between 1,000 and 6,000 feet above sea level. When using the pressure canner method, pressure must be increased. If using a dial-gauge pressure canner, process foods at 12 pounds pressure for altitudes between 2,000 and 4,000 feet and at 13 pounds pressure for altitudes between 4,000 and 6,000 feet. If using the weight-gauge pressure canner, use 15 pounds of pressure rather than 10. |
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