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Canning -- Miscellaneous

Why do you have to pressure can low-acid foods?

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Following proper canning procedures is essential to making food safe. Canning is simply heating food in a glass jar with a lid in place for a specified amount of time. Heating kills microorganisms - bacteria, yeast, and mold - that spoil food or make it unsafe to eat. There are two ways that food can be canned. The method that you use depends on the type of food that you will be canning. High-acid foods can be safely processed in a boiling water canner. High-acid foods include all fruits except tomatoes and figs. Microorganisms in or on high-acid foods are easily killed at 212 F (the temperature of boiling water). If you process tomatoes and figs in a boiling water canner, you must acidify them. Add two tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes. For pints, use one tablespoon bottled lemon juice or 1/4 teaspoon citric acid. Acid can be added directly to the jars before filling with product. Four tablespoons of a 5% acidity vinegar per quart may be used instead of lemon juice or citric acid. However, vinegar may cause undesirable flavor changes. Not properly acidifying tomatoes or figs could make them unsafe to eat. Low-acid foods must always be processed in a pressure canner. Low-acid foods include unacidified tomatoes and figs, all vegetables, meat, fish, and poultry. The bacteria that cause botulism cannot grow in high-acid foods but can grow in low-acid foods. These bacteria (Clostridium botulinum) have spores that survive hours in boiling water. However, they are easily destroyed at 240 F (the temperature reach inside a dial-gauge canner at 11 lbs. pressure). You cannot reach this temperature in a boiling water canner. If low-acid food is processed in a boiling water canner, botulinum spores will survive. In the absence of air, a condition found inside a jar after processing, the spores become living bacteria. As the bacteria grow, they form toxin. Eating even a drop of this potent toxin can be fatal to humans and animals. Most cases of botulism have been caused by low-acid foods that were improperly canned at home. To make sure your home canned foods are safe, carefully follow tested canning instructions.

PREPARED BY: Angela M. Fraser, Ph.D., Associate Professor/Food Safety Specialist, NC State University in July 2004

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