As we've posted before, canning is a great way to use the foods you have grown in your garden or have purchased from your local farmers' market. Canning allows us to enjoy the wonderful tastes of summer all year long, but to keep your food safe, you have to know which method of canning to use.

There are two methods of canning, water boiling canning and pressure canning. The processing method you will use depends on the acidity of the food.

Foods that are acidic (meaning they have a pH of 4.6 or less) can be processed using the boiling-water canning method. Most fruits are naturally high in acid however some foods must have acid added to them, such as pickled foods, tomatoes and figs.

Traditionally tomatoes were considered to be an acidic food; however some now have pH values slightly above 4.6. This means in order to ensure they are safe, they must be acidified. To acidify tomatoes citric acid, vinegar or lemon juice must be added. This will help to ensure a pH of 4.6 or lower is reached, making your tomatoes safe. This process must be done for all tomato base products, regardless if using the boiling-water canning method or the pressure canning method.

Boiling-Water Canning

During the boiling-water canning process water reaches a temperature of 212 °F. At this temperature enzymes are inactivated and molds, yeasts and some bacteria are destroyed. The critical aspect of this processing method is that a vigorous boil must be maintained throughout the entire specified processing time. If the boiling stops at any time during processing, you must bring the water back to a vigorous boil. Then begin the processing timing again starting from the beginning.

The following foods can be safely processed using the boiling-water canning method:

Acidic Foods

• Fruits
• Pickles
• Sauerkraut
• Jams
• Jellies
• Marmalades
• Fruit butters

Low-acid foods will not be acidic enough to prevent the growth of a dangerous bacterium called Clostridium botulinum.

A pressure canner is the only safe method for processing low acid foods. A pressure canner allows food to be processed under pressure, at temperatures higher than boiling. When the pressure canner gauge indicates the pressurized canner has reached 10 PSI (pounds per square inch) the steam circulating within the canner has reached a temperature of 240 °F. At this temperature, spores of bacteria are destroyed that cause botulism, as well as other types of spoilage.

To ensure bacteria are destroyed, the temperature must be maintained for the specified processing time. The length of processing time will vary depending upon the density of the food, its packing liquid and its pH. These processing times have been scientifically tested to ensure the food in the jar has reached the temperature needed to destroy all dangerous microorganisms. And therefore, these processing times cannot be changed.

Low Acid Foods

• Meat
• Poultry
• Fish
• Carrots
• Peas
• Corn
• Beets
• Snap beans
• Red kidney beans
• Baked beans
• Mixed foods (spaghetti sauce, soups, stews)

Clostridium botulinum

It is not just a high temperature that destroys the harmful bacteria. Time is another factor that needs to be considered. In low-acid canned foods, it may take anywhere between 20 to 100 minutes of processing under pressure to destroy the Clostridium botulinum toxin depending upon the type of food being processed. Therefore, always process the jars using the correct time and pressure specified.

Pressure Canner Dial Gauge Testing

Each year, pressure canner dial gauges must be checked for accuracy before being used to ensure safety. Why does it need to be tested?

If a gauge reads high, food is at risk for being unsafe, due to under-processing. If a gauge reads low, food will become over-processed and the quality of the product may be affected. Pressure adjustments can be made if the gauge reads up to 2 pounds high or low. However, you will needs to replace the gauge if there is a difference of more than 2 pounds.

If you need to replace the gauge or other parts of the canner, you can contact the canner's manufacture or often these items are found at local stores selling food preservation equipment. If ordering parts, such as gauges, gaskets or safety plugs, be sure to include your canner model number.

Whether you are just considering getting into canning or if you have been canning for years, it's always good to know the science behind the magic. If you are looking to learn more about home food preservation contact your local Extension office to find out about programming opportunities in your area.

Today's post was written by Diane Reinhold, MPH, MS, RDN. Reinhold is a registered dietitian and Nutrition & Wellness Educator covering Jo Daviess, Stephenson and Winnebago Counties.