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Canning Basics

     Safe Canning Methods

  There are two safe ways of canning, depending on the type of food being canned. These are the boiling water bath method and the pressure canner method. The boiling water bath method is safe for fruits, tomatoes and pickles as well as jam, jellies and other preserves. In this method, jars of food are heated completely covered with boiling water (212F at sea level).

Pressure canning is the only safe method of canning vegetables, meats, poultry and seafood. Jars of food are placed in 2 to 3 inches of water in a pressure canner which is heated to a temperature of at least 240 F. This temperature can only be reached in a pressure canner. The Clostridium botulinum microorganism is the main reason why pressure canning is necessary. Though the bacterial cells are killed at boiling temperatures, they can form spores that can withstand these temperatures. The spores grow well in low acid foods, in the absence of air, such as in canned low acid foods (vegetables and meats). When the spores begin to grow, they produce the deadly botulinum toxins (poisons).

Luckily, these spores can be destroyed by canning the food at a temperature of 240 F or above for the correct length of time. This temperature is above the boiling point of water so it can only be reached in a pressure canner.

Foods that are low acid (have a pH of more than 4.6 - see Chart 1, Preserving Food) include meats, seafood, poultry, dairy products, and all vegetables. Because of the danger of botulism, these foods must be canned in a pressure canner.

High acid foods contain enough acid (pH of 4.6 or less - refer again to Chart l, Preserving Food) so that the Clostridium botulinum spores can't grow and produce their deadly toxin. High acid foods include fruits and properly pickled vegetables. These foods can be safely canned at boiling temperatures in a boiling water bath canner.

Tomatoes and figs have pH values close to 4.6. To can these in a boiling water bath, acid in the form of lemon juice or citric acid must be added to them.

If you happen to find old time tables for processing low acid foods in a boiling water bath canner, do not use them. Research has shown these time tables to present a very real risk of botulism.



         Unsafe Canning Methods

  An old out-dated method of canning - the open-kettle method - is now considered unsafe. In this method, foods were heated in a kettle, then poured into jars and a lid was placed on the jar. No processing was done.

With this method there was often spoilage, because bacteria, yeasts and molds that contaminated the food when the jars were filled were not killed by further processing. The growth of these microorganisms, in addition to spoiling the food, often caused any lids that did seal to later come unsealed. This method resulted in a very real danger of botulism.

Steam canning is a newer method of canning that is not considered safe at this time. This canner looks like an upside-down boiling water bath canner. The base is a shallow pan with a rack that is covered with a dome lid. After the jars of food are placed on the canner's base, the small amount of boiling water in the base is supposed to fill the dome lid with steam. The jars are then heated by this steam. However, safe processing times have not been developed and steam canners are not recommended for either high or low acid foods. Low acid foods canned in these canners are potentially deadly because of possible botulism contamination. Also, both low and high acid foods are often very under-processed and therefore could spoil.

Some people want to be innovative in the use of microwave ovens, electric ovens, slow cookers, crock pots or the sun. These methods can be extremely dangerous, especially with low acid foods and are not recommended. So-called canning powders are useless as preservatives and do not replace the need for proper heat processing.


All information Courtesy the "University of Georgia - So easy to Preserve Guide ".

CanninAfrica compiled and maintained

 by Rosalie Acornley Webmaster.