By Sam Beattie
Food Safety Specialist
Iowa State University Extension
Summer bounty brings questions about what to do with the surplus. Thermally processing fruits and vegetable in jars (cans) will provide a flavorful shelf-stable product for eating enjoyment throughout the year. Food preservation by canning of fruits and vegetables has been practiced for nearly 200 years.
In 1809 and after 14 years of experimentation, a Frenchman, Nicolas Appert, produced the first shelf-stable canned food products. His development included air exclusion (hermetically sealed) and a prolonged thermal treatment that rendered the food shelf stable for years. The endeavor was rewarded with a prize from Napoleon who recognized that an “army travels on its stomach” and that there was a great need for transportation of stable foods to troops that were nutritious, safe and flavorful.
Appert went on to use the winnings to start a commercial canning operation. He subsequently wrote the first canning guide, called “The Art of Preserving All Kinds of Animal and Vegetable Substances For Several Years.” In those days, water bath processing was the only type available and foods were processed for hours in hot water. Glass jars and metal cans (developed in 1810) have become the standard, with little changing except for the types of closures.
During Appert’s time, jars were sealed with a cork, sealing wax, and wire; we now use specialized polymers that act as glues to hold the lid onto the glass jar.
Canning has changed
Our understanding of bacteriology has improved. Pressure vessels shorten the time required to kill the most durable bacteria found in foods such as vegetables and meats. The shortened processing time also improved the quality of the product.
In evaluating recipes for preserving foods by canning, it is important to understand that acidic foods such as fruits require milder processing than acid neutral foods such as vegetables or meats. Thus, a pressure vessel (reaching temperatures of 241°F or higher) is required for vegetables and meats, while fruits (jams, jellies, juices) and many pickles require only a boiling water bath (212° F at sea level). Some recipes that are a combination of acid foods and low acid foods require either additional acid in the form of lemon or lime juice or vinegar and/or are pressure processed.
Recipes that require a pressure vessel are relying upon the higher temperature to kill the common soil bacterium Clostridium botulinum. While water bath processing will not kill botulinum, it does kill many other spoilage bacteria and relies upon the higher acidity of the food to prevent the growth of Clostridium botulinum.
Botulinum produces a very potent and deadly toxin in improperly canned foods. It is extremely important to rely upon scientifically sound recipes when canning foods that are low acid or a combination of acidic and low acid foods. Guides such as the Ball Blue Book “So Easy to Preserve” and the recipes at the National Center for Home Food Preservation at the University of Georgia (www.uga.edu/nchfp) are of assistance when choosing recipes.
This article is condensed from the August 2008 issue of Acreage Living, www.extension.iastate.edu/acreage The newsletter article includes recipes for salsa and tomato sauce.
Other articles in this month’s issue--
• ISU Extension Helps Small Farms Thrive
• What is a Small Farm?
Lynette Spicer, Extension Communications and External Relations, (515) 294-1327, firstname.lastname@example.org