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K-State Today

July 22, 2013



Food scientist urges safe food preservation

By Nancy B. Peterson

Seasonal fruits and vegetables typically cost less, so it's easy to understand why cost-conscious consumers are embracing home food preservation.

Doing so can extend health benefits from fresh foods for future meals and trim grocery bills when out-of-season prices rise, said Karen Blakeslee, K-State Research and Extension food scientist.

Blakeslee supports the growing interest in home food preservation, but as a county and state fair judge of home food preservation, has seen her share of food safety mistakes. Such mistakes can cause foodborne illnesses – and be life threatening.

She has crisscrossed the state this year to offer classes on safe food preservation.

Novices can be successful, she said, but it's important for both new and more experienced home food preservationists to choose tested recipes and follow directions exactly. New and improved equipment and recommended techniques can simplify the process.

Blakeslee noted that safe home food preservation typically involves canning, freezing or drying. Recommendations for the three methods have similarities, such as:

  • Start with a clean kitchen. Wash hands frequently. Clean as you go to prevent cross contamination.
  • Start with good food. Select fresh fruits and vegetables that are free of insect damage, nicks, bruises and mold.
  • Read the recipe in advance, and make sure all ingredients and equipment are on hand.
  • Allow time to complete the process. Home food preservation requires staying in the kitchen, on task.

Blakeslee advises following the canner manufacturer’s recommendations and reading the manufacturer’s instructions for using a smooth top electric range or cooktop with canning equipment. That’s important, she said, as some smooth top cooking surfaces are not recommended for home canning because the weight of a canner can crack the glass cook top.

Home food preservationists should become familiar with the properties of the food because food content will dictate preservation methods, the food scientist said. She cited the differences in recommendations for canning low-acid foods and foods with a higher acid content:  

  • Low-acid foods, such as meats and vegetables, require pressure canning to achieve a safe, recommended processing temperature — 240 F — to reduce potential risks of botulism that can grow in improperly canned low-acid foods.

Pressure canners require an investment, but can be used for several seasons when used and cared for according to manufacturer’s directions, which vary with the type of gauge.

A pressure canner with a dial gauge should be checked for accuracy annually because a variance of 1 pound — up or down — can increase the risk of food-borne illness.

Gaskets, seals and vent holes also should be checked annually, said Blakeslee, who said that many K-State Research and Extension offices have the equipment to test pressure gauges for current brands.

A pressure canner with a weighted gauge can be easier to maintain, as only the gaskets — which should be clean and pliable — and general condition will typically need to be checked each year.

While generally cost-conscious, she does not recommend buying a pressure canner at a garage sale, thrift store or auction, as "you won’t know its history, how it has been cared for and may not be able to buy replacement parts."

  • High acid foods, such as fruits, fruit products, jams, jellies, pickles and tomatoes should be processed in a boiling water — 212 F — bath, which requires a large kettle equipped with a rack to position the jars off the bottom of the pan.

"If jars are not placed in a rack and come in direct contact with the bottom of a kettle during the canning process, jars may crack and compromise food safety and quality," Blakeslee said. "The jar and the intended preserved product will need to be discarded." 

Either way, after following the recipe exactly, freshly canned products should be lifted from the canner with a wire jar lifter, and placed on a wire rack to cool.

Lids will "ping" during the cooling process to signal the jar has sealed.  

Consumers also are advised to choose a jar suited for their intended food preservation method.

Jessica Piper, spokesperson for Jarden Home Brands, which produces Ball, Kerr and Golden Harvest canning jars, visited Kansas State University earlier this year to join Blakeslee in updating extension's Master Food Volunteers on home food preservation.

Canning jars with straight sides can typically be used for canning and freezing, Piper said, and canning jars that have "shoulders" are not freezer safe.

New lids should be purchased each year; canning jars with smooth rims free from nicks and chips and screw-on rings free of rust can be re-used from year to year.

While jars remain a dependable kitchen staple, Blakeslee reminded that former and sometimes faddish home canning methods increase food safety risks and are not recommended. Examples include:

  • Open-kettle canning, in which foods are cooked and then spooned into sterilized jars but not processed with a hot water bath, cannot ensure food safety.
  • Oven canning, either in a conventional or microwave oven, is dangerous. Jars are not made for dry heat and can crack. Oven temperature varies. Oven heat is a dry heat that is slow in penetrating into jars of food.
  • Sun canning, in which foods are placed in sterilized jars in sunlight in varying temperatures, but not processed at temperatures high enough to kill potentially harmful foodborne microorganisms can increase food safety risks.
  • Processing canned foods in a dishwasher, in which water temperature is not high enough to kill potentially harmful foodborne microorganisms, also can increase food safety risks.

Washing and sterilizing canning jars in the dishwasher prior to use, however, can be effective – and convenient.

Blakeslee recommends labeling and dating home preserved foods and using them within one year, and also stresses the importance of checking the local altitude.

"While many will think Kansas is flat, the state’s altitude rises from just under 1,000 feet in the east to about 4,000 feet in the west," said Blakeslee, who explained that as the altitude goes up, the boiling point goes down, and that means that foods processed in a pressure canner need processing at a higher pressure.

The processing time for pressure canning will remain the same as the recommended times for lower altitudes, she said.

For water bath processing, the processing time is increased for higher altitudes.

Altitude is typically listed on maps, said Blakeslee, who advised following directions for adjustments for higher altitudes in recipe directions.

More information about home food preservation is available at K-State Research and Extension offices throughout the state, and online at Extension’s Rapid Response Center. The site also links to the U.S. Department of Agriculture’s "Complete Guide to Home Canning," National Center for Home Food Preservation at the University of Georgia and Ball Blue Book Guide to Preserving.

Karen Blakeslee is director of Kansas State University Research and Extension’s Rapid Response Center, where she spends her working hours answering food and food safety questions.