Food Safety Facts
A Fact Sheet for People Who Prepare Food

University of Maine Cooperative Extension
Bulletin #4277

Can Home-Canned Food Spoil?

You've put up 50 pints of food from this year's garden: tomatoes, pickles, jam, chutney and a bumper crop of beans. But you still have rows of canned goods left from the last two seasons. How do you know they're still good?  Can home-canned food spoil?

Yes, it can, and here are some reasons why, and what you can do about them:

Reason: Fresh food was of poor quality, or unwashed, unpeeled or untrimmed.  This results in a high microbial load.  It may have required a longer processing time for complete sterilization than is usually recommended.

Solution: Prepare food properly before canning.

Reason: Food was packed too tightly in jars. As a result, the temperature in the center of the jar did not get high enough long enough for complete sterilization of the food.

Solution: Pack food loosely; prepare according to the USDA Guidelines (1/2-inch slices, halves, etc.), then use the recommended time, pressure and temperature.

Reason: Jars became unsterile soon after being filled.  If lids are not placed on jars and processing is not started right after jars are filled, microorganisms may grow to very high levels prior to processing.

Solution: Fill jars as quickly as possible and use sterile equipment.

Reason: Under-processing. This may be due to inaccurate heat-processing time or if processing was interrupted (by a power failure, pressure fluctuation, etc.).

Solution: Check to be sure you're using up-to-date processing times and watch closely to be sure processing isn't interrupted. (See sidebar).

Reason: Open-kettle canning, microwave canning or oven canning methods were used.  These methods do not get the canned food hot enough long enough to kill microorganisms.  So the food may spoil, may contain dangerous microorganisms and their toxins, or both.

Solution: Use recommended canning methods: a pressure canner for low-acid foods and a boiling water bath canner for high-acid foods.

Reason: Improper cooling of jars after processing. Jars may have been left in the canner at the end of processing time or when the gauge read "O."  As jars cool, they can suck water (containing microbes or spores) back into the food. Very slow or very rapid cooling may also have interfered with formation of a seal.

Solution: Remove jars from canner after processing to cool and protect from extreme temperatures.

Reason: Use of paraffin to seal jelly jars. Paraffin is no longer recommended for sealing jams, jellies or preserves. Mold, which is the most common spoiler of sweet spreads, can send "roots" down along the edge of the paraffin and produce toxic substances in the spread.

Solution: Can jams, jellies and preserves as you would other foods.

Reason:  Improper storage. Home-canned foods that are exposed to temperatures over 95 degrees F may spoil.  Microorganisms tolerate and will grow at high temperatures. So, if they are still present, they may grow and spoil the food, or alter the food so that other microorganisms can grow.

Home-canned foods stores in the sunlight may get very hot inside, which allows the air in the headspace to expand, breaking open the seal and allowing microorganisms to recontaminate the food.

If very acidic foods (pickled or fermented products, and some juices) were kept for a long time, the acid may have eaten away at the lid, resulting in pinholes that allowed microorganisms to get into the jar.  Discard any home-canned food with damaged or flaking metal on the lid.

Lids on home-canned foods stored in a damp place may also rust through, allowing microbes to get into the food.

Solution: Store home-canned foods in a cool, dry place.  Date all home-canned goods and use within a year.

By Mahmoud El-Begearmi, Extension Specialist, Nutrition and Food Safety

For more information about food safety, call USDA's Meat and Poultry Hotline at 1-800-535-4555 or contact your University of Maine Cooperative Extension county office.

Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment.

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