Canning Mistakes: “But, My Jars Sealed”

The AnswerLine Team receives many phone calls and emails regarding canning mistakes—incorrect processing time, canner wasn’t vented, wrong size jars used, forgot to add acid to tomatoes, incorrect headspace, hot water canner used for low-acid foods, elevation not considered–just to name a few.  Mistakes happen but the biggest mistake of all is the assumption, “BUT, MY JARS SEALED!”

Sealed jar of pickle relish
Sealed jar of pickle relish – Photo: mrgeiger

A SEALED JAR DOES NOT EQUAL A SAFE PRODUCT if a canning mistake has occurred, a recipe has been altered, or if an untested recipe was used.  In the canning process, jars of food are heated to destroy pathogens, expel air, and create a vacuum seal.  While this process provides shelf stability, it is also the perfect environment for food borne bacteria, especially Clostridum botulinum, to germinate and produce toxins when a tested canning procedure is not followed.  In that ‘sealed jar,’ conditions favorable to producing the “perfect bacterial storm” exist:
MOISTURE,
‘DANGER ZONE’ TEMPERATURES that allow for bacterial growth (40⁰F – 120⁰F),
ABSENCE of OXYGEN (anaerobic) resulting from the air being driven out during processing, and possibly a LOW ACID food.  (Foods high in acid, like most fruits, or foods to which acid was added, such as vinegar to pickles, are less susceptible to bacterial growth.)

IF YOU MAKE A MISTAKE, ACT QUICKLY.

Canning mistakes (process deviated from the recipe instructions, incorrect processing time, canner water incorrect for pack) can only be rectified or re-canned in the first 2 hours*.  Within that window, they can be reprocessed, frozen, or refrigerated for quick use.  After 2 hours, the food needs to be disposed as it is no longer safe. 

Safely processed (recipe followed without deviation) home canned food can be re-canned within 24 hours if a jar does not seal.

Reprocessing means following the same processing that would have been done if starting with fresh food—remove the lid, empty and wash jars (check the jar rim for tiny nicks), change jar if necessary, reheat, re-fill jars, use new flat lids, and process with correct time and weight (pressure canning).   Most foods do not tolerate reprocessing very well.  Quality is diminished as they usually end up soft and mushy.  Soft foods, such as applesauce, handle reprocessing better than foods with structure.

When reprocessing isn’t a good option, freezing is.  Remove the contents from the jar and put into freezer containers or bags, label and freeze.  Leaving food in the original canning jars is not recommended unless some of the contents are removed to allow for freezing expansion.

One may also put the jars into the refrigerator and use the contents within 3 days.  This is a good option with small batch canning, but may not be so when 7 quart jars are in question.

Home canning is about following the science to make a SAFE product by preventing foodborne illness.  One can never assume the contents of a sealed jar are safe if there has been any alteration to the recipe or procedure, whether intentional or by mistake.

*National Center for Home Food Preservation.

Reviewed and updated, 4-2024, mg.

Marlene Geiger

I am a graduate of the University of Nebraska-Lincoln with a BS in Home Economics Education and Extension and from Colorado State University with a MS in Textiles and Clothing. I enjoy spending time with family and friends, gardening, quilting, cooking, sewing, and sharing knowledge and experience with others.

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Are Two-Piece Lids Really Necessary?

The USDA and the National Center for Home Food Preservation (NCHFP) recommend and support the two-piece lid system even though one-piece and reusable lids are available.  Why use two-piece lids for canning?  

Lid assembly for canning jars
Lid assembly layers – jar rim, metal lid, metal screw band. Flat lid with sealing compound around the edge. Photo: University of Wisconsin – Madison Extension

The two-piece lid system (flats and screw bands) is the best option for home canners. They are easy to use, seal reliably, and easy to tell if the jars sealed. When researched guidelines are followed, the two-piece lid system safely replaces the vacuum system used for commercially canned foods.

The vacuum system is created by a self-sealing system consisting of a flat metal lid held in place by a metal screw band. A trough around the outer edge of the flat lid holds a rubber-like, plastisol sealing compound which acts like a gasket; heat causes the compound to flow slightly over the rim of the jar.  During processing, the gasket allows air to escape from the jar and then forms an airtight seal as the jar cools. 

To ensure safe home canned foods, follow these important tips for two-piece lids:

  • Use new lids (flats) each time; after the first use the lid will no longer seal effectively. With careful handling, canning jars and screw bands may be reused many times. 
  • Purchase your lids (flats) from reputable suppliers.
  • Buy only the quantity of lids (flats) that will be used in a year’s time—please don’t hoard.
  • Follow the manufacturer’s directions for preparing lids (flats) to make sure to get a good seal. Heating of most flats is no longer needed or recommended by manufacturers. In fact, heating may actually cause lid failure.
  • Carefully observe headspace requirements for the product.
  • Removed air bubbles inside the jar with a plastic or silicone spatula.
  • Make sure the rim of the jar is clean before placing the flat lid on the jar.
  • Tighten the screw band as specified by the manufacturer.  Usually this is fingertip tight, which means the first full resistance is felt using just your fingertips.
  • Check all metal lids carefully. Don’t use old (more than 5 years old), dented, deformed, or defective lids.
  • Do not re-use lids from previously canned foods.

One-piece, reusable, and previous used lids are not approved for home canning by the USDA as they may allow air to be trapped within the sealed jar permitting bacteria to thrive and spoilage to occur which can lead to illness and even death.   While one-piece lids are available for home canning, they were made for use in industry where very strict time and temperature controls are in place.  Because they do not allow air to escape properly in home canning, consumers have reported jar breakage and lids buckling.

Reusable canning lids have been around for decades [1]. Research conducted through the National Center for Home Food Preservation on the reusable lids revealed that the three types of reusable lids they tested had an acceptable seal and removed the necessary amount of air. However, despite these finding, it was still recommended that the traditional two-piece metal lid system be used to “ensure the highest confidence in sealing.” [1] While there is no data to indicate that these lids will not perform satisfactorily if the manufacturer’s instructions are followed explicitly, the USDA and NCHFP cannot recommend their use due to a lack of researched-based information about their performance.

Lastly, recommendations and recipes from the USDA and NCHFP are currently based on the two-piece metal lid system.  The recommendation does not come lightly; it is backed by a body of research documenting how well they work consistently for safe home canning.

Why take a chance?  USE TWO-PIECE LIDS for best results and safe food processing! 

Sources:
1G. Sivanandam. Evaluation and Comparison of the Sealing Performance of Three Major Types of Jar Lids Available for Home Canning. Thesis Project – University of Georgia.
Recommended Jars and Lids. National Center for Home Food Preservations (NCHFP)
Put a Lid on It, Best Practices for Using Closures for Home-based Canning, North Central Food Safety Extension Network.
Steps for Successful Home Canning, GH1452. University of Missouri Extension.
Put a lid on It, Best Practices for Using Closures for Home-Based Canning. University of Wisconsin-Madison Extension.

Marlene Geiger

I am a graduate of the University of Nebraska-Lincoln with a BS in Home Economics Education and Extension and from Colorado State University with a MS in Textiles and Clothing. I enjoy spending time with family and friends, gardening, quilting, cooking, sewing, and sharing knowledge and experience with others.

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