Canning

May canned food be reprocessed if the lid does not seal?

Sometimes a jar in a canner load fails to seal upon cooling. This may happen because of a cracked or nicked jar, or perhaps because food is fouling the jar rim where the lid sits. Due to loss of quality, reprocessing of jars is not recommended.

If you do reprocess, follow these steps for a safe product:

1. Re-process within 24 hours. For a safe final product, you must have followed an up-to-date research-tested recipe on your first attempt, and you must reprocess within 24 hours.

2. Remove the lid and empty the contents of the jar into a large pot and heat to boiling.

3. Fill a clean jar with the heated food, remove air bubbles, and top with a new lid.

4. Reprocess using a tested recipe for a hot-pack product.  If there is no tested hot-pack recipe, the product may not be safely reprocessed.

If you did not follow a tested recipe on the first try, all jars, sealed or unsealed, may be unsafe and should be discarded. 

To preserve quality of food where jars failed to seal (and discovered within 24 hours of initial canning), refrigerate the jars and consume contents within one week. Unsealed jars may also be frozen for up to one year, be sure to adjust headspace to 1” before freezing.

If canned foods have been frozen during storage, are they safe to eat?

Freezing does not cause food spoilage unless the seal is damaged or the jar is broken. These often happen as the food expands during freezing. Frozen foods, however, may be less palatable than properly stored canned food. In an unheated storage place, protect canned foods by wrapping the jars in paper or covering them with a blanket.

If my recipe doesn't call for processing, do I need to do so?

Many recipes passed down through the years or found in older cookbooks do not include instructions for processing. The foods are usually canned by the open kettle method, sealed and stored. Foods prepared in this manner present a serious health risk — particularly low acid foods. To minimize the risk of food spoilage, all high acid foods should be processed in a water bath canner or pressure canner and all low acid foods in a pressure canner.

Do I really need to leave a certain amount of headspace in the jar?

Yes, leaving the specified amount of headspace in a jar is important to assure a vacuum seal. If too little headspace is allowed the food may expand and bubble out when air is being forced out from under the lid during processing. The bubbling food may leave a deposit on the rim of the jar or the seal of the lid and prevent the jar from sealing properly. If too much headspace is allowed, the food at the top is likely to discolor. Also, the jar may not seal properly because there will not be enough processing time to drive all the air out of the jar.

How long will canned food keep?

Properly canned food stored in a cool, dry place will retain optimum eating quality for at least 1 year. Canned food stored in a warm place near hot pipes, a range, a furnace, or in indirect sunlight may lose some of its eating quality in a few weeks or months, depending on the temperature. Dampness may corrode cans or metal lids and cause leakage so the food will spoil.

Is it necessary to sterilize jars before canning?

Jars do not need to be sterilized before canning if they will be filled with food and processed in a boiling water bath canner for 10 minutes or more or if they will be processed in a pressure canner. Jars that will be processed in a boiling water bath canner for less than 10 minutes, once filled, need to be sterilized first by boiling them in hot water for 10 minutes before they're filled.

What should I do if I notice a buckled lid after canning?

A buckled lid is one that has sealed but shows a crease across a section or edge, indicating improper jar filling or lid tightening during the canning process. These lids often have a high failure rate, meaning the vacuum seal is likely to be lost during storage, making the food unsafe for long-term storage.

If you notice the buckled lid right after processing, the food is still safe as long as you handle it properly. You can refrigerate, freeze, reprocess (please read reprocessing guidelines here) within 24 hours. However, if you discover the buckling while the jar is on the shelf, you should assume the food has spoiled and throw it out.

Is it safe to process food in the oven?

No. This can be dangerous because the temperature will vary according to the accuracy of oven regulators and circulation of heat. Dry heat is very slow in penetrating into jars of food. Also, jars explode easily in the oven.

Can two layers of jars be processed in a canner at one time?

Yes, two layers can be processed at one time, in either the boiling water bath or pressure canner. Place a small wire rack between the layers so water or steam will circulate around each jar. Make certain that the water covers the tops of all jars by 1 inch in a boiling water bath canner. The pressure canner should have 2 to 3 inches of water in the bottom.

Is it necessary to exhaust a pressure canner?

Yes, it is very important to allow steam to escape for 10 minutes before closing the valve, or placing the weight on the vent. If the canner is not exhausted, the inside temperature may not correspond to the pressure on the gauge.

Should liquid lost during processing be replaced?

No. Loss of liquid does not cause food to spoil, though the food above the liquid may darken. If, however, the loss is excessive (for example, if at least half of the liquid is lost), refrigerate the jar(s) and use within 2 to 3 days.

Is it all right to reuse jar fittings (lids and bands)?

Lids should not be used a second time since the sealing compound becomes indented by the first use, preventing another airtight seal. Screw bands may be reused unless they are badly rusted or the top edge is pried up which would prevent a proper seal.

Why is open kettle canning not recommended?

In open kettle canning, food is cooked in an ordinary kettle, then packed into hot jars and sealed without processing. The temperatures obtained in open kettle canning are not high enough to destroy all spoilage and food poisoning organisms that may be in the food. Also, microorganisms can enter the food when it is transferred from the kettle to jar and cause spoilage.

Can acid foods be processed in steam canners?

The University of Wisconsin, under the leadership of Dr. Barbara Ingham, has conducted research on appropriate use of atmospheric steam canners for home canning in collaboration with the National Center for Home Food Preservation (NCHFP).  Atmospheric steam canners are used for processing naturally acid or properly acidified foods with natural or equilibrated pH values of 4.6 or below. They are not pressurized vessels used for processing for low-acid foods. 

Sufficient studies and peer review have been completed that we are now able to say that as long as certain critical controls at various steps in the canning process are achieved, USDA and NCHFP process times for canning acid or properly acidified foods (pH of 4.6 or below) at home with properly research based recipes and procedures can be used. The research looked at temperature distribution in the steam environment surrounding the jars in a dome-style steam canner, heating patterns of several different food types during processing in the canner, and the contribution of standardized cooling procedures at the end of the process time.

Some of the key controls in addition to the acidity of the food product are knowing that the canner has had the air vented out of the steam before processing begins, and that the pure steam is at the temperature of boiling water at the start and during processing.  Jars must be preheated before filling with food and cooling prior to processing must be minimized.  Processing times must be adjusted for altitude, and must also be 45 minutes or less, including any altitude modification.  The processing time is limited by the amount of water the canner base will hold, and the canner cannot be opened to add water or for any reason at any time during the process.  Finally, cooling of jars must take place in still, ambient air without any forced, more rapid cooling. The slow cooling of processed jars is important to the overall food safety of the whole canning procedure.

Dr. Ingham provides further instructions and details about carrying out canning in an atmospheric steam canner using USDA acid food processing recommendations at her webpage:   https://fyi.uwex.edu/safepreserving/2017/10/24/safe-preserving-using-a-steam-canner/.

Please also see this update in her blog postings: https://fyi.extension.wisc.edu/safefood/2020/08/18/an-update-on-safe-use-of-steam-canners/

The results of this research were published in a peer-reviewed scientific journal in May 2015.
Willmore, P, Etzel, M, Andress, E. and Ingham, B. (2015). Home processing of acid foods in atmospheric steam and boiling water canners. Food Protection Trends, Vol 35, No. 3 (May-June), p.150–160.

Why do the undersides of metal lids sometimes discolor?

Natural compounds in some foods, particularly acids, corrode metal and make a dark deposit on the underside of jar lids. This deposit on lids of sealed, properly processed canned foods is harmless.

What causes jars to break in a canner?

Breakage can occur for several reasons: 1. Using commercial food jars rather than jars manufactured for home canning, 2. Using jars that have hairline cracks, 3. Putting jars directly on bottom of canner instead of on a rack, 4. Putting hot food in cold jars, or 5. Putting jars of raw or unheated food directly into boiling water in the canner, rather than into hot water (sudden change in temperature-too wide a margin between temperature of filled jars and water in canner before processing).

If I find mold growing inside a jar of canned food, can I just scrape it off and eat the food?

Mold growth in foods can raise the pH of the food. In home canned products, this could mean that the high acid products could become low acid and therefore run the risk of botulism or other bacterial spoilage. Thus, any home canned product that shows signs of mold growth should be discarded. USDA and microbiologists now recommend against even scooping out the mold on jams and jelly products and using the remaining jam or jelly, even though that used to be suggested.

How can I remove scale or hard-water film from canning jars?

Soak jars for several hours in a solution containing 1 cup of vinegar and 1 gallon of water.

What can I process in half-gallon canning jars?

At least one canning jar manufacturer is selling half-gallon canning jars. That manufacturer has a printed note on the top that says half-gallon jars are only used for some highly acidic foods in a boiling water canner, with instructions to call a toll-free number for the instructions. When we last called, the only choices are grape juice and apple juice, as we also recommend.

The only processes that USDA, the National Center for Home Food Preservation and the University of Georgia have to recommend for half-gallon jars are for very acidic fruit juices (and juice only): Apple Juice (http://www.homefoodpreservation.com//how/can/canning-fruits-and-fruit-products/apple-juice) and Grape Juice (http://www.homefoodpreservation.com//how/can/canning-fruits-and-fruit-products/grape-juice). This process time is not to be used for tomato juice, for example.

There are no other research-tested processes for half-gallon jars. Boiling water processes for other foods for jars larger than those published with recipes (usually pints and/or quarts) cannot be extended by any formula to a larger jar.

We are aware that there are historical recommendations for canning foods in half-gallon jars. However, these are not currently accepted or endorsed by the USDA, Cooperative Extension System or U.S. manufacturers of home canning jars.