Canning jars: Choose and use wisely

Canning food is a great way to save money and enjoy the season’s bounty all year long. Take care to choose the appropriate jars to ensure food safety.

Jars should be inspected for any flaws before using them for canning.
Jars should be inspected for any flaws before using them for canning.

Specific equipment is needed to preserve food at home safely. Some of the most important supplies are proper canning jars. The canning process requires the heating and cooling of food to form a vacuum seal. When food is packed into canning jars, the headspace is filled with air. During processing, air is pushed out of the jars and the headspace fills with steam. When processing is completed and the jars cool, the contents shrink. A vacuum forms within the jar and pressure holds down the lid. The sealing compound on the lid prevents air from re-entering the jar, as well as microorganisms that could contaminate the food. Specific glass canning and freezing jars are recommended because glass does not react with its contents.

Canning jars should be inspected and in perfect condition before using. If jars have hairline cracks, chips or nicks on the sealing edge they should not be used. Other types of jars that are not appropriate for canning are:

  1. Bail-type, a jar that has a glass cap and rubber ring that fits on a sealing ledge located about ¼ inch below the top of the jar. The Bail-type jars do not create a strong enough seal to prevent contamination and microorganisms from entering the food.
  2. Commercial jars, the jars you buy from the store with product already in them. Commercial jars are considered one-use jars and may break with high temperatures. The two-piece lids will not screw on commercial jars properly and it will not have a safe seal.

To keep jars in good condition, avoid sudden temperature changes. Do not put a hot jar on a cool surface or put raw packed jars into boiling water. Use a rack in the bottom of the canner to prevent jars from sitting directly on the bottom of canner. Do not use metal utensils when removing air bubbles from hot jars, this can damage or break the glass. Follow recommended headspace from researched recipes, allowing the food to expand in jars when heated and to seal safely. When using a pressure canner, do not lower pressure too quickly. When processing time is completed, remove canner from heat and let stand until pressure is at zero.

Michigan State University Extension, along with the National Center for Home Food Preservation estimates canning jars can be used for about 10 years if handled properly. You can test jars by immersing them in tepid water, bringing to a boil and boiling for 15 minutes. They should not break after this process.

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