How to Cool Jars and Test Jar Lids

Canning is a great way to preserve fruit and vegetables from your garden. It takes some time and effort, but you will reap the rewards of your canning efforts all winter, as well as save money on your grocery bill. There are a lot of things to keep in mind when you do your own canning, and cooling your jars and testing your jar seals are two very important steps in the canning process.

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Cooling Jars

After removing your canning jars from the canner, cool the jars at room temperature for 12 to 24 hours. When removing hot jars from the canner, do not retighten the jar lids. Retightening hot lids may cause seal failures. Cool jars on racks or towels to minimize heat damage to countertops. The food level and liquid volume of raw-packed jars will be lower after the jars have cooled. Air is exhausted during processing and food shrinks. If a jar loses a lot of liquid during processing, do not add more liquid. Check for sealed lids as described below.

Testing Jar Seals

After the jars have cooled, remove the screw bands and test seals by one of the following methods:

Method 1. Press the middle of the jar lid with your finger. If the lid springs up when you release your finger, the lid is not sealed.

Method 2. Tap the jar lid with the bottom of a teaspoon. If it makes a dull sound, the lid is not sealed. If food is in contact with the underside of the lid, it will also cause a dull sound. If the jar is sealed correctly, it will make a high-pitched sound.

Method 3. Hold the jar at eye level and look across the lid. The lid should be concave (curved down slightly in the center). If the center of the lid is either flat or bulging, it may not be sealed.

Reprocessing Unsealed Jars

If one of the jar lids fails to seal, remove the lid and check the rim of the jar for tiny nicks. Change the jar if necessary, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time. Headspace in unsealed jars may be adjusted to 1-1/2 inches and jars can be frozen instead of reprocessed. Foods in unsealed jars can be stored in the refrigerator and consumed within several days.

This document incorporates information from the “Complete Guide to Home Canning,” Agriculture Information Bulletin No. 539, USDA (Revised 2009) and information available from the National Center for Home Food Preservation.

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