The most common and recommended lid type is the self-sealing lid. These lids consist of a flat metal lid held in place by a metal screw band during the canning process. There is a colored gasket compound on the underside of the flat lid, which is crimped around its bottom edge to form a trough for the compound. During the canning process the lid gasket softens and covers the rim but still allows air to escape from the jar. It then forms an airtight seal as the jar cools. These lids can be saved for use for up to five years from the manufacture date as long as they have not been used before; all used flat lids should be discarded after use. Lids over five years from the manufacture date may have gasket compound that fails. To ensure quality lids, only purchase the number of lids you think you will use in a year.
Preparing and Applying Lids
Some recipes suggest that you preheat the lids before processing. To do this, simmer the lids at 180°F without boiling them. This is supposed to help soften the sealing compound but is not necessary with Ball or Kerr lids. To verify that you have prepared the lid correctly make sure to read the manufacturer’s directions for the best seal.
Before placing the lid on the jar examine it to make sure it is not old, dented, deformed, or have any other defects. Make sure to prepare the jar by measuring the headspace and cleaning the jar rim with a dampened paper towel. Cleaning the rim allows for a better seal with the lid’s gasket. Uncleaned rims may cause seal failures and ultimately spoiled foods.
Once the jar is packed and the rim is cleaned, place one of the lids, gasket-side down, onto the cleaned jar-sealing surface. Fit the metal screw band over the flat lid and tighten the lid to the manufacturer’s specifications. If the lids are applied correctly the contents in the jar will contract during the cooling stage which will pull the self-sealing lid firmly against the jar to form a high vacuum, preserving the food.
If a lid’s ring is too loose some of the liquid could escape from the jar during processing and would not provide a secure seal. If a lid’s ring is too tight air cannot vent during processing and food will discolor during storage. Over-tightened lids can also lead to buckled lids and broken jars, especially if the jars are processed in a pressure-canner.
Screw bands should not be stored on jars. After they have been removed to test the seal wash, dry and store them in a dry area. If left on stored jars they become difficult to remove, often rust, and may not work properly again.
Testing Jar Seals
Once the jars have cooled for 12 to 24 hours, remove the screw bands and test the seals with one of the following options:
- Press the middle of the lid with your finger. If the lid springs up when you release your finger the lid is not properly sealed.
- Tap the lid with the bottom of a teaspoon. If there is a ringing, high-pitched sound the jar is sealed properly. If it makes a dull sound, the lid may not be properly sealed. A dull sound may also occur if food is touching the bottom of the lid; if you can see food touching the bottom of the lid try one of the other two methods to verify a quality seal.
- Hold the jar at eye level and look across the lid. If the lid is properly sealed it should be concave (slightly curved down in the center). If the center of the lid is flat or bulging, it may not be properly sealed.
Screw bands should not be stored on jars. After they have been removed to test the seal wash, dry and store them in a dry area. If left on stored jars they become difficult to remove, often rust, and may not work properly again.
Reprocessing Unsealed Jars
If a lid fails to seal on a jar, remove the lid and check the rim of the jar for small nicks or other inconsistencies. If the jar has nicks or other problems, trade it out for a new one and reprocess within 24 hours using the same processing time and procedure. If you don’t have the time or do not want to reprocess the jars the food could be frozen instead. To do this, leave a headspace of 1-½ inches. If reprocessing or freezing will not work, an unsealed jar could be stored in the refrigerator and consumed within several days.
Lids Not Recommended for Use
Glass caps are attractive but ineffective lids for home-canning jars. One-piece zinc porcelain-lined caps and zinc caps that use flat rubber rings for sealing jars are no longer recommended as they do not form a proper seal.