After packing of jam, peanut butter, mayonnaise, fruit preserves and any products packed in jar, a method should be employed to assure there are no leaks among the products. Inverting the jars for two weeks as leakage test may be used. However this inversion method is not applicable for viscous products like jam, jelly, peanut butter and mayonnaise. Inversion method also takes time, two weeks or less.

Three simple methods can be done for testing of jar seal for leaks and airtightness.
1) For pop-up caps. Hot-packed products creates a vacuum when cooled. The vacuum pulls the cap downward. Cap surface should not pop up when press with fingers.
2) For ordinary caps. The cap surface should be slightly concave or slightly curve downward the center due to vacuum pressure. Pop-up caps have deeper curve.
3) Properly sealed cap produce a high pitch sound when tap with a spoon. A dull sound is produced by improperly sealed jar.

Jars that fail the test should be set aside for reprocessing.