Frequently Asked Preservation Question: Pinholes in Lids

What causes holes in jar lids that look like pinholes?

Sometimes they are caused by the manner in which the lids are handled. Lifting lids out of water with a metal tool may scratch the enamel on the lid providing a pathway for acids to work their way through the lid. More often, it is simply a chemical reaction with the metal and the food in the jars. According to the National Center for Home Food Preservation, natural compounds in some foods, particularly acids, corrode metal and make a dark deposit on the underside of jar lids. This is harmless unless holes go all the way through the lid. Salt can also cause corrosion on lids. Using proper headspace can reduce contact of the food with the lid. If food is stored too long, acids may create pinholes. Discard the contents of any jar if the pitting has gone completely through the lid — the jar would be unsealed and the contents could be unsafe.

References:

Blakeslee, K. (2020, July 14). "You Asked It:  What are Pinholes in Canning Lids?".  K-State Research and Extension.

National Center for Home Food Preservation. "Frequently Asked Canning Questions:  Why do the undersides of metal lids sometimes discolor?".