Is mustard lid supposed to pop when opening? I opened a jar of English mustard with a flat plastic lid and it didn't pop. It wasn't bulging or anything and contains citric acid. Is it supposed to pop? I notice French's mustard (North American style squeezable kind) doesn't pop so I think it's the same. It's just that we are so conditioned to expect jars to pop that when they don't I wonder. Mayonnaise doesn't either which I know is okay...
Plastic usually does not pop, it bends slowly under the pressure differential. Metal usually bends quickly and pops when the vacuum in a jar is relieved. Some jars use thicker metal that wont bend, so you have to listen for the rushing intake of air. Sometimes now they replace air with nitrogen before sealing. You won't get a pop or any other sound from a jar canned that way.