Metal lid doesn’t pop when you open jar?

If a jar doesn’t have a button, it’s not supposed to make a distinctive pop

Not all glass jars bear button lids. If the lid is flat or plastic it might still achieve a good seal, but you won’t get a pop.

How do manufacturers determine whether a food gets a button lid or not?

Manufacturers have to meet a number of requirements for food safety. For example, many products need to have a minimum level of acidity to prevent problematic microbial growth. Products (including those with acidity requirements) often need to be heat processed in a way that will kill microbes that could spoil the food or make someone sick. These requirements are particularly stringent for otherwise perishable food that’s processed to be stable while sealed at room temperature.

Requirements can vary based on the product. For example, a food that never leaves the cold chain would not be heat treated (and would have a short refrigerated or room temperature shelf life), so wouldn’t have a button lid. Sometimes high pressure is used instead of or in addition to heat (the pressure also kills microorganisms). Some products may need heat treatment, but perhaps not enough to create the suction needed to pull in a button. Or a product might be packaged in plastic that wouldn’t support a button lid. Finally, a manufacturer simply might not want to use a button lid for aesthetic reasons. The button itself is not a requirement for safety, it’s simply an indicator that a good seal was achieved.

If you don’t see a button, you won’t hear a pop, but you might still hear a hiss or a “shhhhounk”-type noise. That’s a sign that there was some amount of a vacuum at the top of the container or that the air inside the container was otherwise recalibrating with the outside world. In addition to sealing containers while products are hot, some manufacturers deliberately pump nitrogen gas into the airspace at the top of a container. This keeps the product at peak quality longer as it minimizes oxygen exposure, which can speed a food’s decline. Somewhat obviously, you might also hear a hiss if you open a carbonated beverage; that hiss is carbon dioxide that’s built up from the rising bubbles.

Companies might also use a plastic band around the lid to provide reassurance that the product hasn’t been tampered with. You may also see paper, foil or plastic seals across the mouth of a container; those help preserve freshness by blocking oxygen.

What if a glass jar of peanut butter doesn’t pop?

Peanut butter is a food that, whether packaged in plastic or glass jars, typically has a flat lid that doesn’t pop. Peanut butter itself has so little water that it won’t support the growth of food pathogens or spoilage organisms, according to the Institute of Food Technologists. Nut butters don’t even need to be refrigerated after opening because it’s simply not a concern. Nut butters can, however, succumb to rancidity, which is why an oxygen-blocking seal made with plastic and/or foil over the jar mouth can help keep it fresher for longer. I often see such a seal on plastic jars of peanut butter; I’ve never seen such a seal over a glass peanut butter jar. While I haven’t discussed this with a peanut butter manufacturer, my sense is that their metal lids, which have a ring of soft plastic in them, form an extremely tight seal that’s akin to what a peel-off seal achieves on a plastic jar. Sometimes you’ll also see a plastic band around the lid as reassurance that it’s been untouched since it was jarred.

If you’re wondering how to square the fact that peanut butter won’t support microbial growth with the fact that peanut butter has been involved, if infrequently, in recalls related to salmonella, here’s the deal: that’s a question of contamination, not growth within the product.  In those cases, the salmonella wouldn’t be able to grow in the peanut butter, but could simply rest there until someone ate it and got sick. Even though the bacteria can’t grow in peanut butter, the oily environment can, unfortunately, protect it until someone eats it, according to IFT. Getting a foodborne illness from peanut butter would be very rare and would likely stem from an error on the manufacturer’s side (the peanut butter could have touched poorly cleaned equipment) and bad luck on the consumer’s side. 

But back to the buttons. If a jar has a button that doesn’t pop, discard the food. If the container doesn’t have a button lid and everything else looks normal, the food is probably fine.