There are many reasons why you can see bulging lid on yogurt container. Some can be dangerous some just annoying.
When I first visited a grocery store in Evergreen I was surprised to see many yogurt containers with bulging lids. First thing that comes to my mind is that the product is not good anymore or is past its expiration date. In my eyes it was an obvious sign of expired products. At least it was my experience while living in the Midwest. I would never buy yogurt with swollen lid.
I checked the expiration date and it was fine. The expiration dates on packages assume the food has been stored properly. Many yogurt brands today have fruits on the bottom. If they are not adequately refrigerated during the transit or in the store itself, it can cause the fruit to ferment and release gases. That would cause the lid to bulge. That might be one of the reasons anyway. If you don’t know for sure what the problem is it’s safer to toss the yogurt. That would be case in flatland like Illinois.
So what is going on with the yogurt in the mountains ?
Obviously it has something to do with living at high altitude. Bags of potato chips are often swollen and yogurt cups have swollen lids. Same applies to cottage cheese and similar products. So be careful when opening them. 🙂 If chips are sealed tightly at lower elevation, the pressure differential at 7200 feet might cause them to swell or in some cases the bags will burst at the seams.
This is because they were sealed at low altitude where the air pressure was high, and their internal pressure is thus the pressure of the air where they were sealed. At high altitudes, the pressure inside the bag is still the same as sea level air pressure, but the air pressure outside the bag is lower. This means that overall the air inside the bag pushes outward harder than the air outside the bag pushes inward. If that happens to something liquid or any other messy substances, you might not be happy camper!
The products are in the most cases fine. Just take into consideration the fact that this gets worse with altitude. If you take yogurt from 7200 feet to 10000 feet, your yogurt might have an accident.
Most items that are tightly sealed will have this problem.
So when you first move into the mountains and see many packages with puffed tops, DON’T PANIC! It’s normal and it’s most likely caused by change in elevation during the transportation. Just be careful how you handle them to avoid spillage and unnecessary mess.