Thank you for voting We're glad you found this helpful! Would you like to join our email list? You'll get our best cooking tips, ingredient guides, and kitchenware picks every other week, delivered to your inbox every other week.
Opt out at any time. We value your privacy.
Thank you for voting Noted! Would you like to share why? It's anonymous. Reader feedback helps us improve and, where necessary, correct our articles.

We're reader-supported. If you buy through our links, we may earn a commission at no cost to you.

Are Yogurt Containers With Bulging Lids Safe to Eat?

Are Yogurt Containers With Bulging Lids Safe to Eat?
marcomayer /Depositphotos

Sometimes, the foil lid on a yogurt container will bulge at the top, even if you haven’t opened it and it isn’t past its expiration date.

Since you’re here, I take it this happened to a cup or two of yogurt in your fridge. Now, you’re wondering whether or not they’re safe to eat.

I found myself doing the same a few days ago when I opened my fridge, and it caught my attention that the lid on a seemingly good Greek yogurt container had all of a sudden puffed up.

There wasn’t much helpful information out there, so I set out to do some research and then wrote this post. So here’s everything you need to know on the topic of bulging yogurt.

Let’s start with the basics: yogurt is a fermented milk product.

It’s cultured with living bacteria that can survive refrigerator temperature and which feed on the carbohydrates contained in it long after it’s made, packaged, and sold.

Don’t worry; these bacteria are generally harmless. They’re called probiotics, and, as quite a few studies over time have shown, they’re good for you in more than one way. Most containers of yogurt also have many other bacterial cultures, active to a varying extent.

More often than not, a puffed-up lid on your yogurt container is a sign of bacterial activity. It’s also a timely reminder that it’s about to go off sooner rather than later.

Plain yogurt is made by heating raw milk to a temperature of 185°F, which helps to denature the milk protein (called “casein”) and keeps it from curdling.

Next, the milk is cultured with Streptococcus thermophiles (S. thermophiles) or Lactobacillus bulgaricus (L. bulgaricus) bacteria and left out at a temperature of 113°F for 4 to 12 hours.

At this temperature, the bacteria multiply aptly and feed on the sugars (called “lactose”) in the milk, producing greater and greater quantities of lactic acid, diacetyl, butyric acid, and alcohol as byproducts.

The lactic acid reacts to the casein in the milk, making it firm and giving it a tangy taste (hence the distinct thickness and tartness of yogurt). The rest of the byproducts also ameliorate the yogurt’s aroma and flavor.

However, yogurt bacteria are not capable of producing gas. So they can’t cause the lid on the yogurt container to bulge. When this happens, it’s probably a sign that the S. thermophiles or L. bulgaricus cultures are dying off and getting superseded by gas-producing bacteria.

Treat it as an alert that the yogurt is closer to spoiling than you probably think.

What’s the verdict, then? Is bulging plain yogurt safe to eat?

If it isn’t past its expiration date, shows no visible signs of spoiling, and doesn’t smell off in any way, it probably is.

Another and less-apparent reason why yogurt containers can bulge is altitude.

Yes, you read that right.

If you live at a high altitude and the yogurt container was sealed at a low altitude, the altitude difference can cause the lid on it to bulge.

“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,” explains a resident of Evergreen, Colorado, a town at 7,165 feet above the sea.

“This means that, overall, the air in the inside the bag pushes outward harder than the air on the outside of the bag pushes inward.”

If this is the case for you, then most tightly sealed items—be they yogurt containers, milk cartons, and bags of chips—will puff up.

On the other hand, flavored yogurt might bulge for reasons more concerning than probiotic activity or altitude differences and should be discarded.

If the yogurt is a fruit-on-the-bottom type, it may have been improperly refrigerated during transportation or at the store, according to Master of Science and Registered Dietitian Andrea Klausner, who expressed her view on the topic in the UC Berkeley Wellness Letter in response to a reader question.

Bulging flavored yogurt should be discarded, Klausner advises, “since you can’t really know what causes the swelling.”

In June of 1989, the United Kingdom had its largest recorded outbreak of food-borne botulism when an outbreak associated with contaminated hazelnut yogurt affected 27 people. Sadly, one person passed away.

So if you see puffed-up peel-off lids on flavored yogurt, it’s best to throw it in the bin, even if it isn’t past its expiry date.

Better safe than sorry.

Leave a comment
Know your author

Written by

Jim is the former editor of Home Cook World. He is a career food writer who's been cooking and baking at home ever since he could see over the counter and put a chair by the stove.

Leave a comment


  1. what if an non flavoured yogurt, stored outside the refrigerator for 4 hours is bulged inside the pack? shall I discard it. it has 21 days..

    1. Hey there,

      Thanks for stopping by!

      It’s the probiotic bacteria—per the Harvard T. Chan School of Public Health’s article on the matter, Lactobacillus bulgaricus and Streptococcus thermophilus.

      It’s essentially the bacteria that feed on the sugars in the milk (lactose) and convert it into acid, which is what gives yogurt its distinct tang.

      Dim Nikov,

  2. The most likely answer is that the yogurt was contaminated with yeast. Yeast is everywhere but yogurt with an initial load of 100 or more yeast cells/g will probably spoil over time as the yeast cells multiply. Spoilage becomes evident as a swelling of the yogurt package due to gas production by yeast fermentation. Its could happen from something as simple as someone putting their dirty hands in a container of yogurt or foil lid (typical if only one container has the bulging effect) or equipment or seals that were not properly cleaned or rotated (typically effect entire batches or lots of containers)

Join the discussion

Your email address will not be published. Join the discussion with fellow readers. You can leave a comment using your name, initials, or nickname—we don't ask for your email address.