The Real Reason There's a Dent in the Side of Your Milk Jug

Quick: head to your fridge and look at the gallon of milk. Does it have an indentation on the side? Is it an "innie" or an "outie"? This simple little dent has ignited a lot of speculation over the years, with numerous internet threads dedicated to theories about the dent. Some say that it’s there to let you know when your milk has gone bad, but (spoiler alert, ha!), that’s just a myth.

What Is the Dent on Milk Gallons For?

As cool as it would be for your gallon of milk to tell you if it's good or bad to drink, that’s not why the dent exists. Instead, that circular indentation has a much more boring purpose: It’s there to ensure that your container of milk is structurally sound and filled with the appropriate volume of milk, according to Snopes. And, as a bonus for manufacturers, it makes milk jug production a little bit cheaper because the design allows the containers to be made with less plastic.

Still, the myth won't die. There are multiple Reddit threads dedicated to the topic, and countless Facebook posts with tens of thousands of shares from people calling out the “fun fact” that the indent expands when milk is bad. It’s not surprising that this theory persists throughout the years. Theoretically, it makes sense that if your milk goes bad bacteria inside could create gas, causing the bottle to expand, popping out that dent.

But that’s not how milk spoilage works. As food science professor Bob Roberts told USA Today, the bacteria that spoil milk usually don't produce gas. Instead, an indent might pop out if it was held at a warm temperature. (And this has to be rather extreme.) If you've ever kept a gallon of milk a little too long in your own refrigerator, you know it didn't explode after going bad. So, we're calling this theory bunk.

How Can I Actually Tell if My Milk Is Spoiled? 

Fortunately, milk is one of those foods that doesn't hide when it’s gone bad. You’ve already got a built-in milk spoilage detector: your nose. If you smell a sour, rancid scent when you open up the jug, it’s a sure sign that it’s past its prime. That’s true even if it’s still within its “best by” date. Even if you didn't use the "smell test," you'd know immediately from its sour, off-putting taste (and possibly chunky texture) that your milk went bad.

If you’ve been using the indentation on the side of your milk jug to decide whether or not it’s good to drink, let that practice go. Maybe we’ll get lucky and the milk industry will one day introduce a bottle that can immediately let us know whether or not the milk’s gone bad before we pour it into our coffee in the morning.