We found out what this weird little dimple is for, and you’ll definitely be surprised
You’re at the store, and you grab a jug of milk to plop in your grocery cart. You notice there’s a circular indentation in the container, so you take a peek at a few others. This little dent is everywhere you look. Naturally you wonder: What’s up with that?
No matter what brand of milk you drink, if it’s in a plastic container, it likely contains a circular or oval indent on its side. As for why it’s there, that’s one of those grocery store mysteries people can’t stop trying to solve. (There are whole Reddit rabbit holes devoted to the topic.)
As a science reporter (and rabbit hole fan), I just had to get to the bottom of this packaging puzzle. I reached out to Matt Herrick, executive vice president and chief impact officer of the International Dairy Foods Association in Washington, DC, to learn more about this curious milk-jug feature. Read on to learn why the dents are there—and what they reveal about the quality of your milk.
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What types of milk have these indents?
It doesn’t matter if you drink skim or whole, almost all milk sold in plastic jugs has some form of indent, according to industry insider Herrick. “Standard best practice among fluid milk processors who blow the plastic molds for milk jugs is to use a mold that provides for an indentation,” he says.
That means you’ll spy these round dimples not only on standard cow’s milk, but on other milk varieties sold in high-density polyethylene (aka plastic) containers, including some almond milks, buttermilks and even flavored milks, like the love it-or-loathe-it pumpkin spice. You may also spot them on containers of other beverages, such as orange juice, iced tea and apple cider.
What do people think the dents are for?
Rumors have been flying about the meaning of this dairy-aisle oddity. A viral Facebook post this summer claimed the divot is there to tell you if the milk is fresh—if the indent pops out, the post argued, the milk has gone bad. That’s because when milk spoils, it releases gas-producing bacteria, so the idea behind the theory is that the buildup of pressure on the container would force the milk-jug “innie” to become an “outie.”
It sounds reasonable, but it’s a myth, according to The Dairy Alliance, a non-profit advocacy organization representing Southeast dairy farms. Milk would have to be incredibly old and spoiled to release that much gas (and by that point, you’d notice the funky smell and taste). Not to mention, milk can go bad without releasing any gas at all.
Another conspiracy theory? The divot is part of an insidious plot by Big Milk to shortchange us by an ounce or two. If that sounds a tad far-fetched, it’s because it is (more on this below).
So what’s the actual purpose of the dent?
Why There’s A Circular Dent On The Side Of Your Plastic Milk Jug
The dimple on the container ensures you get the full amount of milk you paid for. “The indents allow milk processors precise control over the volume and weight of milk containers,” Herrick explains. Alas, this also means you aren’t getting any bonus glugs.
That little indentation is also helpful after the jugs get filled. Here’s the science behind the dent: As milk travels from the bottler to the grocery store, slight variations in refrigerated temperatures along the way can cause the milk container to expand and contract.
“The indentation allows for this to occur without sacrificing structural integrity of the jug,” Herrick says, which in turn ensures the safety of the milk. Milk packagers must pay careful attention to a container’s shape and form because it affects whether the container will buckle under pressure, according to a study published in Sage Journals.
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