A Redditor's Canned Pineapple Juice 'Turned Jet Black' After Weeks In The Fridge — Here's Why

Some poor soul got the shock of their life after opening their fridge to discover an unfinished can of pineapple juice had transformed into a black liquid more reminiscent of squid ink than of tropical fruit. The photo, which they shared to Reddit's r/mildlyinteresting, turned out to be more than just mildly interesting — gaining tens of thousands of upvotes, with commenters comparing it to black paint, printer ink, and crude oil.

So what exactly is going on here? Well, it's hard to say with complete certainty what happened from a photo alone, but pineapple (both the juice and the fruit) is one of the more unusual canned products on account of being so acidic. Because of that acidity, pineapple products are canned differently from many other foods — they're packed in steel cans with a thin layer of tin instead of the thicker protective coatings on other canned goods. That tin layer reacts with oxygen before the fruit does, helping to stop the fruit and its juice from darkening. 

But once that seal is broken, it's no longer sealed from oxygen or other contaminants, and the conditions inside the can change. There is at least one other documented case of canned pineapple developing black discoloration (although that case was linked to a manufacturing defect) as well as a few commenters on the original Reddit post declaring the same thing happened to them. So while it is impossible to confirm without scientific testing, the fruit's acidity, the length of time it spent in the opened can, and the unusual color change all point to a reaction involving the can.

The viral photo is a good reminder that there are proper ways to store opened canned goods in the fridge, and inside their original can for long periods is not one of the best. The liners on modern canned foods are there so that the metal and the food don't interact too much while the can remains sealed, but once it's opened, the USDA recommends transferring leftovers to a clean glass or plastic container to preserve both quality and flavor. And as the original poster admitted, they'd left the juice in the can "for weeks."

It should also be said that cases like this are quite uncommon, despite what Reddit would have you believe, so don't be afraid to stock up on canned pineapple or pineapple juice. However, if you do happen to have an opened can of pineapple juice that has been sitting in the refrigerator for longer than five to seven days, the USDA says it's probably best to forget about the piña coladas and throw it away.