Why Your Tofu Might Turn Blue (It's Not Necessarily Spoiled Or Expired)

There are several foods that are naturally blue. Off the tops of our heads we can name lobster, corn, and blueberries, the last of which can be sprinkled with a dash of this pantry staple for a flavor pop. But tofu? It's definitely not on the list. The soybean blocks of varying firmness are increasingly beloved by American foodies looking for a healthier source of protein. They usually come in shades of off-white or cream, though smoked and spiced varieties may be a golden-brown shade. Blue tofu? Not so much. Unless, of course, it's been stored in close proximity to red onions ...

Under that circumstance you absolutely can get blue tofu, and although the look of it might make your hair stand on end, your protein is still completely edible. The color change is down to a 100% natural reaction between the anthocyanins in red onion and the alkaline tofu, which produces the startling blue color. Red cabbage, eggplant, and other foods that are purple, blue, or red are typically rich in anthocyanins and will have the same — entirely harmless — effect on tofu and some other alkaline ingredients. One Redditor, who added red cabbage to fried egg rice, found the eggs turned "a weird greenish blue" after a night in the fridge, adding, "It was delish, but looked sooo moldy."

Blue tofu tinted with red onion veggie would make a wicked Halloween food or dinner party conversation starter, whichever type of tofu you choose to serve. That being said, if you know for certain there are no anthocyanin-rich veggies in your fridge or you have any suspicion of bacterial or mold growth, think twice about chowing down on tinted tofu. As demonstrated by doctoral student Marina E. De León, the color could be caused by bacterial contamination.

In a 2020 post on X, she poured out a glass of soy milk and was fascinated by the pale blue shade. De León confirmed in the thread that the milk was "waaay past the expiration date," but — in a move that made our stomachs churn -– said it didn't taste any different. Her study, as published in the Journal of Genomics, suggested the Pseudomonas carnis bacteria (a relatively newly identified species connected to meat) could be responsible for the blue coloration in soy milk and tofu.

It's a handy warning, especially for occasional eaters of tofu. A Facebook post showing a slab of tofu with a vivid purple vein drew lots of comments about proper storage. Unopened tofu will last as long as the expiry date on the packaging, but once opened, it needs to go in an airtight container, where it will keep for three days. If your sealed-off tofu has turned blue in that time, it could be due to bacteria, and it's best to pop it in the trash.