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Why Do Onions Turn Blue?

If you’ve ever cooked onions, particularly red onions, then you’ve probably looked down in the pan to find they’ve turned blue or turned bluish-green.

Red onions are chosen because they’re sweeter than regular white onions and they contain quite a bit more sugar too. However, it’s the pigments called anthocyanins that explain how a red onion can end up bright blue. Here’s what you need to know.


Why Red Onions Turn Blue (Lemon Juice Is Handy To Turn Them Back)

The red in your red onions is due to pigments called anthocyanins and they are the same chemicals that are found in Shiraz grapes (which make a gorgeous red wine color).

Under acidic conditions, your red onions are red and they are a naturally acidic onion which means that most of the time, you see the red color.

However, if the ph scale changes because of an alkaline component in your food then you will see a colour change as the onions become blue in an alkaline environment.

If it is important to you to have red onions rather than blue ones then you want to make them into acidic food again. You can add lemon juice and then once your litmus test goes back to acid, you will find the blue-green disappears and you end up with a pinkish-red again. The good news is that adding lemon doesn’t give you yellow onions.

Vinegar brought to the pan can also reverse the color change but, vinegar has a much more distinct flavor and might not be appropriate for every dish.


Does Red Cabbage Turn Blue Too?

Yes! If you’ve ever been cooking red cabbage and seen a similar reaction that’s for exactly the same reason the pigments called anthocyanin turn blue-green when your sautéed red cabbage starts to go alkaline too.

In fact, anthocyanin turns blue whenever it’s present and you have alkaline foods present too. So purple carrots, for example, will have their red produce a blue when you dump them in alkali, as will purple cauliflower. And the color intensifies with everything turning blue when the alkalinity increases. In fact, it can look a bit like copper sulfate from a chemistry set when taken to extremes.


Is It Safe To Eat Onions And Cabbage That Turn Blue?

Yes, we understand that when your onions turn green blue, they might not look as appealing but from a practical application? They’re completely safe to eat and the change in color will not affect flavor. It’s just the natural color of your food in the presence of an alkali.


Is It Safe To Eat Onions That Have Turned Green?

This has nothing to do with the anthocyanins in your onions. If they’ve turned green and have a green sprout, then your onions are old.

You can eat it – but it won’t taste great. It’s best to cut out the sprout and any damaged or moldy material or just replace the onion entirely.


Why Did My Onion Turn Purple?

It might be that the purple you are seeing is the blue that we’ve described above and if that’s the case – it’s perfectly safe to eat.

However, onions also suffer from a condition known as “purple blotch” and if your onions are covered in purple patches? That’s a fungus and it’s best not to eat the onion at all as it’s slightly toxic.


Final Thoughts On Blue Onion

We love onions and if yours go a little blue during cooking, don’t worry – they do that and if the color really matters to you, get a lemon and switch it back.

We can also recommend onions when you use these amazing Japanese cookbooks, our favorite vegetarian cookbooks and when making onion soup from our best soup cookbooks!

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