Why Do Red Onions Turn Blue? [The Truth Sciences & Benefits)


Have you ever been cooking onions, especially red onions, and suddenly they turn blue? Wondering why?

Important Thing To Remember:

There is a natural colorant in red onions that can turn blue/green if cooked with certain foods. The colorant is entirely safe and will not affect the flavor.

Blue is not a color we are used to seeing when cooking fresh ingredients, so this can be a bit of a shock when it happens.

In this article, we are going to be looking at why do onions turn blue? what can you do to stop it, are they healthy to eat, and others frequently ask questions about the blue onion.

Now, without further ado, let’s dive in.


Why Do Red Onions Turn Blue/Green When Cooked and Refrigerated?

So, you may have noticed that this usually only happens with red onions, and there’s a reason for that.


The main reason that red onions turn green/blue is a natural colorant called anthocyanins. When cooked or refrigerated with alkaline foods, anthocyanins can turn from red to green/blue. [*]

So the reason behind this bizarre color change is actually a little bit of science. Anthocyanins are very sensitive to a change in pH levels, and that chemical reaction is what changes the color during cooking.

Anthocyanins are also what give Shiraz grapes and blueberries their distinctive color.

This chemical change only affects the color. The texture, flavor, and smell will all remain the same.

Although this isn’t very nice to look at, it’s not something you need to worry about.

How to Stop Your Onions From Going Blue

While your onions turning blue will not harm you or taste bad, you might want to avoid them because they don’t look very appealing.

There are two main ways to avoid your onions going blue:

#1. Add an acid

The number one way to solve this problem is by adding acid to the dish.

Onions are naturally acidic, so using lemon juice or vinegar will bring the color back to a bright reddish-purple. 

Try adding an acid that suits the cuisine you are cooking, so it doesn’t interrupt the flavors.

#2. Cook with less alkaline foods

Certain foods are more alkaline than others, and cooking these with red onions is bound to turn them blue.

If you want to avoid this happening, you should avoid foods like nuts, lentils, and eggs which are very alkaline food.

Is it Safe to Eat Onions That Have Turned Blue?

Okay, we know that looking down at your food and seeing bluish-grey onions can be alarming, but we’re here to assure you that they are safe to eat

The color change doesn’t affect the flavor or any other properties of the onions except for the color. If you mix the onions through the dish, you probably won’t notice them much anyway.

Anthocyanins actually have a lot of health benefits, so having them present in your food is not a bad thing.

Anthocyanins have the following health benefits: [*]

  • Antidiabetic 
  • It can help prevent some cancers
  • Anti-inflammatory
  • Antimicrobial 
  • Antiobesity
  • Prevents cardiovascular disease

Consider swapping out regular onions with sweet red onions to add a little extra nutrition to your cooking.

However, if you notice that your raw onions have purple patches, you need to throw them away. This is a fungus called purple blotch, and it is toxic.

How Can You Tell if an Onion Has Gone Bad vs. Going Blue?

You may notice that sometimes your onions turn green; this has nothing to do with anthocyanins. They are only activated when cooked with other foods. 


If your raw onions are anything but the purple or white shade that you bought them, then it’s likely that they have gone off.

Other signs that your onions have gone off include:

#1. Growing green shoots

Onions that have been left in the cupboard for too long will start sprouting. If they do this, we recommend discarding them immediately.

#2. Developing soft spots

If the onion starts rotting from the inside, then the onion will develop soft spots. Black spots of mold will often accompany them.

#3. Unpleasant odor

The onion will start to smell rotten, which indicates that it is probably rotting from the inside.



And so, the mystery of blue onions has been solved using a little bit of science.

What can we take away from this?

  • Anthocyanins are natural colorants that go blue when cooked with alkaline foods.
  • Blue onions are totally safe to eat
  • The color change will not affect the flavor

We hope this guide has reassured you that blue onions are safe, so it doesn’t alarm you the next time it accidentally happens. 

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