Why Do Onions Turn Blue ? Break Out Your Chemistry Set

Did your onions turn blue ? As weird as that might sound, there’s really a very good explanation for it. No, really. This happens very often with red onions or shallots, but also with red cabbage and sometimes even cherries.

And thankfully there’s a way to reverse it ! Yep, if your onions are blue we can make them pink again. So let’s see why that happens, to understand why and how we can reverse it.

red onion turning blue

Why do onions turn blue ?

Red onions turn blue because they contain a pigment called anthocyanin. That pigment reacts with alkaline elements in your food, like magnesium and calcium. There are other alkaline elements but those rarely end up in your food.

Foods high in magnesium and calcium are legumes, nuts, tofu, leafy greens, dairy, and most proteins. Red onions contain the pigment anthocyanin, which produces blue-based red. So it can range from blue to purple to red, and when cooked it can turn blue, especially if the final dish is alkaline.

How anthocyanin works

Anthocyanin is present in lots of foods, not just red onions or shallots or red cabbage. It’s also present in berries, black rice, and even black or purple carrots. If you’re thinking it has to do with cyanide, don’t. It’s very different.

Anthocyanin is made up of ‘antho’ meaning flower-based, and ‘cyanin’ as in the color cyan. If you remember your printer’s seemingly ridiculous request for ‘cyan’ when it needs to print any color, it’s because cyan is a primary color. And anthocyanin is primarily found in flowers, hence the ‘antho’ part.

The thing is, this pigment can show up in different colors depending on the pH of the food item. A very alkaline food with this pigment will look blue, such as blueberries or butterfly pea flower. Yes, the blue tea that turns purple, that one.

A very acidic food will look red, such as red onion or cherries. A middle ground or basic food will appear purple, such as purple cauliflower.

You might have noticed that some foods aren’t perfectly blue or red, and the purple can be bluish or reddish. That’s because the pH scale is a sliding scale, and not three set levels. It’s more of a degree thing.

Back to anthocyanin, in red onions it appears reddish-purple because the onions are mostly acidic. Once something alkaline (from the other end of the spectrum) touches this pigment, it turns more towards blue. The more alkaline the food, the bluer it gets.

This is also why pickled red onions get that beautiful pinkish hue. The acid in the vinegar makes the anthocyanin lean more towards red, producing the pink hue you see.

Read also: Are Onions And Garlic Related ?

Is it safe to eat onions that turned blue ?

When it comes to red onions, you can eat them even if they turned blue. There’s nothing wrong with them, it’s just their natural coloring and how it reacts to alkaline foods.

You can try and remove the onions if they bother you, but that could prove difficult in many cases. A better option would be to try and reverse the color.

How to get the blue out

You can tone down the blue by adding some acidic elements into the foods. For example if you’re cooking red cabbage and it’s turning a bluish purple, you can turn it back to a pinkish red with a dash of vinegar.

The same can work with red onions or cherries. Whether it’s vinegar or lemon juice or lime juice, the end result should be the same. We’re not sure about orange juice, but if you’re feeling very adventurous you could give it a shot.

Conversely, if you want to bring the blue back, add a little bit of baking soda and it will be bright blue again. We’re not sure how tasty that would be, but it would definitely change the color.

Is there any way to avoid the blue ?

The only way to avoid red onions turning blue is to not use any alkaline elements in your cooking. So no nuts, lentils, beans, legumes, egg white, red meat, leafy greens, anything of the sort.

And if you’ve got a hankering for chili con carne, just use regular white or yellow onions ! Those will definitely keep their regular color, unless you expose them to very acidic foods like vinegar or lemon juice. In which case they’ll turn blue-green. Ah, chemistry !

If you’ve got any other food curiosities be sure to check the related articles below, we’re always adding more food facts to make your life that much easier.