Why Do Onions Turn Blue Break Out Your Chemistry Set

Did your beautiful pink-red onions turn blue all of a sudden when you tossed them for sauteing? As weird as it may sound, there is certainly a very good explanation for this phenomenon to occur. This happens very often with red onions, shallots, red cabbage and even sometimes cherries.

But rest assured, thankfully there is a way to reverse it! Yes, you can turn your blue onions pink again! But first, let’s understand why red onions turn blue in the first place.

Why Do Onions Turn Blue?

Red onions contain a pigment called "Anthocyanin" – antho meaning flower based and cyanin meaning cyan colour. This pigment gives onions their blue-based red colour ranging from blue to purple to red.

When onions are cooked with food having alkaline elements like calcium and magnesium, they turn blue. In order to understand why we need to first understand how the pigment- Anthocyanin works.

How Does Anthocyanin Work?

Anthocyanin is present in various foods like berries, black rice, black-purple carrots and not just red onions and cabbage. If by any chance you are linking anthocyanin with cyanide, please don’t. These two are entirely different.

Anthocyanin is primarily found in flowers hence the flower based citation Antho. This pigment can show up in different colours depending on the pH of the food item. Very alkaline food with this pigment will look blue like blueberries, butterfly pea flowers. Yes, anthocyanin is the reason why butterfly pea flower tea turns all purplish on squeezing that lemon in it.

Whereas a very acidic food containing anthocyanin will look red like red onion, shallots, cherries. On the other hand, basic foods will appear purple such as purple cauliflower and carrots. You might have noticed that some foods are not entirely blue nor entirely red and not even entirely purple. They are somewhere in between blue, red and purple. This is because the pH scale is not set just on three levels, namely, alkaline, acidic and basic. There are many in-between degrees in the pH scale.

Now getting back to how anthocyanin works in making red onions blue. Red onions appear reddish-purple because they are acidic in nature. Once something alkaline touches this pigment, it turns the onions more towards blue. The more alkaline the food, the bluer the onion gets. Legumes, nuts, tofu, leafy greens, dairy, and most proteins are high in calcium and magnesium and hence alkaline in nature.

This is also the reason why your pickled onions get that beautiful pinkish tint. The acidic nature of vinegar pushes the anthocyanin pigment making the food appear more reddish producing that pink hue we love.

Is It Safe to Eat Onions That Turned Blue?

Yes, absolutely! As the red onions turning blue is entirely a natural phenomenon related to only the colour change because of the anthocyanin pigment reacting to the alkaline foods, it is entirely safe to consume blue onions.

If it bothers you, you may try and remove them. But this could prove to be very difficult in many cases. Hence, a better option would be to try and reverse the colour from blue back to reddish pink.

How To Reverse From Blue To Red?

As the blue colour in onions comes from the reaction of anthocyanin with alkaline food you can tone down the blue by adding some acidic elements into it. For example, if you are cooking onions with legumes and they are turning blue then add a dash of lemon juice after the cooking is done or vinegar while cooking to reverse the colour change.

Whether it’s lemon juice, lime juice or vinegar that you use, the end result would be the same- colour reversal! We are not quite sure about the orange juice as we haven’t tried it yet. But theoretically, it should work as it is acidic. If you are feeling adventurous do try it out.

On the other hand, if you want to bring back the blue colour, add a little bit of baking soda and it will be bright blue again. We are not sure what taste this step would impart but it surely will change the colour.

Is There Any Way to Avoid the Blue?

Yes, the only way to avoid your onions turning blue is to avoid cooking them with alkaline elements like nuts, lentils, legumes, egg whites, red meat, leafy greens, or anything of that sort. This is very difficult though!

And if the blue colour really irritates you, just use the white or yellow onions instead. Those will definitely keep their original colour unless you expose them to very acidic foods like vinegar, lemon juice, lime juice. In that case, they will turn bluish-green. Ah, chemistry!