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.
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.
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.