Red Cabbage Water Turns Pink or Green, Depending on What You Add

Slice a red cabbage, pour hot water over the shreds and wait ten minutes. The water turns a deep violet, the colour of a good felt-tip pen. Now pour a little into three glasses. Add a spoonful of vinegar to the first and it blushes bright pink. Stir some baking soda into the second and it drifts towards green. Leave the third alone and it stays purple. Nothing went in except things from the kitchen shelf, and yet there are three different colours on the table.

One pigment, many colours

The colour comes from anthocyanins, a family of pigments that dissolve in water. They are the same pigments that colour blueberries, blackberries, cherries, grapes and many flower petals, and red cabbage is full of them. A chemistry guide from the Georgia Institute of Technology, written for exactly this experiment, sums up the result: cabbage anthocyanins are bright pink in an acid, reddish purple in a neutral liquid and green in a base. Vinegar and lemon juice are acids. Baking soda is a base. The cabbage water simply reports which of the two it has met.

How the molecule changes its look

What separates an acid from a base is hydrogen. An acid is crowded with loose hydrogen ions, and a base has very few. An anthocyanin molecule picks hydrogen up or lets it go, depending on how much is around, and every time it does, its shape and its electric charge shift a little. A molecule with a different shape absorbs different light, so the eye sees a different colour. A 2017 review in the journal Food & Nutrition Research sets out the order: in an acid the pigment sits in a red form called the flavylium cation, at neutral it looks purple, and as the liquid grows more alkaline it moves on towards blue.

That is why the purple water is useful as well as pretty. Chemists call a substance like this an indicator, and it is a favourite classroom tool for sorting household liquids into acids and bases. It explains something at the stove, too: a splash of vinegar or a squeeze of lemon keeps cooked red cabbage on the bright, pinkish side. And the change runs both ways. Add baking soda to the pink glass a little at a time and, with a small fizz, the colour travels back to purple.

What people often get wrong

It is easy to assume that a new colour means a new substance, as if the vinegar had brought some pink dye with it. It has not. The pigment is the same pigment all along, only wearing a different shape. The second mistake hides in the name. Red cabbage is not really red, any more than it is purple or green: its colour was never fixed, and depends on what the leaf is sitting in. Nor does every purple plant behave the same way. The Georgia Tech guide notes that anthocyanins from other plants have colour patterns of their own.

Why food scientists care

Blue is the difficult one. In April 2021 a team that included chemists at the University of California, Davis reported in the journal Science Advances that red cabbage holds small amounts of a brilliant blue anthocyanin, and that they had designed an enzyme to turn the cabbage's other anthocyanins into it. Blue colours are rare in nature, the researchers pointed out, and without the right blue it is hard to mix a clean green either. So the humble cabbage that turned your vinegar pink may one day colour blue sweets. Until then, it remains the cheapest chemistry set in the vegetable aisle.