Cooking science

Colours that change with acid

The red, purple and blue pigments in red cabbage, blueberries, blackcurrants, red onions and blood oranges all change colour with acidity. Add acid and they go red; add alkali and they go blue, then green.

One pigment, several colours

Anthocyanins are a family of pigments whose molecular structure changes with the acidity of their surroundings, and each form reflects light differently.

In acid they are red or bright pink. Around neutral they are purple. In alkaline conditions they shift to blue and then to green and yellow-green.

This is not a chemical curiosity confined to a laboratory. It is why red cabbage cooked in plain water turns a dull blue-grey and why blueberry muffins made with too much bicarbonate of soda come out green.

Which foods carry them

Red cabbage

The classic demonstration, and the one where it matters most in cooking.

Berries

Blueberries, blackcurrants, blackberries, cherries, elderberries.

Red onion

Turns bright pink in a vinegar pickle, which is the whole appeal.

Blood orange

The red flesh is anthocyanin, and it fades with heat and shifts with pH.

Purple potatoes, purple carrots, aubergine skin

All the same family.

Rhubarb and red-stemmed chard

Some contribution, alongside other pigments.

Butterfly pea flower

The most dramatic — a blue tea that turns magenta the moment lemon is added.

Keeping the colour you want

• Add acid to keep it red or pink. A splash of vinegar or lemon juice at the start of cooking red cabbage keeps it vivid.
• Braised red cabbage with apple and vinegar is not only a flavour tradition — the acid is what stops it going grey.
• Reduce the bicarbonate of soda in a blueberry cake or muffin, or add buttermilk or lemon juice to counteract it.
• Pickle red onions in vinegar and they turn bright pink within an hour.
• Cook briefly. Prolonged heat degrades anthocyanins regardless of pH, which is why blood orange colour fades in a syrup.
• Avoid iron and aluminium pans, which react with anthocyanins and dull or blacken them. Stainless steel or enamel.
• Accept the fade in preserves. Blackcurrant jam darkens and blood orange marmalade loses its red. Both are normal.

The demonstration worth doing once

Ten minutes, and it makes the principle permanent.

1. Shred a little red cabbage and simmer it in water for ten minutes.
2. Strain off the liquid, which will be a deep purple-blue.
3. Divide it between three glasses.
4. Add lemon juice or vinegar to the first. It turns bright pink or red almost instantly.
5. Leave the second alone. It stays purple.
6. Add a pinch of bicarbonate of soda to the third. It turns blue, then green.
7. Anybody who has done this once never forgets why their red cabbage went grey.

Common questions