Vol. XX · Vegetables, Fruit and Legumes Plate 4 of 12pigments, carotenoids, beta-carotene, lycopene, anthocyanins, red cabbage, pH indicator, blue, purple, beetroot, betalains, colour of vegetablesMF

Red, Orange and Purple.

Red cabbage braised with apple and vinegar stays crimson; stir-fried in an iron wok with no acid, it can turn blue-grey. The cabbage is a pH meter.

Two families of pigment colour most red, orange and purple produce. Carotenoids — beta-carotene in carrots and sweet potatoes, lycopene in tomatoes and watermelon, capsanthin in red peppers — are oily molecules held in the cells' plastids. They survive heat well, dissolve in fat rather than water (so they colour the oil of a stir-fry or a curry and are absorbed better with fat), and cooking actually releases them from the cells: cooked tomato gives more usable lycopene than raw.

Anthocyanins are the opposite: water-soluble, held in the cell sap, and sensitive to pH. They are red in acid, purple near neutral, blue and then green-yellow in alkali — which is why red cabbage needs apple, vinegar or wine when cooked, why blueberries turn green-blue in a muffin made with too much baking soda (XXV.1), and why purple beans and asparagus turn green when boiled as their pigment leaks away. Beetroot's red is a third kind, betalains, stable to pH but bleeding into water and fading with long heat. The workshop changes the pH of red cabbage.

Below: the colour of red-cabbage anthocyanin at the pH you choose (red in acid, purple, blue, green-yellow in alkali), with kitchen liquids marked on the pH scale.

What you will learn

Tell carotenoids from anthocyanins.
Use acid to keep red cabbage red.
Explain why carotenoids colour oil.
Explain blue-green baked blueberries.

Workshop

Colour bands of red-cabbage anthocyanin, the classic kitchen pH indicator: red below about pH 3, red-purple to purple between 4 and 7, blue near 8, green near 10 and yellow above about 12 (published indicator ranges; exact shades vary). pH of kitchen liquids are typical values.

Fat for orange, acid for red

Cook carotenoid vegetables with a little fat and do not fear long heat: a carrot or a red pepper keeps its colour through a long braise, and its pigment, now partly in the fat, is better absorbed. Cook anthocyanin vegetables with acid and for no longer than needed: braise red cabbage with apple, vinegar or red wine; add a squeeze of lemon to purple-skinned potatoes or aubergine; and keep purple and black beans in their cooking water, which holds much of their pigment.

Watch the alkalis in a kitchen: baking soda and baking powder with too little acid (XXV.1), hard water (III.10), the lye-water (碱水) of ramen and mooncake doughs, and iron and aluminium pans, whose metals can bind anthocyanins and turn them grey-blue. Red-cabbage juice makes a vivid home pH test: boil a few leaves, strain, and drip lemon juice or soda into small glasses of it.

At the stove

Boil a few leaves of red cabbage, strain the purple water into four glasses, and add lemon juice, nothing, a pinch of baking soda and a pinch of washing soda. Line up the colours.

The "blue" in blueberries is mostly a waxy bloom on the skin scattering light; the flesh and juice are red-purple, as a squashed berry shows. In alkaline batter the juice turns green — the green streaks in a blueberry muffin made with too much soda.

Terms

Carotenoids: Fat-soluble orange and red pigments: beta-carotene, lycopene.
Anthocyanins: Water-soluble red, purple and blue pigments that change with pH.
Betalains: The red pigments of beetroot, stable to pH but water-soluble.
pH indicator: A substance whose colour shows the pH.

Carotenoids (orange, red) dissolve in fat and survive heat. Anthocyanins (red, purple, blue) dissolve in water and change with pH: red in acid, blue and green in alkali. Cook red cabbage with acid.

Sources

A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.