Vol. XX · Vegetables, Fruit and Legumes Plate 3 of 12green vegetables, chlorophyll, pheophytin, olive green, acid, baking soda, broccoli, green beans, spinach, cooking time, lid, bright greenMT

Keeping Green Vegetables Green.

Green beans are brightest after three minutes in boiling water and drabbest after fifteen. The clock and the acid in the pot decide.

Chlorophyll is a green pigment with a magnesium atom at its centre. Heat lets the vegetable's own acids, released from broken cells, swap that magnesium for hydrogen, turning chlorophyll into olive-brown pheophytin. The reaction is faster in more acid conditions and at higher temperatures, and it never reverses, so every extra minute of cooking, every squeeze of lemon early, every lid that traps acid vapour back in the pot, makes greens duller (XVI.2).

Two things go the other way. The first minute of blanching makes greens brighter than raw, because the air between the cells is driven out and the pigment shows through clearly. And alkali preserves chlorophyll: a pinch of baking soda in the water keeps greens vivid, as Victorian cooks knew — but it also softens them fast (XX.2), makes them taste soapy and destroys vitamins. The cook's answer is speed: a big pot of fast-boiling salted water, small pieces, a few minutes, then serve or plunge into ice water. The workshop boils green beans.

Below: the share of chlorophyll left and the softening of green beans boiled for the minutes you choose at the pH you choose (reaction bench, course kinetics; acid speeds the pigment change, alkali the softening), with the colour they would show.

What you will learn

Explain why greens turn olive.
Cook greens to keep their colour.
Explain the brightening of the first minute.
Weigh baking soda's colour against its texture.

Workshop

Reaction bench at 100 °C: chlorophyll to pheophytin with the course rate, multiplied by 3^(6.5 − pH) for acidity; softening with the course rate (× 0.35 for whole beans) multiplied by 2^(pH − 6.5) (teaching factors in the measured direction). Colour drawn as a blend from green to olive by the share converted.

Fast, open, and served at once

At neutral pH, green beans keep about 0.67 of their chlorophyll after four minutes in boiling water and 0.30 after twelve. In slightly acid water (pH 5) they keep only 0.13 after four minutes. A pinch of baking soda (pH 8) keeps them green but softens them to 0.63 against 0.30 in four minutes. So: boil greens in plenty of salted water so it stays at a boil when they go in; leave the lid off for the first minutes so volatile acids escape; cut them small enough to cook in a few minutes; and stop them in ice water if they are not served at once (XVI.2).

Acid is best added at the table: dress green beans or broccoli just before serving, because even cold, vinegar turns them olive within an hour or two. Long-cooked greens — braised kale, collards, 梅菜, Indian saag — are a different dish, olive by intention and tender and deep in flavour. Spinach and other leaves lose colour less than beans because they cook in seconds; frozen greens were blanched before freezing to stop the enzyme that would otherwise dull them in storage (XIII.3).

At the stove

Boil green beans and take some out at 2, 5 and 12 minutes. Line them up and compare the colour; then dress the 5-minute ones with vinegar and watch them over an hour.

Before chlorophyll was understood, cooks kept greens bright with copper pans or even copper sulphate — copper takes magnesium's place in chlorophyll and makes a stable green. It works, and it is poisonous in the amounts once used; the practice was banned in the nineteenth century.

Terms

Chlorophyll: The green pigment of leaves, with magnesium at its centre.
Pheophytin: Olive-brown chlorophyll that has lost its magnesium.
Blanching: A brief boil, then usually cold water, to set colour and stop enzymes (XVI.2).
Shocking: Plunging cooked vegetables into ice water to stop cooking.

Acid and time turn chlorophyll into olive pheophytin. Keep greens green with fast boiling, an open pot, short times and acid only at the table. Baking soda keeps colour but ruins texture.

Sources

Chlorophyll to pheophytin in heated green vegetables: first order, activation energy about 50–60 kJ/mol, much faster in acid (S. J. Schwartz and T. V. Lorenzo, Crit. Rev. Food Sci. Nutr. 29, 1990). Teaching values within these ranges.
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.