Vol. V · The Molecules of Food Plate 4 of 11cell walls, cellulose, pectin, hemicellulose, fibre, softening vegetables, acid, baking sodaMH

Fibre and Cell Walls.

A raw carrot snaps because its cells are full, firm and glued tight. Cooking dissolves the glue.

A plant is built of cells, each enclosed by a wall of cellulose fibres embedded in hemicellulose and held to its neighbours by pectin, a gel-like cement. Raw, the cells are swollen with water and pressed against each other, and the vegetable is crisp. Heat does two things: it kills the cells so they lose their pressure and go limp, and, more slowly, it breaks down the pectin so the cells come apart — the vegetable softens and, in the end, falls to mush.

Pectin breakdown depends strongly on conditions. In alkaline water — a pinch of baking soda — it is fast, which softens beans quickly but can turn green vegetables to mush; in acid it is slow, which is why vegetables cooked in tomato sauce or with vinegar stay firm; and calcium cross-links pectin (III.10), firming it further. The workshop softens a vegetable at the temperature and pH you choose.

Below: plant cells drawn as they soften, and the fraction of softening after the time you choose at the temperature and pH you choose (course kinetics with a teaching pH factor).

What you will learn
Workshop
Softening as a first-order reaction (course parameters: rate 0.25 per minute at 100 °C, activation energy 90 kJ/mol), multiplied by a teaching pH factor: about twice as fast per pH unit above 6, slower below.

Glue, acid and alkali

In plain water near the boil a vegetable reaches tender (60 % softened, in this model) in about 4.3 min; at 85 °C, below a simmer, softening is much slower (45% after eight minutes), which is why gently poached vegetables keep their bite. At pH 4, as in a tomato sauce, the same eight minutes give only 26%; at pH 8, with baking soda, 99%.

Use the effects deliberately. Add tomatoes, wine or vinegar to a braise only after the vegetables (and beans) have softened, or they may never do so. A pinch of baking soda softens chickpeas for hummus but ruins the texture and the vitamins of green vegetables. Calcium — in hard water, or added as calcium chloride in canned tomatoes and some pickles — keeps pieces whole. Cellulose itself never softens in cooking; it is the fibre that passes through us undigested.

At the stove

Simmer two batches of sliced carrots for ten minutes, one in water and one in water with a spoonful of vinegar. Taste and compare; the acidified ones stay noticeably firmer.

Pickle makers add calcium chloride to keep cucumber pickles crisp, and canned tomatoes often list it for the same reason: calcium bridges between pectin chains hold the cells together.

Terms

Cellulose
The tough fibre of plant cell walls; cooking does not soften it.
Pectin
The gel-like cement between plant cells; heat and alkali break it down.
Turgor
The water pressure inside living plant cells that makes vegetables crisp.
Dietary fibre
Plant material we cannot digest, mainly from cell walls.
Vegetables soften as heat breaks down pectin: faster in alkali, slower in acid or with calcium.

Sources

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