Vol. XX · Vegetables, Fruit and Legumes Plate 1 of 12plant cells, cell walls, turgor, crispness, osmosis, wilting, crisp lettuce, ice water, salting vegetables, cucumbers, vacuole, pectinMT

Plant Cells.

A limp celery stick left in a glass of cold water for an hour snaps again. Nothing was added but water, pushed in by osmosis.

A plant cell is a bag of watery sap inside a box of cellulose, held together with pectin (V.4). The sap is full of dissolved sugars, salts and acids, more concentrated than the water around it, so water is drawn in by osmosis (III.8) until the cell presses hard against its walls. That pressure, turgor, is several times atmospheric pressure, and it is what makes a carrot snap, a lettuce leaf crisp and an apple crunch: biting bursts the pressurised cells and releases their juice.

When a vegetable loses water — picked, kept warm and dry — the pressure falls and it goes limp. Put it in cold plain water and water flows back in; put it in salty or sugary water stronger than its sap and water flows out, which is how salting wilts cabbage for 泡菜 (XIV.4) and draws the water from cucumbers. Cooking destroys the membranes that hold the pressure, which is why even barely cooked vegetables lose their crunch. The workshop soaks a leaf.

Below: the turgor pressure in a vegetable's cells soaked in water containing the salt or sugar you choose (van 't Hoff's law with a cell sap of 0.35 osmol/kg), and a sketch of the cells, swelling or shrinking.

What you will learn
Workshop
Van 't Hoff's law, ideal: osmotic pressure Π = cRT. Cell sap taken as 0.35 osmol/kg (a typical value for vegetable tissue); salt counts two particles per formula unit (ideal), sugar one. Turgor shown as the difference, set to zero when the outside is stronger (the cell then loses water and the wall is no longer pressed).

Water in for crunch, salt out for softness

A cell sap of 0.35 osmol/kg pulls water in with about 0.85 MPa — more than eight times atmospheric pressure. In this ideal model salty water of about 1.0 % balances the sap exactly; anything saltier draws water out. Sugar, a bigger molecule counted once, needs about 12 % to do the same, which is why sugaring fruit for a compote or 糖渍 draws juice out only slowly unless the sugar is heavy (XIII.6).

In practice: revive limp lettuce, herbs, celery and carrots in ice-cold plain water for 15–30 minutes, then dry them well (dressing clings only to dry leaves, X.3). Salt cut cucumbers, cabbage or aubergine with 1–3 % salt to draw out water before pickling, stir-frying or frying, so they stay firm and absorb less oil. Dress salads at the last moment, since salt and acid wilt leaves in minutes (X.3). Store vegetables cold and humid — in the crisper drawer, wrapped — to slow water loss.

At the stove

Put a limp stick of celery in ice water and another in 3 % salt water for an hour. Snap both, and taste the second.

Turgor is how non-woody plants stand up: a sunflower stem or a lettuce has no skeleton, only pressurised cells pressing against their walls. A wilting houseplant is a plant whose cells have lost that pressure.

Terms

Turgor
The water pressure inside a plant cell that keeps it firm.
Osmosis
Water moving through a membrane towards the more concentrated side (III.8).
Vacuole
The large sac of watery sap inside a plant cell.
Wilting
Loss of firmness as cells lose water and pressure.
Crispness is pressure: water drawn into cells by osmosis presses them against their walls. Plain cold water restores it; salt draws water out and wilts; cooking breaks the membranes and ends it.

Sources

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