Vol. III · Water Plate 8 of 11diffusion, osmosis, brine, cure, marinade, salt penetration, Fick's lawHMT

Osmosis and Diffusion.

Salt spreads into meat the way heat does, by diffusion — but a thousand times more slowly. A day buys millimetres.

Put a chicken breast in brine and the salt starts to move in. It does so by diffusion: dissolved ions wandering at random, more of them moving from where they are many to where they are few. The mathematics is the same as for heat (II.1–2) — Fick's law instead of Fourier's — so the same square law holds: twice as deep takes four times as long. But the diffusivity of salt in meat is around 10⁻¹⁰ m²/s, about a thousand times smaller than heat's, so where heat crosses a centimetre in minutes, salt needs a day.

Water moves too. Where salt or sugar is concentrated outside the cells, water is drawn out of them by osmosis — which is how salting draws water from cucumbers and cabbage (XIV.4) and how a dry cure pulls moisture from ham. The workshop soaks a slab of meat in brine.

Below: the salt concentration across a slab of meat in brine after the time you choose (as a share of the brine's), with the depth reached by a tenth of it and the rule of thumb 2√(Dt).

What you will learn

Describe diffusion and Fick's law.
Estimate how far salt travels in an hour and a day.
Distinguish diffusion from osmosis.
Plan brining and marinating times.

Workshop

Salt diffusing into a slab of meat from both faces (Fick's law solved on the water bench; diffusivity about 4 × 10⁻¹⁰ m²/s at 20 °C, slower in the fridge). Concentration is shown as a share of the brine's, from the surface inward.

Millimetres a day

In the fridge, an hour in brine takes salt only about 1.8 mm into the meat; a day takes it about 9.8 mm, and in a thick piece three days take it 16.7 mm against 10.0 mm for one — the square law again, where three times the time buys only about 1.7 times the depth. At room temperature diffusion is a little faster (12.7 mm in a day), but brining should be done cold for safety. For a 30 mm slab, a day brings the whole piece to 33 % of the brine's concentration on average, the centre to 3 %.

So brines and marinades are surface treatments for anything thick. A brine works on a chicken breast or a pork chop overnight, and on a turkey only partly; injecting brine (as hams are cured) or cutting the meat thinner works where soaking cannot. Large molecules — the flavours of herbs, garlic and spices in a marinade — move even more slowly than salt and barely pass the surface; acids and enzymes in marinades act only where they reach (XVIII.5).

At the stove

Cut a thick slice of potato or a piece of chicken in half after a night in a brine coloured with a few drops of food colouring or a little soy sauce. The coloured rim shows how far the liquid travelled — usually a few millimetres, however long the recipe promised.

Adolf Fick, a physiologist, published his law of diffusion in 1855, explicitly by analogy with Fourier's law of heat conduction.

Terms

Diffusion: Molecules spreading from where they are concentrated to where they are scarce by random movement.
Fick's law: The flow of a dissolved substance is proportional to its concentration gradient.
Osmosis: Water moving across a membrane towards the more concentrated solution.
Diffusivity: How fast a substance spreads; for salt in meat about 10⁻¹⁰ m²/s, a thousand times slower than heat.

Salt and flavour spread by diffusion, a thousand times slower than heat: a day reaches millimetres, not centimetres.

Sources

A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.