Vol. III · Water Plate 5 of 11evaporation, wet-bulb temperature, stall, brisket, barbecue, wrapping, evaporative coolingHT

Evaporation and the Stall.

A wet piece of meat in a 110 °C smoker can sit at 65 °C for hours. It is not resting; it is sweating.

Barbecue cooks know the stall: a brisket in a smoker climbs steadily to about 65 °C at its centre, then stops — sometimes for hours — before rising again. The meat is not stuck; it is cooling itself the way a runner does. Juices squeezed from the warming muscle reach the surface and evaporate into the hot, dry air of the smoker, and evaporation carries away so much heat that the surface settles near the wet-bulb temperature of the air, far below the air itself.

Only when the surface finally dries, or the evaporating juices run short, does the surface climb and the centre follow. Wrapping the meat in foil or butcher paper traps the moisture and stops the evaporation — the barbecue trick called the "Texas crutch". The workshop smokes a brisket, wrapped or not.

Below: the centre and surface of an 80 mm brisket in a smoker at the temperature you choose, unwrapped (evaporating) or wrapped (not).

What you will learn
Workshop
An 80 mm beef brisket slab from 5 °C in a smoker (air at the chosen temperature, low humidity), cooked for 12 hours. Unwrapped, its juices evaporate from the surface; wrapped, they cannot (thermal bench). The shaded band marks 60–75 °C.

A cook that sweats

In a 110 °C smoker the unwrapped brisket's surface sits far below the air for hours — after one hour it is at about 52 °C — while its juices evaporate, and the centre takes 1 h 49 min to climb from 60 to 75 °C. Wrapped, with nothing able to evaporate, it climbs the same range in 67 min, and reaches 90 °C in 5.7 hours instead of 8.0. At 135 °C the stall is shorter (63 min), because the hotter air dries the surface sooner.

The wet-bulb temperature is the temperature a wet surface settles at when evaporation balances the heat arriving from the air: the drier the air, the lower it is. The same physics makes a roast in a dry oven brown slowly at first (VIII.2), and is why a covered roasting tin or a splash of water in the oven changes how a surface cooks. Pitmasters choose: ride out the stall for a drier, darker bark, or wrap to save hours at the cost of a softer surface.

At the stove

Next time you slow-roast a large joint, log its centre temperature every half hour with a probe left in. Look for a flat stretch between about 60 and 75 °C. If you want to shorten it, wrap the joint in foil when the flattening begins and note how quickly it moves again.

The stall puzzled barbecue cooks for decades — theories included fat rendering and collagen melting — until the physicist Greg Blonder showed in 2011, with a wet sponge in a smoker that stalled just like meat, that it is evaporative cooling.

Terms

Evaporative cooling
The cooling of a wet surface as its water evaporates and carries heat away.
Wet-bulb temperature
The temperature a wet surface settles at in moving air when evaporation balances the heat received.
Stall
A long pause in the rise of a large piece of meat's centre temperature during slow cooking in dry heat.
Texas crutch
Wrapping barbecue in foil or paper to stop evaporation and shorten the stall.
A wet surface in dry heat cools itself by evaporation; that is the stall, and wrapping 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.