Vol. III · Water Plate 3 of 11altitude, boiling point, air pressure, high-altitude cooking, baking adjustmentsHT

Cooking at Altitude.

On a mountain, water boils before it is hot enough. Everything cooked in it takes longer, and some things never cook at all.

The higher you go, the less air is pressing down, and the lower the temperature at which water's vapour pressure matches it. In Denver, at about 1 600 m, water boils near 95 °C; in Lhasa, at about 3 650 m, near 88 °C. Food in boiling water is cooked at that lower temperature, so it takes longer — and things that need nearly 100 °C, such as softening dried beans or setting the starch in rice, may take far longer or never quite finish without a pressure cooker (III.4).

Baking changes too: water evaporates faster and gases expand more in thinner air, so cakes rise fast and then collapse unless the recipe is adjusted. The workshop shows the boiling point against altitude and how much longer a potato takes to cook through.

Below: the boiling point of water against altitude (standard atmosphere and the IAPWS saturation curve), and the time for the centre of a 40 mm potato to reach 90 °C in boiling water at your altitude.

What you will learn
Workshop
Boiling point against altitude from the standard atmosphere's pressure and the IAPWS-IF97 saturation curve. The potato: a 40 mm cylinder from 20 °C in water boiling at the local boiling point, to 90 °C at the centre (thermal bench).

About a degree every 300 metres

At 1 600 m the air pressure is 84 kPa and water boils at 94.6 °C; a 40 mm potato takes 20 min to reach 85 °C at its centre instead of 17 min, 17 % longer. At 3 650 m water boils at 87.8 °C and the potato takes 71 % longer. The rule of thumb — about one degree less for every 300 m — holds well up to the heights where people live.

Simmered and braised dishes need more time; pasta and rice need more time and sometimes more water, because more evaporates; dried beans may never soften in an open pot at great heights, which is why pressure cookers are standard kitchen equipment on the Tibetan plateau and in the Andes. Bakers at altitude commonly reduce leavening, add a little liquid, raise the oven by 10–15 °C so the structure sets before the gas over-expands, and reduce sugar slightly — the U.S. agricultural extension services publish tables.

At the stove

If you live above about 1 000 m, boil water and measure it with a calibrated thermometer (I.4). Write the number down: it is your kitchen's boiling point, and every boiled or simmered time in a sea-level recipe will need lengthening.

Charles Darwin, cooking potatoes in the Andes in 1835, found that after hours of boiling they were still hard; his guides blamed the pot, which "did not choose to boil them".

Terms

Atmospheric pressure
The weight of the air above; about 101 kPa at sea level, falling with height.
Standard atmosphere
An agreed model of how pressure and temperature fall with altitude.
High-altitude adjustment
Changes to a recipe for thinner air: more time, more liquid, less leavening, a hotter oven for baking.
Pressure cooker
A sealed pot that raises the pressure and with it the boiling point (III.4).
Water boils about 1 °C cooler for every 300 m of altitude; boiled food cooks slower, and baking needs adjusting.

Sources

  1. ISO 2533:1975, Standard Atmosphere (the troposphere barometric formula).
  2. IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).