Vol. XXXIII · Modern Techniques and the Frontier Plate 4 of 11foams, airs, espuma, siphon, cream whipper, nitrous oxide, N2O, CO2, Henry's law, lecithin, gas solubility, pressure, overrun, stability, safetyMT

Foams and Airs.

Two 8 g chargers of nitrous oxide in a half-full siphon raise the pressure to about ten atmospheres. Released at the nozzle, the gas that dissolved under that pressure inflates the cream to more than double its volume.

A foam is gas dispersed in a liquid, held by something that stabilises the thin films between bubbles: proteins (egg white, X.6), fat crystals (whipped cream, X.7), or surface-active molecules such as lecithin. An "air", made famous by elBulli, is the lightest foam: a flavoured liquid — juice, stock, milk — with about 0.3–1 % soy lecithin, whipped at its surface with a hand blender until it froths; the froth is spooned off and served at once, because such thin films drain and collapse within minutes. Airs carry aroma and a sensation of lightness rather than substance.

A siphon (cream whipper) makes denser, more stable foams. The liquid — cream, or a purée or sauce stabilised with gelatin, starch or fat — goes into the canister, which is sealed and charged with nitrous oxide (N₂O) from small 8 g cartridges. Under pressure, the gas dissolves in the liquid in proportion to the pressure (Henry's law); N₂O dissolves far better than air, and well in fat, and has little taste. When the lever opens, the liquid leaves through the nozzle at atmospheric pressure, the dissolved gas comes out as tiny bubbles throughout, and the liquid puffs into a foam. Carbon dioxide cartridges make a fizzy, slightly sour foam (carbonic acid). Siphons are pressure vessels: use only the maker's chargers and number, never a damaged or overfilled canister. The workshop charges a siphon.

Below: the pressure in a 1 L siphon holding the volume of liquid you choose after the number of 8 g chargers you choose, the gas dissolved in the liquid at that pressure (Henry's law with approximate solubilities in water at 20 °C), and the largest foam volume it could make if all the dissolved gas formed bubbles.

What you will learn
Workshop
Ideal-gas and Henry's-law estimate at 20 °C: each charger 8 g N₂O (0.18 mol, about 4.4 L at 1 atm) or 8 g CO₂; gas shared between the headspace (1 L − liquid) and solution, with solubility about 0.6 L of N₂O or 0.9 L of CO₂ per litre of water per atmosphere (more in fat). Maximum overrun = dissolved gas ÷ liquid volume; real foams reach perhaps half to two-thirds of it. Siphons are typically rated well above these pressures when used as directed.

Pressure in, foam out

Two N₂O chargers in a 1 L siphon half full of liquid give, by this estimate, about 10.9 atmospheres, at which some 655 % of the liquid's volume in gas can dissolve — the most the foam could expand. Real foams lose some gas through the nozzle and the walls of bubbles, so a cream that might in principle expand several-fold ends nearer twice its volume. Shaking after charging speeds dissolving; chilling helps too, because gases dissolve better in cold liquids, and cream foams need the fat cold to hold.

Siphon basics: fill no further than the maker's line (usually half to two-thirds); strain purées so they do not block the nozzle; screw the head on clean threads; charge with the maker's cartridges, one at a time, shaking after each; hold upside down to dispense; release remaining pressure before opening. Never use a dented or corroded canister, never exceed the stated number of chargers, and store charged siphons cold, away from heat. Nitrous oxide is also a drug of misuse; inhaling it causes injuries and nerve damage, and many places restrict its sale.

At the stove

Whip a spoon of lecithin into a cup of fruit juice with a hand blender at the surface. Spoon off the air and time how long it lasts.

The cream whipper was invented as a domestic gadget in the early twentieth century; the Austrian company iSi began making them in 1964. Chefs at elBulli turned it to savoury "espumas" in the 1990s — hot potato foam, foams of beans and of smoke.

Terms

Air
A very light lecithin froth, short-lived.
Espuma
A siphon foam.
Henry's law
A gas's solubility is proportional to its pressure.
Overrun
Increase in volume from incorporated gas.
Airs: lecithin froths, gone in minutes. Siphons: N₂O dissolves under pressure (Henry's law) and puffs the liquid on release; stabilise with fat, gelatin or starch, chill, don't overfill. A siphon is a pressure vessel: maker's chargers and number only.

Sources

  1. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.
  2. N. Kurti, "The physicist in the kitchen", Royal Institution Friday Evening Discourse, 14 March 1969; the first International Workshop on Molecular and Physical Gastronomy, Erice, 1992 (Kurti and H. This); G. Pralus's sous-vide foie gras for the Troisgros restaurant, 1974; F. Adrià, H. Blumenthal, T. Keller and H. McGee, "Statement on the new cookery", The Observer, 10 December 2006; N. Myhrvold, C. Young and M. Bilet, Modernist Cuisine, 2011.
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.