Foams are common in the kitchen. Egg whites and meringues, a head of beer, whipped cream, cappuccino etc. What’s modernist about foams is pushing the limits a bit. For example, can it be lighter and airier, and can liquids that don’t foam of their own accord be foamed?
One way to achieve this is to add a surfactant. Think of these molecules as a short rod, one end of which is hydrophilic (water-loving) while the other end is hydrophobic (water-avoiding and gas-preferring).
When the surfactant is dissolved in a liquid and the mixture beaten to introduce air, the surfactants arrange themselves on the surface of the air bubbles such that the gas-loving ends are in the bubbles, and the water-loving ends are pointing outwards into the liquid. Thus the surfactant coats the surface of the bubbles and stabilises them.
An example of a surfactant that is fairly readly available is soy lecithin. You’ll probably find it in health food shops, although it may be granular and perhaps not de-oiled, so an online culinary supplier might be necessary. The point is that it is nothing unusual as an ingredient, just the application.
Not much is needed. I recently did some trials, and 0.3 to 0.4% by weight of lecithin is all that is needed to foam tap water. The amount will differ according to the liquid though. I’ve attempted some videos that show the process for making a Citrus Air, a light foam of lemon and lime juice. I think I might qualify for a Razzie.
125ml lemon juice, 125ml lime juice, 250ml water (all at room temperature), 2g lecithin (0.4% by weight). The water dilution is necessary to reduce the intensity of the acidity, and because acidity retards foaming. Aerate with a stick blender:
Leave about 1 minute for excess liquid to drain out of the foam:
A different textured (lighter) foam, can be made using an aquarium bubbler (that oxygenate fish tanks). Here it is just bubbling in some water:
Now in the citrus mixture.
The ‘air’ is coarser and more ephemeral than the hand-blender foam:
They both have an intense lemon/lime taste, and make an interesting way to add these flavours.
The foam was used to garnish chicken in the dish below (taken from summer-menu-2014), while a little of the air could be draped over an oyster in its shell.