Culinary airs & foams, those frothy darlings of the molecular gastronomic world. Like a sauce, but much lighter, more controllable, and not prone to making things soggy on the plate. And remarkably simple to make: take a suitably dosed liquid, whip it until bubbles form, then sit back amazed as those bubbles don’t pop.
All this magic happens by dosing your liquid with an emulsifier. And even though we aren’t technically making an emulsion, the science at work is nearly the same.
A Recipe for Mayonnaise. Sort of.
Get a bowl of water, and slowly pour a stream of oil into it, while whisking. The oil will break up into tiny droplets, evenly dispersed throughout the water. However, as soon as you stop whisking, those hydrophobic droplets will seek each other out, coalescing into a discrete layer of oil.
Now get an egg, break it into a bowl, and again whisk it to incorporate the white and yolk. While still whisking, slowly pour in the oil. Start with just a drop at a time; as the mixture thickens, you can increase to a trickle, and then gradually work up to a steady stream of oil. This mixture will thicken and look homogenous. More importantly, even though the egg is largely water, once you stop whisking, the oil will not separate. Congratulations, you’ve just made an emulsion! And the world’s blandest tasting mayonnaise.
An emulsion is just a fine dispersion of one liquid (or “phase”) within a second, immiscible (“un-mixable”) liquid. Normally, when you try to incorporate two immiscible liquids, they will begin to separate once you stop mixing them. To prevent this, you need to use an emulsifier — something which is able to bond to both of your liquids. In food, emulsions are nearly always either oil-in-water or water-in-oil, so a culinary emulsifier is something which is soluble in both water and oil. In fancy terms, we say that the emulsifier’s molecule has two different ends: a hydrophillic end, which forms chemical bonds with water but not with oils, and a hydrophobic end, that forms chemical bonds with oils but not with water.
Emulsifiers are everywhere. Protein is one. So is lecithin, a fatty substance naturally occurring in, well, all sorts of plants and animals, including in egg yolks.
That mayonnaise we just made? That was an oil-in-water emulsion: the egg white is our continuous aqueous phase (the base, watery liquid into which the oil phase is dispersed), which was dosed with lecithin from the egg yolk.[1. Egg whites contain protein, which can also act as an emulsifier. This means it’s technically possible to make a yolk-less mayonnaise. It won’t taste the same, but it will be similarly thick and creamy.] As we add and agitate the oil, it gets dispersed into tiny droplets throughout our continuous phase. Before those droplets can coalesce, they are surrounded by emulsifying molecules which line up hydrophobic ends inward, bonding with the oil, hydrophilic ends outward, bonding with the water, locking the droplets of oil into a stable network. Presto emulsion.
Wasabi Air: Like an Emulsion, Only Different
So how do we get from an emulsion to an air? Simple: airs are bubbles of gas dispersed throughout our base liquid. The lecithin in our aqueous solution still surrounds the bubbles, hydrophilic end out. The lecithin molecules also bond to each other, forming a stable network. However, there is no oil for the hydrophobic end to bond to. Because the lecithin is only bonding to one phase, but is still stabilizing two immiscible substances (one liquid, one gas), it is now acting as a stabilizer, instead of an emulsifier.
To create the air, add anywhere between 0.3-0.8% (by weight) soy lecithin to a cold liquid. Place your bowl in an ice bath (lecithin works best cold), and agitate the liquid with an immersion blender. The foam which collects on top will remain stable at room temperature for at least 20 minutes, or can be frozen for longer life.