In my lab - Foams, part 2: Sience

So what is foam? Foam is gas suspended in liquid. The gas may be air, CO2, N2O or just about anything, but these are the ones you will usually find. Whipped cream contains air. Foams created with a whipping siphon contains N2O. Carbonated sodas like Coke (yes, it is a foam according to the definition of gas suspended in liquid) will contain CO2. So, sodas and beer are foams, too. And, from what I read, so is bread. Bread is a set foam. I do feel that this is stretching is a bit, but who am I to judge?

Basically foam is like an emulsion between liquid and gas. If you mix oil and water, they will soon start to separate with the oil going to the top and the water going to the bottom. But if you add an emulsifier like egg yolk in there, some components (lecithin) i the egg yolk will make the two stay together.

So foam is an emulsion between liquid and gas, and as such it needs an emulsifier or foaming agent to be able to stay as a foam and not go back to liquid and gas separated. The foam forms when molecules from the foaming agent start coating gas bubbles forming a network that prevents the individual bubbles from coming together to form bigger bubbles. If you add detergent to water and then add air by whisking, the air that is blended with the water will stay there as bubbles because the foaming agent in the detergent keep the bubbles separate. Now, in this kind of foam the water actually fall downwards and out of the foam, making the foam very light and dry and thin. If you wanted a thicker and wetter foam, you could do that by thickening the liquid with for instance Xanthan. This is called stabilising the foam, and Xanthan is a stabiliser in this respect.

So you need a foaming agent and possibly a stabiliser to thicken the foam. In addition, you need to add the gas. This may be done by whisking, blowing air into the solution or suing a whipping siphon to add gas. Natural foaming agent may be protein (for instance in milk), fat droplets (in whipped cream) lecithin (from egg yolk or soy) etc. The stabiliser may be anything that makes the liquid thicker, such as, as I mentioned, Xanthan that I wrote about in an earlier post here. Ordinary gelatin is great for foams because the proteins both act as a foaming agent and it makes the liquid thicker, so it is also a stabiliser. But since gelatin melts when heated, it can only make cold foams. If you want to make hot foams, you can for instance use lecithin as the agent. This will make a quite thin and delicate foam, so you may want to thicken it with Xanthan which also works when heated.

Basically what I want to find out is which foaming agents to use when, what kind of foams they make and how I can change the appearance of that foam with stabilisers and the like.

Here is a good page on foams:

http://www.modernistcookingmadeeasy.com/info/modernist-techniques/more/culinary-foams-technique