Silk is far from a universal ingredient in artisan soaps, but it is increasingly common. And there are several reasons for that – but they all add up to it making the soap smoother, shinier, and, well, silkier – and but mostly stabilizing the lather, or foam. OK, how does it do that?
The answer? Protein. Proteins are the basic building blocks of all manner of things, and silk is almost entirely composed of protein and smaller amino acids. Dissolving silk into the lye solution at the beginning of the soapmaking process breaks the long proteins into smaller pieces which end up evenly distributed throughout the final soap. Even these smaller pieces, though, are still huge honkin’ molecules with lots of different parts, which is how they manage to do what they do.
And what is it they do? Well, anyone who has ever made a meringue has seen the efficacy of a protein-stabilized foam – and indeed pretty much all culinary foams benefit from the stabilizing effects of proteins in one way or another, from whipped cream to cappuccino to chocolate mousse. But before we can get into why this is the case, we should take a look at what a foam actually is.
In its simplest form, a foam is a whole bunch of little teeny bubbles separated by thin films of liquid. Seems pretty obvious, right? Sure. But liquid being what it is, and gravity being what it is, the liquid is going to want to run downward and eventually collect at the bottom of the pile, leaving nothing in between the bubbles but air. And since air was what was inside them in the first place, this pretty much means they aren’t bubbles anymore, they’re just – well, air. This will essentially always happen eventually, but there are some things we can do to make it happen more slowly.
What allows a foam to form in the first place is usually a surfactant, lowering the surface tension of water and creating conditions favorable to the formation of a film. But wait, I hear you say, isn’t soap itself a surfactant? Why do we need the protein? Well, if all we want to do is make the foam in the first place, we don’t. Soap will lather perfectly well on its own – just add water and suds up. But with the exception of some carefully-formulated shaving soaps and bubble baths, it doesn’t usually stick around very long, because soap is an insufficiently complicated molecule.
Proteins, however, as stated above, are huge honkin’ molecules, and have the potential to interact with themselves and each other in interesting and fairly complicated ways. Once a protein is denatured – for example, by dumping it into a crazy high pH lye solution – all of the parts of its structure that once held it together in the specific shape that let its do its job – the forces that make silk so incredibly strong, in this case – are flailing around without anything to do, like the hook half of a Velcro strip bereft of loops. But wait! That piece over there has loops! It’s not a perfect fit, maybe it only fills 5% of the hooks, but hey, it’s better than nothing. So they end up very weakly bonded, one to another, all through the solution. This trait of proteins is what gives soup stock its “body” and “mouthfeel”, and it’s what helps these bubbles stay put longer.
So that’s why I put silk in my soap. Is that the only source out there for soapy proteins? Absolutely not! Other common protein sources in soap include milk (often goats’ milk) and oatmeal, and I also get a protein boost from the beer I add to my Hops in the Shower line. Some people use other fibers, such as angora wool. Now, not all proteins are created equal, so each of these protein sources will have a subtly different set of effects on the soap. But in nearly all cases, the foam stabilization effects will be in play in the lather they generate.