Oils, and Why They Differ

So what is up with all the different oils used in different products, anyway? I touched on this topic very briefly in “What Makes This Soap So Special” a while back, briefly explaining some of the characteristics of the oils in our SunSoap that led to me including them in that formula. The fact is, different oils have so many different characteristics that sometimes choosing the selection of oils to make up a particular product is the most time-consuming part of the whole product development process.

First, a bit of technical overview. Oils are fats which are liquid at room temperature, technically, which means some of the things I call “oils” for simplicity’s sake are not, really – coconut oil melts at 76F, so is only liquid at the temperature of some rooms; cocoa butter and shea butter, for instance, are decidedly solid at room temperature, and beeswax is higher-melting yet (also, waxes follow different rules anyway, so let’s forget about beeswax for now). But for the purposes of this and similar discussions, I will use “oil” and “fat” interchangeably as terms.

Triglyceride/fat molecule (image from Wikipedia)

A fat molecule is known as a “triglyceride” – a glycerol molecule with three fatty acid chains hanging off of it. The differences in character between types of fats is all down to variations in the fatty acids – the differences between beef tallow and grapeseed oil are all in those three dangly bits of those molecules. Some of the ways in which fatty acids can differ from one another are terms we’ve heard – saturated, unsaturated, cis, trans – and those are all important in their way. There are other differences as well, less relevant to our diets and so not so frequently discussed.

Serious cooks already know a bit about this, of course. One would never deep-fry in extra-virgin olive oil, for instance; partly because most of the distinctive flavors are destroyed long before deep-frying temperatures are reached, and partly because heating olive oil that far above its smoke point is just asking for a kitchen fire. Sometimes it doesn’t matter all that much – if you’re putting a cup of vegetable oil in your brownie mix according to the directions on the box, you can use the bottle labeled “vegetable oil” (which is probably mostly soybean oil), you can use canola oil, corn oil, or just about anything else oily you’ve got in the kitchen, and while it might change the results a little, you’ll still end up with brownies at the end.

Similarly, if you mix any oil with the right amount of lye, you’ll get soap, and it’ll get you clean, well enough, so what’s the big deal?

Soap is, at the most basic chemical definition, a salt. Salts are made from the reaction of an acid and a base – sodium chloride, table salt, is made from sodium hydroxide and hydrochloric acid, for instance. Soap is the result when sodium hydroxide reacts with the fatty acids in the tails of a fat molecule. So as different oils have different fatty acid compositions, the soaps they make are very different from one another. They’re all sodium salts, but then, table salt and borax are both sodium salts, too, and they’re hardly interchangeable!

Fatty acid  categories (image from Wikipedia)

(Oh, and by the way – the sodium hydroxide reacts with the fatty acids, producing soap (and water, which eventually cures out of the finished soap) – but those fatty acids came attached to glycerol in the original triglyceride molecule. Well, you’ve heard me talk before about the glycerin which many large-scale soapmakers have removed from their soap and which handmade soapers leave behind, and now you know where it comes from!)

Now, having conveyed all this groundwork, I’ll confess that I don’t actually spend a whole lot of time thinking about fatty acid profiles in depth. It’s not entirely irrelevant to me – for instance, sunflower oil comes in “high oleic” and “high linoleic” varieties, each of which has different properties, affecting the character of the finished product in noticeable ways. And fatty acid profiles make it easier to choose oils for a soap recipe and predict hardness of finished bar, quantity and quality of lather, and so on. But if I’m making a recipe for a lotion, scrub, conditioner, or other moisturizing product, I’m usually thinking about more immediate oil qualities rather than the fatty acid composition which created them: Is it easily absorbed by the skin? Is it comedogenic? Does it feel heavy and greasy, or is it lightweight? Does it have a smell, and if so, will it harmonize with the scent I have in mind for the finished product?

Future posts on this topic will expand on qualities of various oils that lead me to choose them in product development, and how I know which oils I can substitute for one another in which contexts, and how I go about composing a selection of oils to create exactly the effect I’m looking for – though, of course, usually not on the first try, no matter how much science I put into the choosing. But then, that’s what science is about, right? Trying things, learning from how they go wrong, and then trying new things!

And if I stray too far into scientific esoterica and leave anyone with entirely no idea of what on earth I’m talking about, please a) let me know before I head any further off into the weeds, and b) feel free to ask questions!

Next in this series will discuss fatty acids and soap in detail.

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About Amy Young

Founder of Foam on the Range soaps. View all posts by Amy Young

5 responses to “Oils, and Why They Differ

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