We’ve all seen them; they’re in every soap display I have ever encountered: soaps in assorted shades of tan and café au lait and caramel and chocolate, scattered among their brilliantly colored kin like sparrows nesting with cockatiels. In some cases this may be a deliberate choice on the part of the soapmaker, but odds are that by far the bulk of them are due to vanilla, with a couple of other culprits trailing along behind.
Vanilla? Really? But surely people don’t make so very many vanilla soaps as to have 10 or more kinds in a display of 50 turn dark? Well, no. Or at least, not exactly. The thing is, vanilla is an incredibly common ingredient in fragrance blends. When you want sweet notes, or to smooth out a sharp woodsy combination, or provide a bit of depth to a particularly flighty floral, you turn to vanilla. It’s an indispensable component of any fragrancer’s repertoire, but it’s not without its down side: the aforementioned darkening.
Vanilla in its natural state is a complex assortment of aromatic compounds, and chief among them is one called vanillin. Vanillin is also present in synthetic vanilla fragrances, as it is readily synthesized and goes a long way toward giving the “vanilla” impression. It is also subject to oxidation, and that is where the story gets complicated.
See, I have known for as long as I have been making soap that vanilla-containing fragrances turn soap brown, and have assumed all along from the evidence that it was due to an oxidation reaction. After all, it happens from the outside in, as it would if air had to penetrate the soap, and things turning brown over time is almost always an oxidation reaction anyway; it seemed a safe enough bet. But when I tried looking into the chemistry of it to get some actual answers to feed to my voracious BUT WHY?! monster, the information I found was a frustrating mixture of uninformative and unhelpful. Vanillin, you see, is an aromatic (fancy science speak for “ring structure”, in this case, rather than “smells good”, even though it does) compound with a portion of its structure which indeed oxidizes readily, but it oxidizes to vanillic acid, which is colorless.
Colorless. Well. That certainly doesn’t answer the question of why things turn dark! So I kept looking, and found a few lab protocols with cautions on things to avoid doing when oxidizing vanillin to synthesize vanillic acid, lest it turn a dark brown in the process. Aha! Well, then. I still haven’t gathered enough data (and honestly, unless an cosmetic organic chemist comes wandering in and feels informative, I probably won’t, given the limitations of both my equipment and the time I feel justified in devoting to a piece of, let’s face it, fairly idle curiosity) to determine precisely what element present in soap (and bath salts, and a few other products which darken in the presence of vanilla) causes that effect on the reaction. I have some suspicions, but in any case I now feel fairly safe in declaring that the oxidation of vanillin to vanillic acid is responsible for the effect.
What good does it do me to learn that? Right away, not much, really. I don’t mind the darkening of the soap; I think it provides an interesting challenge to create “looks” for soap with that constraint to their color schemes. So I won’t (at least not immediately) be looking for a way to prevent this oxidation reaction, given that it is perfectly harmless and does not cause the quality of the soap to deteriorate in any way. But if in the future I should change my mind about that, I now know that what I need to stop is an oxidation reaction, and so I would know where to start in on solving the problem.
For now, it’s just a piece of curiosity fulfilled to my satisfaction, and one more thing crossed off my list of “it just happens that way, that’s why” list of things that bug me about the accumulated wisdom of soapmaking. It’s great to know what works, but I like to know why it works, too; it makes it more likely I’ll have success when I decide to do variations on the themes. Science is fun! …even when it’s also frustrating. Which is actually most of the time; ask any scientist. Or don’t; they might throw things at you if it’s been a particularly frustrating day.