Brown happens. Leaves turn brown in the fall. Grass turns brown when it's dry. Bite into an apple; the white flesh turns brown over the course of half an hour. Brown is the natural color of vegetable matter if you just wait long enough, and vanilla is no exception. Vanilla beans are harvested while they are green and odorless. The scent of vanilla comes when they are dried in the sun, where they turn dark brown. The beans are steeped in alcohol, resulting in the familiar brown vanilla extract.
For this month's article I was asked to address the color of vanilla scented soap, which begins as a light tan, but darkens over time. My first reaction to this question was, “What do you expect? Brown is the color of vanilla!” The curious thing about vanilla is not that it turns soap brown, but that we expect otherwise. And the reason for this is that we associate the flavor of vanilla with white-colored foods. So the real puzzle here is not that vanilla soap is brown, but that vanilla cake and ice cream are white.
Natural vanilla consists of hundreds of compounds, but the principle component, and the one most responsible for the scent is vanillin, whose structure is shown in Figure 1. As in previous installments of this column, I show a structural formula, the kind of thing a chemist would draw on a napkin when talking to another chemist. Each vertex in the formula represents a carbon atom. Hydrogen atoms are inferred from the number of carbon atoms, and oxygen atoms are shown explicitly. You don't need to understand all the nuances, but it should be evident that the molecule has four parts. At the upper left is an OH group, which is similar to water (H²O), and makes vanillin soluble in water and alcohol. At the lower left is a methoxy group. In the middle is a hexagonal ring of 6 carbon atoms, variously referred to as a benzene ring, or an “aromatic” ring. And at the lower right is a carbon atom double-bonded to an oxygen atom, a group known to chemists as an aldehyde group.
Vanillin is not the only molecule to combine a benzene ring with an OH group. If you left the aldehyde and methoxy groups off of vanillin, you would have a molecule called phenol (pronounced “funole”), the active ingredient in Chloraseptic throat spray, and the compound responsible for its characteristic scent. Molecules like vanillin and phenol are referred to as phenolics. The important thing about these compounds from a soapmaking perspective is that they are slightly acidic and consequently react with alkalis like sodium hydroxide. When people report that vanilla does not “last” in CP soap, this is probably the reason. The harsh alkaline environment of CP soap may be avoided by adding vanilla to an HP soap after the cook, or to an MP soap, and this may produce a more satisfactory aroma. But the reaction with alkali is reversible, and since CP soap becomes less alkaline as it ages, its scent may improve with time.
Vanilla beans are not the only source of vanillin. It can be easily synthesized from another phenolic compound, eugenol, the principle component of clove oil. It can also be made as a by-product of paper manufacturing or from petroleum. The most economical source depends on market conditions, which change from year to year. When vanillin from a source other than vanilla is sold, it is referred to as “artificial” or “imitation” vanilla. Imitation vanilla contains the same molecule that gives natural vanilla its scent and taste, but is missing the hundreds of other compounds which nuance the natural product.
Is vanillin also responsible for the color of vanilla? No, vanillin is colorless. Caramel color is usually added to imitation vanilla so that it looks similar to natural vanilla, but there is a variant of imitation vanilla called “clear” vanilla that does not contain artificial colors. It is as clear as water. This is the version often used for wedding cakes and other foods intended to be as white as possible.
When natural vanilla is used to scent cold process soap, the original color is a light tan. But as the soap cures, it turns brown, beginning on the surface and penetrating over time to the interior. Figure 2 shows a cross section of a soap caught in the middle of this process. The fact that the color change originates on the surface shows that it is caused by reaction of the soap with air. Since this does not occur in unscented soap, we infer that something in the vanilla is being oxidized. Is it vanillin or is it one of the other components of vanilla?
To answer this question, I added some sodium hydroxide to clear vanilla on a small dish and left it exposed to the air. Over the course of a few hours it gradually browned until it's color was the same as that of natural vanilla. The results of this experiment imply that alkali accelerates the oxidation of vanillin and that the color of the oxidation products is brown. Clear vanilla would not seem to avoid the browning of CP soap, but it might be an option for MP soap, which is typically less alkaline.
So what can be done about the browning of vanilla soaps? Some companies market additives (presumably antioxidants) that will delay the oxidation of vanillin. Instead of avoiding the issue, however, I encourage you to embrace the brown color of vanilla as a feature. Consider it a scent and a colorant. If you think about it this way, you could divide a batch of soap into two portions, scent one of them with vanilla, and then swirl the two together. The result would be white soap with brown streaks—not exactly wedding cake, but certainly not German chocolate. Using this strategy, you can make a vanilla-scented soap whose color meets consumers' expectations for vanilla products.