Chemistry and why our vanilla containing scents change


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Mar 18, 2015
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Before I start, this is a little technical so I wanted to say that before someone starts reading and goes "GAH, I'm so confused! (and I don't care!)" I tried to write it as best as I could to explain things so that they can be understood by most of us who soap! If you don't agree with something I wrote here, I'd love to continue discussing it. This is only what I have personally found after searching for a while. I've included numbers 1-4 in my explanation and they correspond to my sources at the bottom.

I’ve wanted to know 2 things about vanilla in soap very badly for quite a while now. Seeing the “Vanilla test project” thread (and the discoloring challenge we did) really got me thinking. Vanilla does change in our soap and, it’s not just that we see it turn brown. Something else happens to it that changes the scent entirely. So, I decided to do some research on what happens to our vanilla in our CP/HP (and even MP or other bath and body products) to make it go brown and change odor.

At first I really thought that both would be answered easily or with one compound but, after searching for only a few minutes, I found that my answer was probably more complicated than I originally anticipated. I’ve been looking on and off for a few months an finally feel confident enough in my answer to post.

Here were the things I knew:
1. The specific component that is responsible for all of our issues is vanillin (and sometimes ethyl vanillin). It is the main component of characteristic vanilla flavor and scent.
2. When we make soap we are working in an alkaline environment.
3. The reaction does not occur spontaneously, but only on exposure to air (think of when you cut a cured bar of vanilla soap in half)

I used that as the basis of my search. I found that in highly alkaline solutions, vanillin forms vanillic acid.(1) Vanillic acid is used for flavor and is a “sweeter” vanilla like compound. I did not find any information on it being used by perfumers or even a characteristic odor on an SDS (safety data sheets). This led me to believe that vanillic acid does not have much of an odor at all and is likely responsible for the change in scent we notice when using high vanillin fragrances. However, it turns out it is a colorless, to light yellow compound. That meant it couldn’t account for the color change we observe.

After searching for a while (longer than I’d like to admit…) it turns out that someone else had come to the exact same conclusion as I had on a soap blog (only 3 years prior!).(2) However they hadn’t found what turns the soap brown.

Now I started to look for why our vanillas turn brown. I knew it was linked to oxidation but I did not know how or even what was being formed. You wouldn’t believe how little information I could find on why vanilla turns brown chemistry wise. All I could find was “well, it’s oxidized”. Yes, but to what exactly??

I found my answer in a soil chemistry book after seeing a post (3) about dark colored quinone formation from polyphenols. Polyphenols are a class of compounds that include vanillin; quinones are another class of compounds. It turns out that in the presence of highly basic conditions- like when we make soap- polyphenols, like vanillin, can react spontaneously to form dark colored quinones (4). They react with themselves or other organic molecules (which we have a huge abundance of when making soap) to form these quinones. I’m posting a picture from the book below. I labeled it with vanillin and vanillic acid so It’s a bit easier to read.

I don’t really know how much vanillic acid or quinones are forming when we make our soaps and I doubt I ever will but, I feel better now knowing how vanilla containing fragrances change and what they change into.

If someone feels this is better to move to the general chat section, I understand!

Where I found my information:

(1) of vanillin to vanillic acid&f=false
(4) oxidation to quinones&f=false

Vanillin and dark colored quinones.JPG

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