Are we sure you can control superfat composition with HP?

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topofmurrayhill

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Here's a potentially important chemistry question.

Given a soap with no excess lye or oil, if you add a superfat of free fatty acids to it in some proportion, apparently you would come back sometime later and discover that the soap no longer contains the same proportion of FFAs that you put into it. They will have migrated according to the overall composition of the soap, such that some of the FFAs you added will have become soap, while fatty acids that were originally soap will have become FFAs.

The question is, will the same thing happen with triglycerides added to the soap instead of free fatty acids? I think that's liable to be a different matter, but I'm not certain. If the same migration happened with added oils, you wouldn't be able to control the composition of your superfat even with HP.

This question was inspired by a patent application, the relevant portion of which is below. Regardless of the answer, as a soap geek I found this interesting and maybe others will as well.



 

FlybyStardancer

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The thing with fats added after the cook with HP is that the fatty acids are still firmly attached to the glycerol molecule. You need the energy of the reaction to lye to break the bonds between the fatty acid portion and the glycerol portion of the triglyceride. Not enough energy input, no broken bonds. That's what keeps the superfat added after the cook a superfat instead of mixing it up with the other fatty acids.
 

galaxyMLP

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This really got me thinking. Although I agree with Flyby about the triglycerides not being able to "swap" like the fatty acids do, it did get me to wonder about the mechanisms which triglycerides decompose by.

With age, and oxidation, fatty acids become rancid, yes? Well, triglycerides bread down via hydrolysis. In this case, when we make soap we are doing a hydrolysis reaction. However, hydrolysis can even occur in plain water. And I would guess even more so in the basic environment of soap. I don't think it would happen very quickly or to a very large extent but this might explain why soap that has been wet before will go rancid faster. Maybe a small but appreciable portion of the left over free triglycerides are actually decomposing to free fatty acids which then go rancid in an oxygen rich environment?

In that case very old soaps that have their oils breaking down, they may be reaching an equilibrium as posted by TOMH.

Just a thought. I could be totally off base.
 

topofmurrayhill

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Yes, I got to wondering the same thing after Flyby's post. It would pertain to any free fatty acids in the superfat oil (dependent on the type of oil and its condition) and presumably any that were produced by hydrolytic rancidification. As you say, it would be interesting to know how much of a consideration the latter might be.
 

The Efficacious Gentleman

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It is interesting, as I know we have spoken about the base needing to be fairly powerful in order to actually make the soap.

Another thing that is mentioned but not in detail, is time - it doesn't say how much time it took (and under what conditions) for this to happen.
 

galaxyMLP

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In the case of plain water hydrolysis, you won't be left with soap, only the free carboxillic acid. It cleaves the bond at the glycerol bond.

Eta: And TEG, I was wondering the same thing about "some time"
 

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