How is development of ash and oxidation of oils in cold process soap related?

  • #1

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I posted this on thedishforum:

On the German soapmaking forum, there are several posts noting that there seems to be a correlation between ash and oxidation of oils. Can you speak to the presence of ash and soap going rancid?

And got this reply:

Yes, I could see this being an accurate observation. Rancidity is an oxidative process. When fatty acids oxidize, some of the products of those reactions are aldehydes and ketones. Aldehydes with fewer than 6 carbons, in particular, will form soluble carboxylic acid salts when treated with sodium hydroxide (lye). Because of their solubility, those salts are probably some of the precipitates that form when soap ashes. This might explain an observed correlation between rancidity and ash.

Additionally, saturated fats, in particular, can form crystals under certain conditions (grains in butters, anyone?). Because they lack the reactive double bonds needed for tight crosslinking during polymerization into the soap colloid, they sometimes precipitate out, and because of their solubility when converted to soap, can migrate out of the soap as water moves to the surface. This happens in in old oil paintings, for example. http://cool.conservation-us.org/waac/wn/wn...1/wn20-108.html [Broken] The trick is to identify what those conditions are, that cause crystal formation.

I might dig into the chemistry of this further, after New Year's, when I have time to go peruse the chemistry library. I have limited access to articles from home. I'm quite sure that commercial soap manufactures know *exactly* what causes efflorescence, and how to prevent it.


Could someone spell out what happens to the oils in the soap, and the soap itself, when soda ash develops? If soda ash is possibly a sign that the soap is going bad, then preventing soda ash is more important than many who make soap believe.

Thanks very much for your help!
 
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  • #2
chemisttree
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Drying can interrupt the process by moving the water phase and the lye dissolved in it to the surface. Once on the surface, that fraction of the lye doesn't react with fat or free fatty acids elsewhere within the soap. In that case, I would expect the soap to be more prone to rancidity since there is an imbalance between the free fatty acids and the lye.

Oxidation can occur as well, especially in fats high in polyunsaturates. Perhaps there is a correlation between the rate of saponification in polyunsaturates and the rate of saponification but I don't know of any.

Additionally, saturated fats, in particular, can form crystals under certain conditions (grains in butters, anyone?). Because they lack the reactive double bonds needed for tight crosslinking during polymerization into the soap colloid, they sometimes precipitate out, and because of their solubility when converted to soap, can migrate out of the soap as water moves to the surface. This happens in in old oil paintings, for example. http://cool.conservation-us.org/waac.../wn20-108.html [Broken] The trick is to identify what those conditions are, that cause crystal formation.
This is very suspect. I don't believe any of it. I don't believe that unsaturated oils crosslink to stabilize colloids in soap. If it were true, soaps would behave very differently than they do.
 
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  • #3
Chemisttree, thanks very much. I had a hunch that the lye becoming soda ash on the surface made it unavailable to become soap.

Thank you, too, for your comments about the section of the post on the soaping forum about the crystals. It didn't sit right and I didn't know what to look up to question it.
 

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