ResolvableOwl
Notorious Lyear
First a peek on the perfect match in viscosity:
Now to the stinky details:
At various places, the usage of butter (from cow's milk) for soap has caused the most dramatic descriptions of an obnoxious odour nuisance. I already expressed my surprise about this earlier on. I don't want to warm up that story, yet I won't give up the faith in the validity of my personal experience, just to appease some doctrine that lacks the empirical founding to credibly disprove my own sensual impressions. I went back to the drawing board, made a bit of research, and found an exciting remark in a 1953 dairy encyclopedia, heirloom from my dairy master grandfather):
There are short-chain (= stinky) fatty acids in milk fat without doubt, and upon adding acid to dairy butter soap, they are liberated and smell obnoxious. BUT the *)footnote describes these (C4:0 = butyric acid, along with C6:0 and C8:0 as volatile („flüchtig”)). What does this mean in practice? Well, there is the Reichert-Meißl test for volatile fatty acids in butter, where these fatty acids are removed via steam distillation for measurement via titration. But that means that steam distillation is also a viable means to remove volatile FAs from soap! As a side remark, the entry also mentions that the amount of volatile FAs is lower in winter, than in summer or when the cows are fed with turnip. Coconut oil is noted to have on average 1/4 of volatile acids.
My stage 1 recipe: 45 g clarified butter, 23 g palm kernel oil, 30 g HO sunflower oil, HP-saponified with 26.5 g KOH to positive clarity test. So far nothing special, except for some 5% lye excess. The result is a bright yellowish, clear gel that, oddly enough, faintly smells like maize/corn (maybe some volatile aroma components that made it over from the cows' food?).
As an unusual second stage, I add 15.8 g citric acid to the soap. It instantly seized/split the soap into a murky whitish-yellow oil layer (holding soaps and free fatty acids) and an aqueous bottom layer (holding potassium citrate, excess citric acid, glycerol, and much of the water). It first smelled like milk gone bad/sour. It took some time until the characteristic stink of butyric acid has become dominant in all its repulsiveness: somewhere between sweaty feet, diapers, and vomit. I burned the hell out of it, kept it at a strong boil for about an hour (window wide open, door closed; you could smell the butyric acid in the whole kitchen for two days after that
).
Ideally, I want this reaction to happen: potassium butyrate (from finished soap) + citric acid → potassium citrate + butyric acid↑
Pure butyric acid boils at 164°C, but is volatile enough to be removed by water vapour already, so if I only let it boil long enough, most of the butyric acid should be removed.
After this acidic step (pH 5), I added another 16.4 g of KOH as the third stage, to neutralise any free acids (citric acid, free long-chain fatty acids that should form soap again), and to get back into alkaline regime. Now I had the problem that I don't know how much butyric acid I have “lost” through boiling. And LS doesn't forgive errors in amount of oils or lye beyond some 1% – except for castor oil! So I added 15.5 g castor oil (a theoretical 5% excess, from which the butyric losses are subtracted). It turned out to work well: clear LS without stringy skin feel, and not zappy.
After letting this settle for two days, I cautiously diluted it, and had that perfect honey-like viscosity after just adding the first 50 mL of water.
Performance: I love this LS! It is so pleasingly syrupy, feels gentle yet cleansing on the skin, and is very bubbly. Gorgeous dense & stable lather when diluted 1:1 and put into a frother. It has a slight smell, but this is just that odd chemical, rubber-like odour like any other LS has, absolutely no hint of butyric acid. To address the issues that others have reported of dairy butter soap making their skin stink – I can confirm that there is a faint butyric smell when I rub my hands in this soap, the lather does smell a tiny bit, but I really have to keep both hands close to the nose and inhale consciously. After rinsing off, not the faintest trace of smell is left (This is in contrast to my LS based on poppy seed oil, that still leaves my hands with a slight poppy smell).
So either I had terrible luck with that very brand of butter, or the acidic distillation method really is suitable to remove the butyric smell from dairy butter soap (or both). This result, however, does by no means justify the technique and all the effort I put in there. Dairy butter is expensive, of arguably inconsistent technical performance (look alone at the spread of saponification values and contenst of PUFAs!), cumbersome to work with, has a questionable eco-balance, and last but not least, it is replaceable by oils that just make more sense in soap. This experiment was purely a proof of concept.
Bottom line: It can well pay off to not resign against seemingly overwhelming opinions, as long as you know science on your side. Stay skeptical. Question authority. Make errors. Know it better.
Edit: It seems that my keyboard has a bad day today.
Now to the stinky details:
At various places, the usage of butter (from cow's milk) for soap has caused the most dramatic descriptions of an obnoxious odour nuisance. I already expressed my surprise about this earlier on. I don't want to warm up that story, yet I won't give up the faith in the validity of my personal experience, just to appease some doctrine that lacks the empirical founding to credibly disprove my own sensual impressions. I went back to the drawing board, made a bit of research, and found an exciting remark in a 1953 dairy encyclopedia, heirloom from my dairy master grandfather):
There are short-chain (= stinky) fatty acids in milk fat without doubt, and upon adding acid to dairy butter soap, they are liberated and smell obnoxious. BUT the *)footnote describes these (C4:0 = butyric acid, along with C6:0 and C8:0 as volatile („flüchtig”)). What does this mean in practice? Well, there is the Reichert-Meißl test for volatile fatty acids in butter, where these fatty acids are removed via steam distillation for measurement via titration. But that means that steam distillation is also a viable means to remove volatile FAs from soap! As a side remark, the entry also mentions that the amount of volatile FAs is lower in winter, than in summer or when the cows are fed with turnip. Coconut oil is noted to have on average 1/4 of volatile acids.
My stage 1 recipe: 45 g clarified butter, 23 g palm kernel oil, 30 g HO sunflower oil, HP-saponified with 26.5 g KOH to positive clarity test. So far nothing special, except for some 5% lye excess. The result is a bright yellowish, clear gel that, oddly enough, faintly smells like maize/corn (maybe some volatile aroma components that made it over from the cows' food?).
As an unusual second stage, I add 15.8 g citric acid to the soap. It instantly seized/split the soap into a murky whitish-yellow oil layer (holding soaps and free fatty acids) and an aqueous bottom layer (holding potassium citrate, excess citric acid, glycerol, and much of the water). It first smelled like milk gone bad/sour. It took some time until the characteristic stink of butyric acid has become dominant in all its repulsiveness: somewhere between sweaty feet, diapers, and vomit. I burned the hell out of it, kept it at a strong boil for about an hour (window wide open, door closed; you could smell the butyric acid in the whole kitchen for two days after that
).Ideally, I want this reaction to happen: potassium butyrate (from finished soap) + citric acid → potassium citrate + butyric acid↑
Pure butyric acid boils at 164°C, but is volatile enough to be removed by water vapour already, so if I only let it boil long enough, most of the butyric acid should be removed.
After this acidic step (pH 5), I added another 16.4 g of KOH as the third stage, to neutralise any free acids (citric acid, free long-chain fatty acids that should form soap again), and to get back into alkaline regime. Now I had the problem that I don't know how much butyric acid I have “lost” through boiling. And LS doesn't forgive errors in amount of oils or lye beyond some 1% – except for castor oil! So I added 15.5 g castor oil (a theoretical 5% excess, from which the butyric losses are subtracted). It turned out to work well: clear LS without stringy skin feel, and not zappy.
After letting this settle for two days, I cautiously diluted it, and had that perfect honey-like viscosity after just adding the first 50 mL of water.
Performance: I love this LS! It is so pleasingly syrupy, feels gentle yet cleansing on the skin, and is very bubbly. Gorgeous dense & stable lather when diluted 1:1 and put into a frother. It has a slight smell, but this is just that odd chemical, rubber-like odour like any other LS has, absolutely no hint of butyric acid. To address the issues that others have reported of dairy butter soap making their skin stink – I can confirm that there is a faint butyric smell when I rub my hands in this soap, the lather does smell a tiny bit, but I really have to keep both hands close to the nose and inhale consciously. After rinsing off, not the faintest trace of smell is left (This is in contrast to my LS based on poppy seed oil, that still leaves my hands with a slight poppy smell).
So either I had terrible luck with that very brand of butter, or the acidic distillation method really is suitable to remove the butyric smell from dairy butter soap (or both). This result, however, does by no means justify the technique and all the effort I put in there. Dairy butter is expensive, of arguably inconsistent technical performance (look alone at the spread of saponification values and contenst of PUFAs!), cumbersome to work with, has a questionable eco-balance, and last but not least, it is replaceable by oils that just make more sense in soap. This experiment was purely a proof of concept.
Bottom line: It can well pay off to not resign against seemingly overwhelming opinions, as long as you know science on your side. Stay skeptical. Question authority. Make errors. Know it better.
Edit: It seems that my keyboard has a bad day today.
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