sour milk

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"...Is the conversion of lactic acid to Sodium lactate what keeps the soap from turning into a moldy sour milky wad of funk?..."

I don't have a clue, really. Might be.

What I wonder is this -- what keeps soap with ~any~ food or botanical additive from going "sour"? Folks are putting some unusual stuff into soap, and supposedly it doesn't spoil if reasonable care is taken. I haven't tried a lot of additives -- just milk and beer -- so I can't speak to this one.

"...Also is there enough lactic acid in dairy that I should adjust my NaOH to compensate? Soapcalc does have a milk fat option for calculation. Butter milk has 4grams per cup would I enter that there to keep my lye working?..."

Okay, this I can answer.

First off, don't confuse the fat (Soapcalc's milk fat aka butterfat) that is in milk or a milk product with the lactic acid that might also be in the product. Two completely different animals.

If you want to figure the amount of lye required to saponify the fat in your buttermilk, then you are correct -- put in the actual grams of fat (4 g) in your cup of buttermilk rather than the total weight of the buttermilk (probably weighs more like 225 g or so).

Lactic acid is an additive, not a fat. Yes, it does consume some lye, but you would not include it in Soapcalc's saponification calculations -- it does not have a saponification value in the same sense that a fat does. In another thread "Sodium lactate and yogurt", I wrote:

"...yogurt only contains about 0.9% available acid after fermentation. Even if we assume that ALL the acid in yogurt is in the form of lactic acid (a common alpha hydroxy acid) that's still a very low concentration for an active exfoliant...."
Source: http://www.self.com/blogs/flash/2012/03/can-you-exfoliate-with-yogurt.html

"...the final lactic acid content [in the yogurt samples] ranges from 0.66 to 1.10% for all samples...."
Source: http://www.ijens.org/vol_12_i_01/124901-8585-ijbas-ijens.pdf

If you use, say, 400 g of yogurt in your soap, the yogurt would add roughly 2.5-4.5 grams of lactic acid to the soap batch. When the lactic acid reacts with the lye (NaOH), the soap would contain roughly 3-5.5 grams of sodium lactate as supplied by the yogurt...."
Source: http://www.soapmakingforum.com/showthread.php?t=40136

End quote ... back again...

It seems reasonable to assume buttermilk and sour milk and yogurt might well have similar levels of lactic acid (I've not taken the time to check that). Doing the math, I figure the 2.5 to 4.5 grams of lactic acid supplied in 400g of buttermilk or yogurt or sour milk would consume 1.1 to 2.0 g of NaOH in the conversion to sodium lactate.

That's not a lot, but it will raise the superfat a bit and it's certainly worth keeping in mind as you design your recipe.
 
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