Residue from yoghurt

[Saintlysoaper]

Hi all

I have noticed in my recent batches of soap ( now 4-6 weeks old) that if I repeatedly wash my hands with them, say, three times in a row ( while I was assessing lather etc) they leave a small amount of smooth white residue on my skin. The only way I can describe it is that it's like when you add a bit too much clay to a soap. Has anyone experienced this from adding a fair amount of yoghurt (50% water substitution) to a CP soap?

It's not drying as such it just leaves a whitish cast to the skin and feels just a little bit 'coated'.

 

[IrishLass]

My experienced guess is that it is most likely soap scum, which is typical with lye-based soaps, especially if you have hard water. The minerals in hard water react with soap to produce an insoluble soap (known as scum), which shows up as a white powdery film. I myself have hard water and am quite familiar with soap scum. To combat it, I add a little tetrasodium edta, a chelator, to my soap formulas. I use it at a rate of .5% as per the total weight of my entire batch. It hasn't totally eliminated the scum 100%, but it sure has eliminated a good portion of it.


IrishLass

 

[Saintlysoaper]

Thanks Irishlass

I haven't experienced it with any of my previous soaps just these with yoghurty ones. I definitely live in a hard water area though and these are the least cleansing soaps I have ever made. Will that increase the effect?

 

[BrewerGeorge]

When you say "yogurt residue" do you mean the leftover acidic whey?

If so, all that should really do is combine the lactic acid in the whey with a bit of the lye to create sodium lactate (which many of us add intentionally to harden soaps) while increasing your superfat just a bit. The effect should not be much - especially with only 50% substitution - because acidic whey only typically runs about 7% lactic acid at most. For instance, in my 40oz loaf using 370g of water normally, substituting 50% with whey would provide about 9 grams of lactic acid to "use up" lye an increase superfat. That translates into around 3/4% superfat difference.

TL;DR - It's probably not the whey.