There a several ways to incorporate milk into your normal cold process soaps. You can:
- make your lye solution with milk instead of water
- use a double strength lye solution and add in the milk at trace
- use powdered milk
But your question on the Forum was about "hot process" soap - where you are cooking the soap paste (either for bar soap or liquid soap) until it's fully saponified. The reason that we go through so much trouble with the cold process techniques cited above is to keep the milk from caramelizing and turning tan or orange. Putting the soap through a hot process just adds insult to injury and cooks the milk even more! If you were to make soap just substituting milk for water in the lye solution, even if you kept your initial mixing temperatures cool, by adding the heat of cooking, you're going to have a color shift, or worse. I've heard of people having separation, curdling, and mysterious layers of oil on the top of their soap in addition to tan or brownish soap.
So...if you're going to hot process your bar soap, using the two methods other than direct substitution of the milk for the water work better.
Using powdered milk would be the easiest, and least likely to cause problems or discoloration. However, you can still use liquid milk in a 50:50 ratio.
The idea is to let the oils take the brunt of the lye - and then add the milk so that it has less lye to fight with. What you do is divide the amount of liquid in your recipe in half, and use half water and half milk. For example, if your recipe calls for 4 ounces of lye and 8 ounces of water, make your lye solution with 4 ounces of lye and 4 ounces of water. Bring the oils and lye to trace...(it's going to get to trace FAST) and then add the additional 4 ounces of milk right before the cook.
But wait...there's one more method if you want to use milk...and that's in a rebatch.
A little bit of milk in a rebatched soap really helps to make the soap smoother and more moldable...but just a little bit. The danger of putting in too much milk after the cook is that it doesn't get "cooked" into the rest of the soap...you have finished soap, with some milk mixed into it...it's much more likely to go sour/rancid. Plus...a certain amount of water/liquid is needed to facilitate the oil and lye mixing and cooking.
To 12 ounces of grated pre-made basic soap, add:
- 9 ounces of goat's (or other) milk
- 2 teaspoons of powdered benzoin
- several drops of fragrance
The benzoin is (I believe) being used as a preservative - probably (as I said above) because the milk doesn't completely incorporate into the soap. However...benzoin is a controversial ingredient - it has been found to be a sensitizer. You could use a normal cosmetic type preservative like Germaben or Liquipar Optima in the milk before you added it into the soap. That should reduce the chance of any microbial action.
So yes...milk can be added to hot process soaps...either right before the "cook" or you can do a rebatch of a basic batch of soap, and add in milk at that point...but you run the risk of the milk going sour.
Mailbag Monday is a feature on the site where I share readers' questions and the answers. You can ask questions about candle making, soap making, or any of the related crafts that we engage in. Just email me with "Mailbag Monday" in the subject line.