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I thought of a soap "flavor" that smells like morning cofee. Therefore, I thought alongside the vegetable oil to use full fat milk as fat source.

Would the strong base required also break down the fats that exist in the milk as well? Also, what chances are that the milk part of the soap to go bad as well?

I will use the method that requires no heat.

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Yes, you can, but there are some things you need to consider.

In summary:

  • Using milk in soap will always create an unpleasant smell for about a week after cutting the soap. After that first week, the smell mostly disappears.
  • If you burn the milk (because the chemical reaction gets too hot) the burned smell won't disappear.
  • The milk will not color the soap opaque white, you'll need to add white pigments for that.
  • The smell of the milk will definitely change. So I'm not sure you can get a "milk coffee" fragrance.
  • Milk cannot replace any fat in the recipe. Most milk has < 4% fat.
  • The fat that's in the milk is supposed to be very healthy and caring for the skin.

There are in general 2 ways how you can add milk to your soap:

Milk powder

You dissolve the lye in water and let it cool down to room temperature. Add milk powder to the oils before adding the lye. If you want an opaque color, you need to add white pigments.

You can add 1 tablespoon of milk powder for each pound of oil.

An example (including recipe) can be seen in this video by Kiwanna’s Kitchen.

Fresh milk

I found that this video (by Bramble Berry) sums up the process of using liquid milk in soap very well and explains the pros and cons.

In this method you prepare the lye with milk instead of water. The problem is that dissolving lye produces a lot of heat, enough to actually burn milk, which produces an unpleasant odor and yellow color.

So you need to freeze the correct amount of milk in an ice cube tray. Put the milk ice cubes into a mixing bowl, sprinkle a little lye on top and stir really well. Keep adding a little lye at a time and stir constantly. The lye will soon start melting the milk, but hopefully not burn it. The slower you add the lye to the milk ice, the better result (color and smell) you'll get. You can further cool the milk by putting the mixing bowl into an ice bath.

Stir well past the point the milk is all melted. Because of the opacity of the milk you cannot see whether all the lye crystals are dissolved, so stir longer just to be sure.

You also need to keep the oils rather cool before combining everything and put the soap into a fridge or freezer to keep it from burning after it's been poured.

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  • My guideline will be Nile Red's video: youtube.com/watch?v=uMBeXHnWhsE but with more oil and instead of coconut Oil I was thinking to use condensed milk.
    – Dimitrios Desyllas
    Commented Jul 19, 2021 at 20:31
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    Beat me to the answer. This mostly covers what I would have written. A few supplementary comments: 1. If you want the fat to come from milk fat, you need to use butter, which contains about 80% fat (still substantially adulterated from pure oil, and affects the mix; cream is only about 35% fat, so a messy starting point). 2. Milk powder is non-fat, mostly casein. The lye converts that to casein paint or glue (mixed with the soap from the fat). The same thing happens to the non-fat components of milk. 3. To reiterate, you won't get a milk fragrance or color.
    – fixer1234
    Commented Jul 19, 2021 at 20:32
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    @DimitriosDesyllas, condensed milk has less water, but the same ratio of fat to casein. It makes a messy starting point. You need to use an accurate mix of alkali to fat to saponify all the fat and react all of the alkali so the soap isn't caustic on your skin. Milk or milk products complicate figuring that out. People sometimes use or include milk products for the supposed skin benefits. But if your goal is just the aroma, it won't help with that.
    – fixer1234
    Commented Jul 19, 2021 at 20:46
  • So prepare the fat let react with the base and add any milk substance later right?
    – Dimitrios Desyllas
    Commented Jul 20, 2021 at 7:50
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    @DimitriosDesyllas If you use only a very small amount of milk, you can do that. If you want to use more, you either mix milk powder with the oils before adding the lye or you substitute water with liquid milk to dissolve the lye before mixing it with the oils.
    – Elmy ♦
    Commented Jul 20, 2021 at 7:55

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