Updated: Jul 20, 2019
Last week a student messaged me and was perplexed about why her beautiful goat milk recipe "didn't quite feel as mild as I had hoped." When she showed me her recipe, I noticed she had used a lye excess, or more lye than was needed. When I asked her why she formulated her recipe this way, she stated "because I used goat milk instead of water and didn't want to have too much fat that would cause DOS." I didn't quite understand what she meant, as a lye excess would mean that there would not be any remaining fats, but rather remaining lye. She tagged me on a post where someone had wrote that they "superfatted this recipe with goat milk after the cook" and wanted to do the same. She was under the impression that by simply using goat milk in her recipe, it would add an additional 6% fat. This is not the first time that I have worked with someone who has tried to superfat their recipe with goat milk, and probably not the last. You will often see remarks in different Facebook groups like "I will superfat my soap with 6% goat milk," but what does this really mean? Is it possible to superfat your soap with 6% goat milk?
Contrary to popular belief, although goat milk does contain fats that may remain unsaponified in your recipe, it probably doesn't add as much superfat as you think it does. There is a common and perpetuated myth that adding goat milk to your recipe significantly increases the superfat, "raises your superfat by 5-6% to make an extra creamy bar," or that "if your soap is made at superfat of 5% and you use goat milk you will really have 11% superfat." Although that would be nice, it simply isn't true. Because of this misinformation, my student thought her recipe would have 5% superfat in it, when in reality, it probably had less than 1% or may have even been lye-heavy.
On average, goat milk is reported to be comprised of 3.9% fat. This concentration may be higher or lower, depending on where you source your goat milk from. If goat milk is comprised of 3.9% fat, this means that there is about 4g of fat for every 100g of goat milk.
Lets explore the math to see how this works out when trying to superfat by including goat milk:
If we used a recipe that calls for 100g of coconut oil and set our calculator to 0% superfat, it would require 13.5g of NaOH to completely saponify the recipe. Fats that remain unsaponified in your recipe, or fats that are added after the saponification process has been complete, are commonly called the superfat. Very simply put, the superfat is a "a little extra fat/oil in your soap."
If we made our recipe with a 0% superfat, assuming all SAP values are 100% correct and that our scales and measurements were 100% accurate, by using 13.5g of lye, we would not have any free fats and all would be soap.
Lets pretend that we used a 1:1 lye to water ratio for our recipe's lye solution. Because we want to use milk instead of water, we would use 13.5g of goat milk. If 4% of that 13.5g of goat milk is fat, that would mean there would be an additional 0.54g of fat added to our recipe or n additional 0.5% fat.
For a 2:1 goat milk to lye ratio, or 27g of milk, it would provide an additional 1.08g of fat or 1% .
For a 3:1 goat milk to lye ratio, or 40.5g of milk, it would provide an additional 1.62g of fat or 1.5%.
A 3:1 ratio of goat milk is what we would consider as a "full water recipe". That is more than a 40% water concentration, even higher than most fluid hot process recipes. Even with all of that goat milk, it still only adds about 1.5% extra fat the composition. In order for us to get a full 6% from the average goat milk, we would have to use over 140% of the total oil weight in milk. That is a LOT of goat milk! We would end up with a caustic oil and goat milk soup that would take forever to saponify.
So although goat milk does add fat to our recipe, it probably doesn't pack as powerful as a punch as originally proclaimed. Unless you plan on adding a lot of goat milk to your recipe, it may be wise to consider including additional fats from your oil sources if your goal is to include a superfat. This goes for other dairy products and yogurts too!
With so much information out there, it can be hard to interpret what it all means and how it applies to our topic. Many people will read an article that says "goat milk is made of 4% fat" and immediately think that "goat milk adds 4% superfat," but that doesn't exactly add up, as we just saw. As a beginner, it is important to understand the difference between the two in order to create informed recipes, with the soap qualities and properties we desire, and hopefully this article can help bring this to light.
Does this mean that you shouldn't include milks as part of your recipe? Absolutely not! Milks are an amazing way to create unique and personal recipes, and include additional components like proteins and sugars. There is a reason why goat milk soap is so popular- it can be formulated to create a creamy, bubbly and very gentle cleansing bar!
Just for fun, let's take a look at some of the common milks and dairy products often included in soap recipes. These are average reported numbers, your milk may have a higher or lower fat content depending on where it is sourced and how it is processed.
Goat milk- 4g
Cow milk (1%)- 1g
Cow milk (Whole)- 3.25g
Camel milk- 1.72g
Buffalo milk- 7g
Sheep milk- 7g
Heavy cream- 19g
Coconut milk- 24g
Whale milk- 40g
Seal milk- 50g
(Although whale and seal milks aren't commonly used in soap making, it is still fun to learn about and see the differences between land mammals and large sea mammals.)
With milks and dairy products that are higher in fats, like seal milk and coconut milk, you can recalculate your recipe by A. Adding additional lye and using the correct SAP value or B. Including it as a excess fats or superfat. When formulating a liquid soap recipe, it is important that free fats be kept to a minimum, so always be sure to take this into consideration and adjust your recipe with additional lye as needed!