Superfat is any excess fat that remains in a soap after saponification is done. Most handcrafted soap intended for bathing almost always contains some superfat for two reasons.
The first reason for superfat is safety. A soap must include enough fat to make sure all of the lye is used up. Since our recipes are based on estimates of the saponification values and alkali (NaOH or KOH) purity, the general policy is to add a little more fat than is strictly necessary to ensure all of the alkali is used up. A superfat of 1% to 3% is good safety margin.
The second reason is to increase the mildness of the soap on the skin. Soap sometimes cleans so well that it dries or irritates the skin. Increasing the superfat above the 1% to 3% safety margin can help tame this tendency.
This idea that superfat increases the gentleness of soap has created some misleading but persistent myths. Here are a few of the fictions about superfat --
Fiction -- More superfat is always better.
Higher superfat might seem like the ultimate, easy answer to making a mild, gentle soap. Unfortunately, it's not. If you learn to tailor the superfat to the recipe, your soap will perform better.
A too-high superfat will definitely reduce lather and increase the softness of bar soap. It can leave a greasy feel on the skin and may even affect your plumbing.
What is "just right" and what is "too much" superfat will vary with the soaper and with the recipe. If you are not sure, the default of 5% superfat is a good place to start if you are using a classic blend of fats.
To learn what superfat you prefer for a given blend of fats, make one of your favorite recipes in three versions -- one with 2% superfat, one at 5%, and one at 8%. See which version feels best on your skin, which one lathers best, and which one lasts longest. If you have been automatically using a high amount of superfat, the answers may surprise you!
Fiction -- Soap with a low superfat will be drying or irritating to the skin.
It is true that superfat can be used to increase the mildness of some types of soap that would otherwise be too harsh on the skin. A 100% coconut oil soap is one example of a strongly cleansing soap that can leave skin dry and tight. If you are going to make this recipe, then the only way to tame the soap is to use a high superfat of 15% to 20%.
A soap made with a classic blend of fats, however, will typically be mild to the skin even with a superfat as low as 2% to 3%. What I mean by a classic blend is a small amount of fat high in lauric and myristic acid (coconut, palm kernel, or babassu); a moderate amount of fat high in palmitic and stearic acid (lard, tallow, palm, or butters); and a moderate amount of fat high in oleic acid (olive, avocado, high oleic safflower, etc.). Mix in a bit of castor or another interesting fat or two, and the result is a classic blend for soap making. Learn more: What do the Soapcalc numbers really mean?
Fiction -- Fat added at trace will remain as superfat in CP (cold process) soap.
"Trace" is a stage at which the fat and lye solution are chemically emulsified, meaning these ingredients are able to stay blended together on their own. It is a visible sign that says you no longer have to stick blend or whisk the mixture to keep it well mixed.
At trace, the saponification reaction is just barely getting started -- perhaps only 10% of the NaOH has been used up. With 90% of the NaOH still active, any fat added at trace is just as likely to be turned into soap as any other fat in the recipe.
The bottom line is there is truly no practical benefit to adding a fat at trace. The book Scientific Soapmaking by Kevin Dunn has an excellent chapter on this topic.
Fiction -- Fat added after the cook will remain as superfat in HP (hot process) soap.
During saponification, an alkali such as NaOH partly or completely breaks the triglycerides (fat molecules) into fatty acids, and the fatty acids the react further with the NaOH to become soap.
It turns out something as simple as water can break a fat molecule apart too. The chemical reaction of fat with water is called "hydrolysis" (hi-draw-luh-sis). Hydrolysis creates a mix of free fatty acids, glycerin, and partly deconstructed fat.
Due to saponification and hydrolysis and due to the chemically flexible nature of soap molecules, an exotic miracle superfat will not remain permanently intact in soap. It will gradually change into chemicals that aren't at all like the original oil.
How fast does this transformation happen? As with so many soapy things, the answer is "it depends." Saturated fats and monounsaturated fats are more likely to remain mostly intact for a longer time; polyunsaturated fats probably won't last as long. (Many exotic oils are polyunsaturated.)
Anything that slows down rancidity (DOS) will also slow down hydrolysis, so antioxidants and chelators (ROE, EDTA, citrate, etc) and cool, dry, dark storage conditions may help the superfat oil remain intact a little longer.
The bottom line is superfat added to HP soap after the cook will remain more or less in its original form for awhile, but it is not guaranteed to remain intact forever.
Fiction -- Enough superfat will cling to the skin to be an effective moisturizer.
Some soapers add exotic or expensive oils to their soap, and they expect these superfat oils to provide deep conditioning and moisturizing benefits and "nourish" the skin.
As I have just explained, superfat does not remain fully intact in soap. The superfat will sooner or later become a mixture of tri-glycerides (the original fats), partly dismantled fat molecules (di-glycerides and mono-glycerides), and fatty acids. This blend of fatty chemicals will not behave like the original superfat.
Even if the superfat could remain in its orginal form, only tiny traces of fat will be able to cling to the skin. (1) There are several reasons for this --
You only use a few grams of soap when bathing. The superfat is only a small fraction of the soap. That means there is very little fat available to cling to your skin.
The soap lather (and thus the superfat) does not remain on the skin. It is there for only a few moments, and then is rinsed off.
Any fat, including superfat, is emulsified by soap. Emulsification means the fat becomes water soluble and wants to stay mixed with the bath water.
So ask yourself -- how can a short exposure to a small amount of soap with a tiny bit of superfat in it do much to soothe and condition a person's dry or delicate skin? Skin is far more likely to benefit from putting those lovely fats into a lotion or body butter that remains on the skin for hours.
(1) How much superfat might actually remain on the skin after washing with a superfatted soap? Here is a thought experiment to show why superfat in soap does not do much to condition and sooth the skin --
Being a engineer-ish kind of soapy geek, let's say I weigh my soap before and after bathing. I determine the soap weighs 5 grams less after my shower. This is based on a 140 g bar of soap, 30 days of use, 1 shower per day, so 140/30 = 4.7 g per shower. I will use 5 grams per shower -- that's an easy number.
Let's also say I take the bar to my soapy science lab and determine the soap is superfatted with a true 10% excess of fat (aka superfat.) I break out my trusty calculator and determine the 5 grams of soap with which I bathed contains a total of 5 X 10/100 = 0.5 grams of fat.
If I were an overly optimistic sort, I might assume every molecule of that fat will stick to my skin during bathing. Knowing what I do about how soap emulsifies fats to make them water soluble, I know only a fraction of that tiny amount of fat actually sticks to my skin. Most of the superfat simply rinses down the drain.
For the sake of this argument, however, I will use with the overly-optimistic idea that all of the superfat sticks to my skin.
According to Cornell University, the surface area of my skin is about 1.9 square meters (m2). The amount of superfat theoretically sticking to my skin is thus 0.5 g / 1.9 m2 = 0.26 g / m2. One square meter (m2) is about one square yard for us Americans.
Let me compare this to using my favorite lotion.
I estimate I put 1-2 tablespoons (0.5-1 ounce or 15-30 grams) of lotion on my skin after showering. The lotion contains 15% fat.
If I use the least amount of lotion possible (1 tablespoon), I would apply 15 grams X 15 / 100 = 2.25 grams fat to my skin. That is 2.25 / 1.9 = 1.18 g of fat / m2 of skin.
That still does not seem like a lot of fat, but a mere 1 tablespoon of my lotion will put FIVE TIMES more fat on my skin than the soap theoretically will. And I am confident all of the fat in the lotion will stick to my skin, unlike the superfat in the soap.
A good lotion is going to do a much better job of making my skin feel smooth and soothed.