SAP values – why they are important in soap making and what makes a mild soap

To create a good quality soap, your soap formulation will need a selection of different hard and soft oils with a 5-10% superfat (the percentage of oils that will not be turned into soap). Hard oils contribute hardness, cleansing and lather to your soap. Soft oils can make a soap milder, but can also leave a soap soft. Each oil is made up of different fatty acid composition, which determine these qualities in a soap. This is why a combination of both hard and soft oils are needed. But not only differ the oils in composition, they also differ in the amount of fatty acids, which is why each oil requires a different amount of caustic soda to turn it into soap.

In short, oils are made up of triglycerides, and each triglyceride consists of a glycerol and three fatty acids. The glycerol will be used to make the glycerin, and the three fatty acids are used to make three soap molecules. To make the soap molecules, you need caustic soda, or sodium hydroxide (NaOH) to be precise, and you need three sodium hydroxide molecules for a triglyceride – one for each fatty acid. How many fatty acids are contained within a soap, depends on the shape of the fatty acids, which come in three different shapes: flat (saturated), with one bend or kink (monounsaturated), or with two and more kinks or bends (polyunsaturated).

In my workshops, I like to compare it with a ream of paper. Normally, a ream will hold 500 sheets of paper. This is because the sheets of paper are flat and you can stack them up nicely. But imagine if you crumpled each paper into a ball. Now, try fitting as many balls of paper in the same box that held the 500 sheets. There is no way you’d be able to fit 500 balls of paper in there. Maybe if you’re lucky, you’ll manage 100.

This is similar to the fatty acids. Flat-shaped fatty acids can stack neatly and tightly, and because of that, they appear solid at room temperature. These are your saturated fats, saturated with fatty acids, such as coconut oil, palm oil, shea butter, cocoa butter, tallow, lard, etc. Unsaturated fats are those liquid at room temperature, and they are liquid because their fatty acids are a bent shape, just like the balls of paper. These fatty acids don’t stack nicely, and they keep moving around, leaving the oil liquid. Unsaturated fats are your liquid vegetable oils, such as olive oil, rice bran oil, canola oil, castor oil, etc.

Your saturated fat is your ream of paper, the fatty acids neatly stacked together.

Your unsaturated oil is the same box, but instead of containing tightly stacked flat-shaped fatty acids, you have crumpled up bendy balls of fatty acids.

So despite having the same volume, i.e. the same box, there are more fatty acids (aka sheets of paper) in the box with the saturated fats than there are in the box with the unsaturated fats (aka crumpled balls of paper). And as each fatty acid gets turned into a soap molecule, those unsaturated fats produce less soap molecules as do the saturated fats.

Important: it’s not the type of oil that makes for a mild soap, but the number of soap molecules contained within a soap. Unsaturated oils contain less fatty acids, and hence, produce soaps with fewer soap molecules.

The best soaps contain a number of different oils, fats and butters. I usually like to use between 4 and 5 oils. Again, this is due to the different fatty acids each oil contains, as each fatty acid brings different properties to soap. Single oil soaps usually don’t perform as well or have undesired qualities like being soft, too cleansing, or poor lathering qualities. Combining several oils adds more different types of fatty acids, allowing you to create soaps with what I like to call the holy trinity of a soap:

  • a hard bar of soap
  • mild cleansing and not drying
  • lots of bubbles (bubbles are very important!)

For example, coconut oil and palm oil will create hard bars of soap with good lather properties, but on their own, they can be drying to the skin, due to the high of fatty acid content (which get turned into soap molecules). Adding a ‘soft’ oil, an unsaturated oil, will bring down the amount of fatty acids/soap molecules in a soap. Other oils, such as castor oil, can be added for their lather boosting qualities.

How to calculate the caustic soda amount in your formula

For each oil, you need a saponification value, or in short SAP value, which is the ratio of caustic soda to oil. Luckily, we don’t have to figure out the SAP values as they are readily available on the internet nowadays. For ease of use, I’ve compiled a list of SAP values and properties of the most common used oils, fats and butters in soap making for you to use and download. Alternatively, you can also use soapcalc.net, a free online soap calculator.

Once you have the SAP value for each oil in your formula, multiply the amount of each oil by its SAP value. This will give you the amount of caustic soda that you will need for each oil.

Then add up all the amounts of caustic soda, and this will be the amount needed to turn ALL the oils into soap. However, you don’t want want to turn all the oils into soap, unless you’re making a household soap. Handmade soaps will always contain a superfat, a small percentage of oils left as oils in the soap and NOT turned into soap. There are two main reasons for the superfat.

A minimum 2% superfat will act as a buffer in your soap and prevent your soap from being caustic. Your soap can remain caustic for various reasons: your scales might be slightly off; if you pour oil from one container to another, a small amount of oil will be left in the first container; you might make a mistake when measuring… – all these errors will be caught by the buffer and prevent you from making a caustic soap.

Another reason for the superfat is to make the soap milder in a sense. Extra oils reduces the amount of soap molecules in your soap, but the extra oils also use up some of the soap molecules when you are using the soap, because these soap molecules will attach themselves to the oil molecules from the soap rather than to the sebum (oil) molecules on your skin. But don’t think that these oils will condition or moisturise your skin. The superfat is only between 2 and 10%, any more and you will risk a soft and sticky soap. And if you consider the tiny amount of soap you use each time you wash, it will contain an even tinier amount of oil, as in a few molecules, and it is more likely that these oil molecules will bind to the soap molecules in the water/soap mixture than end up on your skin, and even less likely to penetrate into your skin, because you are planning to rinse the soap off again, right? I’m just saying! Even if you don’t believe in science, this is just basic maths. (End of rant)

To calculate the superfat in your formula, you can either add an extra 2 to 10% oils to your recipe, or you can do a lye discount, by reducing the amount of caustic soda in your formula. This is my preferred method. Instead of using the total amount of caustic soda I calculated (=100%), I will only use, for example, 95% of the caustic soda, which will leave exactly 5% of the oils unsaponified (not turned into soap).


Here’s a worked example:


500 g olive oil
350 g coconut oil
100 g shea butter
50 g castor oil

SAP
x 0.134 = 67 g
x 0.178 = 62.3 g
x 0.128 = 12.8 g
x 0.128 = 6.4 g
—————
148.5 g

148.5 grams is the total amount of caustic soda (NaOH) I would need to turn all the oils to soap. However, I want a 5% superfat in this recipe, so I’ll be discounting my lye by 5%. Or in other words, instead of using 148.5 g, I’ll only use 95% of 148.5 g of caustic soda.

95% of 148.5 g = 141 g of caustic soda

The amount of water I’m using is 2:1 water to caustic soda ratio, and this can be rounded. (Water calculation is a whole other discussion for later!)

141 x 2 = approximately 280 g of water

Here’s the final formula :

BASIC SOAP RECIPE

500 g olive oil
350 g coconut oil
100 g shea butter
50 g castor oil

141 g caustic soda
280 g water


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