Superfat vs. lye discount – the difference to know
Grab a cup of coffee or a hot tea, or whatever beverage you prefer, take a short pause and sit down comfortably. I will show you, that the difference can become quite important and that we should care.
I did not think about it until I got a question on how to approach superfatting of the finished hot processed soap.
The difference is subtile and to explain, I will define the two terms:
- Lye discount means that you calculate the amount of lye needed to saponify all the oils in your recipe and than discount part of it – e.g. lye discount of 5% means that you make your soap with 5% less lye than the required amount.
- Superfat means you first mix your soap and than add some more oil/fat either at trace or once the soap is finished. There is an important difference between the two as I will explain later.
After both procedures, we say that we superfatted your soap, which is true. There is indeed some extra / super fat that was not saponified.
However, there are two important differences :
- By superfatting you can sometimes control which oils will saponify and which not
- There is an important and non-trivial difference in the calculation of the superfat percentage between the two methods
Superfat – how to control which oils will saponify?
There is a general belief that by adding a particular oil at trace, this oil will be the last to saponify and that this is the method to control which oils will create the main part of your superfat. According to work of K. Dunn, author of the book Scientific Soapmaking, who has made laboratory experiments on this subject, this is not true:
The composition of unsaponified oil in finished cold-process soap does not depend on the order in which the oils are added. The oil component that reacts most slowly with the lye will be more concentrated in the unsaponified oil than in the original oil blend.
This is the book !!! (I cannot stress enough how important I find it for each soapmaker…)
In other words, if you add for example coconut oil at trace, its saturated fatty acids will saponify quicker than for example unsaturated fatty acids of the olive oil that was in the mix from the very beginning. At the end, there will be more of the olive oil fatty acids unsaponified in your soap than fatty acids of the coconut oil, even if this was added last.
The only way how to control the non-saponification of your superfat oils is to add them after the saponification is completed – e.g. in the hot process soap.
How much did you really superfat your soap?
The thing is that while in the lye discount the percentage of superfatting is straightforward to calculate, it is not always the case if you superfat at trace or after the soap was finished (for hot processed soap).
Now lets take an example recipe:
950g (95 %) olive oil
50g (5%) coconut oil
If I calculate my lye with 5% lye discount and saponify my oils with it, I will have 5% superfatted soap, because 5% of my oils in my oil mix could not saponify. Easy!
However, let’s say I want the coconut oil remained unsaponified and so I HP my soap first. This means I have to saponify 950g of olive oil with lye calculated at 0% and only after the saponification is completed I add 50g of coconut oil. In this case, I do not have anymore 5% superfat… how is that?
Let’s calculate the superfat of my soap under these two scenarios:
Scenario 1: I apply lye discount of 5%
This means I use only 95% of the lye needed to saponify all the oils in the recipe. I would need (according to SoapCalc) ~ 137.9g of NaOH for 100% of oils, therefore I use 0.95 * 137.9g = 131g of NaOH.
I mix both oils with my lye until trace. 95% of my oils react with all the lye and 5% of them remain unsaponified. My superfat is 5%.
Scenario 2: I apply superfat by adding 50g of coconut oil to 0% discounted soap from 950g of olive oil.
This means I calculate my lye for complete saponification of 950g of olive oil. SoapCalc estimates 128.7g of NaOH. I mix my soap and add 50g of coconut oil at trace, or after the saponification is finished (hot process). In my recipe I used 128.7g of NaOH which represents 93.3% (128.7g/137.9g) of the lye required to saponify both oils together, therefore my superfat is 6.7% !
However, if I would add 50g of olive oil as superfat, I woud finish with 5% superfatted soap! 1000g of olive oil requires 135.5g of NaOH, 128.7 / 135.5 = 95%.
Now this difference is due to different saponification values (SAP) of both oils. Simply said, 50g of coconut oil contains more fatty acids than 50g of olive oil, therefore it represents more than 5% of all the fatty acids, concretely, 50g of coconut oil represents 6.7% of fatty acids from our example recipe.
Imagine you had a recipe with 900g (90%) of olive oil and 100g (10%) of coconut oil. The lye amount needed to saponify both oils is 140g of NaOH, but only 122g of NaOH for 900g of olive oil. Adding 100g of coconut oil would resolve in 13% superfat, instead of expected 10%.
Adding oils at trace does not really help in selection on which oils will remain unsaponified, so there is no need to make scenario 2 for cold processed soap. Its just more work (and what if you forget to add your oils at trace?). The same applies for fist mixing my oils, taking out 50g of the mixture and calculating lye for the remaining 950g of oils. This is the same thing as the lye discount, just much more work with calculations…
Superfat % formula for HP soap
So we can conclude that scenario 2 makes sense for superfatting hot processed soaps only, after the saponification was completed, if we want to control the oils which will create our superfat. The final % of superfat of your soap can be calculated as follows:
1) Calculate the lye needed to saponify all the oils in the recipe (including those added after the soap is finished), at 0% lye discount, let’s denote it A
2) Calculate the lye needed to saponify only the oils in the basic recipe (without superfat oils), let’s denote this value B
3) The final superfat is calculated as 1 – B/A
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