Those of us that have heard of the INS value of soap, first learned of it from Dr. Robert McDaniel (Dr. Bob). He mentions it in a brief paragraph of his book “Essentially Soap” without attributing the original source, saying only that the ideal is 160 and, after some study of the INS values of good published soap recipes, he adjusted all of his own recipes to be closer to this ideal number. His book lists the INS values of several common oils, but he doesn’t reveal the actual calculation.
Dr. Bob acknowledges that he doesn’t know what the initials represent; possibly “Iodine iN Soap” or “Iodine aNd SAP”, since we don’t know the origination of the calculation it is any one’s guess.
Now, you can just go get Dr. Bob’s book and use his published INS values, but we’re on the inquisitive side and don’t like to just take someone’s word that their numbers are correct; especially since when Dr. Bob demonstrates the use of the INS value, he uses a number completely different than the one in his table. We had to wonder, which was the correct number??
Frustrated with the lack of information about INS values, we came across a worksheet on the Miller Soap website. This worksheet had the INS calculation, except for the flaw (in our opinion) of assuming that Dr. Bob’s calculation is correct if the worksheet result differs from Dr. Bob’s table.
The calculation is actually a simple one, making us wonder why there is such an elusiveness around it. For those of you that like math and want to know how to experiment with INS values in your recipes continue on…
Obtain the KOH SAP value of each of your oils from your supplier. Two things are important here, first you should get the numbers from your oil supplier so you know the number is absolutely correct for your ingredients; second, make sure to get the KOH number. Yes, the SAP listed for potassium hydroxide – do not use the SAP for sodium hydroxide (NaOH) or your numbers won’t be right. The KOH SAP value is multiplied by 1000 to bring it into a whole number. For example, Olive Oil has a KOH value of .190.
.190*1000 = 190
Next, look up the iodine values for each of the oils in your recipe. The iodine values are usually provided as a range with a high and low value. Olive Oil has an iodine range of 79-95. These numbers are averaged together to get a single number.
(79+95)/2 = 87
Subtract the iodine average from the SAP to get the INS value for the oil.
(190*1000) – ((75+94)/2)= 103
Once you have the INS values for each of your oils, you need to calculate the percentage of each oil in the recipe. Divide the weight of the single oil, by the sum of the weights of all of the oils. For example, a recipe with 12 ounces of coconut, 12 ounces of olive oil and 16 ounces of lard would be calculated as:
The coconut and olive would be: 12/(12+12+16)= .30 or 30%
The lard would be: 16/(12+12+16) = .40
The INS contribution of the oil in the recipe would be the INS value multiplied by the percentage. So our Olive oil would have its INS contribution figured as follows:
Add each of the INS contributions of each of the oils in the recipe to get the total INS value. The ideal is said to be 160.
Our example recipe above has an INS of 163.9 – fairly close to the ideal. We can adjust the recipe by replacing a portion of high INS oils (such as coconut, palm and lard) with oils with low INS values (such as jojoba, sunflower, and grapeseed), trying to lower the value to get it closer to the 160 ideal.
Remember, INS values are only one consideration when creating your recipes. Lather, texture, and hardness are a few other considerations. It would be easy to concentrate on only the INS calculation and end up with an exact 160, but if you ignore the other considerations you could end up with a soap that has qualities that you just don’t like.
So why should you even consider using the INS value at all? By adjusting our recipes to be closer to the INS values, we have experienced a drastic reduction in the time it takes to trace – less than 5 minutes. In addition, prior to the INS calculation, our recipes seldom ever completed a “gel” stage so saponification was completed over the course of several weeks during the drying time. After adjusting the INS values, both our lard and palm based recipes gel every time. When completing the gel phase, we’ve noticed that our soaps are extremely mild straight out of the mold. While we still feel that the bars need to complete a curing process it is more to allow the bars to harden rather than for completion of the saponification process.
So our experience with using the INS calculation to adjust recipes is that it creates a mixture of oils that more readily traces, goes into the gel phase, and completes the entire saponification process faster.
If you have used the INS values to adjust your recipes, we’d love to hear how your recipes turned out.