Commercial soap makers want soap that saponifies reasonably quickly, forms a hard soap that is easy to manufacture, and has a reasonably long shelf life. Both the Iodine Number and the INS value were originally created to identify blends of fats that are likely to meet those requirements.
For hand crafted soap makers, the INS value and Iodine Number is not quite as meaningful, because we usually focus on other qualities in our soap that have little or nothing to do with the requirements needed for large scale, high volume soap manufacturing.
The Iodine Number is the result of a chemical test that measures how much of a particular iodine solution reacts with the fatty acids in a particular fat or blend of fats. The Iodine Number indicates the amount of of unsaturated fatty acids present. Soap made with mostly unsaturated (liquid) fats will tend to have a high Iodine Number and soap made with mostly saturated (solid) fats will have a low Iodine Number.
If you look at cooking oils in a grocery store, the nutrition information will list the amounts of saturated, monounsaturated, and polyunsaturated fats in the oils. Liquid oils high in mono- and polyunsaturated fats will consume more iodine solution and will thus have a higher Iodine Number. Solid fats high in saturated fats such as shortening, palm, palm kernel, coconut, tallow, lard, and the nut butters (shea, cocoa, etc.) will consume less iodine and thus have a lower Iodine Number.
A soap that is made mostly with fats that have high Iodine Numbers may be softer than one would like at the time of unmolding. The soap may also have a greater chance to become rancid (DOS, Dreaded Orange Spots.) A high Iodine Number doesn't absolutely mean the soap will be soft or become rancid; it's just an indicator that it might.
The conclusion that a person might draw from this is that a soap recipe with a low Iodine Number is better, but that is not necessarily true either. Soap performs best with a blend of saturated and unsaturated fats. To give you a point of reference, The popular SoapCalc recipe calculator suggests a range for the Iodine Number between 41-70, and Iodine Numbers within this range will come from blends of unsaturated and saturated fatty acids.
The INS value has been in use since the early 1900s. The letters "INS" stand for "Iodine Number Saponification" as defined by E.T. Webb in his 1927 book Soap and Glycerin Manufacture. (1)
INS is equal to the Saponification Value minus the Iodine Number. INS was originally developed as a quick check to identify blends that saponify easily and make a hard soap suitable for high volume manufacture. For makers of handcrafted soap, the INS value is not as meaningful, since most small-scale soap makers have priorities for our soap recipes other than being suitable for large-scale mass-production.
There is a hypothesis in the handcrafted soap world, however, that suggests soap recipes should meet some ideal INS number. Robert McDaniels in his book Essentially Soap, published 2000, may have been the first to popularize this idea. He suggested an ideal soap recipe should have an INS value of about 160.
An INS of 160 can be reached only if the soap contains a large percentage of fats with high saponification values, such as coconut oil. When a recipe has an INS near McDaniel's ideal of 160, the soap will almost certainly be too harsh and drying for many people to use for regular bathing.
This "ideal INS of 160" concept also doesn't explain the success of classic soap recipes with low INS values. One example is a 100% olive oil soap (castile) with a measly INS of 105.
Anne Watson, in her book Simple Soapmaking, drops the bar to a more realistic level; she suggests the INS should fall between 145 to 160. The SoapCalc recipe calculator suggests a similar INS range of 136 to 165. These ranges for INS are far more realistic for handmade soap.
What INS values do some common soap-making fats have?
Fats high in myristic and lauric acids have high INS. Coconut is at the top of the list with an INS of about 260 followed by palm kernel and babassu at about 230.
Tallow, cocoa butter, palm, and lard -- fats high in stearic and palmitic acids -- are next with INS ranging from 140 to 160.
Fats with INS of 95 to 115 include shea butter, castor, and high-oleic fats, such as olive, high oleic sunflower, high oleic safflower, high oleic canola, avocado, and sweet almond.
Fats with INS from 45 to 85 include beeswax, lanolin, and fats high in linoleic and linolenic acids, including rice bran oil, corn oil, rapeseed, grapeseed, pumpkin, regular sunflower, soybean, regular canola, regular safflower, hempseed, and flaxseed (linseed).
Even if you follow the more relaxed recommendations from Sapcalc or Anne Watson, however, it still does not make sense for small-scale soap makers to focus mainly on INS when designing a soap recipe. Even if two recipes meet an INS requirement, they can perform quite differently in the bath.
For example, a 100% lard recipe and a second recipe with a blend of coconut and olive can be formulated to have the same INS number. The lard soap will last a long time in the bath and will produce modest amounts of dense long-lived lather. The coconut-olive soap, however, will have a much shorter life in the bath and will produce a larger amount of fluffy lather.
Soap recipes can be designed without ever looking at the Iodine Number or INS value. I know of two rules of thumb that soap makers use to guard against rancidity and softness --
Some soap makers check the fatty acid profile as they design a recipe. They keep the combined percentage of linoleic and linolenic acid under 15%.
Others limit the amount of polyunsaturated oils in their recipes. Oils high in polyunsaturated fatty acids include grapeseed, hemp, corn, and soy oils, as well as regular canola, sunflower, and safflower oils.
In summary, the INS value or Iodine Number are meant for evaluating commercial soap making formulations that rely on a limited number of fats. These properties have limited value when designing handcrafted soap recipes.
References
(1) Webb, ET. Soap and glycerine manufacture: A modern treatise on the production of soaps of all kinds, and on the recovery and distillation of glycerine. Davis Bros, London. 1927. Locator service: https://openlibrary.org/books/OL6716630M/Soap_and_glycerine_manufacture.
Copyright © 2002-2025 - All rights reserved by Classic Bells Ltd.
Template by OS-templates.com
