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 needs. 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 than whether it is suitable for commercial 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 book Modern Soap and Glycerin Manufacture, 1927.
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 processing and packaging. For makers of handcrafted soap, the INS value is not as meaningful, since most of us are not in the game of mass-producing large amounts of soap.
What INS values do some of the more 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).
Unfortunately, there is a school of thought in the handcrafted soap world that says soap recipes should meet an "ideal" INS value. 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.
A soap with an INS this high is almost certainly too harsh and drying for many people to use for bathing. And it ignores the success of classic recipes, such as 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.
Even so, it does not make sense to focus mainly on the INS value when designing a soap recipe.
For example, a 100% lard recipe and second recipe with a blend of coconut and olive might have the same INS number. Both soaps might firm up quickly so they are easy to unmold, but the two soaps will perform quite differently in the bath.
The lard soap will last a long time and will produce low amounts of dense long-lived lather. The coconut-olive soap will have a much shorter life in the bath and will produce a generous amount of fluffy lather.
In summary, the INS value or Iodine Number can be helpful, but they should only be a small part of the whole picture when designing a soap recipe.
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.
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