What are the Iodine Number and INS?
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 oil, tallow, and lard will consume less iodine and thus have a lower Iodine Number.
A soap made from liquid fats with 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. Soap performs best with a blend of saturated and unsaturated fats. Oleic acid in particular is a desirable unsaturated fatty acid. It is important to have a reasonable amount of oleic acid in a soap recipe because it enhances the lathering ability and mildness of soap.
The INS value has been in use since the early 1900s. 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 do not produce large amounts of soap for the mass market.
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, 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, canola, regular safflower, hempseed, and flaxseed (linseed).
Unfortunately, there is a school of thought that says soap recipes need to reach 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. The problem is that a soap with an INS this high is almost certainly too harsh 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 popular SoapCalc recipe calculator recommends 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 on INS as the main criteria for designing a soap recipe. For example, a 100% lard recipe and second recipe with a blend of coconut and olive might have the same "ideal" INS. They both might be easy to unmold and might have long shelf lives, but the two soaps will perform quite differently in the bath.
An excellent soap recipe can be designed without looking at the Iodine Number or INS value. Some soap makers keep the sum of linoleic and linolenic acid under 15% to help guard against DOS and softness. And others have a rule of thumb about limiting the polyunsaturated oils such as canola, corn, soy, sunflower, safflower, etc. to modestly low amounts for much the same reasons. These rules of thumb can be just as useful as INS or the Iodine Number.