The INS value in a soap recipe is related to iodine as it relates to the saponification value in the entire recipe of soap.This is a number that was formulated in the 1930’s with vague origins. The first time the term INS is widely sourced it is in Dr. Robert McDaniel’s book, Essentially Soap who says that the INS is a combination of the iodine and SAP value in a weighted average.
According to Dr. McDaniel the ideal INS number to aim for is 160 with a range of 147 – 170.
Other sources describe the INS as the saponification value minus the iodine number. Iodine number is a measure of the unsaturated bonds in the fats. As a general guideline, the more soft oils the more unsaturated, and so the higher iodine number. Because of the rough classification of soap ‘quality’ and and more predictable outcome with fewer oil combinations and ingredients, the INS number was over all a more useful guideline with early large industrial batches, while less important with small hand crafted batches where an artisan soap maker is carefully reviewing the balance of oils and butters while making a smaller amount and focusing on a higher quality result. The INS number was a way to evaluate the ease of saponification and stability in a recipe when batches were made on large scale and possible less human scrutiny over the entire process.
For the hand craft soap maker the INS number can be used to predict the shelf life of a batch. Higher numbers are attributed to saturated fats. These oils saponify more easily, and harder, and make a more cleansing bar. These oils are more stable and will last on the shelf longer than unsaturated fats once turned into soap. Coconut is at the top of the list at an INS of about 258, followed by palm kernel, tallow, cocoa butter, palm oil, lard, and shea with INS of 115-230.
Lower INS numbers are attributed to unsaturated fats and soft oils. Therefor, the lower the INS number the less resistant the batch will be to the DOS (Dreaded Orange Spots) phenomena that can happen as the floating oils degrade over time during storage. The help protect against DOS the INS number should be evenly balanced (around 160 or higher)Oils that contribute to DOS are lower in iodine, sch as: polyunsaturated oils such as canola, corn, soy, sunflower, safflower. These oils should remain lower in percentage in the total weight of the recipe.