A little bit more chemistry for a lovely December morning!
The iodine value of an oil or butter is a measure of the saturation of the fatty acids. As we go up on the iodine value scale, we'll see more double bonds or more unsaturation in the oils. Something like coconut oil, which has a high degree of saturated fatty acids, will have a lower iodine value (10.4) than grapeseed oil (135), which has a lot of unsaturated fatty acids. For the most part, looking at the iodine value of your oil or butter can give you a lot of information about its shelf life. The lower the iodine value, the longer this oil is likely to last.
Disclaimer: The iodine value offers very valuable information, but there are some freak oils out there that mess up the whole system. An oil that is very high in anti-oxidants - say, soybean oil (130.5) - will have a longer shelf life than a similar oil that is low in anti-oxidants. Soybean oil has a shelf life of about a year, whereas grapeseed oil has a shelf life of 3 to 6 months. This has to do with the way Vitamin E and other anti-oxidants retard rancity. For the most part, the system works, but there are some exceptions.
As the iodine value increases, the titer point decreases, meaning the lower the iodine value, the more likely the oil is to be a butter and be more solid. So if I see an iodine value of 15 (babassu oil), I'm fairly sure this is will be solid at room tempearture. An iodine value of 100 leads me to believe it's an oil (like castor or sweet almond oil).
How do the scienticians determine the iodine value? From Wikipedia: "The iodine value (or "iodine adsorption value" or "iodine number" or "iodine index") in chemistry is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. An iodine solution is yellow/brown in color and any chemical group in the substance that reacts with iodine will make the color disappear at a precise concentration. The amount of iodine solution thus required to keep the solution yellow/brown is a measure of the amount of iodine sensitive reactive groups."
It's all very interesting, but why do we care about this? It's helpful to know the iodine value of your oils so you can determine how long they will live on your shelf and in your products. If you know the iodine value of regular sunflower oil is 132.5, you can predict this will have a short shelf life and add your anti-oxidants or choose another oil. (Noting the exceptions, of course.)
You can also look at an exotic oil and figure out how thick it might be. If I see a new ingredient - let's call it toasteroven butter - with an iodine value of 36.5, I know it's unlikely to be a liquid and it's more likely to be a butter like cocoa butter (which has an iodine value of 36.5). I also know that it should have a long shelf life. If you see an oil - let's call this one cutiedog oil - with an iodine value of 140, I know this is definitely an oil and likely has a short shelf life.
If you're looking to add more unsaturated fats in your diet or your products (wonderful fatty acids like oleic or linoleic acid), you can look at the iodine value and determine how saturated it is!
And finally, it's cool to be able to look at something like an ester and figure out that it's a solid or liquid and its shelf life based on the iodine value (yes, non-natural oils like esters have iodine values as well). So if you see something like cetyl alcohol and see it has an iodine value of 1.0 max, you know it has a long shelf life and is definitely not a liquid!
So now you know about iodine values! Yay!