Let us look at the iodine value definition in analytical chemistry. The iodine value, also known as the iodine number, is a measure of the degree of unsaturation of fat, wax, or oil; it is expressed in gms, the amount of iodine, which is taken up by 100 gms of the fat, wax, or oil.
Iodine is not taken up by saturated oils, waxes, or fats, because their iodine value is zero; however, iodine is taken up by unsaturated fats, oils, and waxes.
About Iodine Value
Unsaturated compounds have molecules either with double or triple bonds, which are very reactive towards the iodine. The more iodine attached, the higher is the value of iodine and the more reactive, softer, less stable, and more susceptible to the oxidation process, and rancidification is the fat, oil, or wax. In performing this particular test, a known excess quantity of the iodine, which is usually in the iodine monochloride form, is allowed to react with a known amount of fat, oil, or wax, and thereafter, the remaining unreacted iodine amount can be determined by the process of titration.
Drying oils, which are used in the varnish and paint industry, have relatively high iodine values (up to 190). Semi drying oils such as soybean oils have intermediate iodine values (up to 130). Non Drying oils such as olive oil, which are used for food products and in soapmaking, have relatively low iodine values (up to 80).
Significance of Iodine Value
Iodine is one of three trace elements of the body that are vitally important for the human body. Whilst the body only contains approximately 14 mg of iodine, it is vital for thyroxine formation, which is a hormone secreted from the thyroid gland in the neck that plays an important part in regulating the metabolic rate of the body.
In order to meet the requirements of the body, we need only 1mg per week. Now, most of the salts are fortified by iodine salts which is one of the reasons that goitres are less common than they were once.
The end-product is transported around the blood bound to the proteins (primarily thyroid, binding globulin). It is released to cells slowly in an active form, and from here, it regulates the excitability of the nerve fibres to the metabolism rate.
Need of Iodine
Iodine is much needed by the thyroid gland to produce thyroid hormones. The deficiency of iodine is being known as a "silent epidemic" in some medical communities. Its extreme importance is taught rarely by the Doctors, and patients alike routinely overlook it. "Endocrine mineral" is not only important for the thyroid gland, but it is also important for our adrenal gland, reproductive system, and entire hormone system. A primary worldwide study has found that between 92-96% of the developed world are iodine deficient.
Peroxide Value vs. Iodine Value
The peroxide value is defined as the amount of oxygen consumed in the reaction, which reduces all the unsaturated bonds (C=C) in a given amount (or mass) of a lipid mixture during the autoxidation process, where the oxygen forms peroxides ultimately. The peroxide value is always proportional to the iodine amount formed when that peroxide is released to an acidic solution that contains the known amount of either the unsaturated fat or oil mixed in with a known excess starch (or amylose) and iodide solution. It means a known saturated iodide-starch solution - as an indicator. The amount of iodine (I2), which is formed (from the iodide, I−) in this reaction, is called the iodine value or iodine number.
"Unsaturated" oils or fats mean the lipophilic compound contains C=C bonds. In the presence of a C=C bond and Oxygen (O2), water will be converted to hydrogen peroxide (H2O2) through the autoxidation process. While the units of mEq oxygen per gram fat are often used, the correct SI unit would be given as mmol oxygen per gram fat. (It is to note that 0.5 mEq O2 = 1 mmol O since every "equivalent" of O2 contains two oxygen atoms).
The peroxide method is one of the ways to measure how unsaturated fat or oil is. We can also find the Peroxide value, and the related references for the reaction that occurs can be found from many sources. The iodine value is given as the mass of iodine, either in g or mg, released from the mixture at the colour change time. It contains the units of g/100g fat (or the mg I/kg fat).
It is recommended that if we had a different indicator (for example, Bromide), then the peroxide value would be proportional to the value of bromine and not the value of iodine.
Importance of Iodine Value in Fats and Oils
There is much importance of iodine value in fats and oils. The Iodine value is defined as a measure of unsaturation. It means the number of double bonds present in the oil or fat will, in turn, depend on what the oil or fat is! For example, nonane (c9) is the oil, but it has zero double bonds, and when we get to c11-c12, we are starting into waxes.
In general, oils tend to have more double bonds, but it can simply be the various chain lengths of the alkanes in the mix.
So we cannot really say the exact iodine value of fat and oils.