Saponification number is defined as the number of milligrams of KOH required to saponify completely 1 gm of fat. Since fats are mixture of triglycerides, most of which are of mixed type, so saponification number is measure of average molecular weight of the fatty acids comprising the fats (i.e. the measure of the average chain length of fatty acids). Saponification number is an important constant particularly in distinguishing or identifying certain oils. Iodine number is defined as the number of grams of iodine absorbed by 100 gm of fat. Halogens, e.g. iodine or bromine are taken up by the fats because of the presence of double bonds present in the fatty acid of the fat. Iodine number is the measure of the degree of unsaturation of a fat. The higher the Iodine number, the more is the unsaturation present in the fat. Iodine number is a useful characteristic for assessment of both purity and nutritive value of the fat.
The physical attributes of oil or fat are dependent on factors such as seed or plant source, unsaturation degree, length of carbon chains, isomeric forms of fatty acids, molecular structure of glycerides and processing. Unsaturated fats, which are at room temperature liquid, are unsaturated in comparison to the solid unsaturated fats. With the increase of the length of chain of fatty acid, melting point also increases.
Therefore, lower melting point is of the saturated fatty acid with short chain like butyric acid whereas long chain saturated fatty acid have higher melting point. The molecular structure of triglyerides can also affect the properties of oil and fat. For oils and fats, which are esters of long-chain fatty acids, higher saponification value means the high number of triglycerol bonds in the test sample. To break larger amount of bonds, large number of KOH is used, thus leading to high saponification value. Various saponification values for oil and fat have been obtained ranging from minimum 63 to maximum 267.4 for oil and minimum 60.2 to maximum 204.4 for fat. Three moles of KOH saponifies one mole of fat completely. Sap as low as 63 could be of jojoba oil and 267.4 could be of coconut oil. The degree of unsaturation of fat or oil is defined in terms of the iodine value of the fat. Iodine value is the number of iodine in grams which reacts with 100 gm of fats double bonds.
The higher the iodine value, the more is the unsaturation of a specific oil or fat. The typical iodine value of crude soybean oil is in the 125-135 range. A typical semisolid shortening made from partially hydrogenated vegetable oils will have an iodine value of 85-95.
The saponification and iodine number of the oils and fats specifies the number of bonds present in the oil and fat. With the help of sap and iodine number, the oil and fat can be identified as the iodine value and sap value are defined for majority of oils and fats.
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