# Iodine Number, Saponification Number and Essential Fatty Acids The iodine number and saponification number are important chemical characteristics used to determine the nature and properties of fats and oils. In biochemistry and Biology for Chemists, these values provide useful information about the degree of unsaturation and the average molecular size of the fatty acids present in a fat. If a fat has a low iodine number and a high saponification number, certain definite conclusions can be drawn about its fatty acid composition. The concepts of essential fatty acids are also closely related to the biological importance of lipids and their role in human nutrition. The iodine number of a fat or oil is defined as the number of grams of iodine absorbed by 100 grams of fat. It is primarily a measure of the degree of unsaturation of the fatty acids present in the fat. Unsaturated fatty acids contain one or more carbon-carbon double bonds, and these double bonds can react with iodine or iodine-containing reagents. Therefore, the greater the number of double bonds, the greater is the amount of iodine that can be absorbed. A high iodine number indicates a high degree of unsaturation, whereas a low iodine number indicates that the fat contains predominantly saturated fatty acids or relatively few double bonds. Thus, a fat having a low iodine number contains a comparatively high proportion of saturated fatty acids. Such fatty acids have no carbon-carbon double bonds. Examples include palmitic acid and stearic acid. Saturated fats generally have higher melting points than highly unsaturated fats and are often solid or semi-solid at room temperature. The low iodine number therefore indicates that the fat is relatively saturated and has a lower degree of chemical unsaturation. It also suggests that the fat is less susceptible to oxidative rancidity because double bonds are particularly vulnerable to oxidation. The saponification number is defined as the number of milligrams of potassium hydroxide (KOH) required to saponify one gram of fat or oil. Saponification involves the hydrolysis of ester bonds in triglycerides in the presence of an alkali, producing glycerol and the potassium salts of fatty acids, which are commonly called soaps. The saponification number is inversely related to the average molecular weight or average chain length of the fatty acids present in the fat. Fats containing shorter-chain fatty acids have a greater number of fatty acid molecules per gram. Consequently, more ester bonds are present in a given mass of fat, and more KOH is required for complete saponification. Such fats therefore have a high saponification number. Conversely, fats containing long-chain fatty acids contain fewer fatty acid molecules per gram and generally have a lower saponification number. Therefore, when a fat has a high saponification number, it indicates that the average molecular weight of its fatty acids is relatively low and that the fat contains a comparatively high proportion of short- or medium-chain fatty acids. When this is considered together with its low iodine number, the fat can be inferred to contain predominantly saturated fatty