Some of the reactions of lipids that are used for assessing the quality of lipids are listed below.

1. Free fatty acid value (FFA value)

2. Saponification Number

3. Iodine number

4. Peroxide value (PV)

5. Thiobarbituric acid Value (TBA Value)

1. Free fatty acid value (FFA value): It is defined as the number of mg of KOH required to neutralize the free fatty acids present in 1g of fat and it is also expressed as acid number. Quality of lipid can be assessed by determining the free fatty acid present, and it is of value in determining rancidity due to free fatty acids.

The FFA value is determined by titration with a standard alkali. The free fatty acid value is an indication of the extent of hydrolytic rancidity.

2. Saponification Number: The saponification number is defined as number of the mg of KOH required to saponify one gram of fat or oil. The glycerides of the fat may be readily decomposed into glycerol and salts of the constituent fatty acids (soaps) by boiling with strong bases such as sodium or potassium hydroxide. Since fats are insoluble in water, the process is facilitated by the addition of alcohol which dissolves the fat.

The saponification of fats is important in the chemical examination of fats. Since fats are mixtures of glycerides and the glycerides in turn contain various chain length fatty acids, the saponification number is an index of the average molecular size of the fatty acids present. It is inversely proportional to the average molecular weight of the fatty acids in the fat.

3. Iodine number: It is defined as the amount in grams of iodine absorbed by 100 g of fat. The triglycerides of unsaturated fatty acids react with a definite amount of iodine, which adds across the double bond. In commercial practice it usual to measure the degree of unsaturation in oil which is reported as the iodine number.It is often used to classify an unknown oil or fat into a particular class by determining the degree of unsaturation. e.g. cod liver oil have an iodine value between 155 and 170.

4. Peroxide value (PV): It is defined as the number of milliequivalents of peroxide per kilogram of fat.

The peroxide value gives an indication of the extent of oxidative rancidity. It is a measure of the amount of oxygen absorbed at double bonds in unsaturated fatty acids contained in the oil/fat.

The most common method of estimation is based on iodometric titration that measures the iodine liberated from potassium iodide by the peroxide present in the oil.Fresh oil usually has peroxide values of less than 1 meq/kg and may increase upto 10 meq/kg. Values of the order of 10 to 20 meq/kg usually indicate rancidity.

5. Thiobarbituric acid Value (TBA Value): The TBA-reactive substances provide an indication of quality. The carbonyls and other reactive substances are distillated from acid media will yield a red colored compound upon heating with TBA reagent, It is measured spectrophotometrically. If the value exceeds 1-2µ mole malonaldehyde/g of fat, the fat is considered as rancid.