Theory

Acrolein test:

Acrolein test is used to detect the presence of glycerol and fat. This test is based on the dehydration reaction, in which the water molecules are removed from the glycerol by adding the reagent potassium hydrogen sulphate. The reaction between glycerol and potassium hydrogen sulphate results in acrolein formation, which is characterized physically by the release of a pungent smell.

Sudan IV Test

Sudan IV test is used to detect the presence of lipid in a solution. This test is based upon the principle of binding and solubility of lipid in non-polar compounds. As Sudan IV is a non-polar stain, the lipid will bind with it and retain the stain’s colour by giving a red-orange colour. Sudan IV does not stain or bind to the polar compounds.

Furfural and Baudouin Test:

The Furfural and Baudouin tests are specifically used to monitor the adulteration in the ghee. The common adulteration found is from sesame oil or any hydrogenated vegetable oil. The furfural test detects sesame oil while the Baudouin test detects the presence of hydrogenated oil. These tests give crimson colour.

Huble's Test:

This test is used to know the degree of unsaturation in the given sample. Unsaturation test is used to detect the unsaturated fatty acids or double bond in a lipid sample. All the neutral fat contains glycerides of fatty acids. Double bonds are found in the structure of unsaturated fatty acids, which becomes saturated by taking up either bromine or iodine. If the lipid contains more unsaturated fatty acids or more double bonds, it will take more iodine. Count the number of drops added to chloroform and Huble’s iodine solution until the pink colour disappears. The number of drops determines the taking up of iodine by the unsaturated fatty acid of lipids.

Saponification value:

The saponification value of an oil or fat (lipids) is defined as the number of mg of potassium hydroxide required to neutralize the fatty acids resulting from the complete hydrolysis of 1 g of the sample. Soap is formed during saponification, for example:

C₃H₅(C₇H₃₅COO)₃ + 3KOH = C₃H₅(OH)₃ + 3C₁₇H₃₅COOH

Stearin Glycerol Potassium stearate

The esters of the fatty acids of low molecular weight require the more alkali for saponification, so that the saponification value is inversely proportional to the mean of the molecular weights of the fatty acids in the glycerides present.

When a fat is boiled with an excess of alcoholic potassium hydroxide (KOH), the triglycerides hydrolyse, and glycerol and soap are formed. The alkali consumed by this hydrolysis is a measure of the Saponification Value (S.v.). It is defined as the number of milligrams of potassium hydroxide (KOH) required to saponify completely one gram of the oil or fat. The saponification value is sometimes used to check for adulteration. The higher the saponification number, the more capable the oil is of making soap. Higher triglyceride saponification values suggest more medium-chain fatty acids.

Saponification value (S.V)= 56.1xMmx (Vo-V1)/m
Mm: Molarity of HCl
m: mass of oil
Vo: Volume of the HCl to titrate blank sample cm3
V1: Volume of the HCl to titrate test sample cm3

Acid Value:

The acid value determines the amount of free fatty acids in fat. Usually, the sample which is dissolved in a solvent (AOCS method: isopropyl alcohol/toluene; DGF method ethanol/ether) is titrated with 0.1 N potassium hydroxide solution against phenolphthalein.

The acid value is defined as the number of milligrams of potassium hydroxide required to neutralize the free fatty acids present in one gram of fat. It is a relative measure of rancidity as free fatty acids are normally formed during the decomposition of oils that contain triglycerides.

Oils and fats become acidic with the oxidation or hydrolysis of moisture in the atmosphere leading to fatty acid formation.

Oxidized oils and fats containing large amounts of acid cause metals to corrode.

Iodine Value

Iodine value can be used to measure the degree of unsaturation of oils and fats. The results are normally expressed as the number of grams of iodine absorbed by 100 g of oil or fat, considering the conditions of the test.