Vegetable oils are mixtures of chemicals called glycerides or esters of glycerides and long chain fatty acids. Oil is obtained by crushing oil bearing seeds such as groundnut, sunflower seeds, or rapeseed, and then pressing the oil through expellers. These mechanically extracted oils contain impurities like gums and free fatty acids (FFA), which have to be removed to make the oils suitable cooking media. Refining of oil is done by first treating it with alkali which forms soap with the FFAs. The soaps settle out carrying with them some colouring matter. The colour and odour are removed by treating the oil with some absorbing material like Fuller’s earth. Refining can also be done by extracting the pure oil with a suitable organic solvent like hexane and then removing the solvent by distillation.
