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When extracting lipids or fats from foods, both the method as well as the solvents chosen to perform a complete, or close to complete extraction are important. If these two elements are not taken into consideration, the extraction may not be complete, or the extract may contain a large quantity of undesired impurities.
As lipids are relatively non-polar molecules, they can be pulled out of a sample using relatively non-polar solvents. With a non-polar solvent, only non-polar molecules in the sample dissolve while polar ones do not. Problems arise however, in cases where lipids are bound in animal or plant cell membranes. Animal and plant cell membranes are made up of molecules that have both polar and non polar regions such as triglycerides (molecule with polar glyceride heads and non-polar fatty acid tails) and phospholipids (similar to triglycerides, but a phosphate group replaces the fatty acid tail). These molecules end up grouping together with their polar heads sticking outwards and non-polar tails inwards making it difficult for non-polar solvents to interact with the non-polar tails and extract them. As these molecules are part non-polar and part polar, we need a solvent that presents some of these same characteristics. This is why we use a mixture of two solvents such as hexane and isopropanol. The isopropanol is polar enough to interact with the polar regions and help "pull apart" the cell membrane while also being non-polar enough to help in extracting non-polar fats and being soluble in the hexane.
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