Fat is determined by extraction with solvents. Most methods use a dried sample that has been ground or homogenized, because fine particles increase the efficiency of extraction. The classical approach is the Soxhlet method, wherein a weighed sample is continuously extracted in a specialized apparatus to heat, condense, and cycle the solvent over the sample in a continuous process for 18 24 hours. The sample is then dried; the difference between pre- and postextraction weights is the lipid weight. Diethyl ether is the standard solvent for these extractions, but petroleum ether may be substituted to reduce the danger of fire and explosion. Wet samples are usually extracted with chloroform-methanol or sometimes petroleum ether-diethyl ether. The solvent dictates the efficiency and composition of the extracted lipids. Chloroform-methanol completely extracts all neutral lipids (triglycerides, cholesterol, etc.), plus most phospholipids. Extraction with petroleum ether, chloroform, or methylene chloride alone will not remove the more hydrophilic phospholipids.

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