Principle
Lipids are one of the major constituents of foods, and are important in our diet for a number of reasons. They are a major source of energy and provide essential lipid nutrients. Nevertheless, over-consumption of certain lipid components can be detrimental to our health, e.g. cholesterol and saturated fats. In many foods the lipid component plays a major role in determining the overall physical characteristics, such as flavor, texture, mouthfeel and appearance. In the proximate system of analysis, `fat' is measured as the fraction of the food that is soluble in lipid solvents. For nutritional purposes the measurement of `total fat' has limited value; nevertheless, it still is widely reported and is retained in many requirements for food labeling and the regulation of food composition. The “Soxhlet” method described here is recognized by the Association of Official Analytical Chemists (AOAC) as the standard method for crude fat analysis. A dried, ground sample is extracted with ether which dissolves fats, oils, pigments and other fat soluble substances. The ether is then evaporated from the fat solution. The resulting residue is weighed and referred to as ether extract or crude fat. The sample is contained in a porous thimble that allows the solvent to completely cover the sample. The thimble is contained in an extraction apparatus that enables the solvent to be recycled over and over again. This extends the contact time between the solvent and the sample and allows it time to dissolve all of the fat contained in the sample. In order for the solvent to thoroughly penetrate the sample it is necessary for the sample to be as finely comminuted as possible. Both the solvent and the sample must be free of moisture to avoid coextraction of water-soluble components in the sample such as carbohydrates, urea, lactic acid, glycerol, etc. Low temperature is used to evaporate the ether and remove residual moisture to prevent oxidation of the fat. Fat includes fatty acids, triglycerides, esters, long chain alcohols, hydrocarbons, other glycol esters and sterols determined by the method. It is expressed as g fat per 100g sample.