Lipid extraction is a fundamental step in food analysis for nutritional labeling, quality assessment, and research purposes. The choice of extraction method depends on the food matrix, the type of lipids present, and whether total fat or specific lipid classes are of interest.
Soxhlet Extraction
The Soxhlet method is the classical reference technique for total fat determination. A dried sample is placed in a cellulose thimble and repeatedly extracted with a boiling organic solvent, typically petroleum ether or n-hexane, over 4–24 hours. The solvent is then evaporated, and the residual fat is weighed. Soxhlet extraction is simple, reproducible, and well-established for regulatory compliance. However, it is time-consuming, uses large volumes of solvent, and extracts only non-polar lipids — polar lipids such as phospholipids are poorly recovered.
Folch Method
The Folch method (chloroform-methanol extraction) is widely used for total lipid extraction, particularly for tissues with high water content. The sample is homogenized with a 2:1 (v/v) mixture of chloroform and methanol. After addition of water or saline solution, the mixture separates into two phases: the lower chloroform phase containing the lipids and the upper aqueous phase containing non-lipid contaminants. The lipid phase is collected, dried, and weighed. This method efficiently extracts both polar and non-polar lipids, including phospholipids and glycolipids.
Bligh-Dyer Method
The Bligh-Dyer method is a modification of the Folch procedure that uses a lower solvent-to-sample ratio, making it more practical for large sample volumes. The proportions of chloroform, methanol, and water are adjusted to create a single-phase extraction system, which is then split into two phases by dilution with water and chloroform. This method is particularly suitable for samples with high moisture content, such as fish and seafood.
Supercritical Fluid Extraction
Supercritical fluid extraction (SFE) using carbon dioxide (CO2) is a modern, environmentally friendly alternative to solvent extraction. CO2 above its critical point (31.1 °C, 73.8 bar) has liquid-like density and gas-like diffusivity, enabling efficient lipid extraction. The addition of modifiers such as ethanol can increase the extraction of polar lipids. SFE eliminates the need for organic solvents, reduces analysis time, and can be coupled directly with chromatographic analysis. The main limitation is the high initial equipment cost.
Acid Hydrolysis
For samples containing bound lipids, such as baked goods, pasta, and snack foods, acid hydrolysis is required to release lipids trapped within starch or protein matrices. The sample is boiled with hydrochloric acid to break bonds between lipids and other food components, followed by filtration and extraction with organic solvents. The extracted fat is then dried and weighed.
Dairy-Specific Methods
The Mojonnier method is the official method for fat determination in dairy products. It uses sequential extraction with ethyl ether and petroleum ether, followed by solvent evaporation and gravimetric measurement. The Gerber method, specific for milk and cream, involves digestion of the protein with sulfuric acid and separation of fat by centrifugation in a butyrometer, where the fat percentage is read directly from a calibrated scale. The choice of extraction method depends on the food matrix and the intended use of the extracted lipids. Moisture content must be known to express results on a consistent basis.
