Gravimetric methods

Principle: Fats are soluble in organic solvents (such as ether, petroleum ether, hexane or chloroform), but insoluble in water. This property is used for the separation of fats from other components in foods such as proteins, carbohydrates and minerals. After extraction, the solvent is evaporated and the residue (= total fat) is weighed. Any method of analysis in which the final stage of the analysis involves weighing is called a gravimetric method.

The efficiency of the extraction depends on the solubility of the fats in the solvent. Polar lipids, e.g. glycolipids or phospholipids, are more soluble in polar solvents, e.g. methanol, while non-polar lipids, e.g. triglycerides, are more soluble in non-polar solvents, like hexane.

Therefore, total fat extracted by using one solvent may be quite different from a method using another solvent. Some foods require a treatment with chemicals prior to the extraction to disrupt fats bound to proteins and carbohydrates, e.g. treatment with acid.

Methods: A number of methods are available to extract fats from foods. Particular methods were developed and applied to specific foods: e.g. Extraction of cereals with water saturated butanol, or Röse-Gottlieb procedures for milk products. For each described method a lot of modifications have been reported. Specific procedures which should be applied to certain foods are laid down in AOAC, ISO or other official methods. Often, more than one method can be used.

It is necessary to be aware that some methods can lead to incomplete extraction, e.g. Soxhlet with ether, or that some methods, e.g. acid hydrolysis, are not suitable to characterise lipid classes, like phospholipids, triglycerides, glycolipids, mono-acylglycerol etc., or that some mild methods (i.e. without acid or alkaline hydrolysis) are preferred when a fatty acid profile is required.

Below you find four gravimetric extraction methods that are

often used :