Soxhlet extraction

Method

The Soxhlet method is an example of semi-continuous solvent extraction, mostly done with petroleum or diethyl ether. The sample is placed in an extraction thimble, which is connected to a round bottom flask containing the extraction solvent, and to a condenser. When the flask with solvent is heated, the solvent evaporates, and moves up into the condenser.

The evaporated solvent will condensate and the liquid solvent formed will drip on the sample until the extraction thimble is completely filled. Then the solvent siphons back into the round bottom flask, together with the dissolved fats. After many cycles of evaporation, condensation and extraction, the solvent in the round bottom flask is evaporated. The dissolved fat will remain in the round bottom flask.

The mass of the extracted fat is measured on a balance. The advantage of this system is that instead of many portions of warm solvent being passed through the sample, just one batch of solvent is recycled.

Characteristics

• Applicable for dry foods (dry matter > 95%)

• Applicable for readily extractable fats (storage fats).

• Incomplete extraction when amount of structural fats is considerable or occluded, e.g. baked products. Pre-digestion with acid (HCl) can solve this, e.g. application in meat samples.

• Samples containing sugar need to be pre-washed with water and dried again. Mono-and disaccharides in the sample will be co-extracted and increase the yield.

Extraction of fats with diethyl ether can lead to artificially high values because diethyl ether also extracts certain low molecular polar components, such as urea and mono- and disaccharides.

Hexane and petroleum ether are non-polar solvents and for this reason hardly extracts low-molecular polar compounds and hence the yield of an extraction is lower compared to a diethyl ether extraction.