The method described by Soxhlet in 1879 is the most commonly used example of a semi-continuous method applied to extraction of lipids from foods. According to the Soxhlet’s procedure, oil and fat from solid material are extracted by repeated washing (percolation) with an organic solvent, usually hexane or petroleum ether, under reflux in a special glassware.
In this method the sample is dried, ground into small particles and placed in a porous cellulose thimble. The thimble is placed in an extraction chamber (2), which is suspended above a flask containing the solvent (1) and below a condenser (4). The flask is heated and the solvent evaporates and moves up into the condenser where it is converted into a liquid that trickles into the extraction chamber containing the sample. The extraction chamber is designed so that when the solvent surrounding the sample exceeds a certain level it overflows and trickles back down into the boiling flask. At the end of the extraction process, which lasts a few hours, the flask containing the solvent and lipid is removed. In some device a funnel (3) allows to recover the solvent at the end of the extraction after closing a stopcock between the funnel and the extraction chamber. The solvent in the flask (1) is then evaporated and the mass of the remaining lipid is measured. The percentage of lipid in the initial sample can then be calculated.
Despite disadvantages of this procedure (poor extraction of polar lipids, long time involved, large volumes of solvents, hazards of boiling solvents), several methods involving automatic solvent extraction were described. Different automated or semi-automated extraction instruments may be found on the market.
Several solvent extraction systems based on the Soxhlet device are on the market to allow fast and safe determination of total lipids in food, soil, ….
As an example, FOSS has launched several types of “Soxtec Systems” including automated or semi-automated analyzers, which extract lipids rapidly and accurately.
These instruments perform boiling, rinsing and solvent recovery. Similarly, Soxtherm extractors from Gerhardt GmbH was developed to reduce extraction times.
The sample to be analyzed is weighed into cellulose thimbles and inserted in the extraction device. Except diethyl ether, all solvents may be used (about 15 ml per sample), with a 75% recovery of the solvent after the extraction which is completed in 30 to 60 min, depending on the application.
Another device by ViscoALPHA enables the user to have a 2, 4 or 6 place system in 2 versions (micro or macro). An electronic unit can control and monitor up to 4 extraction units individually.
Compact and simple systems with one to six samples are sold by Behr Labor-Technic GmbH.
The XT10 Extractor by ANKOM provides an Official Procedure (AOCS Am 5-04) for efficient and economical solvent extractions of various sample types. It allows batch processing for the extraction of up to fifteen samples at a time and up to 100 samples per day. A process temperature of 90°C accelerates extraction kinetics, reducing most extraction times to under 40 minutes. Samples are placed into filter bags which prevent sample transfer error. The XT10 automatically recycles solvent and makes it ready for reuse. Solvent is manually added after each extraction. The XT15 Extractor provides a totally automatic operation for up to 150 samples.
The Büchi Extraction System B-811 is an automated system which can be used to perform an extraction according to the original Soxhlet principle. Four different extraction methods are possible without making any changes to the unit : Soxhlet standard, Soxhlet warm, hot extraction and continuous extraction. The system has an inert gas supply to avoid oxidation during extraction and to accelerate the evaporation and drying process even with high boiling point solvents (up to 150°C). Several application notes may be downloaded from the Büchi site.
A comparison of different extraction methods for total lipid quantification in meat and meat products was reported (Perez-Palacios T et al., Food Chem 2008, 110, 1025). The Soxhlet method with previous acid hydrolysis had the same efficiency as that of the method described by Folch.
A microwave-assisted Soxhlet extraction of seed oil (sunflower, soybean, rape) was described using a cellulose cartridge placed into a quartz extraction vessel inserted in a modified Microdigest 301 device (Prolabo). This procedure is slightly different from that described for the extraction of dry materials. Despite a 3 h analysis, the time reduction and the lack of need for seed grinding makes this procedure a suitable competitor of the previously described methods (Garcia-Ayuso LE et al., Anal Chem 1998, 70, 2626; Garcia-Ayuso LE et al., Chromatographia 2000, 52, 103). Comparative experiments have shown that no significant differences between the extract obtained by the Folch reference method without fat alterations and the microwave-assisted Soxhlet extraction were detected. These results demonstrated the applicability of that method for the extraction step in routine analysis of a great quantity of food samples (Ruiz-Jimenez J et al., Anal Chim Acta 2004, 525, 159).
This extraction technique, called microwave-assisted Soxhlet extraction, uses two sources of energy, namely microwaves, applied on the extraction chamber of a modified Soxhlet, and electrical heating applied on the distillation flask. This system has been used for the determination of oil content and fatty acid composition of various biological materials and foodstuffs. To overcome some limitations of the analytical process (water content), a new and convenient process was designed and developed (Virot M et al., J Chromatogr A 2007, 1174, 138; Virot M et al., J Chromatogr A 2008, 1196-1197, 57).
A modification of that extraction procedure was proposed taking into account the use of a “green solvent”, limonene, instead of hexane (Virot M et al., J Chromatogr A 2008, 1196-1197, 147). The proposed method is effective and valuable since no significant difference was obtained when using hexane or limonene for the extraction of oleaginous seeds.
A critical evaluation of various solvents systems used for the quantification of microalgal fatty acids with lyophilized samples has been reported (McNichol J et al., Lipids 2012, 47, 195).
The bio-based solvent 2-methyloxolane can be considered also as an excellent alternative to the petroleum-based solvent n-hexane for edible/cosmetic oil extraction with a Soxhlet device (Gharby S et al., OCL 2020, 27, 27). Furthermore, the utilization of 2-methyloxolane for oil extraction can drastically reduce the health and environmental impacts associated with n-hexane.
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