Microwave oven extraction
Microwaves have been reported first to produce acceleration of rates of a large array of chemical reactions, the feasibility of microwaves for extracting lipids from seeds, foods and soil was reported later (Ganzler K et al. J Chromatogr 1986, 371, 299). We improved and applied this physical treatment to the extraction of food and tissue lipids. We chose one of the most difficult problems: the extraction of dry biological preparations, samples of animal food and powdered rat carcass (Leray C et al. Analusis 1995, 23, 65).
We used a laboratory microwave oven in a classical solvent mixtures and obtained maximal recoveries and accurate analyses after a 2 min extraction. The fatty acid composition of glycerolipids were similar to those obtained with the classical Folch’s procedure.
One gram dry sample is placed in a 30ml glass open vessel with 2 ml of water. 10 ml of methanol are added and the vessel is fitted in a Microdigest A 301 or equivalent (Mars-X) (CEM-Prolabo) . The microwave digester power was run 2 min at 20 Watt. A second addition of 20 ml of chloroform is made and a second 2 min extraction step is run. The vessel content is filtered through filter paper, rinsed and the lipid extract evaporated to dryness.
This procedure can be used as the basis of quantitative extraction of lipids from various biological materials, even those rich in polyunsaturated fatty acids. An adaptation of this procedure for milk and egg powder was also described (Paré JRJ et al. J AOAC Int 1997, 80, 928). An appplication was described for the extraction of ergosterol in grains (wheat, rice, barley, corn) (Zhang H et al., J Agric Food Chem 2008, 56, 11077).
Microwave-assisted extraction was compared with several other methods involving automatic instruments for the determination of the oil content in seeds (Matthäus B et al. JAOCS 2001, 78, 95). The results were comparable with those of a standard method.
A technology combining microwave and Soxhlet extraction was designed in 1998 (Garcia-Ayuso LE et al., Anal Chem 1998, 70, 2626). 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).
A procedure of microwave extraction was optimized for the analysis of fatty acids in Chinese medicinal herb Aconitum szechenyianum (Qu WX et al., Phytochem Anal 2011, 22, 199). After being soaked with 90% acetone/water, the blending was treated for 8min at 500W. Three extractions made the fatty acids almost completely extracted. The analytical results showed that abundant unsaturated fatty acids are present at the extract. Compared with Soxhlet extraction, microwaves have the advantages of much higher fatty acid yield and much lower cost.