Vacuum Microwave Hydrodistillation VMHD

VMHD was elaborated and patented by Archimex in 1994 [40]. This technique is based on selective heating by microwaves combined with sequential application of a vacuum. The plant material is placed in a microwave cavity with water to refresh the dry material. The plant material is then exposed to microwave radiation to release the natural extract. Reducing the pressure to between 100 and 200 mbar enables the evaporation of the azeotropic water-volatile oil mixture from the biologi- 

According to the patents, VMHD provides yields comparable to those obtained by traditional hydrodistillation but with extraction times only one tenth those required with hydrodistillation. The thermally sensitive crude notes seem to be preserved with VHMD, in contrast to conventional hydrodistillation. VMHD is suggested as an economical and eflRcient technique to extract high-quality natural products on a large scale [52-54]. 

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In the last decade there has been an increasing demand for new extraction techniques, amenable to automation, with shortened extraction times and reduced organic solvent consumption, to prevent pollution and reduce the cost of sample preparation. Driven by these goals, advances in microwave extraction have resulted several techniques such as microwave-assisted solvent extraction (MASE) [32, 36-39], vacuum microwave hydrodistillation (VMHD) [40, 41], microwave hydrodistillation (MWHD) [42, 43], compressed air microwave distillation (CAMD) [44], microwave headspace (MHS) [5], and solvent-free microwave hydrodistillation (SEME) [45, 46]. Table 22.3 summarizes the most common microwave extraction techniques for plant matrices and lists their advantages and drawbacks. Over the years procedures based on microwave extraction have replaced some of the conventional processes and other thermal extraction techniques that have been used for decades in chemical laboratories. 

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