Understanding heat and mass transfer processes during microwave-assisted and conventional solvent extraction

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Solvent extraction is a mass transfer process. In this paper, we investigate the role of heat transfer in solvent extraction: in particular, how the heat transfer properties of the solid and the heating method (conventional heating and microwave heating) drive this mass transfer process. Water-based solvent extraction of pectin from orange peel, apple pomace, mango peel and carrot pulp was carried out. The thermal conductivity and dielectric loss were shown as good predictors of extraction performance, with step change increases in mass transfer rates when microwave processing was applied to biomass with dielectric loss significantly higher than water (e.g. 120 mins reduced to 45 mins for optimal pectin extraction from apple pomace). When the loss factor was lower there was no difference in extraction performance between the two technologies (e.g. carrot pulp extraction time was 60 mins in both cases). Further investigations were carried out at different heating rates for both conventional and microwave extraction in order to decouple the effects of microwave volumetric and selective heating. It was shown that below a certain power threshold (within the range of 100–120 W in these experiments), microwave and conventional extraction are equivalent, while above the threshold, microwaves achieved a step-change in extraction time. These findings are the first experimental confirmation of recent theoretical advances in microwave biomass processing, in which Temperature-Induced Diffusion drives mass transfer. It is also the first paper to allow identification of biomass characteristics that will be most amenable to microwave extraction.