Traditional or hydro-diffusion and gravity microwave coupled with ultrasound as green technologies for the valorization of pomegranate external peels

To exploit the full potential of pomegranate marcs, this study explores different conditions of ‘green’ extraction of ellagitannins and other bio-active phenolic compounds from the ‘external by-product’ of pomegranate industrial processing. Significant recoveries of ellagitannins, free ellagic acid and radical scavenging compounds were obtained by coupling both the traditional (MH) or the hydro-diffusion gravity (MHG) microwave assisted drying to a direct ultrasound assisted extraction (UAE) using a food-grade solvent (EtOH/H2O). The amounts of the ellagitannins after microwave drying were always significantly higher than with the traditional oven drying. Moreover, microwaves reduced the drying times from several hours to few minutes reaching comparable residual moisture content. The microwave hydrodiffusion gravity system allowed the recovery of bio-active compounds in the condensed water too. The comparison of UAE with traditional maceration and decoction highlighted the higher efficiency of UAE in the quicker extraction of bio-active compounds, except for the total ellagitannins which were better recovered after the longer maceration.