Continuous microwave processing of olive-pomace for active compounds recovery

Abstract

Microwave assisted extraction of natural products has been studied over the past decades as an alternative process for high-value bioactive compounds. The main advantages claimed for this technique are high extraction rates, high extraction yields and low degradation. Despite the high number of publications on this topic, most of them are considered only for analytical purposes, and only a few ones are studied for industrial implementation. Olive-oil industry produces a by-product that corresponds to the residue of the olive-pulp after the first press, known as olive-pomace. This is a low-value residue, mainly to produce olive-pomace oil, but the price paid for this by-product barely covers the cost of transport. However, olive-pomace has a high content of active compounds (hydroxytyrosol, oleuropein…) that make it valuable for the extraction of bioactives. Most of the recovery methods reported in literature make use of high temperature conditions, which can degrade some thermolabile compounds, or use non-authorized solvents for foodstuffs purposes (benzene, hexane…). In this work a microwave assisted extraction pretreatment has been developed to enhance the extraction recovery of bioactive compounds from this residue, while reducing the residence time for extraction. Operating conditions are initially studied in a laboratory microwave oven. The analysed operating variables were material/solvent ratio and specific energy. Microwave pretreatment is followed by a fast conventional solid-liquid extraction process. Extraction kinetics are determined not only for polyphenol content or specific compounds, but also for the total solid extract residue. This way, in addition to recovery, also the richness of the final dry product can be optimized to obtain simultaneous maximum extraction yield and final product concentration. Extraction richness (mgGAE/gDry Extract) may be improved more than 40% by this technique, compared to conventional extraction processes.