Polyphenol composition and antioxidant and metal chelating activities of the solid residues from the essential oil industry

Essential oils of aromatic plants have numerous applications in fields like medicine, cosmetic and food industry. Nevertheless, the average yield of essential oil distilled from aromatic plants is below 5% (w/w) and consequently a considerable amount of solid residues is generated. These residues are especially rich in polyphenols that can be exploited as health-promoting compounds in food and feed or as anti-aging ingredients in cosmetic products, and thus to enhance the overall profitability of the aromatic plants. In this work, we have tentatively identified the polyphenols present in solid residues from the steam distillation of Cistus ladanifer, Lavandula×intermedia, Santolina rosmarinifolia and Thymus mastichina, and evaluated their antioxidant and chelating activities by means of several in vitro methods, such as the β-carotene/linoleate model system, reducing power, DPPH radical scavenging, and iron and copper ion chelation. A Soxhlet extraction and an ultrasound-assisted extraction have been used in terms of comparison. Most of the extracts whether they were obtained by Soxhlet system or by ultrasonic method showed good antioxidant activities. Soxhlet ethanolic extract of T. mastichina showed an antioxidant activity comparable to the standard red grape pomace. The influence of the total polyphenol content on the chelating activities was not as important as on the antioxidant activities and the predominant role of certain polyphenols and/or the presence in the extracts of other compounds with a favourable chemical structure to metal complexation can be assumed. This was the case of the ultrasound ethanolic extract of C. ladanifer, which was the most effective in iron chelating activity despite its medium-to-low content of total polyphenols. All these data point to profit from these aromatic plant solid residues as a valuable source of bioactive compounds.