Hydroalcoholic extracts from the bark of Quercus suber L. (Cork): optimization of extraction conditions, chemical composition and antioxidant potential

Cork is the bark of the tree Quercus suber L. which finds use in diverse applications. However, a significant percentage is still rejected and burned for energy production, despite containing valuable molecules for materials processing and with important biological activities. Herein, the optimization of the extraction process to obtain these molecules, using mild solvents and conditions, is described within a biorefinery perspective. The extracts were obtained by direct contact solvent extractions with water, ethanol and its mixtures for different time and temperatures, and evaluated for chemical composition, total phenolic content (TPC) and antioxidant properties [by DPPH radical scavenging, ferric reducing antioxidant power (FRAP), Trolox equivalent antioxidant capacity (TEAC) and oxygen radical absorbance capacity (ORAC) assays]. The results showed that the extraction process is accelerated and higher yields are achieved with the increase in temperature without chemical degradation or compromising the antioxidant capacity. For all solvents, at reflux temperature, more than 90% of the extractable material is obtained within 6 h (80% within 1 h). The highest TPC and antioxidant capacity are observed for the extracts obtained with mixtures of water and ethanol of similar volumes. The antioxidant capacity measured by DPPH, FRAP and TEAC assays was found to be proportional to the extract TPC, while ORAC is favored for higher percentages of ethanol on the extracting solvent. The main constituents of these extracts are the ellagitannins, vescalagin, castalagin and β-O-ethylvescalagin, along with other phenolic acids (mainly ellagic and gallic acids) and various flavonols. The extracts stability was monitored up to 1 year of storage with neither reduction in the antioxidant capacity nor chemical degradation. These results show that extracts with strong antioxidant potential and high content of bioactive molecules can be obtained from the processing of waste streams. Cork is a sustainable forest product and the development of new fields of application contributes toward a zero waste cycle for a complete material biorefinery.