**1. Introduction**

The cork oak (*Quercus suber* L.) forest is a unique production system developed by men that has a substantial ecological role, for example, against desertification and in maintaining animal and plant biodiversity in their restricted area of occurrence in the Western Mediterranean countries, covering a total of 2.1 million hectares. Portugal has 34% of the world's area, which corresponds to about 720,000 hectares and 22% of the national forest [1]. Cork is obtained from the bark of the cork oak, and the production is made in a sustainable process carried out during the tree's lifetime. It is one of the world's important nontimber forest products, supporting a dedicated industry of cork products directed toward global markets.

World cork production reached 201,000 tons annually. Portugal is the leader, concentrating 49.6% of the cork production, acting also as an importer of cork raw material, therefore increasing its share on the world production of cork products. In 2017, Portugal exported 197,000 tons of cork products representing 986.3 million euros [2]. Cork stoppers lead the exports, accounting for 72% of the total value (710.7 million euros). Within the cork stopper segment, natural corks come first with 60% of the total (428.6 million euros), followed by other types of stoppers with 21%, including technical stoppers, microstoppers, and others (149.8 million euros), and champagne stoppers with 19% [2].

Cork is a world-renowned material used for sealing wine bottles. Cork is a cellular material with chemical inertia and a set of specific physical and mechanical properties that provide an outstanding performance when in-bottle wine aging is wanted, by combining the required minute oxygen transfer with mechanical sealing of the bottle, durability, and chemical stability [3, 4]. Cork is the closure material preferred by wine consumers, and a bottle corked with a natural cork stopper is indicative of high-quality or very high-quality wine, as shown by recent surveys (Wine Opinions, CTR, Iniciativa Cork, Gfk, Opinion Way).
