## 2 Quercus suber and the Cork Industry  
Quercus L is a genus that belongs to the Fagaceae family and includes several species of trees, around 450 different species scattered throughout the world [11; 12]. This genus can be divided into two subgenera: one composed of species that live in temperate regions generally in the Northern Hemisphere, named _Quercus_, and another group that includes trees typical from subtropical regions in Asia, mostly in the east and southeast, called _Cyclobalanopsis_[13]. Quercus suber, the cork oak, is a slow-growing evergreen tree that can live up to 200 years or more and is native to Mediterranean countries, such as European-like Portugal, Spain, France, Italy and North-African-like Tunisia, Algeria, and Morocco [14; 15]. _Q. suber_ has a unique bark, thick and porous and with fissures, that protects the tree cells from the aggressions of the outside environment, such as forest fires [16]. This bark, known as cork, is exploited by humans without endangering the tree vitality since it has the ability to regenerate as it is being harvested through the years [17]. Additionally, cork oak has the capacity to prevent soil erosion and desertification, to regulate the hydrological cycle, and to reduce CO\({}_{2}\) emissions, protecting the biodiversity around it. For those reasons, this tree is part of an agroforestry system called \"montado\" in Portugal and \"dehesa\" in Spain that brings together forests, livestock, and agriculture [17; 18; 19; 20]. Cork oak was highly used for its wood to build ships or manufacture tools, although nowadays, the primary use of this tree is the extraction of cork, the common name given to the cork oak outer bark that has outstanding properties, making it an important raw material for numerous applications [12; 16]. Cork is a light material, is impermeable and compressible, is a good acoustic insulator, has low thermal conductivity, and has a high capacity to absorb energy and to resist impact and friction [17]. Thus, the main applications of cork are insulation, flooring, cork agglomerates and natural cork stoppers [21; 22]. As several studies have shown, cork is also a source of bioactive compounds [13; 23; 24; 25; 26; 27; 28; 29]. Cork extracts revealed the presence of substances with biological activity that could be incorporated in pharmaceutical and cosmetic formulations [4; 13; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Therefore, we can consider that cork is a renewable and sustainable resource, currently gaining more interest as a raw material [7; 9]. Nowadays, the cork industry is important in the economy of countries, such as Portugal and Spain. Portugal detains about 55% of the world production of cork [22]. The whole process begins with the first cork harvest. When the tree reaches between 25 and 30 years, the process of extraction of cork starts [12; 16; 17; 22]. The first harvested cork is called \"virgin cork\", has poor quality, and cracks easily; thus, it is not used in most applications [16; 37]. Then it takes 9 to 12 years to extract subsequent layers of cork from the same tree; thus, the bark obtains the adequate thickness [16; 37]. The second extracted cork is called \"first reproduction cork\" and has better quality; however, it is used essentially to produce cork agglomerates [16; 37]. Only in the third extraction, the \"second reproduction cork\", cork is used for its major purposes, such as to produce cork wine stoppers [16; 22; 37]. To avoid any damage to the tree, cork stripping is only done manually when the tree has perfect physiological conditions to remove the cork, being a seasonal activity in the transition from spring to summer [17]. On the other hand, as can be seen, this process is fully sustainable since the tree remains intact, and throughout each extraction it produces more and more cork [19]. Subsequently, the cork industry produces a lot of waste to manufacture the final products.