## 4 Bioactive Compounds on Cork and Cork By-Products
In fact, many epidemiological studies point to the health benefits of fruit and vegetable intake, owing to the presence of many antioxidant phytochemicals [71]. They represent the second most abundant group of organic compounds in the plant kingdom produced as a response to the influence of biotic and abiotic factors and whose main functions include support, hormonal regulation, seed germination, protection against pathogens, with herbivores and UV radiation also being involved in flavour, smell and colour [72]. These compounds have at least one hydroxyl group attached to the aromatic ring with great structural diversity ranging from simple to complex structures obtained through the shikimate pathway [73]. Phenolics, also known as phenylpropanoids, can be classified into essentially two large groups named as flavonoids and nonflavonoids. Chemically, flavonoids have a fifteen-carbon backbone (C6-C3-C6) with two phenyl rings, A and B, attached through a three-carbon chain that normally arises as a heterocyclic pyran ring [74]. On the other hand, nonflavonoids encompass phenolic compounds, usually with a relatively simpler structure than flavonoids, such as phenolic acids, coumarins, stilbenes, hydrolysate, and condensed tannins and lignnas [75]. The antioxidant, anti-inflammatory, antimicrobial, and anticancer properties of phenolic compounds make them very attractive for pharmaceutical, cosmetic, or food applications [76]. Hence, the valuation of this raw material and its by-products increasingly involves the identification of their phenolic composition [27]. Cork phenolics are obtained by polar solvent extraction, and even though its composition is variable within trees and geographic location, it essentially includes phenolic acids and aldehydes, coumarins, flavonoids, and tannins [31]. Methanol/water mixtures are the most frequently employed methods to extract cork phenolics, often followed by an organic solvent [32; 76]. The cork extracts can also be prepared through sequential extraction with increasing polarity solvents [77]. In 2015, [PERSON] and co-workers reported a microwave-assisted extraction method, using different proportions of water, methanol, and ethanol, demonstrating that the use of these alcohols promotes a significant improvement of polyphenol recovery. Among them, p-coumaric (**7**), syringic (**8**), and sinaptic (**9**) acids are the major compounds on bark extracts (Figure 3) [78]. In the same year, a new method for the extraction of phenolic compounds from cork granulates using a mixture of water with propylene glycol was reported [76; 79]. Hydrolysable tannins and low molecular weight phenolic compounds are suggested as the potential bioactive compounds from _Quercus super_ bark [27]. The most common phenolic acids in cork are ellagic (**10**), protocatechuric (**11**), gallic (**12**), vanillic (**13**), ferulic (**14**), and caffeic (**15**) acids, whereas phenolic aldehydes include vanillin (**16**), protocatechuric aldehyde (**17**), and coniferyl aldehyde (**18**) (Figure 3) [80; 81]. Tannins can be monomeric or polymeric and even condensed or hydrolysable, which are related to bitterness and astringency of wines as a result of binding to salivary proteins [82]. Several studies indicate that these phenolics, namely castalagin (**19**), grandinin (**20**), vescalagin (**21**), and roburin (**22**) are capable of migrating from cork stoppers to wine solutions after bottling and may interfere with its organoleptic properties, such as taste, colour, or bitterness or participate on wine oxidation (Figure 4) [83; 84; 85]. The first reported HPLC analysis of cork extract prepared with a methanol/water mixture showed that the most abundant phenolic compounds are the phenolic acids, ellagic (**10**), and protocatechuric (**11**) [32]. Aldehydes, such as protocatechuric aldehyde  
Figure 4: Chemical structures of major ellagitannins from _Quercus super_ bark. Figure 3: Chemical structures of bioactive phenolic and aldehyde acids from _Quercus super_ bark.