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Aromatic herbs and spices such as oregano are especially rich in their phenolic content and are good sources of natural antioxidants

Aromatic herbs and spices such as oregano are especially rich in their phenolic content and are good sources of natural antioxidants, antimicrobials and antifungals in human diet (Chun et al., 2005). A tabulated overview of phenolic composition of O. vulgare subspecies from different origins and different extraction methodologies are summarized in Table 3 and Figure 2-3. Phenolics include four basic structures: coumaric acid, caffeic acid, ferulic acid and sinapic acid. In nature, they are usually esterified with other compounds such as chlorogenic acid, which is the link between caffeic acid and quinic acid, and rosmarinic acid wich is a caffeic acid ester. The major phenolic acid that have been identified in O. vulgare species is rosmarinic acid (Gutiérrez-Grijalva et al., 2017; Kikuzaki and Nakatani, 1989; Kolda? et al., 2015; Miron et al., 2011; Radušien? et al., 2008; Vallverdú-Queralt et al., 2014; W’glarz et al., 2006). Two classes of flavonoids are recognized in plants of the Labiatae: free flavonoids and the flavonoid glycosides. Both classes are present in Origanum (Kintzios, 2003). Free flavones, flavonols, flavanones and dihydroflavonols have been identified in many members of the genus Origanum. The major flavonoids of oregano include flavons. In addition, 6-substituted and 6,8-disubstituted flavonoids that are uncommon elsewhere, present in the genus Origanum (Gutiérrez-Grijalva et al., 2017; Skoula et al., 2008; Tomás-Barberán et al., 1988). A number of O-glycosides and C-glycosides have been found in O. vulgare. Luteolin is the most common aglycone followed by apigenin; most sugar moieties are glucosides and glucuronides (Kintzios, 2003). The content and distribution of flavonoid and phenolic acid in O. vulgare can vary depending on the cultivar, geographical and environmental factors. The levels of rosmarinic acid have been reported to vary between the chemotypes within the species of O. vulgare ssp. hirtum and European O. vulgare ranging from 13.73 to 63.69 mg/g on a dry weight basis (Shen et al., 2010); these results showed a broader range of rosmarinic acid in comparison with Austrian O. vulgare ssp. vulgare selected plants of 19 populations (9.4 to 37.2 mg/g dry mass) (Lukas et al., 2013). Phenolic profile in forms of decoction and infusion, that traditionally used for therapeutic applications, and hydroalcoholic extract of O. vulgare were evaluated and compared by Martins and coworkers in 2014. Decoction presented the highest concentration of flavonoids and total phenolic compounds, followed by infusion and hydroalcoholic extract, respectively. Phenolic acids content (found in lower amounts in comparison with flavonoids) did not vary among different samples. Rosmarinic acid was the most abundant phenolic acid in all the preparations, while luteolin 7-O-glucoside (hydroalcoholic acid) and luteolin O-glucuronide (infusion and decoction) were the most abundant flavonoids (Martins et al., 2014). Zhang et al. have isolated six new phenolic compounds (18, 19, 57-60) along with five known ones (3,4,12,15 and maltol 6′-O-(5-O-coumaroyl)-?-D-apiofuranosyl-?-D-glucopyranoside) from the ethanol extract of the whole plants of O.vulgare (Zhang et al., 2014). Liang et al. (2010) also identified a new phenolic glucoside, origanoside, from the ethyl acetate soluble part of the methanol extract of O. vulgare (Liang et al., 2010). Two protocatechuic acid ester derivatives, origanol A and origanol B, had been reported from the methanolic extract of O. vulgare collected from India (Rao et al., 2011). Liu and coworkers identified three novel polyphenolic compounds, origanine A?C, using the hyphenated LC-DAD-SPE-NMR/MS methods (Liu et al., 2011). In 2003, a new dihydrobenzodioxane derivative, origalignanol (23), together with nine polyphenolic compounds, salvianolic acid A (24), salvianolic acid C (25), lithospermic acid (26), apigenin 7-O-D-glucuronide (51), apigenin 7-O-D-(6-methyl)glucuronide, luteolin, (36), luteolin 7-O-D-glucopyranoside (45), luteolin 7-O-D-glucuronide (50), and luteolin 7-O-D-xylopyranoside, were isolated from the aqueous ethanolic extract of O. vulgare