Although the earlier studies focused on the antioxidant

Although the earlier studies focused on the antioxidant capacity of tocopherols, ascorbic Amiloride HCl dihydrate and carotenoids, then it was noticed that phenolic compounds were more powerful than others (Rice-Evans, Miller, & Paganga, 1996). Phenolic compounds, the most numerous and ubiquitous groups of plant secondary metabolites, could be subdivided into 6 general groups: flavonoids, phenolic acids, phenolic alcohols, stilbenes, lignans, and tannins (Table 1). It was demonstrated that they had possible antioxidant properties because of their aromatic ring, which could stabilize and delocalize an unpaired electron in its structure (Rice-Evans, Miller, & Paganga, 1997). It was noticed that despite the same basic aromatic structure of phenolic compounds, bearing one or more hydroxyl group substituents bring about diversity in their structure and antioxidant activity (Table 1) (Bravo, 1998). It was revealed that as the largest group of phenolic compounds, flavonoids had side group variations in their B and C rings that resulted in the diversity of the flavonoid classes such as flavones, isoflavones, flavonols, flavanols and flavanones. Most common flavonoid members and their structures are demonstrated in Table 1. It has been conducted several studies to elucidate the relationship between structure of phenolic compounds and their antioxidant activity, and reported that the number of hydroxyl groups and the position of substituents determined their antioxidant activity (Balasundram et al., 2006, Heim et al., 2002, Rice-Evans et al., 1996, van Acker et al., 1996). The other group of phenolic compounds was described as phenolic acids consisting of a benzene ring bonded to a carboxylic group (benzoic acid) or to a propenoic acid (cinnamic acid) and were divided into two main subgroups according to their structure: hydroxycinnamic and hydroxybenzoic acid (Table 1). The smallest group of phenolic compounds, stilbenes, were also come to fore with their antioxidant activity even in very low concentration. More specifically, it was demonstrated that resveratrol, one of the most common stilbenes in nature, were able to prevent oxidative stress in human body and cardiovascular diseases (Frombaum, Le Clanche, Bonnefont-Rousselot, & Borderie, 2012). Moreover, the French paradox was also associated with beneficial effects of stilbenes (especially resveratrol) in wine; however, it was emphasized that it was not possible to absorb recommended therapeutic doses by only drinking wine. Recently, the history of resveratrol, some examples of proposed function and pharmacological aspects have been reviewed comprehensively by Weiskirchen and Weiskirchen (2016). As high molecular weight polyphenols, lignans and tannins, were complex phenolic compounds, their structures could not be completely elucidated yet. However it was known that they were highly effective antioxidants with hydroxyl groups in their structures. With these developments, beverages (e.g. wine, tea, fruit juices) (Gil et al., 2000, Lakenbrink et al., 2000), fruits (e.g. berries, apple, pineapple, grapes) (de Souza et al., 2014, Robards et al., 1999, Seeram et al., 2006), vegetables (e.g. celery, broccoli, beans, artichoke, spinach) (Tomás-Barberán & Espín, 2001), spices and herbs (Zheng & Wang, 2001) have been demonstrated for the screen of their phenolic contents for many years (Table 2). Tea (black, green, oolong), as a widely consumed beverage, and its extracts containing dominantly catechins have been attracted much attention due to its high antioxidant capacity (Perumalla and Hettiarachchy, 2011, Wang and Helliwell, 2001). In another study, it was shown that grapes, wines and grape byproducts contained large amount of phenolic compounds and they were able to inhibit the lipid peroxidation (Kanner, Frankel, Granit, German, & Kinsella, 1994). Sánchez-Moreno, Larrauri, and Saura-Calixto (1999) also claimed that antioxidant activity of grape juices, wines and related polyphenolic compounds were based on their free radical scavenging capacity. Different types of berries, as antioxidant rich fruits, were investigated to screen their profiles and selected flavonoids (kaempferol, quercetin, myricetin) with phenolic acids (p-coumaric, caffeic, ferulic, p-hydroxybenzoic, gallic and ellagic acids) were detected in these berries (Häkkinen et al., 1999, Häkkinen and Törrönen, 2000). Deng et al. (2013) investigated the antioxidant potential of 56 vegetables and quantified the main phenolic compounds and contents of those vegetables. Lipophilic and hydrophilic extracts of each vegetables showed different amount of antioxidant compounds and as major phenolic compounds chlorogenic and gallic acid were widely detected in these vegetable extracts. In the literature, it was easily realized that most studies have focused on the determination of bioactive compounds of fruit and vegetable extracts and measuring their extractable lipophilic or hydrophilic antioxidant capacity. Foods containing relatively low amounts of soluble antioxidant compounds, such as cereal grains, were underappreciated for many years and there was little information about their insoluble-bound phenolic constituents (Krygier et al., 1982, Maillard and Berset, 1995). Afterwards, the studies showed that most of phenolic acids that are the major antioxidant compounds in cereals grains found in the insoluble bound forms (Acosta-Estrada, Gutiérrez-Uribe, & Serna-Saldívar, 2014). After noticing that cereal grains contain unique phytochemicals exist in free, soluble conjugate, but most are in insoluble bound forms, they have come into prominence as natural antioxidant sources (Adom & Liu, 2002). Researchers reported that cereal grains or their products showed antioxidant activity as much as well-known antioxidant foodstuffs such as red wine, tomatoes or peaches (Esposito et al., 2005, Miller et al., 2000). Wheat, as one of the major cereal grains in human diet, and its different fractions were characterized according to their phytochemical profiles, total phenolic and carotenoid contents, or total antioxidant activities of different wheat varieties, which ultimately influence their nutritional and health benefits (Adom et al., 2003, Esposito et al., 2005).