**3. Flavonoids**

The flavonoids constitute the largest group of polyphenols (of low molecular weight) and are considered to be responsible for the color and taste of many fruits and vegetables [65, 66]. Since first identified in the mid-1800s, more than 9000 flavonoid structures have been described, with formulas, references and biological information [67, 68] and the list is constantly increase. These include over 600 different naturally occurring anthocyanins [69] that are widely distributed among at least 27 families, 73 genera and innumerable species.

Many of these compounds are yellow in color, as the Latin root suggests. They are present particularly in the epidermis of leaves and the skin of fruits [9, 70]. Flavonoids can accumulate in vacuolar compartments, or be secreted, for example, as part of root exudates. Most attracting is the accumulation of flavonoids on the surface of leaves and flowers [70]. Flavonoids play roles in many facets of plant physiology [71] and their influence on the transport of the plant hormone auxin is one of their most important roles.

Around 5000 of the flavonoids studied have antioxidant activity. Because the number of phytochemicals already identified is only a small part of those that exist in nature, there is  considerable interest in new methods [37, 72–78] of separation, isolation and characterization of polyphenol structures from foods.

are increasingly recognized due their nutritional value, since they may help reduce the risk of chronic diseases [50–53]. The capacity of phenolic compounds to trap free radicals depends upon their structure, in particular of the hydrogen atoms of the aromatic group that can be transferred to the free radicals [54, 55] and of the capacity of the aromatic compound to cope with the uncoupling of electrons as a result of the surrounding displacement of the electron-π

Although the percentage of absorbed natural polyphenols is usually quite low [56], researchers have seen a large quantity of metabolites of polyphenols in the form of simple phenolic acids in the blood. The amount and form in which plant phenolic substances are administered influence greatly the physiological effects connected with their consumption [57]. About 1 g of polyphenols per day is commonly ingested with foods, being the most abundant antioxidant in the diet (about 10 times higher than the intake of vitamin C and 100 times that of vitamin E) [28, 29, 58]. The daily intake of polyphenols is difficult to estimate and depend on several factors. In the literature there are about 1000 peer-reviewed publications [28] concerning the polyphenol content in food. Recently, the construction and application of a database with polyphenols content in foods has facilitated this task [59]. The Institute Nationale de la Recherche Agronomique [60] has developed a new Phenol-Explorer database [59] covering over 60,000 foods useful to epidemiologists, food scientists and food manufacturers. The content of polyphenols of the 100 richest dietary sources can range from 15,000 mg per 100 g in cloves to 10 mg per 100 mL in rose wine [61]. EuroFir [62] is another database to build national food composition in different countries within the framework of the European Food Information Resources Network (i.e., Spanish

The flavonoids constitute the largest group of polyphenols (of low molecular weight) and are considered to be responsible for the color and taste of many fruits and vegetables [65, 66]. Since first identified in the mid-1800s, more than 9000 flavonoid structures have been described, with formulas, references and biological information [67, 68] and the list is constantly increase. These include over 600 different naturally occurring anthocyanins [69] that

Many of these compounds are yellow in color, as the Latin root suggests. They are present particularly in the epidermis of leaves and the skin of fruits [9, 70]. Flavonoids can accumulate in vacuolar compartments, or be secreted, for example, as part of root exudates. Most attracting is the accumulation of flavonoids on the surface of leaves and flowers [70]. Flavonoids play roles in many facets of plant physiology [71] and their influence on the transport of the

Around 5000 of the flavonoids studied have antioxidant activity. Because the number of phytochemicals already identified is only a small part of those that exist in nature, there is

are widely distributed among at least 27 families, 73 genera and innumerable species.

plant hormone auxin is one of their most important roles.

system [27]. The polyphenols are still gaining attention.

120 Phenolic Compounds - Natural Sources, Importance and Applications

[63] and Irish [64] databases).

**3. Flavonoids**

Concerning the chemical structures of flavonoids in which two aromatic rings are present that linked by three carbons in an oxygenated heterocycle, i.e., a flavan (2-phenyl-benzo-γ pyran) nucleus consisting of two benzene rings combined with an oxygen-containing pyran ring, the parent compound bearing a tricyclic (C6-C3-C6) skeleton. The heterocyclic benzopyran ring is known as the C ring, the fused aromatic ring as the A ring and the phenyl constituent as the B ring. The structural differences in each flavonoid family result from variations in the number/substitution pattern of the hydroxyl and methoxy groups [28, 79, 80], as well as different glycosylation patterns and the presence of a C2-C3 double bond in the heterocycle pyran ring.

Compounds are classified according to differences in their heterocycle (C ring): flavonols, flavones (catechins), flavanones, chalcones, dihydrochalcones and dihidroflavonols, anthocyanins and isoflavonoids (isoflavones) [29, 55], varying in the oxidation state (degree of saturation) of the heterocyclic central pyran ring. When unsaturation is present, the geometry of the molecule is planar, as in the case of anthocyanins, flavones and flavonols.

Flavonoids usually occur as glycosides in plants, reflecting a biological strategy increasing their polarity and necessary for storage in the plant cell vacuoles and decreasing their reactivity to interact with macromolecules [81–83]. While flavan-3-ols (catechins and theaflavins) are present in either a free form or as gallic acid esters (e.g., in tea). The glycosidic linkages appear to be important for the absorption of flavonoids [44].

Williams and Grayer [84] have stated: "Flavonoids continue to capture the interest of scientists from many different disciplines because of their structural diversity, biological and ecological significance (e.g. the colored pigments in many flower petals) and health promoting and anticancer properties." Recent advances in genomics, proteomics and metabolomics provide new approaches in the field of flavonoids in plant: protection against oxidative diseases, ability to modulate the activity of various enzymes and interactions with specific receptors are among the most significant health benefits [67, 85].

Flavonoids of dietary significance are present in edible plants in widely varying combinations [86, 87]. Unlike traditional vitamins, flavonoids are not essential for short-term wellbeing. The daily intake is almost at the same level as the sum of other antioxidants, including carotene, vitamin C and vitamin E, can range from several hundred mg up to 1–2 g [88] and although flavonoids are not essential for short-term may exhibit potential health benefits at modest long-term intake [85, 89, 90]. **Table 1** [91–98] shows the dietary sources of flavonoids. Great differences in flavonoid intake and food sources were observed between a large Mediterranean cohort and non-Mediterranean populations (U.S. and Finland as non-Mediterranean countries) [99]. The mean intake for a Spanish population was 313 mg/day [99]. Estimated per capita daily flavonoid intake is 182 and 177 mg for the UK and Ireland, respectively [100].


**Table 1.** Dietary sources of flavonoids [28, 91–98].
