# An Overview of Bioactive Flavonoids from _Citrus_ Fruits
## 4 Flavonoid Composition in Different Parts of _Citrus_ Fruits  
It has been found that _Citrus_ flavonoids are present in almost all the parts of _Citrus_ fruits in different species. Several studies showed that peels are the main sources of polyphenols in _Citrus_ fruits [19]. Seed and peel extracts from numerous _Citrus_ species have proved to be an important source of flavonoids, such as polymethoxyflavones (PMFs), flavanones, and glycosylated flavanones [20]. More than 5000 flavonoid molecules have been identified [21]. _Citrus_ juice is also a major source of flavanone glycosides. Moreover, the contents and types of flavonoids vary among different _Citrus_ species and fruit parts [21]. Flavanones are the most important flavonoids in Citrus species. Hesperedin and naringein are the predominant flavanones in Citrus fruits [16]. They can be found in all plant parts: stem, branches, bark, flowers, leaves, roots, rhizomes, seeds, fruits, peels, etc. Flavones are the second major group of flavonoids in _Citrus_, with flavonols in third place because their content is much lower than the other flavonoids [17; 18]. The content of these components depends upon the age of the plant; most _Citrus_ species accumulate substantial quantities of flavonoids during organ development. Another study showed that flavonoids content might be affected by the maturity of the fruit, the post-harvesting treatments and the extracting processes [22]. Juice preparation and the processing of fresh _Citrus_ fruits may decrease flavonoids content by 50% due to their water washing methods [23; 24], or by elimination of the richest parts of the fruit [25]. In this review, we gather some relevant data for the most abundant flavonoid component in sweet orange (_C. sinensis_), sour orange (_C. aurantium_), mandarin orange (_C. reticulatata_), Clementine (_C. Clementina_), lemon (_C. limon_), and graprefruit (_C. Pardisi_) juices and peel. It should be noted that _Citrus_ peel exhibit higher contents of flavanones than other parts of the fruit [11]. Depending on their varieties, graprefruit and sour orange are very rich in naringin. Other _Citrus_ species such as sweet orange, and lemon have low quantities of naringin (Table 3). [PERSON] et al. [26] studied the composition of flavanone glycosides in various orange juice varieties and concluded that narirutin and hesperidin are the most abundant compounds. Hesperidin is present in high levels in Clementine, sweet orange, mandarin orange and lemon (39.9-20.5 mg/100 mL juice), while naringin (23 mg/100 mL juice) and narirutin (7.6 mg/100 mL juice), glycosides of naringenin, are especially more abundant in graprefruit (Table 3) [27]. In lemon, erioctrin is present in high level (16.17 mg/100 mL juice) [26]. Didymin is a typical flavonoid glycoside also known as neopocirin used in Asian countries as a dietary antioxidant [28]. This flavonoid is mainly present in orange, mandarin and graprefruit (1.89-0.30 mg/100 mL juice) (Table 3). The most abundant flavanones in _Citrus_ peel are naringin andhesperidin. These flavanones are all glycosylated either by rutinose (6-O-a-L-rhamnosyl-D-glucose) or by neohesperidose (2-O-\(\alpha\)-L-rhammosyl-d-glucose) linked in position 7 [28]. Hesperidin (0.002 to 9.42 mg/g peel dry basis) [29; 30] is the main flavanone in all lemon cultivars while levels of diosmin and eriocitrin are the lowest [31]. Mandarin peel is rich in hesperidin (3.95 to 80.90 mg/g peel dry basis) [32; 33], naritutin (7.66 to 15.3 mg/g peel dry basis) [22; 34], and naringin (0.54 to 0.65 mg/g peel dry basis) [32; 33]. Naringin is the most abundant flavonoid in grapefruit and bitter orange peel, conferring a characteristic bitter taste (10.26 to 14.40 mg/g peel dry basis) [29; 35]. _Citrus_ peels contain also polymethoxyyl flavones, such as sinensistein (0.08 to 0.29 mg/g dry basis), nobiletin (0.2 to 14.05 mg/g dry basis), tangeretin (0.16 to 7.99 mg/g dry basis) and heptamethoxyflavone [26; 36; 37; 38].