**3. Flavonoids in foods**

Flavonoids are synthesized in all parts of the plant. They provide colour, fragrance and taste to the fruits, flowers and seeds, which make them attractants for insects, birds and mammals, which aid in pollen or seed transmission [2]. Flavonoid distribution in plants is affected by different factors, including variation and exposure degree of light. The production of high oxi‐ dized flavonoids is accelerated by light [21]. Major classes of flavonoids based on abundance in food are flavonols, flavones, isoflavones, flavanones, flavandiols, anthocyanins, proantho‐ cyanidins and cathechins. Other classes of flavonoids include flavan‐3‐ols, anthocyanidins, chalcones, and other biosynthetic intermediates of the flavonoid biosynthesis are aurones, bioflavonoids and dihydrochalcones [22].

The content of flavonoids in food varies, which depends on the source, vegetable, fruits or seeds, as well as processed food. The content of some important flavonoids in foods such as cabbage, spinach, carrots, peas, mushrooms, peaches, strawberries, orange juice, white wine or brewed coffee is low or <10 mg/kg or L. In some other vegetables and fruits, such as lettuce, beans, red pepper, tomato, grapes, cherries, red wine and tea, their content is < 50 mg/kg or L. On the other hand, their content in broccoli, kale, French beans, celery and cranberries is high, over 50 mg/kg or L. Apple contains flavonoids such as quercetin (184 mg) and epicatechin (180 mg) in approximate 120 g of flesh with 80 g of skin [24]. Citrus fruit such as oranges and grapefruits contain flavonoids such as nar‐ ingin and hesperidin. Naringin is mainly present in grapefruits and sour oranges, while hesperidin is present in sweat oranges, mandarins and lemons. In addition, 500 ml of orange juice contains 292 mg of hesperidin [25, 26]. Berry fruits (strawberries, blueberries and cranberries) contain high levels of anthocyanins in their skin and flesh; strawberries contain 33.63 mg per 100 g, whereas blueberries contain 13.52 mg [27]. Onions contain up to 22 mg of quercetin per 100 g [28]. **Table 2** shows various classes of flavonoids and some of their food sources.


**Table 2.** Food sources of flavonoids.

**2.3. Flavanones**

356 Flavonoids - From Biosynthesis to Human Health

**2.4. Flavanols**

**2.5. Anthocyanidines**

**2.6. Isoflavones**

coumestane, pterocarpan) [20].

**3. Flavonoids in foods**

bioflavonoids and dihydrochalcones [22].

The basic structural model of flavanones has 2‐phenylbenzopiran‐4‐one‐skeleton. These com‐ pounds are of great interest due to the fact that they play an important role in the metabolic pathway of the other flavonoids. Their metabolic precursors are chalcones, flavones, dihydro‐ flavonols; isoflavones are biosynthesized from the flavanones [18]. Their chemical structures have a hydroxyl group in 3 position of C ring [19]. Physically, some flavanones are colourless (Hesperidin), pale (naringenin, eriodictyol) and others yellow (neohesperidin) [15, 16].

Flavanols, well known as flavan‐3‐ols or catechin, have a hydroxyl group in C3 of C ring, with no carbonyl group. These flavonoids are colourless (catechins, gallocatechin, epicatechin, epi‐ gallocatechin, gallate) and some of them show yellow colouration (procyanidin, theaflavines).

This is the other subgroup, which structurally has a hydroxyl group in 3 position and also a double bond between carbons 3 and 4 of C ring. These molecules are water soluble, and are

This is a distinctive subclass of flavonoids. They structurally have a 3‐phenylchromone skel‐ eton with same three phenolic rings referred to as A, B and C rings. But also, in some of their derivative compounds, they can form an additional heterocyclic ring (D ring) (rotenoid,

Flavonoids are synthesized in all parts of the plant. They provide colour, fragrance and taste to the fruits, flowers and seeds, which make them attractants for insects, birds and mammals, which aid in pollen or seed transmission [2]. Flavonoid distribution in plants is affected by different factors, including variation and exposure degree of light. The production of high oxi‐ dized flavonoids is accelerated by light [21]. Major classes of flavonoids based on abundance in food are flavonols, flavones, isoflavones, flavanones, flavandiols, anthocyanins, proantho‐ cyanidins and cathechins. Other classes of flavonoids include flavan‐3‐ols, anthocyanidins, chalcones, and other biosynthetic intermediates of the flavonoid biosynthesis are aurones,

The content of flavonoids in food varies, which depends on the source, vegetable, fruits or seeds, as well as processed food. The content of some important flavonoids in foods such as cabbage, spinach, carrots, peas, mushrooms, peaches, strawberries, orange juice, white wine or brewed coffee is low or <10 mg/kg or L. In some other vegetables and

the pigments responsible for the red to purple color in plants.

### **4. Antioxidant activity**

The best described property of almost all groups of flavonoids is their powerful antioxidants, which act as free radical scavengers as they are potential reducing agents and protect from oxidative reactions taking place inside the body [22]. The antioxidant activity of flavonoids is due a combination of their iron chelating and free radical scavenger properties, also the inhi‐ bition of oxidases enzymes such as lipoxygenase, cyclooxygenase, myeloperoxidase, NADPH oxidase and xanthine oxidase; avoiding thereby the formation of reactive oxygen species and organic hydroperoxides [19]. Moreover, flavonoids can inhibit enzymes by indirectly involv‐ ing in the oxidative process, such as phospholipase A2, at the same flavonoids can stimulate other enzymes with antioxidant activities like catalyse and superoxide dismutase.

This feature depends of their molecular structure. This free radical scavenging activity is pri‐ marily attributed to the high reactivity of hydroxyl substituents. For example, by scavenging free radicals, the hydroxyl group on the B‐ring donates hydrogen and an electron to hydroxyl, peroxyl and peroxynitrite free radicals, stabilizing them, because of the high reactivity of the hydroxyl group of the flavonoids, radicals are made inactive according to Eq. (1), where R\* is a free radical and O\* is the oxygen free radical) [17, 29–31].

$$\text{F-OH} + \text{R} \bullet \rightarrow \text{ F-O} \bullet \text{+RH} \tag{1}$$

The significance of other hydroxyl configuration is less clear, but beyond increasing total number of hydroxyl groups, A‐ring substitution correlates little with antioxidant activity. Similarly, the difference in antioxidant activity between polyhydroxylated and polymethox‐ ylated flavonoids is most likely due to differences in both hydrophobicity and molecular pla‐ narity [30].

A lot of studies have realized the antioxidant activity of flavonoids. Majewska et al. [41] evalu‐ ated different flavonoids and compared their antioxidant activity with DPPH radical. The flavonoids such as quercetin, rhamnetin, isorhamnetin, luteolin and apigenin were tested and the results showed that quercetin has the highest antioxidant potential at lower concentration (0.1–5 µg/sample). In other studies, Choi et al. [42] tested the inhibitory activity of xanthine oxidase, linoleic acid peroxidation and scavenging capacity of DPPH with some flavonoids, and obtained the following results at a concentration of 100 µg/ml: for xanthine oxidase, cate‐ chin, morin, naringenin and quercetin showed 100% of inhibition, and rutin only 43%; for lin‐ oleic acid peroxidation, quercetin showed the highest inhibition (82%), followed by catechin (71%), rutin (63%), morin (56%), naringenin (53%); and finally for DPPH catechin, morin and quercetin obtained a high activity (100%) than rutin (95%), naringenin did not show any data.

On the other hand, using an aqueous emulsion of linoleic acid/β‐carotene at 50°C, and com‐ pared with synthetic antioxidant BHT, ᴅ,ʟ‐α‐tocopherol and its acetate, Burda and Oleszk [43] examined the antioxidant activity of some flavonoids and observed the highest values were obtained by synthetic ones (88–95%) and slightly lower, but still high antioxidant activity was shown by a group that included fisetin (61%), kaempferol (65%), galangin (64%), quercetin (63%), robinetin (61%), morin (63%) and kaempferide (60%), all of these are flavonols with a free hydroxyl group at the C‐3 position.
