**1. Introduction**

Flavonoids are secondary metabolites of plants; this group of phenolic compounds includes approximately 4500 compounds [1]. They are classified into different subgroups (**Figure 1**). Their nuclear structure includes carbons C6-C3-C6, and the diversity of flavonoids depends on the position of the aromatic ring [2].

Flavonoids and flavones are the most common in plants, whereas flavanones, flavanols, dihydroflavones, and dihydrochalcones have a limited distribution. Flavonoids are present in plant tissues in relatively high concentrations in free forms (aglycones) or conjugated with sugar molecules (glycosides) [5].

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**Figure 1.** Structure of the different types of flavonoids (modified from Refs. [3, 4]).

Flavonoids are polyphenolic compounds found in all vascular and non-vascular plants [6]. They are important in the diet of humans because they possess a high nutritional value; besides, they are used as effective medicines in the treatments of certain diseases, and therefore, they are called nutraceuticals. Nowadays, a diet rich in fruits and vegetables is recommended to contribute to the prevention and treatment of cardiovascular diseases, diabetes, cancer, chronic inflammatory disorders, and degenerative diseases [7].

Some examples of nutraceutical flavonoids are the following: isoflavones (genistein, daidzein) obtained from celery, soybeans, and other legumes. Isoflavones have antitumor, anticancer, and antioxidant action, and they improve immune response, lower cardiovascular risk, and menopausal symptoms. Quercetin is contained in onion, citrus fruits, broccoli, red grapes, apple, and cherries. Kaempferol is present in broccoli and radishes [8]. Anthocyanins are present in red wine and fruits. These flavonoids are important due to their antioxidant, anticancer, and antithrombotic properties, as well as their ability to lower blood cholesterol [9].

Their concentration can vary among species: in fruits, vegetables, and medicinal plants, ranges from 0 to 6125.6 mg kg−1 have been reported. For example, 1720.5 mg kg−1 has been observed in spinach, 3575.4 mg kg−1 in strawberry, and 459.9 mg kg−1 in apple [10].
