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Over 4000 chemically unique flavonoids have been identified in plant sources. These low-molecular-weight substances, found in all vascular plants, are phenyl- benzopyrones (phenylchromones) with an assortment of basic structures. Primarily recognized as the pigments responsible for the autumnal burst of hues and the many shades of yellow, orange and red in flowers and food (see Chapter 13), the flavonoids are found in fruits, vegetables, nuts, seeds, stems, flowers as well as tea and wine and are important constituents of the human diet.
Middleton et al. (2017) studied this question.