During the last decades the scientific and industrial interest on natural occuring compounds such as flavonoids increased. As a result, research has identified more than 7.000 different flavonoid compounds, which are separated in various classes (Harborne & Williams, 2000, Williams & Grayer, 2004).

Flavonoids are widely spread natural compounds found in different plant tissues as flowers, roots, leaves and stems. They constitute the largest group of secondary metabolites involved in various functions in plants, e.g tissue colouration, pathogen defense, attraction of pollinators, signal compound in plant-microbe interactions, protecting against UV-radiation and aiding seed dispersal. Additionally, they are important part of the human and animal diet and of biomedical interest since they inhibit cell proliferation, are antioxidants and display for example antimutagenic, antinflammatory, antithrombic and antihypertensive effects.

Remarkable bioactivities have been ascribed to several flavonoids. They represent an active principle of various medicinal plants and have been associated with potential beneficial health effects. These include the prevention of cardiovascular diseases, cancer and inflammation leading to an increasing interest in these compounds, e.g. as potential constituents of functional food. But their medicinal value, their impact and targets on human nutrition and health has still to be further investigated and there is a need to know more about their structure, metabolism and bioavailibility.

Flavonoids are synthesized from precursors of the phenylpropanoid and acetate-pathways. One of the first flavonoid of the main pathway, the flavanone (2S) naringenin (NAR), is formed from 4-coumaroyl-CoA and three units of malonyl-CoA by the consecutive activities of chalcone synthase (CHS) and chalcone isomerase (CHI). Three branch point enzymes convert (2S) NAR to flavones, dihydroflavonols and anthocyanidins (Fig. 1) : flavone synthase I/II (FNS I/II) catalyzes the desaturation to apigenin (Ap), while flavanone 3ß-hydroxylase (FHT) leads to dihydrokaempferol (DHK) which can be reduced subsequently by dihydroflavonol 4-reductase (DFR) to a leucoanthocyanidin on the route to catechins and anthocyanidins. Further two enzymes, flavonol synthase (FLS) and anthocyanidin synthase (ANS), may oxidize dihydroflavonols to flavonols or leucoanthocyanidin to anthocyanidin.