All plant phenolic compounds
arise from a common intermediate, phenylalanine, or a close
precursor, shikimic acid. Primarily they occur in conjugated
forms, with one or more sugar residues linked to hydroxyl
groups, although direct linkages of the sugar (polysaccharide or
monosaccharide) to an aromatic carbon also exist. Association
with other compounds, like carboxylic and organic acids,
amines, lipids and linkage with other phenol is also common.6
Polyphenols may be classified into different groups as a function
of the number of phenol rings that they contain and on the basis
of structural elements that bind these rings to one another. The
main classes include phenolic acids, flavonoids, stilbenes and
lignans.