Phenolic compounds have been divided into three major categories: phenolic acids,
flavonoids, and tannins (Chung et al. 1998). Phenolic acids are derivatives of benzoic or cinnamic
acid (Fig. 2). Flavonoids consist of two units: a C6-C3 fragment from cinnamic and a C6
fragment from malonyl-CoA. The major groups of flavonoids in sorghum are the flavans—
flavan-3-en-3-ols with double bond between C3 and C4 and hydroxylated at C3 are
anthocyanidins. Tannins are polymers of 5–7 flavan-3-ol units (catechin) linked through acid
labile carbon-carbon bonds (Hahn et al. 1984; Mehansho et al. 1987b; Butler 1990).