Abstract

Sorghum bicolor is cultivated worldwide as a staple food and forage, and is receiving a renewed interest as a bioenergy crop.  The objective of this study was to understand the relationships between the function (redox and metal complexation) and structural properties (aromaticity and ionizable functionalities) of (poly) phenolic pigments in the pericarp of sorghum grain which will ultimately control its antioxidant and bird/mold resistance behaviors. As compared to white seeds lacking condensed tannins, brown seeds (with and without pigmented testa) contained (i) 0.08-0.01 wt% cyanidin-equivalent condensed tannins, (ii) higher aromaticity (which decreases the reduction potential of polyphenols), and (iii) as much as 6-fold greater Fe^III reduction capacity.  The degree of aromaticity was determined by (i) UV absorbance at 360 nm and (ii) fluorescence excitation-emission (EEM) peak position. Basic (0.1 M NaOH) extracts of all seeds contained EEM peaks (230/330 and 280/330) attributable to protein. Addition of Fe^III resulted in a new aromatic EEM peak (320/440) only for the brown seeds which could be used as a fingerprint for the redox and coordination chemistry of sorghum grain pigments.

Key words: Proanthocyanidin, cereal, sweet sorghum, colorimetric method, transition metal.