Effects of drying temperature and long-term storage conditions on black rice phenolic compounds

This study aimed to investigate the influence of drying temperatures (20, 40, 60, 80, and 100 degrees C) followed by 12 months-storage under normal-atmosphere (conventional), nitrogen-atmosphere, and vacuum-atmosphere on black rice phenolics. Increase in drying temperature reduced the total content of free-phenolics, free-flavonoids, and anthocyanins. An increase in bound phenolics extractability was observed at 60 and 80 degrees C, suggesting some extent of phenolics polymerization and complexation at these drying temperatures. The free fraction of ferulic, caffeic, p-coumaric, and gallic acids were the most thermally unstable compounds. After storage, no reductions were observed only for free caffeic and protocatechuic acids. For total flavonoids, an increase in the free fraction and reduction of the bound fraction were observed in the grains stored under conventional-and vacuum-atmosphere. A reduction of total phenolics occurred regardless of the storage condition. However, storage under nitrogen-atmosphere is the most recommendable condition for maintenance of phenolics in black rice.