Phenolics, flavonoids, proanthocyanidin and antioxidant activity of brown rice with different pericarp colors following storage

Two rice cultivars with different pericarp colors were selected for investigating the effect of storage on the concentration of rice phenolic compounds and antioxidant activity (AOA). The concentration of total phenolic compounds in the free fraction of the dark brown rice (DBR) was 355.4mg GAE per 100g rice, which was 9.3 times higher than light brown rice (LBR), and a similar trend was also found in AOA between the two cultivars (32.4 versus 4.0mmolTE per g rice). The concentration of phenolic compounds present in the free fraction was higher than that in the bound fraction in DBR, whereas the proportion of phenolic compounds between the free and bound fractions was the reciprocal in LBR, indicating that the concentration and distribution of phenolic compounds in rice are phenotype-specific parameters. Phenolic compounds present in the free fraction showed significantly higher AOA compared to the phenolic compounds in the bound fraction based on the same mass, which implies that the structure and profile of phenolic compounds between the two fractions may differ. The ageing process induced changes in the amount of phenolics, flavonoids and proanthocyanidin extracted from both cultivars, with storage at 37°C leading to a 15-20% reduction in the concentration of these compounds present in free and bound fractions. This suggests that phenolic compounds in the two fractions are both involved in the chemical reactions that occurred during the rice ageing process. There was also a significant reduction in AOA (P<0.001) when rice was stored at 37°C for 6 months. The plausible mechanism for the reduction in AOA might be due to the decomposition of the antioxidant compounds during rice ageing.