Luke Howard
Blueberries and blackberries are a rich source of phenolic compounds that are reported to have numerous health benefits. The levels of phenolics in the berries are influenced by many factors including genetics, fruit size, maturation, harvest date and growing season. In our laboratory we have determined how some of these factors influence the phenolic content and antioxidant capacity of blueberries and blackberries. In blueberries, variation in oxygen radical absorbing capacity (ORAC), total phenolics, total anthocyanins, hydroxycinnamates, flavonols and fruit weight among genotypes was much greater than that observed between growing season, indicating that genetics plays a more important role than growing season in influencing ORAC and phenolic content. However, our results indicate that environmental growing conditions can impact levels of phenolics and ORAC in blueberries and that certain genotypes vary in their capacity to synthesize phenolics under different growing conditions. Berry size can also impact phenolic content and ORAC, with smaller berries generally having higher ORAC values, anthocyanins, and total phenolics than large berries. The concentrations of total phenolics, polyphenols and ORAC changed markedly in Ozarkblue and Bluecrop cultivars during fruit maturation. Levels of ORAC and total phenolics generally decreased from green to 50-75% blue stages of maturity and then increased with further ripening. Total anthocyanin content increased from green to the 100% blue stage of maturity, with the largest increase occurring from 75% to 100% blue stages of maturity, Green fruit contained much higher levels of flavonols and hydroxycinnamates than more mature fruit. Across all maturity stages ORAC values correlated significantly with total phenolics and hydroxycinnamates (R2 = 0.93 and 0.77, respectively), but not with total anthocyanins or flavonols. The effect of harvest date on phenolic content and ORAC was found to be genotype dependent and a minor factor compared with genetic differences.
In blackberries, variation in total anthocyanins and total flavonols among genotypes was much greater than that observed between growing season, while variation in total ellagitannins and ORAC was greater between growing seasons than that observed between genotypes. Significant effects observed for genotype and growing season interactions for all variables indicate that some genotypes varied in their capacity to synthesize phenolics under different growing conditions. The concentrations of total phenolics, polyphenols and ORAC changed markedly in Apache and Chickasaw cultivars during fruit maturation. Levels of ellagitannins decreased, while anthocyanins increased as fruit ripened. Flavonols were highest in immature fruit and generally declined with ripening. Total phenolics showed a similar trend as ellagitannins, but not with anthocyanins and flavonols. Both photochemiluminescence antioxidant capacity (PCL) and ORAC values decreased substantially from the green to mottled stage, but PCL values increased with further ripening in both cultivars, while ORAC values increased in overripe fruit of Apache, but not in Chickasaw. Across all maturity stages ORAC values correlated significantly with ellagitannins and total phenolics (R2= 0.90 and 0.83, respectively), but not with anthocyanins or flavonols. PCL values correlated significantly with ellagitannins, flavonols, and total phenolics (R2=0.66, 0.76 and 0.59), but not with anthocyanins.
Our results indicate that plant breeders can select for increased levels of total phenolics to increase antioxidant capacity in blueberry and blackberry genotypes, but the genotypes should be screened over multiple growing seasons in order to account for variation in environmental growing conditions.
Keywords: Blueberry, blackberry, polyphenols, antioxidant capacity, flavonols, hydroxycinnamates, ellagitannins, maturation, growing season