Polyphenols and antioxidant capacity of vegetables under fresh and frozen conditions

The phenolic and oxygen radical absorbance capacity (ORAC) values have been measured in six fresh and frozen vegetables (beet green, spinach, broccoli, carrot, onion, and celery) from the same cultivar by analyzing the whole juice (WJ) and the acetonic extract of the squeezed pulp. To exploit the effect of the acid environment on the stability and recovery of the phenolics, perchloric acid (PCA) was added directly to WJ and to the pulp before the extraction with acetone. In both fresh and frozen vegetables, PCA markedly increased the recovery of phenolics extracted from the pulp, but PCA had no effect on the WJ. Four of six frozen vegetables showed lower phenolic and ORAC values than the fresh vegetables, whereas in the other two cases, values were significantly higher compared to fresh samples. Among the fresh vegetables, beet green showed the highest ORAC and phenolic values; however, when measured in two different cultivars of beet green, the ORAC value showed as much as 4.5-fold variation, whereas total phenolics and flavonoids showed 1.2- and 3.5-fold variations, respectively. The results show that total phenolics and ORAC, compared in fresh and frozen vegetables, represent an index of the mildness of blanching in the industry of frozen vegetables and provide a measure of the gap in antioxidants in the diet of people who consume frozen instead of fresh vegetables. The plant genotype is an important source of variability in the ORAC value, which can be conveniently used to increase the intake of antioxidants from vegetables.