Effect of Superatmospheric Oxygen on Anthocyanins, Phenolics and Antioxidant Activity of Blueberries and Strawberries

The effects of superatmospheric oxygen levels on anthocyanins, phenolics, antioxidant activity and fruit quality of blueberries (Vaccinium corymbosum L.) and strawberries (Fragaria × ananassa Duch.) during storage at 5°C were investigated. High oxygen treatment increased the levels of total anthocyanins, total phenolics as well as several individual flavonoid compounds during the first 21 days of storage for blueberries and first 7 days of storage for strawberries. Antioxidant activity, measured as oxygen radical absorbance capacity or ORAC, also increased in these fruits during the same period after exposure of the fruit to superatmospheric oxygen concentrations. However, these effects diminished with prolonged storage duration. No significant differences in total anthocyanins, total phenolics, ORAC values or individual flavonoid compounds were observed between high oxygen and air-treated fruit during the later part of storage. Little differences were also found in titratable acidity, total soluble solids or surface color among the fruits treated with various concentrations of oxygen throughout the storage period. These results suggest that high oxygen treatments may improve the antioxidant activities of blueberry and strawberry fruits during the initial stage of storage. INTRODUCTION Antioxidants are compounds that are capable of performing a number of functions including acting as free radical scavengers, peroxide decomposers, singlet and triplet oxygen quenchers, enzyme inhibitors, and synergists (Larson, 1988). Antioxidants can also delay or inhibit the oxidation of lipids or other molecules by inhibiting the initiation or propagation of oxidizing chain reaction (Velioglu et al., 1998). Fruits contain a great variety of phytonutrients and are good sources of antioxidants (Block et al., 1992; Ness and Powles, 1977). The phytonutrients in fruits responsible for antioxidant activity can largely be attributed to phenolic compounds such as anthocyanins, carotenoids, and other flavonoid compounds. These compounds may act independently or in combination as anticancer or cardio-protective agents through various mechanisms. Berry fruits have high content of anthocyanins and phenolics. These compounds protect plants against damaging photodynamic reactions by quenching the excited state of active oxygen species (Larson, 1988; Lewis, 1993). Anthocyanins have received attention as important dietary constituents that provide health benefits and contribute antioxidant capacity beyond that provided by essential micronutrients such as ascorbate and tocopherols. The increased interest in healthy diet and the development of functional foods are stimulating the need for more information concerning the bioavailability and efficacy of phenolics and anthocyanins. Changes in the content of phenolics and anthocyanins and the antioxidant activity in fruits during postharvest period have received much attention during recent years. Proc. IX Intl. Contr. Atmos. Res. Conf. Ed.: R.M. Beaudry Acta Hort. 857, ISHS 2010 476 Increasing the carbon dioxide concentration in the storage atmosphere inhibits the postharvest increase of anthocyanin in strawberries (Gil et al., 1997; Holcroft and Kader, 1999). The total antioxidant activity increased overall by about 45% in cranberry fruits stored in air, but this increase was prevented by storage in 30% CO2 plus 21% O2 (Gunes et al., 2002). Mareczek et al. (2000) reported that the level of total phenolics in the peel of ‘Golden Reinders’ and ‘Gala Must’ apples were similar in fruit stored for 7 months at 0C in air and in CA (3% O2 +5% CO2), but phenolics were reduced after an additional 8 days at 16°C in the peel of ‘Gala Must’ fruit previously stored in air. On the other hand, Mazza and Miniati (1993) observed that anthocyanins in apples were relatively stable when stored at 2°C in air, but decreased in low O2 and high CO2. High O2 treatment has recently been demonstrated as an alternative to traditional low O2 and high CO2 controlled atmosphere technique to maintain quality and safety of fresh produce (Day, 1996). The elevated O2 atmospheres have been proven to be particularly effective in inhibiting microbial growth and enzymatic discoloration and preventing anaerobic fermentation reactions (Amanatidou et al., 2003; Day, 1996; Jacxsens et al., 2003; Wszelaki and Mitcham, 2000). However, little information is available concerning the effect of superatmospheric oxygen on antioxidant system in fruits and vegetables during storage. This study was undertaken to determine the changes of phenolics, anthocyanins and antioxidant capacity in blueberries and strawberries in response to high oxygen exposures. MATERIALS AND METHODS Chemicals Kaempferol, (R)-phycoerythrin (R-PE) and quercetin were purchased from Sigma Chemical Co. (St. Louis, MO). 2’, 2’-Azobis (2-amidinopropane) dihydrochloride (AAPH) was obtained from Wako Chemicals USA Inc. (Richmond VA). Trolox (6hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) was purchased from Aldrich (Milwaukee, WI). Acetonitrile, methanol, acetone, and water were of HPLC grade and were purchased from Baxter (Muskegon, MI). All anthocyanins and their aglycons were obtained from Indofine Chemical Co., Inc. (Somerville, NJ). Other authentic standards were obtained from Sigma and Fisher Scientific (Pittsburgh, PA). Plant Materials and Treatments Blueberries (Vaccinium corymbosum L. cv. Duke) and strawberries (Fragaria × ananassa Duch. cv. Allstar) were hand-harvested at commercially mature stage from Butler’s orchard, MD and sorted to eliminate damaged and unripe fruit and selected for uniform size and color. Three kilograms of samples were placed in each 18 L jar and three jars were used for each treatment. The jars were placed at 5°C and connected to continuous flow (120 ml/min) of humidified air (control), 40, 60, 80 or 100% O2 (balanced with N2 in all high O2 treatments). The gases were checked regularly with an O2/CO2 analyzer (AMETEK, Pittsburgh, PA) and maintained at ±2% for the duration of the experiment. Samples were taken initially and during storage for decay evaluation and other analysis. Fruit Decay Fruit decay was visually evaluated during the course of the experiment. Any berries with visible mold growth were considered decayed. Fruit decay was expressed as percentage of fruit showing decay symptoms. Sample Preparation for Various Assays Ten berries from each replicate were cut into small slices and mixed. Batches of 5g samples were stored at -80°C until analyzed. To prepare the fruit extracts, three 5-g samples of berries from each replicate were extracted twice with 15 ml of 80% acetone containing 0.2% formic acid using a Polytron (Brinkmann Instruments, Inc., Westbury,