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Antioxidant components and color characteristics of blueberries dried by different methods Vaghri, Ziba


Blueberries have recently become the subject of focus for some scientists owing to their high antioxidant activity and potential health benefits beyond nutrition. The components likely to be responsible for this activity are the phenolic compounds and vitamin C. Dehydration can be successfully applied to this fruit to make it available all year round. While freeze-drying and air-drying are two conventional methods used for dehydration, vacuum microwave dehydration is a relatively new method of food dehydration, which has the potential of producing high quality novel dehydrated products. This study was designed to investigate the effects of vacuum microwave, air, freeze drying and a combination of air/vacuum microwave drying on vitamin C, anthocyanin, phenolic content and antioxidant activity of two blueberry cultivars, Bluecrop and Hardy Blue. In addition the color characteristic of the berries were assessed. Vacuum microwave dried blueberries retained higher amounts of vitamin C than the air and air/vacuum microwave dried ones. The vitamin C content of air-dried and combination treatments were too small to be detected by the method employed in this study. Total anthocyanin level of the vacuum microwave dried sample was higher (p< 0.05) than those of the air-dried and combination treatments in one of the two cultivars, and was comparable to the level of anthocyanin in the frozen berries. Vacuum microwave and freeze-dried berries had the highest concentration of phenolic compounds (p≤ 0.05). All blueberry extracts exhibited antioxidant activity, which was positively correlated with total anthocyanin, total phenolic (p< 0.05) and vitamin C content (p≥ 0.058). There was no significant difference between the antioxidant activities of various dehydrated blueberries. In Hardy Blue berries, all dehydration methods caused loss of antioxidant activity. In Bluecrop berries, some dehydration methods such as vacuum microwave drying, air/VMD, and freeze-drying were able to preserve the antioxidant activity to the level which was comparable to that of the frozen berries of that cultivar. In terms of color characteristics the puffed structure of vacuum microwave and freeze-dried berries likely influenced the Hunter lab values. In these treatments despite higher anthocyanin content, due to the distribution of the pigment over a bigger surface, they had higher Hunter L (indicating lightness), and lower Hunter a (indicating redness) and Hunter b (indicating blue color) values than the air-dried samples. No clear trends were observed between Hunter a, b, and L values, and the extent of heat and oxygen exposure. Therefore, vacuum microwave drying showed a better efficiency in retaining some bioactive components, when compared to air-drying. Vacuum microwave drying shortens the drying time considerably, and by doing so, it has a definite advantage over methods such as air-drying which has a prolonged drying time.

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