Effect of Processing and Storage on the Content of Selected Antioxidants and Quality Parameters in Convection and Freeze-dried Bilberry (Vaccinium myrtillus L.)
Abstract
Fresh and dried b y convection and lyophilisation bilberries (Vaccinium myrtillus L.) were examined for content of water, vitamin C, vitamin E, total anthocyanins, total polyphenols and level of antioxidant activity against the DPPH radical. Both products were analyzed immediately after drying and after 4, 8 and 12 months of storage at 2 ± 1 ºC and 20 ± 2 ºC. Fresh fruit had a dry matter content of 14.74%, 100 g of which contained 208 mg vitamin C; 36.4 mg vitamin E; 2064 mg anthocyanins; and 5000 mg polyphenols, with antioxidant activity of 1750 µM Trolox eq/1 g dry matter. After 12 months’ storage, retention rates in products dried by convection and sublimation were respectively: vitamin C 38 –47% and 49 –53%; vitamin E 73 –77% and 76 –82%; anthocyanins 47 –54% and 55 –59%; polyphenols 68 –74% and 72 –74%; and antioxidant activity 63 –70% and 73 –77%, the lower value referring to storage at room temperature and the higher to chilled storage. There were no effect of storage temperature on rehydration of both products. Storage led to a significant increase in the proportion of red and yellow colors in both types of dried product compared to the raw material and product after drying.
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Introduction
Fruits are recognized as an excellent source of antioxidants in the human diet. These substances form a large group, which comprises polyphenols (including anthocyanins), vitamins C and E and β-carotene [1, 2]. Many studies have shown a correlation between the consumption of antioxidants and decreased risk of cardiovascular disease and some cancer types [3, 4]. In view of the seasonal availability of fresh fruits, there is a need to find relatively inexpensive methods of preservation that will result in a product with a similar nutritive value to that of the raw material.
Although dried fruits have long been a part of the human diet, there is little in the literature on the levels of antioxidant compounds they contain, especially in the case of wild fruits Borowska et al. [5]. One such species known for its high levels of antioxidants is bilberry (Vaccinium myrtillus L.) Prior et al. [6] a low-growing shrub found in forest undergrowth in temperate regions whose berries have traditionally been consumed directly as well as processed for later use. Dried bilberries are also used in herbal medicine for their anti-diarrhoeal and anti-inflammatory properties, and their ability to reduce the permeability of blood vessels, improve night vision and increase the adaptive capacity of the eye to darkness [7].
Convection drying (by air circulation) is more widely used in industrial processing than freeze-drying due the high costs of the latter, both in terms of equipment and the process itself. Although convection drying is a cheaper process, the resulting product is less abundant in nutritive compounds and more difficult to rehydrate owing to the higher drying temperature and intensive aeration of the material, among other factors [8]. Apart from the drying method applied, the quality of the dried product may also be affected by the conditions and length of storage, two factors which have received little attention in the literature.
The aim of this paper was therefore to compare convection dried and freeze-dried bilberries in terms of the antioxidant content in each product and the extent to which antioxidant levels are affected by the conditions and length of storage.
Conclusion
Retention rates in dried products stored for 12 months were about 50% for vitamin C and anthocyanins and over 70% for vitamin E, total polyphenols, and antioxidant activity against the DPPH radical. Retention in freeze -dried products was superior to that in convection -dried products for all indicators except total polyphenols in products kept in chilled storage. In addition, retention rates were significantly higher at the lower storage temperature. Average losses of vitamin C, anthocyanins and total polyphenols were higher during drying than over the 12 -month storage period, while for vitamin E and antioxidant activity the losses were lower: slightly in the case of the former, and distinctly so for the latter.
There were no effect of storage temperature on rehydration of both products. There were no significant differences in L* value between convection and freeze -dried products, either immediately after drying or throughout the storage period. 12 -month storage led to a significant increase in the proportion of red and yellow colors in both types of dried product compared to the raw material; however, compared with the product immediately after drying, the differences found were significant in most cases except for the convection dried product kept in chilled storage.
