Original Research ArticleCold storage of blueberry (Vaccinium spp.) fruits and juice: Anthocyanin stability and antioxidant activity
Introduction
The blueberry (Vaccinium spp.) is a fruit native to North America and Europe, where it is widely cultivated and commercialized. It is known for its elevated antioxidant capacity against free radicals and reactive species, being considered as one of the best sources of antioxidants in the diet (Prior et al., 1998, Vrhovsek et al., 2012). This activity is related to the presence of bioactive compounds such as flavonoids (especially anthocyanins), tannins and phenolic acids, as well as to various beneficial health properties attributed to blueberry (Heinonen et al., 1998, Smith et al., 2000, Seeram, 2008). Anthocyanins are natural pigments present in fruits and vegetables and widely distributed in nature. They are water-soluble, belong to the flavonoids group, and contain the flavylium cation or 2-phenylbenzopirilium as part of their basic structure (Bridle and Timberlake, 1997). They can be found in the form of glucosides, hydrolyzed in sugars, and as aglycones, known as anthocyanidins (Schwartz et al., 2008).
The anthocyanins are differentiated from one another by the number and position of hydroxyl and/or methyl groups and by the nature, number, position and acylation of the sugars present in their structure. Due to this enormous variety, there are reports of more than 500 anthocyanins and 23 anthocyanidins, of which only 6 are frequently found distributed in nature and in foods: cyanidin (50%), delphinidin, pelargonidin and peonidin (12%) and petunidin and malvidin (7%) (Castañeda-Ovando et al., 2009).
However, the anthocyanins are extremely unstable and easily degraded in the isolated form (Giusti and Wrolstad, 2003), thus restricting their use as food dyes. An increase in hydroxylation increases their instability, whereas an increase in glucosylation confers greater stability (Schwartz et al., 2008). The anthocyanins are normally stable at pH values between 1 and 4. At pH 1, the predominant structure corresponds to the flavylium cation, conferring red and purple colors, whereas at values between pH 2 and 4, blue quinoid bases predominate. They are generally degraded at pH values above 7 (Castañeda-Ovando et al., 2009). In addition to pH and type of chemical structure, the following factors also influence their stability: temperature, light, oxygen, ascorbic acid, sulfur dioxide, the presence of enzymes (peroxidase, polyphenoloxidase and glucosidase), metal ions, proteins and other flavonoids (Bridle and Timberlake, 1997), as also food processing and storage (Schwartz et al., 2008).
Freezing is characterized by a reduction in temperature of the food to below its freezing point, with the formation of ice crystals, thus increasing the preservation time by reducing the water activity. Cooling corresponds to a reduction in temperature of a food to between −1 °C and 8 °C, so as to decrease enzymatic and microbial activities, conferring a longer shelf life on the product (Fellows, 2000).
Many studies have been carried out on the behavior of the anthocyanins and of the antioxidant capacity of blueberries during cold storage (Kalt et al., 2000, Connor et al., 2002, Srivastava et al., 2007, Piljac-Žegarac et al., 2009, Piljac-Žegarac and Šamec, 2011), but there are still some doubts about a suitable storage time and temperature for such products in relation to their bioactive compounds.
Thus the objective of the present study was to analyze the stability of the antioxidant activity and anthocyanins in whole blueberry juice stored under refrigeration at 4 °C for 10 d, and in fruits stored frozen at −18 °C for a total period of 6 months, so as to verify possible changes occurring during domestic storage and thus avoid significant losses of the bioactive compounds and properties of these products.
Section snippets
Samples
Samples of fresh blueberries (Vaccinium spp.) when fully mature were pre-selected for color and size uniformity, and berries with visible injuries and infections were discarded. The fruits were acquired in March 2010 on the retail market in the city of Porto Alegre in the state of Rio Grande do Sul (RS), Brazil, commercialized by the Berrygood Company (São Paulo, SP, Brazil). The samples were washed with water and 1% sodium hypochlorite, then divided into portions of 250 g, stored frozen at −18
Effect on antioxidant activity
Blueberries show high antioxidant activity, correlated especially with their anthocyanin and other phenolic compounds content, and may be considered as one of the highest antioxidant sources among fruits and vegetables (Prior et al., 1998, Vrhovsek et al., 2012). The mean values found in the in natura fruits for antioxidant activity were 29.20 μM TE/g fruit by the ABTS method, and 3322.98 g fruit/g of DPPH by the DPPH method (Table 1), values higher than or close to those found in most fruits
Conclusions
Both the blueberry fruits and whole juice presented good stability with respect to their antioxidant capacity during cold storage, after remaining frozen for a long period and under refrigeration for shorter periods, with little loss of their antioxidant properties. On the other hand, there were significant losses of anthocyanins, possibly due to oxidation and/or condensation reactions with other phenolic compounds. This difference allowed for the inference that the other phenolic compounds, or
Conflict of interest
The authors declare that there is no conflict of interest.
Acknowledgements
The authors are grateful to the CNPq (National Scientific and Technological Development Council) and Capes (Coordinator for Upgrading Graduate-level Personnel) for the financial support.
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