Combined effect of heat treatment, UV-C and superatmospheric oxygen packing on phenolics and browning related enzymes of fresh-cut pomegranate arils
Introduction
Ready to eat or fresh-cut fruit and vegetables have become very popular due to their convenience and health benefits. They are particularly interesting for fruits like pomegranate due to special difficulties in peeling. To preserve whole fresh and fresh-cut plant-based commodities, and considering that consumers are progressively concerned about the use of synthetic additives, there are some alternative methods. Heat treatment, modified atmosphere packaging and UV-C light which have been proven to prevent fresh-cut deterioration (Bruhn, 2000). Shelf life of fresh-cut arils like other fresh-cut fruit and vegetables is short because they are very susceptible to shriveling, microbial growth, loss of nutritional value and, mainly, to enzymatic browning (Gil, Martínez, & Artés, 1996; López-Rubira, Conesa, Allende, & Artés, 2005; Maghoumi et al., 2012). Enzymatic browning is caused by the oxidation of phenolic compounds by the polyphenol oxidase (PPO; EC 1.14.18.1) enzyme so adversely affecting shelf life of fresh-cut arils (Gil et al., 1996).
Mild heat treatment, in particular hot water, has the potential to reduce microorganisms and to inactivate enzymes (Mirdehghan et al., 2006), but it is not always compatible with fresh-cut plant foods since it may reduce consumer acceptability due to undesirable changes of flavor, texture, color and nutritional quality (Orsat, Gariepy, Raghavan, & Lyew, 2001). At the same time, the effectiveness of high O2 (≥70 kPa) atmospheres in preventing enzymatic browning, inhibiting microbial growth and decay incidence on different fresh-cut commodities has been proved (Day, 1996). Finally UV-C radiation offers several advantages since it does not leave any residue, acts both directly by damaging the microorganisms and indirectly by stimulating defense mechanisms in the product (Allende & Artés, 2003; Chisari, Barbagallo, Spagna, & Artés, 2011; Erkan, Wang, & Wang, 2008; Stevens, Khan, & Lu, 1999).
Pomegranates are very rich in phenolic compounds, such as hydrolyzable tannins (punicalagins and punicalins) condensed tannins (proanthocyanidins), anthocyanins (water-soluble polyphenolic pigments which are the primary source of the red color), catechins and phenolic acids (gallic, ellagic and chlorogenic, among others) (Gil, Tomás-Barberán, Hess-Pierce, Holcroft, & Kader, 2000). Phenolics serves as substrate for the oxidizing enzymes, thus they can be affected by an increase in those enzymes activity.
Heat shock, UV-C and high oxygen may induce reactive O2 species, followed by the production of oxygen radical scavengers such as superoxide dismutases (SOD; EC 1.15.1.1), peroxidases (POD; EC 1.11.1.7) and catalases (CAT; EC 1.11.1.6), as a defense system against oxidative stress (Moller, 2001).
At the present time, there is not information available in the literature related to fresh-cut arils on the effect of these physical treatments on phenolic compounds, anthocyanins and the antioxidant enzymes.
The main objective of this study was to investigate the effects of hot water, high oxygen and UV-C treatments applied alone or in combination on the activity of the antioxidant enzymes CAT, SOD and POD, as well as PPO, of fresh-cut arils throughout 14 days of storage at 5 °C.
Section snippets
Plant material
Sweet ‘Mollar de Elche’ pomegranate fruits, the most widely produced Spanish commercial cv., were open air cultivated under the Mediterranean climate of Elche (Alicante, Spain) and supplied by Cambayas Soc. Coop. The same morning of harvesting, fruits were selected in a packinghouse from uniform size (9 ± 0.9 cm diameter) and external color, free from defects, decay, mechanical damages and sunburn. Sound pomegranates were then transported about 60 km to the Pilot Plant in the Universidad
Superoxide dismutase activity
Immediately after the treatments, and compared to control (31 ± 0.7 U mg−1 protein) HW increased the initial SOD activity (39 ± 1.4 U mg−1 protein) while UV-C decreased it (26 ± 1.6 U mg−1 protein) (Table 1). The initial drop in SOD could be due to the UVC-induced high initial concentration of reactive oxygen species, acting SOD as an oxygen radical scavenger. During storage all treatments reduced SOD activity, mainly HW. The highest levels (19–21 U mg−1 protein) were observed for the UVC
Conclusions
The antioxidant activity of pomegranate is well-known (Wang et al., 2004). In this research anthocyanins and phenolic acids, either alone or in combination, are shown as important components responsible of that antioxidant activity.
After 14 days at 5 °C, and compared to control, UVC + HO treated arils showed higher anthocyanin, phenolic compounds, SOD and CAT activities, and lower PPO and POD activities. That combined treatment induced better quality and longer shelf life for fresh-cut arils
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