The main quality defect in fresh-cut potato is enzymatic browning that develops on the cut surfaces of the tissue. Peeling and slicing of tubers cause cellular disruption leading to decompartmentalization of substrates and enzymes (Brecht et al., 2004). This disruption liberates PPO enzyme from mitochondria, allowing it to contact phenolic substrates in the vacuole and oxidize them to quinones, which then polymerize to dark pigments (Friedman, 1997; Martinez and Whitaker, 1995). ‘Russet Burbank’, which is the major commercial potato cultivar in the United States, is very susceptible to enzymatic browning compared with other cultivars (Coetzer et al., 2001) as a result of its high phenolic content and PPO activity (Sapers et al., 1989).
Therefore, in contrast to products with low phenolic content for which inactivation of phenylalanine ammonia-lyase (PAL) enzyme is the most effective way to minimize enzymatic browning (Saltveit, 2000), for products that are rich in phenols such as fresh-cut potatoes, it is necessary to inhibit PPO to prevent the oxidation of pre-existing phenols and their subsequent transformation into melanins. Various treatments have been applied to fresh-cut potato for reducing browning such as the use of antibrowning compounds like sulfites, L-cysteine, ascorbic acid, and/or citric acid (Rocculi et al., 2007; Sapers and Miller, 1995), storage under controlled atmosphere conditions (Angós et al., 2008), modified atmosphere or vacuum packaging (Beltrán et al., 2005), or combinations of these (Limbo and Piergiovanni, 2006; Ma et al., 2010). Until now, sulfites have proven to be consistently the most effective browning inhibitors, but their use for this purpose is controversial as a result of the risk of adverse health effects. It is commonly accepted that alternative technologies for the prevention of enzymatic browning need to be developed that will be effective and safe (Coetzer et al., 2001).
Heat treatments have been shown to prevent the wound-induced synthesis of phenols by inhibiting PAL activity and, thus, reducing browning development in fresh-cut vegetables such as celery (Viña and Chaves, 2008) and lettuce (Loaiza-Velarde and Saltveit, 2001). It is questionable, however, whether heat treatments can reduce enzymatic browning on tissues such as potato, in which the phenol content is already high before wounding. Sapers and Miller (1995) inhibited discoloration of peeled potatoes during storage for 14 d at 4 °C by using a double treatment, ascorbic/citric acid solutions plus heat followed by dipping in a solution containing ascorbic and citric acids plus sodium acid pyrophosphate. Furthermore, the use of a heated onion extract, applied either by spraying or immersion of slices, exhibited a marked inhibitory effect on potato PPO and, indeed, this inhibitory effect was dependent on the heating temperature (Lee et al., 2002). However, the individual role of heat treatment application has not been studied yet on fresh-cut potato and particularly the effect it has on enzymatic browning during storage as well as on phenolic accumulation and oxidation by PPO.
According to Martinez and Whitaker (1995), heat inactivation of PPO is feasible by applying temperatures of greater than 50 °C, but no further details are cited. According to Koukounaras et al. (2008), compared with an unheated control, dipping peaches in hot water at 50 °C for 10 min before slicing had no effect on PPO activity during storage of fresh-cut peaches in modified atmosphere packaging (MAP) with 2% to 2.5% CO2 at 5 °C for 6 d. Inhibition of PPO was achieved by heating crude enzyme extracts from apples using high temperatures (greater than 68 °C) (Yemenicioğlu et al., 1997) or by blanching potatoes in boiling saline solutions (Severini et al., 2003). However, kinetic characteristics of the enzyme in whole apples heated at the tested conditions might differ from those in vitro and additionally blanching temperatures cannot be applied in minimal processing of horticultural products. Indeed, immersion of potatoes at water bath temperatures of 60 °C or higher resulted in tuber surface blackening and rapid decay (Ranganna et al., 1998). In studies aiming to prevent sprouting of potatoes during storage, the latter authors demonstrated that the tubers can tolerate HW treatments at 57.5 °C for 20 to 30 min and be safely stored for 12 weeks at either 8 or 18 °C without suffering from heat damage, whereas Kyriacou et al. (2008) reported that tuber tolerance at the same temperature is limited to 20 min. It would be interesting to investigate whether less severe heat treatments could reduce the accumulation of phenolics in stored fresh-cut potato slices as well as their oxidation resulting from the activity of PPO.
It has also been shown that the duration between heat treatment and processing also has a significant effect on enzymatic browning development during storage of fresh-cut products. Particularly, the beneficial effect of heat treatment of peach (Koukounaras et al., 2008) and lettuce (Loaiza-Velarde and Saltveit, 2001) on browning development on the fresh-cut products was further enhanced when the treatment was applied 4 or 6 h, respectively, before processing, indicating that a pre-processing storage interval can significantly affect the success of heat treatment.
The objective of this study was primarily to evaluate the efficacy of heat treatment in preventing enzymatic browning on fresh-cut potatoes during storage and additionally to examine whether storage delay between heat treatment and processing affects browning development on peeled slices.
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