Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

CHANGES IN NUTRITIONAL QUALITY OF FRUITS AND VEGETABLES 453
oxygen availability, and enzyme-substrate contact, also determine the extent of browning
in fruits.
POX may also contribute to the enzymatic browning in fruits (Nicolas et al., 1994). These
enzymes are involved in oxidation of phenolics in the presence of hydrogen peroxide. POX
can rapidly oxidize 4-methylcatechol in the presence of H 2 O 2 . In pears, PPO can elevate
POX activity by generating H 2 O 2 during oxidation of phenols. POX further oxidizes phenols
by using quinones as a substrate (Richard-Forget and Gauillard, 1997). Thus, POX activity
depends on PPO for its role in enzymatic browning. Zhang et al. (2005) reported that during
storage of litchi, POX activity was increased, which enhanced enzymatic browning in the
fruit pericarp. In addition, anthocyanin concentration decreased with the enhancement of
browning and POX activity in litchi. This indicates that POX plays an important role in
anthocyanin degradation with pericarp browning in litchi pericarp (Zhang et al., 2005).
Postharvest handling and transportation induce wounding, resulting in cell disruption
and loss of compartmentalization. This damage can stimulate the leakage of phenolics from
vacuoles enabling the contact of enzymes with their substrates. Higher POX activity was
found in damaged mangosteen fruits with decreased phenolic content and enhanced lignin
synthesis in the presence of oxygen (Ketsa and Atantee, 1998). However, a significant
increase in the activity of PAL, the first committed enzyme in the biosynthesis of phenolics,
was found during wounding in lettuce that subsequently increased phenolic content (Saltveit,
2000). Higher phenolic contents in lettuce enhanced PPO and POX activity, which induced
browning.
21.5.2 Storage temperature and phenolic compounds
Storage at low temperature is a good method to reduce the activities of PPO and POX
enzymes. However, depending on the commodity and storage temperature, cold storage has
positive and negative effects on phenolic compounds. PPO and POX activities vary from one
fruit to another and even within different cultivars of the same species. It has been reported
that low temperature (5 ◦ C) and normal atmospheric storage of apricots for 10 days decreased
the browning rate and increased the POX (Vámos-Vigyázó et al., 1985). However, there
was an irregular change in PPO activity. This suggests that there are other factors besides
enzyme activity and substrate concentration that influence browning in fruits.
Phenolic contents of fruits are highly influenced by the degree of ripeness, cultivars,
storage conditions, and environmental factors. Dissimilar changes in the pattern of individual
phenolic content have been observed in different fruits. In general, an increase in
anthocyanins has been found during storage of fruits at low temperature. Fruits such as
strawberries (Gil et al., 1997; Sanz et al., 1999), blueberries (Kalt and McDonald, 1996),
and pomegranate (Holcroft et al., 1998) have shown high anthocyanin contents at lowtemperature
storage. Gonçalves et al. (2004) reported that the total phenolic content increased
in sweet cherry during storage at both 1–2 ◦ C and 15 ± 5 ◦ C. However, the levels
of total phenolics were higher in cherries kept at room temperature (15 ± 5 ◦ C) than cold
storage (1–2 ◦ C). They also found a variation in the levels of individual phenolics, but this
variation was lower in cold storage than room temperature (Gonçalves et al., 2004).
In Canada, the marketing season for fruits such as sweet cherry lasts from mid-June to
August, which is rather short. Due to the short marketing season, it is consumed in several
forms including frozen, canned, and juice. Recently, it has been found that anthocyanin