Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

CHANGES IN NUTRITIONAL QUALITY OF FRUITS AND VEGETABLES 457
Antioxidant enzymes remove ROS before their participation in oxidation reactions,
which subsequently prevent initiation of oxidation. In the plant cells, the first line of defense
enzyme is SOD converting the superoxide radical (O −.
2 )toH 2O 2 . Since excessive
amount of H 2 O 2 is toxic to the cells, it is further decomposed by CAT and POX, which convert
H 2 O 2 to water. By using two molecules of ascorbate, APX reduces H 2 O 2 , and generates
monodehydroascorbate (MDHA) and dehydroascorbate (DHA), oxidized forms of ascorbate
(Larrigaudière et al., 2004). However, during postharvest handling and storage, fruits
sustain stress, such as senescence and superficial scald, which alter the levels of antioxidant
constituents. In general, the levels of antioxidants increase during moderate stress conditions,
while, with the advancement in stress, a decline in antioxidant constituents has been
observed, which results in the development of postharvest disorders (Barden and Bramlage,
1994; Purvis et al., 1995; Shewfelt and Purvis, 1995). Various postharvest factors,
such as duration and temperature of storage, can influence stress in fruits and vegetables
and eventually alter the levels of antioxidants.
21.5.6 Harvest maturity and antioxidants
Harvest maturity is also a critical factor for antioxidant constituents. An enhancement in
the lipid-soluble antioxidants was found in apples during storage, which was related to
their levels at harvest (Barden and Bramlage, 1994). In addition, scald development during
storage was negatively correlated with the concentration of antioxidants at the time of
harvest. Tomato fruits ripened off the vine showed higher levels of antioxidants (lycopene,
β-carotene, phenolic, and ascorbic acid) during storage than vine-ripened fruits (Giovanelli
et al., 1999).
Early-harvested “Braeburn” apples showed higher SOD activity, which reduced in the
later-harvested fruits (Gong et al., 2001). However, Golden Smoothee apples showed higher
total antioxidant activities (SOD, CAT, and POX) in late-harvested fruits (Molina et al.,
2005). This indicates that nutritional components in fruits are affected by harvest maturity,
which may vary from one fruit to another. Therefore, optimal harvest date should be
determined for individual fruit.
21.5.7 Storage temperature and antioxidants
Generally, changes in environmental conditions result in changes in antioxidant metabolism.
Low-temperature storage decreases metabolism in fruits and vegetables and minimizes the
risk of damage during storage. Delay between harvesting and cooling can decline the
nutritional quality of the products. A 10% reduction in ascorbic acid content was found in
leafy vegetables stored at 6 ◦ C for 6 days. In contrast, storage at room temperature for only
2 days resulted in a 20% reduction in ascorbic acid content (Lee and Kader, 2000). Similarly,
lycopene content in tomatoes kept at 7 ◦ C was lower than tomatoes kept at 15 and 25 ◦ C.
Ascorbic acid content was found to be stable in tomatoes during storage potentially due to
the high acidity of the fruit (Toor and Savage, 2006).
Kiwifruit slices stored at 0, 5, and 10 ◦ C for 6 days showed 8, 12.8, and 20.6% reduction
in total vitamin C content, respectively (Agar et al., 1999). Since ascorbic acid is the
predominant form of vitamin C, the reduction was higher in ascorbic acid content. A decline
in ascorbate content was found in raspberries and low-bush blueberries during storage from