Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

456 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
late-harvested apples (MacLean et al., 2006). Therefore, plant developmental stage must
be considered to achieve maximum effect from the treatment.
Naturally occurring compounds such as hexanal, released from lipoxygenase pathway,
may prevent browning in fruits. Apple slices treated with hexanal have shown fruit
color stability and reduction in browning and PPO activity during storage (Lanciotti et al.,
1999; Corbo et al., 2000). In tissues, hexanal is converted to hexanol (aliphatic alcohol)
that would help in the prevention of browning by reducing PPO activity. Besides providing
color stability, this natural compound, hexanal, has antimicrobial activity (Corbo
et al., 2000). It has been noticed that the effect of hexanal on color retention was improved
by increasing the storage temperature. Nevertheless, hexanal treatment in table
grapes did not show any significant effect on phenolic content (Artés-Hernández et al.,
2003).
One of the most effective PPO inhibitor is sulfur dioxide (SO 2 ), which is widely used
in food industry. However, due to its negative effects on health, the need for alternatives
increased. Compounds, such as ABA, and formulation of ascorbic and citric acids, have
proved their potential role as antibrowning agents (Rocha and Morais, 2005). ABA has been
reported to enhance anthocyanin accumulation and PAL activity. Exogenous application of
ABA on cherry and strawberry promoted anthocyanin synthesis in the fruits (Hartmann,
1992; Jiang and Joyce, 2003). In apples, a dip treatment in 1% ascorbic acid and 0.2% citric
acid resulted in 90–100% inhibition in PPO activity (Rocha and Morais, 2005). Reduction
in POX activity was also noticed in ascorbic acid–treated cantaloupe melons (Lamikanra
and Watson, 2001). It could be due to the antioxidant property of ascorbic acid, which may
reduce oxidative stress in fruit.
Methyl jasmonate derived from lipoxygenase-dependent oxidation of fatty acids is a naturally
occurring volatile compound in plants that is involved in plant growth, development,
and stress conditions. Preharvest treatment with MJ significantly increased the anthocyanins
and total phenolics in raspberry (Wang and Zheng, 2005). MJ-treated fruits also showed
stimulation in ethylene synthesis, which might provoke enzymes such as PAL. Exogenous
application of MJ also induces defense compounds, such as polyphenols, preventing fruits
from fungal attack during storage (Wang and Zheng, 2005). The combined treatment of MJ
and ethanol also enhanced phenolic and anthocyanin contents in strawberry during storage
period (Ayala-Zavala et al., 2005).
21.5.5 Changes in antioxidant activity
Metabolism of fruits continues even after detachment from the plant. During storage,
fruits and vegetables undergo several stress conditions including wounding, chilling, heat,
pathogens, and senescence. These changes generate reactive oxygen species (ROS), such
as singlet oxygen ( 1 O 2 ), hydroxyl ( . OH), and peroxyl radicals and superoxide anion (O −.
2 ),
which result in cellular deterioration including lipid peroxidation, enzyme inactivation, and
mutation (Halliwell, 2006). Plant cells are protected or alleviated from ROS by an antioxidant
system that works as ROS scavenger. In plants, two types of antioxidants are found:
enzymatic antioxidants and nonenzymatic antioxidants. Enzymatic antioxidants include superoxide
dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX),
and glutathione reducatse (GR), and nonenzymatic antioxidants include flavonoids, ascorbic
acid, α-tocopherol, and β-carotene (Blokhina et al., 2003).