Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

452 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
reduces the risk of developing cancer, cardiovascular, and several other diseases (Kang
et al., 2003; Zhao et al., 2004; Scalbert et al., 2005). Polyphenols have antioxidant properties
protecting cells from damaging effects of ROS that are produced during metabolic
reactions. The antioxidant and free radical-scavenging properties of polyphenols depend
on their molecular structures (such as the position of hydroxyl groups and other features).
An imbalance between antioxidants and ROS results in oxidative stress, which leads to the
development of cancer, aging, atherosclerosis, cardiovascular disease, and inflammation
(Byers and Perry, 1992).
Oxidative damage in cells due to the generation of ROS can be prevented by enzymes
such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase
(APX), and glutathione reductase (GR). These damages can also be prevented by
antioxidant compounds such as ascorbic acid, α-tocopherols, and carotenoids.
Fruits undergo several changes during harvesting, transportation, and postharvest storage,
which affect the nutritional compounds and enzymes involved in the metabolism of
those compounds. The changes during prolonged storage periods are related to the taste,
nutritional quality, and shelf life of the product. Since every commodity shows different
response through storage, it is difficult to preserve nutritional quality of all fruits by a single
technology. Thus, it is extremely important to develop sustainable technologies to maintain
the quality and shelf life of fruits.
21.5.1 Changes in phenolics and anthocyanins
Polyphenols not only give color and taste to fruits and vegetables, but they also contribute
health benefits to human. Qualitative and quantitative compositions of polyphenols differ
from one fruit to another. Even within the same species, a large variation in polyphenols can
be found in the cultivars. For example, in sweet cherry, cyanidin-3-rutinoside is the predominant
anthocyanin. Sweet cherries “Bing” contain high level (180 mg/100 g flesh weight) of
cyanidin-3-rutinoside, whereas “Summit” contains low levels (72 mg/100 g flesh weight)
of this anthocyanin (Gao and Mazza, 1995). However, in case of sour cherries, cyanidin-3-
glucosylrutinoside is a major anthocyanin. A higher level of cyanidin-3-glucosylrutinoside
(227 mg/100 g fresh weight) was found in “Sumadinka” sour cherry, while “Balaton”
had only 88 mg/100 g fresh weigh cyanidin-3-glucosylrutinoside (Kim et al.,
2005).
Plant phenolics are highly unstable and they undergo various changes throughout storage.
These changes are associated with taste and nutritional quality of fruits. Fruits like
cherries, strawberries, and litchi are highly perishable, and they start to develop brown pigments
within 2–3 days of harvest, at ambient temperature. Postharvest browning of fruits
is mainly due to the breakdown of anthocyanins and oxidation of phenolics. In the presence
of oxygen, phenols are oxidized by polyphenols oxidase (PPO), which catalyzes two
reactions: (i) the hydroxylation of monophenols to o-diphenols and (ii) the oxidation of
o-diphenols to quinones, slightly colored compounds, condensed to form brown pigments
(melanins) (Macheix et al., 1990). The free phenolic compounds are predominately localized
in the vacuole, and PPO is localized in chloroplast and cytoplasm. During storage,
subcellular decompartmentalization leads to the enzyme and substrate coming into contact,
which triggers browning in fruits and vegetables (Macheix et al., 1990; Tomas-Barberan
and Robins, 1997). Several factors, such as nature and substrate content, enzyme activity,