The Role of Polyphenols in Quality, Postharvest Handling, and Processing of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 270

duration (Jiang and Joyce, 2003), all of which are important considerations in the postharvest
storage of pome fruits.
12.7 Role of polyphenols in enzymatic browning
during fruit processing
During postharvest storage and processing, the metabolic changes of fruit contents are
mainly responsible for the changes in quality of the fruit (color, taste, texture, shelf life)
or their processed products. Polyphenols and their interaction with other fruit components
have a significant influence on the quality and processing of fruits (Nicoli et al., 2000).
Therefore, the role of polyphenols in enzymatic browning in fruits is considered one of
the most important phenomena in postharvest and fruit processing. Also, polyphenols are
highly sensitive to heat, light, air, and moisture; thus, exposure to such conditions can result
in significant loss of many of polyphenolic compounds, reducing the nutraceutical value of
fruit products (Nicoli et al., 1999).
The exposure of the cut surfaces of certain fruits to air results in browning due to the
polyphenol oxidase (PPO)-catalyzed oxidation of certain phenols to orthoquinones. The
orthoquinones form dark-colored pigments or melanin due to subsequent rapid nonenzy-
matic polymerization (deMan, 1990). Peroxidase (POX), just as PPO may be involved in
browning reactions and production of melanin compounds. The actions of PPO and POX
could also result in off-flavor generation in horticultural products. Both phenomena are of
vital importance to the manufacturer as they impair not only the sensory properties and mar-
ketability of a product, but also lower the nutritional value of fruit. The activity of PPO and
POX can significantly reduce the quality of the fruits after harvesting. Postharvest browning
losses in litchi resulted from the oxidation of phenolics by PPO and POX enzymes (Zhang
and Quantick, 1997). The presence of active PPO in plant tissues can also cause significant
loss of anthocyanins and result in the production of quinones and loss of color, flavor, and
nutritive value (WescheiEbeling and Montgomery, 1990; Kader et al., 1998).
PPO belongs to the group of enzymes called oxidoreductases. PPO is present in almost
all the plants, but the enzyme is most abundant or active in fruits such as apples, peaches,
banana, and avocados. The distribution of PPO in the different parts of fruits may be consid-
erably different, and the ratio of particle-bound and soluble enzymes varies with maturity.
In fruits such as pears and apples, PPO was found to be distributed in almost all of the fruit
parts (Vamos-Vigyazo, 1981). A wide range of PPO is characterized from plants with char-
acteristic substrate (phenolic compounds) specificity, for example, tyrosinase, cresolase,
phenolase, catechol oxidase or o -diphenol oxidase, laccase, or p -dipenol oxidase. (Sapers,
1993; Martinez and Whitaker, 1995). PPO catalyzes two basic reactions: hydroxylation at
the o -position adjacent to an existing hydroxyl group of the phenolic substrate (monophenol
oxidase activity) and oxidation of diphenol to o -benzoquinones (diphenol oxidase activity)
(Fig. 12.6). Both reactions utilize molecular oxygen as a cosubstrate. The protein copper-
oxygen complex is formed by combining one molecule of oxygen with the protein to which
two adjacent cuprous atoms are attached.
All PPOs possess catecholase activity, that is, they can convert o -dihydroxyphenols to o -
benzoquinones, but not all PPOs can oxidize monophenols. However, much attention is paid
to the diphenol oxidases due to their high catalytic rate and their role in the production of the
brown pigment, melanin. Most importantly, once o -quinones are formed, they can undergo