Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE ROLE OF POLYPHENOLS 275
washing, trimming or slicing, shredding, and packaging. Fresh-cut fruit may undergo
surface browning, tissue softening, loss of flavor, and other deterioration reactions. Surface
discoloration is considered to be the most important quality defect and the factor
mostly limiting shelf life of fresh-cut fruits (Lule and Xia, 2005; Toivonen, 2006). During
peeling and cutting, fruit cells are ruptured and thus, PPO located in the cytosol comes
in contact with phenolic substrates that are stored in the vacuole. Furthermore, secondary
reactions results in the formation of complexes between quinones and proteins causing
changes in physical, chemical, and nutritional characteristics, and reducing the shelf life of
the fresh-cut fruit products (Lindley, 1998). Also, these reactions in the advanced phases
produce strongly oxidized phenolic compounds with a subsequent loss of antioxidant capacity
of these compounds (Nicoli et al., 2000). In addition, PAL, a key enzyme in the
phenolic synthesis of phenylpropanoid biosynthesis, can be stimulated (Lattanzio, 2003)
after wounding or injury. As mentioned earlier, cutting or bruising of minimally processed
fruits also increases the activity of POX.
Control of enzymatic browning can be achieved through the use of physical (reduction
of temperature and oxygen, and use of modified atmosphere packaging or edible coatings)
and chemical methods (treatment with compounds that inhibit PPO, remove its substrates
(oxygen and phenolics) or, function as preferred substrates). There has been an extensive
research on investigating ways to prevent or minimize enzymatic browning, especially since
the banning of the use of sulfites to prevent the browning of fresh-cut fruits and vegetables.
Commercial antibrowning agents such as FreshXtend TM (FreshXtend Technologies Corp.,
Vancouver, British Columbia, Canada) and NatureSeal TM (Mantrose-Haeuser Co. Inc., CT)
are available in the market, but their high cost is a limitation for their application in many
value-added fruit-processing operations. In addition to search for low cost and environmentally
friendly antibrowning agents, the development of cultivars with a low potential
for enzymatic browning can significantly contribute toward the prevention of browning in
minimally processed fruits.
12.7.4 Enzymatic browning of fruit juices
Juice and beverage products represent a significant portion of the processed fruit industry.
However, the enzymatic browning caused by fruit pressing and juice extraction is the most
critical step that imparts unfavorable quality to most fruit juices (Macheix et al., 1991).
Processing steps such as cutting, crushing, and pressing enhance PPO activity (Lozano
et al., 1994). Therefore, it is recommended that juice extraction should be done as quickly
as possible and passed through filtration process to the pasteurization step, which can
inactivate PPO enzyme. Also, crushed fruit can be passed through larger bore, tubular heat
exchanger (50–60 ◦ C) to minimize enzymatic browning. The application of high-intensity
pulsed electric fields (PEF) treatments has been also investigated by some researchers. The
PEF treatment depleted PPO and POX activities of grape juice although it was observed
that grape POX was less sensible than PPO to PEF technology (Marselles-Fontanet and
Martin-Belloso, 2007).
In certain cases, PPO is considered beneficial for oxidation of phenolics as it may help
reduce the haze problem in juices. Oxidation of phenolic compounds such as procyanidins
can generate highly reactive intermediates that can complex irreversibly with each
other and with proteins to form insoluble complexes, which are not dissolved on warming