Agriculture Reference
In-Depth Information
Chapter 12
The Role of Polyphenols in Quality,
Postharvest Handling, and
Processing of Fruits
H.P. Vasantha Rupasinghe
12.1 Introduction
Phenolics are a chemically heterogeneous group of over 12,000 plant secondary metabo-
lites that are ubiquitous in plants. A phenolic is a chemical compound characterized by at
least one aromatic ring (C
6
) bearing one or more hydroxyl group. Some of them contain
carboxylic acids and glycosides and are water soluble. Some are insoluble in water because
of complex structure. In general, the principal ecological functions of plant polyphenols
are to adapt the plant to changing biotic and abiotic environment; to defend plants against
herbivores and pathogen attacks; and to act as signal molecules to interact with the environ-
ment.
Pigments such as many colorful polyphenols, especially anthocyanins, serve as attrac-
tants for fruit- or seed-dispersing animals. Among pigments, a wide range of anthocyanins,
other flavonoids, and carotenoids have been characterized in fruits. Plant pigments also
provide protection against environmental stress preventing cellular damage from free rad-
icals generated due to stressful conditions. For example, some phenolic compounds can
absorb light at shorter wavelengths and therefore can protect plants from ultraviolet (UV)
irradiation. Since polyphenols possess strong antioxidant properties, they also play a role
in mitigating the oxidative stress during postharvest storage and handling, and thus con-
tribute to extending the shelf life of fruits. Polyphenols such as lignins contribute to the
firmness of fruits, protecting the fruits against mechanical injuries during postharvest han-
dling. Polyphenols present in fruit skin is implicated as a factor, which contributes toward
the resistance of some apple cultivars to the postharvest physiological disorder, superficial
scald.
Pigments are important feature of fruits both from the ecological perspectives as well as
from human use, since pigments are indicators of fruit maturity and ripeness. Polyphenols
are also among the major determinant factors for the quality, that is, color, aroma, bitterness,
and astringency of fresh fruits as well as fruit products such as juices, beverages, wines,
jellies, and jams. Polyphenols such as tannins interact with proteins and polysaccharides
of processed food products and sometimes form hazes or precipitates and also impact on
the nutritional value. The taste (bitterness) and astringency of fruits and their products are
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