Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE ROLE OF POLYPHENOLS 261
mainly influenced by many polyphenols especially oligomeric proanthocyanidins, certain
hydroxycinnamic derivatives, and flavanones.
Recently, phenolics present in fruits and vegetables have attracted increased attention
in the field of nutrition, health, and medicine largely because of their antioxidant properties
and potential health benefits (Graf et al., 2005). As a result, during the last decade, the
consumer demand for fruits has been increasing significantly due to the understanding of
health-promoting properties of polyphenols and other phytochemicals present fruits. Most
importantly, value-added fruits such as fresh-cut fruits have been introduced to the market,
and the demand for such products is steadily increasing. Conversely, the fresh-cut produce
industry is challenged with postcut enzymatic browning of many fruits due to the oxidation
of specific polyphenolic compounds by polyphenol oxidase (PPO).
Also, fruit phenolic compounds have applications in the food industry as natural colorants,
fragrants, antioxidants, and antimicrobial agents (Cowan, 1999; Muthuswamy and
Rupasinghe, 2007). There is an increasing demand for natural products such as polyphenols
for replacing the synthetic food additives that are implicated with possible toxic effects at
certain concentrations. Polyphenols are also considered as food supplements or value-added
food ingredients (Rozek et al., 2007). Recently, the potential to incorporate polyphenolics
extracted from fruit and vegetable wastes in cosmetic products has also been explored
(Peschel et al., 2006).
In this chapter, an overview is provided on the nature of polyphenols in fruits, their
biosynthesis and regulation, and selected roles of polyphenols in postharvest and processing.
The information provided in this chapter will give an insight to the postharvest biologists
and food processors working on different aspects of fruit polyphenols.
12.2 Structural diversity and classification of plant polyphenols
Plant phenolic compounds can be classified into different classes according to the nature
and the number of carbon atoms of their carbon skeleton (Table 12.1). The structure of
natural polyphenols varies from simple molecules, such as phenolic acids, to highly polymerized
compounds, such as condensed tannins (Harborne, 1980). In general, phenolic
biosynthesis occurs through several different routes, and therefore, produces a heterogeneous
group of compounds. However, two basic pathways are involved: the shikimic acid
pathway and the malonic acid pathway. Most of the plant phenolics are derived from the
shikimic acid pathway. In contrast, malonic acid pathway is less significant in the biosynthesis
of polyphenols in higher plants, but is predominant in fungi and bacteria. The shikimic
acid pathway converts simple carbohydrate precursors derived from glycolysis and the
pentose phosphate pathway to the aromatic amino acids. One of the pathway intermediates
is shikimic acid, which has given its name to this whole sequence of reactions. The
shikimic acid pathway is present in plants, fungi, and bacteria but is not found in animals.
Animals are not able to synthesize the three aromatic amino acids: phenylalanine,
tyrosine, and tryptophan, which are therefore essential nutrients in animal diets. Although
plant phenolics exist in a large variety, most of them are typically derived from phenylalanine
and share the common C 6 C 3 carbon backbone of the phenylpropanoid unit. By
incorporating one or more hydroxyl group(s) into the phenyl ring, different phenolics are
formed.