Phenolic Compounds

The term "phenolic compounds" embraces a wide range of compounds that possess an aromatic ring bearing a hy-droxyl substituent, including their functional derivatives. Phenolic compounds are present in many plants. They are directly related to food characteristics such as taste, pal-atability, nutritional value, pharmacological and toxic effects, and microbial decomposition. Among the natural phenolic compounds, of which approximately 8000 are known to occur in plants, the flavonoids and their relatives form the largest group with more than 5000 known structures (1). This is considered only a fraction of the total number that are likely to be present in nature, since only a small percentage of plant species has been properly examined for their phenolic compounds. Phenolic compounds range from structures that are very lipophilic (eg, tanger-etin) to those that are very water soluble (eg, quercetin 3-sulfate). The size of molecule varies greatly, ranging from monomer, catechol with molecular weight of 110, to the complex heavenly blue anthocyanin pigment of Ipomaea coerulea, which has molecular weight of 1759 (2,3). Considerable numbers of simple monocyclic phenols, phenolic quinones, lignans, and xanthones, as well as polymeric materials such as lignins, melanins, and tannins are considered to be important phenolic compounds.

Only a relatively few polyphenols are considered to be important to foods and feeds. They are p-coumaric, caffeic, ferulic, sinapic, gallic acids and their derivatives, and the common flavonoids and their glycosides. Anthocyanins and flavonols are important pigments in a variety of fruits and vegetables. While many common polyphenols are not pigments that enhance quality, some are important food constituents because they are responsible for off-colors, usually browning, that develop during the storage and processing of fruits and vegetables. Many phenolic compounds participate in both enzymatic and nonenzymatic browning reactions. The retention of fruit and vegetable color has become a matter of great importance because color is one of the primary factors in consumer acceptance of plant foods. In this respect the need to prevent the formation of undesirable color during processing has always posed a challenge to food technologists.

In addition to color, polyphenols also contribute to food flavor and other qualities. For example, astringency of polyphenols and its ratio with sugar and acid are important and useful criteria for determining the overall quality of fresh fruits, fruit beverages, and wines. Some polyphenols, such as chalcones and related compounds found in citrus fruits, are exceedingly sweet or bitter. Both bitterness and astringency of wine are due to phenolic compounds present in grapes. There have been many claims for the adverse effect of polyphenol compounds on dietary proteins. Tannins have the potential to affect many aspects of digestion due to their affinity for proteins, and the tannin-protein complex decreases its digestibility (4). Another important function of polyphenol compounds in terms of the human health benefit is the growing evidence that suggests that polyphenol compounds in the diet have a long-term health benefit and may prevent or reduce the risk of some chronic diseases. Certain naturally occurring phenolic compounds, often referred to as flavonoids, seem to function via one or more biochemical mechanisms to interfere with, or prevent, carcinogenesis. Due to limited space, only those common phenolic compounds found in fruits and vegetables and other plant foods reported in recent years will be discussed here.

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