INTRODUCTION Phenolic Acids in Fruits and Vegetables

I. INTRODUCTION

The several thousand polyphenols that have been described in plants can be grouped into distinct classes, most of which are found in fruits and vegetables [1,2]. Distinctions among these classes are drawn first on the basis of the number of constitutive carbon atoms and then in light of the structure of the basic skeleton. Phenolic acids belong to two different classes, hydroxybenzoic acids (HBA) and hydroxycinnamic acids (HCA), which derive from two nonphenolic molecules, benzoic and cinnamic acid, respectively. In contrast to other phenolic compounds, HBA and HCA present an acidic character because of the presence of one carboxylic group in the molecule. They are widely represented in plants, although their distribution may strongly vary with species, cultivar, and physiological stage. They clearly play a role both in the interactions between the plant and its biotic or abiotic environment and in the organoleptic and nutritional qualities of fruits, vegetables, and derived products, e.g., fruit juices, wines, and ciders. Furthermore, their antioxidant properties are essential in the stability of food products and in antioxidant defense mechanisms of biological systems. These last aspects are largely developed elsewhere in this volume.

Plant organs consumed by humans as vegetables have various botanical origins, e.g., leaves, stems, shoots, flowers, roots, rhizomes, tubers, bulbs, seeds, pods, and even some fleshy fruits. In some cases, it is not easy to distinguish between fruits and vegetables, as there is no concordance between the botanical definitions and the common use of plant organs by the consumer. For instance, bean pods, tomatoes, eggplant fruits, and sweet peppers are fruits in a botanical sense, whereas they generally are commercially marketed as vegetables.

Qualitative and quantitative determinations of phenolic acids, especially the combined forms, have been significantly improved during the last two decades, allowing one to draw a general picture of their distribution in fruits and vegetables and their importance as food constituents. In the comprehensive reviews on these topics that have already been published [1–5] most of the oldest references may be found. In the present review, our attention is focused on the presence and content of phenolic acids in fruits (mainly fleshy fruits with their seeds) and vegetables, and on the main parameters that can modify them.