Sample preparation in the determination of phenolic compounds in fruits

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Phenolic compounds occur as secondary metabolites in all plants.1 They embrace a considerable range of substances possessing an aromatic ring bearing one or more hydroxy substituents, although a more precise definition is based on metabolic origin as those substances derived from the shikimate pathway and phenylpropanoid metabolism.2 A convenient classification of the plant phenols distinguishes the number of constitutive carbon atoms in conjunction with the structure of the basic phenolic skeleton (Table 1). The range of known phenolics is thus vast and also includes polymeric lignins and condensed tannins. Some plant phenols may be involved in primary metabolism whereas others have an effect on plant growth or protect the more vulnerable cell constituents against photooxidation by ultraviolet light by virtue of their strong UV absorption.3 Plant phenols also play an important role in disease resistance in the plant. Intense interest in fruit phenolics is also related to their physiological activity which depends on their antioxidant activity, the ability to scavenge both active oxygen species and electrophiles, the ability to inhibit nitrosation and to chelate metal ions, the potential for autooxidation and the capability to modulate certain cellular enzyme activities.4–7 Thus, knowledge of the levels of these compounds in plants is of considerable interest but is limited by problems of analysis. The structural diversity of the phenolics and its effect on physicochemical behaviour such as solubility and analyte recovery presents a challenging analytical problem. Moreover, a number of phenolic compounds are easily hydrolysed and many are relatively easily oxidized, which further complicates sample handling.8,9 This review emphasises the importance of sample preparation in the determination of phenolic compounds in plant materials particularly fruits. Fruits are an important dietary source of phenolic substances although interest is also shifting to other parts of the plant as potential commercial sources of phenols. Sample preparation is a critical step in analysis and this is even more significant with real samples where the matrix components are biologically active and the analytes represent a diverse spectrum of numerous compounds, many having an unknown identity. Thus, methods of extraction of phenols from fruits are generally dependent on several factors while the usual quantification procedures involve the separation sciences and are universally applicable. Soleas et al.10 illustrated this point. They developed a derivatization procedure for determination of 15 phenolic constituents in solid vitaceous plant materials and concluded that the method ‘should be suitable to measure polyphenols in fruit, vegetables, and other foods provided that efficient extraction techniques are employed’. Such statements are seen frequently in the analytical literature but they tend to belittle the importance of this step (or perhaps they serve to underline its critical importance). Rhodes and Price11 observed that the determination of phenolic species in foods is an important outstanding problem and reviewed methods for the extraction and purification of phenolic antioxidants as the conjugated forms that exist in plant foods. Knowledge of the extraction of phenolics is also desirable outside the analytical context for it has important practical applications in the food industry. For instance, the mechanism and kinetics of phenolic extraction from wood to wine during ageing in barrels12 has significant consequences for the production of quality wines.