Phenolic compounds: from plants to foods

Phenolic compounds are a large class of plant secondary metabolites, showing a diversity of structures, from rather simple structures, e.g. phenolic acids, through polyphenols such as flavonoids, that comprise several groups, to polymeric compounds based on these different classes. Phenolic compounds are important for the quality of plant based foods: they are responsible for the colour of red fruits, juices and wines and substrates for enzymatic browning, and are also involved in flavour properties. In particular, astringency is ascribed to precipitation of salivary proteins by polyphenols, a mechanism possibly involved in defence against their anti-nutritional effects. Finally, phenolic compounds are considered to contribute to the health benefits associated to dietary consumption of fruits and vegetables. During food processing and storage, plant phenolics are converted to a variety of derived compounds. While methods to analyse lower molecular weight phenolic compounds are well developed, analysis of polymeric compounds remains a challenge. Indeed, strong interactions of polymeric phenolics with plant cell wall material limit their extraction. Besides, their polydispersity results in poor resolution and detection, especially of derived structures such as oxidation products. However, recent advances of the analytical techniques have allowed some progress in their structural characterisation. This review summarizes the current knowledge on methods to analyse polyphenols. It presents their reactions in foods and beverages and the resulting structures, and highlights some aspects related to their impact on colour, flavour and health properties, with examples taken mostly from wine research.