Polyphenol is a plant secondary metabolite containing more than one hydroxy group at the benzene ring. They are known to exhibit a range of various bioactivities, including strong antioxidant activity. The exhibiting of antioxidant activity, namely the oxidation reaction of polyphenols, produces unstable intermediates and drastically changes their chemical structure. This reaction occurs not only under biological conditions but also under food processing and storage conditions. In this review, some examples of polyphenol oxidation reactions elucidated using model reaction systems will be introduced. In particular, the chemical structure changes of polyphenols and the oxidation mechanisms inferred from these changes will be described.

Natural antioxidants act as reducing agents in living organisms to protect them from oxidative injury caused by the overproduction of reactive oxygen species. Electrochemical measurements are a very effective tool to evaluate the fundamental physicochemical properties of antioxidants. Among the parameters obtained by electrochemical measurements, the authors paid particular attention to the number of oxidized electrons (n), and discovered a reaction mechanism whereby n increases due to the dimerization or polymerization reaction associated with oxidation. This is a property that can very well explain the antioxidant activity in bulk solution. On the other hand, it is extremely important to observe reactions at the immiscible liquid/liquid or liquid/membrane interface when considering reactions in vivo. Electrochemical observations of electron transfer near the liquid/liquid interface were also performed, showing that the reaction mechanism differs depending on the degree of hydrophobicity. A new spectroscopic method devised by the authors to observe electron transfer at the liquid/membrane interface will also be presented.