Caffeic acid decomposition products: Antioxidants or pro-oxidants?

The potential of phenol antioxidants to suffer decomposition reactions leading to the formation of products exerting pro-oxidant activity was studied. A hydroalcoholic solution containing caffeic acid was assessed for antioxidant and pro-oxidant activity during heating at 90 °C to simulate the heat maintenance of the coffee brews in thermos. Decomposition products were also evaluated by HPLC analysis. In the early steps of caffeic acid decomposition, a decrease in antioxidant capacity was detected, associated to a significant increase in pro-oxidant activity because the development of pro-oxidant compounds. On further heating, an increase in antioxidant activity associated to a decrease in pro-oxidant molecules previously formed and the formation of polymers with higher antioxidant activity was observed. A mechanistic route of caffeic acid decomposition under thermal conditions according to the HPLC analysis was proposed. This study clearly showed that caffeic acid, a well known antioxidant, may also act as pro-oxidant due to thermal decomposition.