Sonochemical effects on free phenolic acids under ultrasound treatment in a model system

Sonochemical effects on seven free phenolic acids under ultrasound treatment in a model system have been investigated. The degradation products have also been tentatively identified by FTIR and HPLC-UV-ESIMS. Five phenolic acids (protocatechuic acid, p-hydroxybenzoic acid, vanillic acid, p-coumaric acid, and ferulic acid) proved to be stable, while two others (caffeic acid and sinapic acid) were degraded under ultrasound treatment. The nature of the solvent and the temperature has been identified as important factors in determining the degradation reaction. Liquid height, ultrasonic intensity, and duty cycle of the ultrasound exposure affected only the degradation rate and did not change the nature of the degradation. The degradation rates of caffeic acid and sinapic acid decreased with increasing temperature. The degradation kinetics of these two acids under ultrasound conformed to zeroth-order reactions at -5 to 25 °C. Both decomposition and polymerization reactions occurred when caffeic acid and sinapic acid were subjected to ultrasound. Degradation products, such as the corresponding decarboxylation products and their dimers, have been tentatively identified.