Thermal Degradation of p-Hydroxybenzoic Acid in Macadamia Nut Oil, Olive Oil, and Corn Oil

p -Hydroxybenzoic acid (PHBA) plays a significant role in sustaining the oxidative stability of macadamia nut oil (MNO). However, PHBA undergoes thermal decarboxylation and loses its bioactive antioxidant properties. In this study, we determine PHBA degradation kinetics in oils at various heating temperatures, which provides fundamental understanding of PHBA thermal degradation in oils and oil quality changes during high-temperature processing. PHBA degradation kinetics in MNO, olive oil, and corn oil were evaluated at temperatures typical for cooking and frying. PBHA headspace concentration was measured using selected ion flow tube mass spectrometry. PHBA decarboxylation followed a zero-order reaction, where degradation could be affected by factors such as the type of oil matrix having different FA compositions, antioxidants, and component interactions. PHBA degradation activation energies ( E a ) showed that PHBA was more stable against thermal decarboxylation in MNO (85 kJ mol –1 ) than in olive oil (40 kJ mol −1 ) or corn oil (22 kJ mol −1 ). The higher enthalpy ( ) of decarboxylation in MNO (82 kJ mol −1 ) indicates that PHBA is more inhibited from decomposition than olive oil (37 kJ mol −1 ) or corn oil (19 kJ mol −1 ). Moreover, the negative entropy values ( ) of PHBA degradation from MNO (−192 J mol −1 K −1 ), olive oil (−277 J mol −1 K −1 ), and corn oil (−325 J mol −1 K −1 ) indicates that these oils impart some inhibitory properties against PHBA thermal decarboxylation.