Heat treatment is a common method to improve cloud stability of high acid food products such as sour orange juice. Inactivation of pectin methylesterase enzyme (PME), naturally found in citrus, is known as a heat treatment index for these products. In this work, the kinetics of PME thermal inactivation and cloud stability in sour orange juice were investigated. The fresh sour orange juice was heated in a controlled water bath at 60, 70, 80 and 90 degrees C for different times depending on applied temperature. Time temperature data was simultaneously recorded using a data-logger. The inactivation kinetics of PME was calculated in terms of thermal resistance parameters (D & Z values), activation energy, enthalpy, entropy and free energy. The obtained Z-value (36.90 degrees C), activation energy (62.61 +/- 0.84 kJ/mol) and free energy (93.52-96.37 kJ/mol) revealed the high stability of PME during the heat treatment. Based on thermal resistance of PME, heating times were corrected by considering the come-up time effectiveness. The results showed that cloud value of heated juice increased by PME inactivation during heat treatment. (C) 2016 Elsevier Ltd. All rights reserved.