The stability and inactivation kinetics of exogenous Aspergillus niger pectin methylesterase in fruit juices was studied in the temperature interval of 52-66 degrees C. The temperature treatment was applied to two kinds of non-pasteurized apple juices, two cloudberry juices and 0.1 M acetate buffer for the assessment of the influence of juice components on the enzyme stability. The enzyme was very thermally labile in the studied temperature range with the z-value around 7 degrees C. The composition of juices caused a considerable difference in the enzyme susceptibility to thermal inactivation. The enzyme inactivation however followed first-order kinetics with approximately the same value of the activation energy of 300 kJ/mol for each material. (c) 2007 Elsevier Ltd. All rights reserved.