The main purpose of using alternative current (AC) heating treatment is to inactivate enzymes and microorganisms. The inactivation of pectin methylesterase (PME) and Aspergillus niger, and the changes in total pectin content by AC heating with different electric field strengths in the range of 36-108 V/cm for different treatment times in tomato fruit were investigated. PME activity decreased with an increase in treatment time at 108 V/cm. However PME activity increased by AC treatment at 48 V/cm for 5 s and at 36 V/cm for 5, 10, 15, and 30 s. compared to untreated control. The highest amount of total pectin was determined as 3.56% at 89 degrees C by AC treatment at 36 V/cm for 80 s. The A. niger inactivation was increased with increase in electric field strength and increased temperature. The concentration of A. niger (cfu/g) was determined to be <10 at 108, 68, 48, and 36 V/cm electric field strengths for the treatment times of 6, 12, 20 and 30 s, respectively. (c) 2005 Elsevier Ltd. All rights reserved.