Apple pectin was deesterified by plant pectinmethylesterase (tomato) or fungal pectinmethylesterase (Aspergillus aculeatus) yielding two pectin solutions with the same degree of esterification of 35% but different patterns of methyl esterification (blockwise or random distribution of methyl ester groups). The optimal temperature for initial tomato polygalacturonase activity in the presence of pectin deesterified by tomato pectinmethylesterase (blockwise) and pectin deesterified by A. aculeatus pectinmethylesterase (random) at atmospheric pressure and at pH 4.4 (pH of tomato-based products) is situated around 50 and 45 degrees C, respectively. The catalytic activity of tomato polygalacturonase during combined pressure-temperature treatments is always higher on pectin deesterified by tomato pectinmethylesterase (block) as compared to pectin deesterified by A. aculeatus pectinmethylesterase (random) as substrate. A decreasing enzyme activity on both types of pectic substrates at the same degree of esterification of 35% is observed with increasing pressure at all temperatures tested and this decrease is more pronounced at higher temperatures. At all temperatures tested, more monomer is formed and at a higher rate using pectin deesterified by tomato pectinmethylesterase as compared to pectin deesterified by A. aculeatus pectinmethylesterase. Initially, a larger oligomer (e.g. degree of polymerisation of 7) is formed at a higher rate and in larger amounts, but was further degraded to smaller oligomers when using the former substrate as compared to the latter substrate, from which this oligomer was formed continuously within the 24-h reaction period. (c) 2005 Elsevier Ltd. All rights reserved.