Juices made from McIntosh and Red Delicious apples were used to investigate how enzyme treatment influences fouling behavior during membrane clarification. When microfiltered, untreated McIntosh juice typically had a permeate flux that was an order of magnitude lower than that observed with the Red Delicious juice control. However, both polygalaturonase (PG) and pectin lyase (PL) treatment increased the McIntosh flux to values similar to those observed with Red Delicious. In contrast, the addition of pectin esterase (PE) to McIntosh or Red Delicious juice produced a fouling layer with a higher flux resistance than the heat-treated control. Scanning electron microscope images showed that PG and PL treatments increased bonding colloidal interactions, leading to more dense clustered aggregation patterns and higher macroporosity. PE treatment increased non-bonding repulsion that inhibited aggregation and decreased flux. However, the lowest specific resistances were obtained by combining PG or PL with PE, which created a gel with both high macro- and microporosity. (C) 2003 Elsevier Ltd. All rights reserved.