24-4

J. YU and R. W. Lencki. Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada

It is well known that pectin, protein, phenolics and starch all play a role in fouling layer development during apple juice membrane filtration. Pectic enzymes are typically used to improve yield and filterability in apple juice manufacture. The presence of phenolic compounds also increases fouling flux resistance due to tannin-protein interactions. The purpose of this study was to investigate the influence of enzymatic pectin hydrolysis and oxidative modification of phenolics on apple juice fouling layer structure.

Apple juice was made from either MacIntosh or Red Delicious apples. Enzyme hydrolysis was performed at 40°C for various treatment times. Polygalaturonase (PG), pectin lyase (PL) and pectin esterase (PE) were added to unclarified apple juice separately or in combination. The oxidation of apple juice was performed at room temperature using a controlled aeration rate. The specific resistance of the juice fouling layer was measured by a dead-end microfiltration unit equipped with a PVDF membrane (0.2 µm pore size) using a constant transmembrane pressure of 345 kPa. Fouling layer microstructure images were obtained by both environmental (ESEM) and field emission scanning electron microscopy (FESEM).

Both PG and PL added separately lowered the specific resistance of MacIntosh juice fouling layer, whereas PG did not alter the flux of Red Delicious juice unless pectin was first demethylated using PE. Unoxidized juices had high filtration resistances and oxidation of apple juice typically lowered fouling layer resistance by over an order of magnitude. ESEM and FESEM analysis showed that both depectinization and oxidation treatments significantly changed fouling layer microstructure. Pectinases increased porosity whereas oxidation affected the degree of aggregation of the gel network.

These results indicate that pectic enzymes and oxidation treatments principally affect membrane flux by altering the microstructure, and consequently, the flux resistance of the fouling layer that forms on the membrane surface.