ABSTRACT
The application of ultrafiltration (UF) as an alternative to conventional processes for the clarification of apple juice was demonstrated by Heatherbell et al. (1977). However, the acceptance of UF in the fruit processing industry is not yet complete because there are problems with the operation and fouling of membranes. During UF two fluid streams are generated: the ultrafiltered solid free juice (permeate) and the retentate with variable insoluble solids which, in the case of apple juice, are mainly remains of cellular walls and pectin. Permeate flux (J) results from the difference between a convective flux from the bulk of the juice to the membrane and a counter diffusive flux or outflow by which solute is transferred back into the bulk of the fluid (Fig. 10-1). The value of J is strongly dependent on hydrodynamic conditions, membrane properties, and the operating parameters. The main driving force of UF is the transmembrane pressure (APTM), which in the case of hollow fiber ultrafiltration systems (HFUF) can be defined as:
where Pj is the pressure at the inlet of the fiber, PQ the outlet pressure, and Pcxt the pressure on the permeate side. In practice, the J values obtained with apple juice are much lower than those obtained with water only. This phenomenon is attributable to various causes, including resistance of the gel
(10-1)
layer, the concentration polarization boundary layer [defined as a localized increase in concentration of rejected solutes at the membrane surface due to convective transport of solutes (Porter, 1972)], and plugging of pores due to fouling. Some of these phenomena are reversible and disappear after cleaning of the UF membranes and others are definitively irreversible.
