Since the development of the first cellulose acetate membrane for reverse osmosis desalination of seawater in 1960, the application of membrane separation processes such as reverse osmosis (RO) and ultrafiltration (UF) has been steadily growing. Currently, membrane separation processes cover a wide range: seawater desalination, wastewater treatment, waste recovery, food processing, medical application, application to biotechnology, gas separation, and waste recovery from nonsolvents. Among these, application in food processing is one of the most important and the most promising. Particularly, in the process of concentrating liquid food, many volatile food flavors escape by the conventional method of evaporation, whereas in the membrane process those flavor components are preserved in the food because no heat needs to be supplied in the latter process. Moreover, because it involves no phase change, the membrane process is intrinsically an energy-saving process compared with the evaporation process. For these two

reasons, serious consideration has been given to various aspects of membrane food processing from the very beginning of the development and application of membranes, and some of them have had considerable industrial success. However, there are also problems in membrane application in food processing industries. The liquid food usually contains dispersed particles, colloidals, and aqueous macromolecular solutes, which precipitate on top of the membrane surface or plug the pores on the membrane surface, resulting in a drastic decrease in the membrane flux. This phenomenon is commonly called fouling and is one of the most serious problems inherent in membrane food processing. Engineering skill in the design of the module is often required to prevent fouling problems. Furthermore, the choice of membrane materials, the design of the membrane pore size, and the pore size distribution can reduce the fouling caused by the blocking of the pore. This latter aspect relates to the rational design of the membrane and is described later in this chapter.