Introduction

The use of membrane technology in the food industry is increasing. Established membrane processes in this field include reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF) and electrodialysis (ED) and, to some extent, nanofiltration (NF). The application of new membrane processes, such as pervaporation (PV), is under development for the food industry. In 1991 the annual sale of membranes and membrane modules for the food industry was estimated at about 235 million US $ or about 10% of the annual sales, distributed as follows: MF 130, UF 60, RO 25 and ED 20 million US $ [1], It should be observed that about half of the annual sales is accounted for by devices for blood purification and about 30% for water purification.

Most of the microfilters installed to date are run in the dead-end mode. The main applications are found in the beverage industry, where microfiltration is used to achieve sterile filtration and clarification. The use of crossflow microfiltration is, however, increasing, leading to better performance and a substantially longer membrane lifetime. In this chapter only the crossflow applications will be discussed.

Ultrafiltration often presents a unique separation solution, for example the use of ultrafiltration for fractionation. In other applications, ultrafiltration is an alternative to other separation processes. When used for concentration, ultrafiltration sometimes competes with evaporation, for example.

Reverse osmosis is normally used for the concentration of liquid foods and food process effluents, and, in some cases, for fractionation purposes, e.g., in the beverage industry for production of beer with reduced alcohol content.

Partial demineralization can be obtained by using membranes in the intermediate range between UF and RO, often called nanofiltration. High-molecular-weight compounds are concentrated simultaneously. A higher degree of demineralization can often be obtained using electrodialysis. In this case, however, the product is not concentrated simultaneously.

Membrane separation technology offers a number of advantages in the food industry. Some of these are gentle treatment of the product at the chosen temperature, (resulting in significant improvements in the product quality compared with other techniques where more severe heat treatment is necessary), unique separation properties, concurrent fractionation and concentration, desalination and purification of solutions, low energy consumption, increased efficiency and simple plant layout. Motives to utilize membrane technology were as follows according to a survey of Japanese companies [2]: improvement of product quality (82.1%), reduction of production costs (32.1%); fortification of products against the products from competitors (28.6%).

Among the disadvantages is fouling, the deposition of material on the membrane surface and/or in its pores, leading to a change in the membrane performance. Fouling causes a reduction in flux and sometimes also changes in the separation properties. Methods of reducing fouling, especially in crossflow microfiltration, will be discussed later in this chapter. The maximum degree of concentration is restricted by viscosity and osmotic pressure, for example. To partly overcome this, plate-and-frame modules with specially designed flow channels have been developed. Reverse osmosis modules and membranes withstanding pressures well above 10 MPa are being developed.

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