Ultrafiltration (UF) membrane technology has been successfully applied to food processes for more than 20 years. Opportunities abound for new UF food applications. Some of these opportunities are described in previous literature1 and in a comprehensive report of possible UF applications in several food industries by the US Department of Energy in 1989.2 UF can save energy, reduce waste and improve product quality.
Fouling remains a critical issue in the development of new UF food applications. Foulants interfere with UF by reducing product rates--sometimes drastically--and altering membrane selectivity. The story of a successful UF food application is in many respects the story of how fouling was successfully controlled. Fouling must be considered at every step of UF process development in order to achieve success.
In this paper, basic concepts of UF and fouling are reviewed and a six step process suggested for selecting the best available membrane, membrane element, and operating conditions. A case study involving the concentration of soy protein is used to illustrate the six step process.
UF can improve both the economics and the quality of soy protein production. UF can reduce costs by reducing energy-intensive drying steps, and improve profitability by recovering whey protein from extract streams. Demand for soy products has grown dramatically in recent years. Continued growth will depend upon the ability of producers to improve the sensory and functional properties.3 Researchers have found that UF improves the taste of soy protein4 and the colloidal stability of soy proteins in simulated milk beverages.5 Hollow-fiber UF systems for soy protein processing have been described in the literature.6, 7 This paper demonstrates that soy protein can also be processed successfully on a commercial scale using spiral-wound UF elements.
Ultrafiltration: The Basics
UF fits between nanofiltration and microfiltration in the filtration spectrum and involves separations of constituents ranging from about 1-100 nanometers in size, or about 500 to 500,000 daltons in molecular weight. UF separations involve proteins, polysaccharides and other macromolecules important to the food industry. separation is primarily according to size, but surface forces are important in determining the separation as well. UF is different from conventional filtration, also called normal or dead-end filtration, in that it operates in the crossflow mode; that is, the feed stream flows parallel to the filtration media (membrane). The difference between crossflow and dead-end filtration is illustrated in Figure 1 (attached). Crossflow acts as a sweep stream to continuously cleanse the surface of the membrane from accumulated retentate. There are two products of UF: the permeate, containing components small enough to pass through the membrane, and the concentrate, containing the retentates.