In a membrane separation process, a feed mixture is partially separated by the membrane, which process are based on diffusion or solution diffusion and sieving principle. The general membrane separation model is shown in figure 1, where the feed mixtures is separated into a retentate (the components do not pass through the membrane) and a permeate.
Similar with diffusion process, can be regarded as mass transfer coefficient, and driving force can be pressure difference, concentration difference and mole friction difference. To be effective for separating a mixture of chemical components, a polymer membrane is required high permeance (the ratio of permeability to the membrane thickness).
Membrane materials can be generally characterized by different pores of microscopic dimensions. Different size of membrane is using to separate different size of particles. Microporous membranes contain interconnected pores and are categorized by their use in microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF).
In dairy industry, basing on sieving principle, MF is used for separating particles with a diameter of 0.05 m to 10 m. It can filter proteins and retains fat globules, microbes and somatic cells (Valentina, 2008). Nanofiltration membrane that is ultra-low pressure reverse osmosis membrane, also known as loose reverse osmosis membrane. The pore size is less than 2 nm to filter salt from whey. Also, the tightness of the nanofiltration membrane is higher than that of the ultrafiltration and the retention of lactose is significant. There is another method, which named reverse osmosis (RO), used for the concentration of milk and whey concentrated separation in dairy industry. Its pore size is 0.1 to 1nm. At pressures higher than the solution’s osmotic pressure, only the water in the solution permeates the membrane and all the macromolecules, small organic molecules and inorganic salts in all solutions are trapped.
The figure 2 can clearly show different applications of membranes in dairy industry. In this paper, UF will be mainly discussed.