Membrane Separation

The separation of components of liquid milk products can be accomplished using semipermeable membranes by either ultrafiltration (UF), reverse osmosis or hyperfiltration (RO), microfiltration (MF), or nanofiltration (NF). The materials from which the membranes are made is similar for all processes and include cellulose acetate, polyvinyl chloride, nylon, polysulfone, and polyamide. Ceramic and mineral membranes are also widely used.

Ultrafiltration. Membranes capable of selectively passing molecules (molecular weights <10,000) are used. Pressures of 0.1 to 1.4 MPa (up to 200 psi) are exerted over the solution to overcome the osmotic pressure while still providing an adequate flow through the membrane for use. UF separates protein from lactose and mineral salts, the protein being the concentrate. Also, UF is used with skim milk to obtain a protein-rich concentrate from which cheese is made. The whey protein obtained by UF can be spray dried. UF is also used for removing minerals from whey and buttermilk.

Reverse Osmosis (Hyperfiltration). Membranes are used for separation of smaller components (molecular weights <500). They have smaller pore size than those used for UF. High-pressure pumps, usually of the positive-piston type or multistage centrifugal type, provide pressures up to 4.14 MPa (600 psi).

RO is used to remove only water from milk or whey; hence a concentrate similar to condensed milk is produced. RO has numerous applications in the dairy and food in

Table 5. Pasteurization Times and Temperatures for Dairy Products



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