Since the introduction of membrane processing to the dairy industry in the late 1960's, separation of dairy fluids using semi-permeable membranes has been used to clarify, concentrate and fractionate a variety of dairy products. The time honored and tested applications involving whey processing, the by-product of cheese making, have introduced a plethora of refined proteins and commercial uses for a once wasted by-product. The membrane applications for fluid milk have been understood for as long as the whey applications but have been underutilized in commercial applications. The same concentration method of Reverse Osmosis used in whey processing is often a favorable alternative to evaporation techniques used in milk processing. The fractionating of whey proteins from the lactose in whey processing also has an attractive benefit in the fractionating of the same lactose from the milk proteins in fluid milk using the same Ultrafiltration techniques. Microfiltration fills the separation profile of membranes out by further fractionating the specific milk proteins of casein and serum proteins away from each other.

In a sense, membrane processing of fluid milk is acting like a harvester of specific milk components without imparting a phase change by the addition of heat, as is typical for evaporation, or an enzyme, as done in most cheese making techniques. The milk is modified by separating, clarifying, or fractionating a selected component in milk from other components using differences in their relative molecular weights and pore sizes of the membranes. Here we talk about specific milk membrane filtering applications that are currently in use in the dairy industry and the multifaceted reasons behind their use both in technical as well as market driven, economic terms.

Membrane Spectrum

No useful discussion of membrane filtration can begin without a look at the filtration spectrum. As you move from the right to the left of the spectrum you are following from coarse to fine filtration. Coarse particle filtering starts the spectrum and can most easily be associated with screen or sock filters. These separate the rocks from the water. The next is microfiltration. Here is where we can separate the large components that are in milk. Fat, for example, can be removed by using a pore size that restricts only the cream from milk but allows the skim milk to pass. You can also use yet another, even smaller pore size to allow a separation of the casein from the milk serum or "Whey" proteins. Moving further along we get to Ultrafiltration where we reject all the large milk components, proteins and fat, but allow the lactose, minerals and water to pass. It is here that with the addition of water, or diafiltration, we can wash even more lactose from the milk. Nanofiltration is the next membrane used for dairy. This membrane rejects all components except for select salts and water. The final membrane used for dairy applications is Reverse Osmosis. This membrane allows only water to pass through the membrane, thus acting as pure concentration membrane.

Cross-flow membrane filtration has opened the doors to a variety of new and innovative dairy products. You don't have to look back too far to the days of the mechanical separator being the only means of harvesting a component of milk. Today, not only can we separate the cream; we can separate virtually every major component of milk through membrane filtration. Membrane filtration technology has rapidly gained prominence in the processing of dairy ingredients. Microfiltration, Ultrafiltration, Nanofiltration and Reverse Osmosis, is making it possible to produce products with very unique properties and functionalities.

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