Permeate is here defined as what passes through the membranes if milk or whey from cheese production is ultrafiltrated. Permeate contains water, lactose, salts, and non-protein nitrogen and is typically discharged from the ultrafiltration plant at 4-5 % total solids, the majority being lactose. The permeate can be used for fermenting, production of glucose-galactose or lactylurea, or concentrated, crystallized, and then dried.
Concentration can either take place in a hyperfiltration plant and an evaporator, or in an evaporator alone for 60% solids. Special attention should however be paid to the Ca3(PO4)2 complex, as it is a well-known fact that it will precipitate at high temperatures, especially if the concentration is high. If the Ca++ is not inactivated, the precipitation will occur as deposits on the heat surface in the evaporator, especially in the first effect. It is therefore necessary to make an intermediate cleaning of the evaporator after 6-8 hours with acid.
In the industry, Ca-phosphate is precipitated in whey/permeate by adjusting the pH to 7.2 by addition of caustic Mg(OH)2 or NaOH, after which the product is heated to 80 °C. The precipitated Ca-phosphate can then be removed by centrifugation or membrane filtration. The discharged product can be dried and sold as "milk calcium". After the de-calcium process the product can be evaporated and dried directly, but it is recommended to "neutralize" it by adding citric or acetic acid, as it will otherwise have problems with deposits in the drying chamber.
It is not possible to spray dry this concentrate directly due to the hygroscopic tendency of the resulting powder and crystallization of the lactose is required. It is necessary to define the requirements for the end product before the spray dryer system is selected.
A standard product, where the lactose is only partly crystallized
The final product will be hygroscopic, as it will absorb moisture for the remaining crystallization forming a very lumpy and caking product when exposed to humid air. For this type of product, the dry process is chosen, where precrystallized permeate concentrate is dried in a COMPACT DRYER™. The moisture content in the integrated fluid bed, however, is too low (≈5 % free moisture) to allow for further crystallization of the lactose. The mobility of the water needed to form lactose crystals of the remaining lactose molecules is simply not present. Further, there is not enough time for post-crystallization to take place.
A non-hygroscopic and non-caking permeate product, where all the lactose is crystallized This can only be produced if there is sufficient mobile water present during the process. This is the case if the dryer (typically a TALL FORM DRYER™) is equipped with a conveying belt. The powder is discharged from the drying chamber at 8-12 % moisture. The belt acts as a timing belt where the moist powder is kept for 10-15 min. during which time the remaining lactose can crystallize. The product is then finally dried in a VIBRO-FLUIDIZER™.