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
The concentration can be done either in a hyperfiltration plant and an evaporator, or in an evaporator alone to 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.
De-calcification
In the industry the Ca-phosphate is precipitated in the 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 80oC. 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 decalcium process the product can be evaporated and dried directly, but it is recom-mended to "neutralize" it by adding citric or acetic acid, as it will otherwise give problems with de-posits in the drying chamber.
Spray drying
It is not possible to dry this concentrate direct due to the hygroscopic tendency of the resulting powder, and crystallization of the lactose is necessary.
It is necessary to define the requirement to the end product, before the dryer system is selected:
A standard product, where the lactose is only partly crystallized. The final product will be hy-groscopic, 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. In this 'dry process' the 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 water needed to form lactose crys-tals of the remaining lactose molecules is simply not present. Further there is not time enough for a post-crystallization to take place.
A non-hygroscopic and non-caking permeate product, where all the lactose is crystallized, 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®.
Another process, which also allows for all the lactose to crystallize, is in the TIXOTHERM™ process. Here the concentrate from the
evaporator is transferred to an agitated film conce ntrator, where the solids content reaches 85%. The viscous product now enters a mixing crystallizer , where the product – due to its thixotropic behavi our is transferred into a fluidizable product enhanced by mixing of the fines from the subsequent fluid bed drying.
To produce a non-caking and non-hygroscopic permeate powder by the TIXOTHERM™ process does not require crystallization tanks – cooling and reheating the product is thus avoided. Neither does it require a spray dryer to produce a powder – a very cost effective process also in energy requirements and building volume. Link Brochure read also pages MilkPowderTechnology.