The main proportion of solids in milk is the lactose or milk suga'. Lactose is a disaccharide made up from glucose and gallactose. Lactose is produced from whey from cheese production or permeates from ultrafiltration of whey or milk. Whey or permeate is pre-concentrated to 60-65 % total solids and transferred to crystallization tanks. Here, a gradual cooling of the concentrate from 50-60 °C to 15-20 °C takes place over a 48 hour period to produce big lactose crystals.
The pre-crystallized concentrate is then passed through a decanter where the main portion of the lactose crystals are separated. The crystals are then washed in order to remove residuals of mother liquor, after which they are dried in a specially designed fluid bed Lactose Dryer. The fluid bed is divided into three sections: the first section is a stationary back-mix bed into which the wet crystals with 8 % moisture are fed by a rotating disc. To avoid lump formations this section is equipped with a rotating rake. From the back-mix section the powder enters the plug-flow fluid bed section for final drying. Hot air of about 100 °C is used for the drying.
The average crystal size (200-250µ) is of importance for the fluidization velocity that can be used and thus influences the performance of the fluid bed, as smaller crystals would require lower fluidization velocity or too big carry-over of product to the bag filter would be the result. Lactose powder is very often classified with different crystal sizes. As a result, milling of the powder after the fluid bed and final sifting is done.