The main proportion of solids in milk is the lactose or 'milk sugar'. 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
oC to 15-20
oC 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 is separated. The crystals are then washed in order to remove residuals of mother liquor, after which it is 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 the final drying. Hot air of about 100
oC is used for the drying.
The average crystal size (200-250µ) is of importance for the fluidization velocity that can be used, and it therefore 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. Milling of the powder after the fluid bed and final sifting is therefore used.