A sub-zero crystallization process for the recovery of lactose
Ruben Halfwerk, Doekle Yntema, Jaap van Spronsen, Albert van der Padt
Abstract
In industry, lactose is generally produced by concentrating whey permeate by evaporation followed by a slow cooling process where lactose is crystallized. Here, an alternative method is presented whereby the concentration and crystallization steps are combined at sub-zero temperatures, so-called eutectic freeze crystallization. It was discovered that simultaneous crystallization of lactose and water (ice) is possible. The obtained lactose crystals had an average size of 10 μm and a thin triangular or tomahawk morphology. The process was analyzed in detail in two steps: freeze concentration and lactose crystallization at sub-zero temperatures. Freeze concentration experiments showed that concentrating to supersaturation was possible without excessive lactose crystallization. In the second step, lactose was crystallized at temperatures below zero from a 30 wt% lactose solution, without observation of significant primary or secondary nucleation. The amount of seed material had a large influence on the final yield, crystal size and morphology. The optimum seed amount was found to be at 0.08% of the total lactose; the resulting crystals had an average size of 26 μm and a tomahawk morphology. Although highly supersaturated conditions are present in the sub-zero crystallization of lactose, crystal growth is found to be the predominant process rather than nucleation.