(88e) Determination of the Dynamic Metastable Zone Width for Lactose Crystallization

In lactose refining, the quality of the lactose crystals can be controlled by the two major kinetic steps: nucleation and growth. Nucleation is strongly related to the metastable zone width (MSZW), defined as the area between the saturation curve and labile zone (where nucleation occurs spontaneously). Inside the MSZW, crystal growth is maximized and secondary nucleation is minimized. Theoretically, operating a seeded industrial crystallization process in the MSZW will ensure the production of large lactose crystals with narrow size distribution. However, the MSZW reported previously for lactose was not customized for industrial lactose refining, resulting in inferior process efficiency and product quality. Therefore, this study aims to refine the MSZW for lactose crystallization, making it suitable for designing an industrial operation. First, the MSZW under continuous stirring and seeded conditions was measured experimentally. The nucleation process was monitored by microscope and spectrophotometer. Then, the induction time for catastrophic nucleation was also determined. The microscopy data showed that the temperature of first nuclei detection was independent of the agitation speeds (rpm). However, after the production of the first nuclei, the spectroscopy data showed that the extent of secondary nucleation increased rapidly as the rpm increased. Thus, to effectively avoid secondary nucleation and promote growth, a lactose crystallization process should operate at minimal agitation rate, in the upper MSZW recommended in this study. In addition, the holding time at each temperature should be less than 40 min.