Focused beam reflectance measurement as a tool for in situ monitoring of the lactose crystallization process

Lactose accounts for about 75 and 85% of the solids in whey and deproteinized whey, respectively. Production of lactose is usually carried out by a process called crystallization. Several factors including rate of cooling, presence of impurities, and mixing speed influence the crystal size characteristics. To optimize the lactose crystallization process parameters to maximize the lactose yield, it is important to monitor the crystallization process. However, efficient in situ tools to implement at concentrations relevant to the dairy industry are lacking. The objective of the present work was to use a focused beam reflectance measurement (FBRM) system for in situ monitoring of lactose crystallization at supersaturated concentrations (wt/wt) 50, 55, and 60% at 20 and 30°C. The FBRM data were compared with Brix readings collected using a refractometer during isothermal crystallization. Chord length distributions obtained from FBRM in the ranges of <50 µm (fine crystals) and 50 to 300 µm (coarse crystals) were recorded and evaluated in relation to the extent of crystallization and rate constants deduced from the refractometer measurements. Extent of crystallization and rate constants increased with increasing supersaturation concentration and temperature. The measured fine crystal counts from FBRM increased at higher supersaturated concentration and temperature during isothermal crystallization. On the other hand, coarse counts were observed to increase with decreasing supersaturated concentration and temperature. Square weighted chord length distribution obtained from FBRM showed that as concentration increased, a decrease in chord lengths occurred at 20°C and similar observations were made from microscopic images. The robustness of FBRM in understanding isothermal lactose crystallization at various concentrations and temperatures was successfully assessed in the study.