EFFECT OF SORBET FREEZING PROCESS ON DRAW TEMPERATURE AND ICE CRYSTAL SIZE USING FOCUSED BEAM REFLECTANCE METHOD (FBRM) ONLINE MEASUREMENTS

摘要

During sorbet and ice cream manufacturing is desired to obtain, a narrow ice crystal size distribution with a small mean size, in order to get a smooth texture in the final product. We studied the influence of the mix flow rate, the evaporation temperature of the refrigerant fluid and the dasher speed the draw temperature and on the ice crystal size during sorbet freezing in order to identify optimal operating conditions. We used the focused beam reflectance method (FBRM) to follow the evolution of the ice crystal size. The FBRM probe, provides accurate and repeatable information about the chord length distribution (CLD) of ice crystals. With this probe it is possible to follow online the development of the ice crystals in sorbets or slurries containing up to 40% of ice. The effect of refrigerant fluid temperature was very important; by decreasing refrigerant temperature we reduce significantly the ice crystal size and we obtain as expected lower draw temperature. High dasher speeds slightly decrease the ice crystal chord length, due to the attrition of the bigger ice crystals, which produces new smaller ice nuclei by secondary nucleation. Also, an increase of the dasher speed slightly warms the product, due to the dissipation of frictional energy into the product. As expected low mix flow rates result in lower draw temperatures because the product remains longer in contact with the freezer wall, extracting thus more heat from the product. Nevertheless, the mix flow rate did not show a significant effect on the ice crystal size.