Online ice crystal size measurements by the focused beam reflectance method (FBRM) during sorbet freezing

Abstract

The ice crystal size distribution determines in part the textural properties of sorbet and ice cream. During sorbet and ice cream manufacturing, a narrow ice crystal size distribution with a small mean size is desired, in order to obtain a smooth texture in the final product. This research studied the influence of the mix flow rate, the evaporation temperature of the refrigerant fluid and the dasher speed on the ice crystal size and the draw temperature during sorbet freezing, so as to identify optimal operating conditions. The evolution of the ice crystal size was followed by the focused beam reflectance method (FBRM), which uses an in situ sensor that provides accurate and repeatable information about the chord length distribution (CLD) of ice crystals. Our results showed that the FBRM sensor is a promising tool which makes it possible to monitor online the development of the ice crystals in sorbets containing up to 40% of ice. Decreasing the refrigerant fluid temperature allows us to reduce the ice crystal size and to lower the product's temperature, due to the increase of the supercooling driving force. 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. 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.