Study of crystal size evolution by focused-beam reflectance measurement during the freezing of sucrose/water solutions in a scraped-surface heat exchanger

This study examines the freezing step that occurs in a scraped-surface heat exchanger during the manufacturing of sorbet. During this step, the product enters the exchanger as a liquid, then it is cooled and partially crystallized before exiting the exchanger as a mixture of liquid and ice, also called sorbet. The freezing step governs the final quality of the product, particularly its texture. Most existing studies have focused on the product after freezing at the exchanger exit. The aim of this work was both to follow the evolution of the ice crystals' granulometry during the freezing of sorbet in the exchanger and to relate this evolution to process parameters such as refrigeration temperature, scraper speed and initial sucrose concentration. Few in situ sensors exist to follow granulometry, and this fact is especially true for rapid kinetics and concentrated suspensions. Focused-beam reflectance measurement (FBRM), an original tool, was used in this study. FBRM is currently used in the chemical and pharmaceutical industries to follow product granulometry, but it is not used in food-related applications. In our study, an experimental protocol was developed to assure identical initial thermal and crystallization conditions. First, the sensor sensitivity and the repeatability of the results were verified. The measurements performed with the FBRM probe showed that this technique can be used to follow crystal granulometry in a sorbet consisting of up to 30% of ice. The effect of process parameters was then analyzed. It appears that a decrease in refrigeration temperature accelerates ice crystallization and yields slightly smaller crystals. The same effect is observed with increasing scraper speed. Additionally, when the initial sucrose concentration in the solution is increased, the ice fraction increases more slowly but the mean chord length is smaller. (C) 2010 Elsevier Ltd. All rights reserved.