Rheological characterisation of sorbet using pipe rheometry during the freezing process

Sorbet produced without aeration is a dispersion of ice crystals distributed randomly in a freeze-concentrated liquid phase. The rheological properties of this suspension will be affected by the viscosity of the continuous liquid phase and the volume fraction of ice crystals. The knowledge of the viscosity of sorbet is essential for the improvement of product quality, the selection of process equipment, and for the optimal design of piping systems. This work aimed firstly, at studying the influence of the ice volume fraction (determined by the product temperature) on the apparent viscosity of a commercial sorbet, and secondly, to propose a rheological model that describes the evolution of the viscosity of the product as a function of the ice volume fraction. The rheology of sorbet was measured in situ by means of a pipe rheometer connected at the outlet of a continuous scraped surface heat exchanger (SSHE). The pipe rheometer was composed of a series of pipes in PVC of different diameters, making it possible to apply a range of apparent shear rate from 4 to 430 s⁻¹. The flow behaviour index of sorbet decreased as the temperature of the product decreased, the effect of which indicates that the product becomes more shear thinning as the freezing of sorbet occurs. The consistency coefficient and therefore the magnitude of the apparent viscosity of sorbet increased with the decrease in product temperature and with the increase of the ice volume fraction. Results also showed that the rheological model described the experimental data within a 20% error.     

Experimental study and modelling of the residence time distribution in a scraped surface heat exchanger during sorbet freezing

Scraped surface heat exchangers (SSHEs) are widely used for crystallization applications in several food processes (i.e. crystallization of margarine, tempering of chocolate, freezing of ice cream and sorbet). The final quality of these food products is highly related to crystal size distribution and apparent viscosity, both of which are determined by the operating conditions of the process. During the freezing of sorbet, the increase in the ice volume fraction leads to an increase in the apparent viscosity of the product. This effect modifies the fluid flow behavior, the residence time distribution (RTD) and the temperature profile inside the equipment. This work aimed at studying the influence of the operating conditions on the RTD and the axial temperature profile of the product in a SSHE, so as to characterize the product flow behavior. RTD experiments were carried out in a continuous laboratory pilot-scale SSHE by means of a colorimetric method. Experiments showed that high product flow rates led to a narrowing of the RTD and thus to less axial dispersion, due to the enhancement of the radial mixing with the decrease in the apparent viscosity of the product. Spreading of the RTD was obtained for lower refrigerant fluid temperatures, due to a higher radial temperature gradient between the wall and the center of the exchanger, leading to a higher gradient of the apparent viscosity. This effect increased the difference in axial flow velocities and thus the axial dispersion. These results can be useful for the optimization and modelling of crystallization processes in SSHEs.