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.     

Residence time distribution measurements and simulation of the flow pattern in a scraped surface heat exchanger during crystallisation of ice cream

Scraped surface heat exchangers are used in the food industry to process highly viscous fluids, and ice-cream in particular, but most of the time, the influence of operating conditions on product quality is poorly understood. The objective of the study is to develop simple tools to help industrials understand, and then optimise their fabrication process. Residence Time Distribution (RTD) has been characterised in an industrial pilot system during real ice cream production, after the method had been validated in an experimental set-up with a simple mixture of water and sucrose. It has been shown that in its dimensionless form, RTD depend slightly on flow rate and scraper rotational speed. A simple model of flow pattern applicable to SSHE during crystallisation was developed to reproduce the observed RTD. It distinguishes two zones: the volume of fluid near the cooling wall where ice is generated and which is swept by the blades and the volume of fluid near to the rotor. Therefore, the model considers two parallel plug flow reactors with axial dispersion, and which exchange fluid by radial mixing. After adjustment of the model parameters, a good agreement was obtained with experimental results. The flow rate is lower in the zone near the cooling wall; this can be due to a higher ice concentration leading to higher viscosity. This approach can contribute to better understand, optimise and control SSHE used for ice cream production.     

TEMPERATURE VARIATIONS IN the OUTLET FROM SCRAPED SURFACE HEAT EXCHANGERS

The radial temperature differences and the stability of the temperatures in the product outlet from scraped surface heat exchangers (SSHE) have been investigated over wide ranges of operating conditions. When the flow was vortical, the temperature were stable and no radial differences occurred. When the flow was laminar, channelling effects occurred. the radial temperature differences of the outlet from SSHEs were sometimes very large. the flow was unstable; this was most obvious from the temperature variations close to the blades. In some cases the central parts of the product passed straight through the SSHE without being heated or cooled. Thus, efficient mixing of the product after the SSHE is of primary importance in many applications with high-viscosity products, e.g., aseptic processing, or the modelling of the performance of SSHEs. A static mixer after the SSHE eliminated the radial temperature differences, but in some cases unstable temperatures remained after the mixer due to unstable flow conditions in the SSHE. the degree of stability was influenced by the design of the SSHE and by the flow rate.     

Online ice crystal size measurements during sorbet freezing by means of the focused beam reflectance measurement (FBRM) technology. Influence of operating conditions

In ice cream and sorbet manufacturing small ice crystals are desired to deliver a product with a smooth texture and good palatability. This research studied the influence of the operating conditions on the ice crystal size and the draw temperature of the sorbet during the freezing process. The evolution of ice crystal size was tracked with the focused beam reflectance measurement (FBRM) technique, which uses an in situ sensor that makes it possible to monitor online the chord length distribution (CLD) of ice crystals in sorbets containing up to 40% of ice. The refrigerant fluid temperature had the most significant influence on the mean ice crystal chord length, followed by the dasher speed, whereas the mix flow rate had no significant influence. A decrease in the refrigerant fluid temperature led to a reduction in ice crystal size, due to the growth of more small ice crystals left behind on the scraped wall from previous scrapings. Increasing the dasher speed slightly reduced the mean ice crystal chord length, due to the production of new small ice nuclei by secondary nucleation. For a given refrigerant fluid temperature and dasher speed, low mix flow rates resulted in lower draw temperatures, due to the fact that the product remains in contact with the freezer wall longer. High dasher speeds warmed the product slightly, due to the dissipation of frictional energy in the product, the effect of which was in part moderated by the improvement in the heat transfer coefficient between the product and the freezer wall.     

SCRAPED SURFACE HEAT EXCHANGERS

In this literature survey flow patterns, mixing effects, heat transfer and power required for rotation in scraped surface heat exchangers (SSHE) are thoroughly discussed, with the emphasis on assumptions and results, while the principal design of different SSHEs are only briefly discussed. The flow patterns control the desired radial mixing and the undesired axial mixing. the flow in a SSHE can be regarded as the sum of an axial flow and a rotational flow. the axial flow is laminar and the rotational flow is laminar or vortical. With laminar flow the radial mixing is poor, which causes poor heat transfer and allows the axial flow profile to control the residence time distribution. the precise onset of vortical flow in a SSHE is hard to predict. the vortical flow makes the radial mixing very efficient, giving good heat transfer and perhaps plug flow behavior. However, vortical flow also causes axial mixing which reduces the apparent heat transfer coefficient and increases the residence time distribution. The power required to rotate the shaft and blades is mainly determined by the design of the blades.     

MODELLING of LAMINAR HEAT TRANSFER IN SCRAPED SURFACE HEAT EXCHANGERS

Heat transfer experiments with starch pastes have been performed in pilot plant scraped surface heat exchangers (SSHE). Effects of the following variables were considered in the modelling of the laminar heat transfer: rotational speed; viscosity, flow rate, number of blades, radius ratio and heat transfer direction. The poor radial mixing restricted the heat transfer coefficients. When the radius ratio increased, the heat transfer rate increased. This was probably due to increasing mixing effects from the blades. the flow rate and the heat transfer directions did not affect the heat transfer coefficients. The scatter in the heat transfer coefficients was considerable, due to unstable flow conditions in the SSHEs. However, the scatter was reduced, compared with previous investigations, by the use of a static mixer after the SSHEs, since the radial temperature differences were eliminated. Better design of rotor and blades may improve the performance of SSHEs operating with laminar flow.     

MODELLING of VORTICAL HEAT TRANSFER IN SCRAPED SURFACE HEAT EXCHANGERS

Heat transfer experiments have been performed in pilot plant scraped surface heat exchangers, using starch pastes and water as model products. When the flow was vortical, the backmixing effects were considered using the plug flow and axial dispersion model; the true surface heat transfer coefficients and axial dispersion coefficients were determined from temperature measurements. These coefficients were modelled and the following variables were considered: rotational speed, viscosity, flow rate, number of blades, radius ratio and heat transfer direction. Using these models, the overall heat transfer coefficients and the axial temperature profiles can be predicted quite precisely for SSHEs over a wide range of operating conditions. the first blade improved the heat transfer coefficients while additional blades did not; and heat transfer was higher when the radius ratio was 0.5 than when it was 0.75. the differences between previously proposed models are explained. The most favorable flow pattern in SSHEs occurred when the flow was vortical, but close to the transition point to laminar flow.     

Scraped surface heat exchangers

Scraped surface heat exchangers (SSHEs) are commonly used in the food, chemical, and pharmaceutical industries for heat transfer, crystallization, and other continuous processes. They are ideally suited for products that are viscous, sticky, that contain particulate matter, or that need some degree of crystallization. Since these characteristics describe a vast majority of processed foods, SSHEs are especially suited for pumpable food products. During operation, the product is brought in contact with a heat transfer surface that is rapidly and continuously scraped, thereby exposing the surface to the passage of untreated product. In addition to maintaining high and uniform heat exchange, the scraper blades also provide simultaneous mixing and agitation. Heat exchange for sticky and viscous foods such as heavy salad dressings, margarine, chocolate, peanut butter, fondant, ice cream, and shortenings is possible only by using SSHEs. High heat transfer coefficients are achieved because the boundary layer is continuously replaced by fresh material. Moreover, the product is in contact with the heating surface for only a few seconds and high temperature gradients can be used without the danger of causing undesirable reactions. SSHEs are versatile in the use of heat transfer medium and the various unit operations that can be carried out simultaneously. This article critically reviews the current understanding of the operations and applications of SSHEs.     

Scraped Surface Heat Exchanger Orientation Influence on Particle Residence Time Distributions

Four levels of particle concentration for an “Interaction” experiment and three levels of carboxymethyl cellulose concentration, flow rate, mutator speed and particle size were used for a “Tracer Only” experiment to determine particle residence times in horizontal and vertical scraped surface heat exchangers (SSHEs). Minimum normalized particle residence time (NPRT) was not significantly affected by SSHE orientation in either type in the experiment. However, maximum NPRT was significantly higher with vertical SSHE except in the “Tracer Only” experiment. Average particles traveled faster than average bulk fluid with less deviation in horizontal SSHE whereas mean particle velocity was less than average bulk fluid velocity and had greater deviation in vertical SSHE.     

METHODS to DISTINGUISH BETWEEN LAMINAR and VORTICAL FLOW IN SCRAPED SURFACE HEAT EXCHANGERS

The transition between laminar and vortical flow has been investigated in scraped surface heat exchangers (SSHEs), using new methods based on viscosity and temperature measurements. The transition occurred close to the critical Reynolds number for annular flow of Newtonian fluids. Thus, the methods used to predict the viscosity can be successfully applied to SSHEs despite the very complex flow behaviour of the products, the complex geometry of the SSHEs, and the heat transfer in the SSHEs. In industrial applications, the transition can be detected using the new method based on temperature measurements at the outlet of the SSHEs together with some calculations. This is a very important result, since with this “sensor” the flow pattern can be controlled towards the optimal flow pattern in SSHEs, which in turn is a fundamental requirement when the taste of aseptic products containing particulates are to be improved by the introduction of continuous sterilization.