Abstract:
The aim of this thesis was to study the fundamental principles of the application of spray crystallisation to food production. This involved the investigation of the heat and mass transfer, crystal growth, nucleation and microstructure formation within individual droplets. A single droplet experimental rig was developed which proved to be a versatile method for investigating the freezing of droplets. It was used to obtain experimental data to analyse the temperature transition, mass transfer, crystal growth, nucleation and microstructure of individual droplets undergoing freezing. Modelling regimes of the different processes were investigated and analysed for accuracy by comparing the model predictions with the experimental data. A simple energy balance model was found to be accurate for predicting freezing times of water droplets. However, for modelling the freezing of sucrose droplets, the application of the model was limited to equilibrium freezing conditions (low freezing rates). It was found that for the application of spray Crystallisation to a particular solution, the influence of the solute on the mass transfer from the droplets needs to be well defined in order to achieve accurate model predictions of freezing times. In respect of the nucleation of ice within sucrose solution droplets, it was observed that there are two distinctive nucleation locations: at the droplet surface and within the droplet volume. Statistical data on nucleation was gathered which showed that established models could be used to predict the nucleation temperatures for a droplet population. The microstructure development of freezing droplets was observed and a number of interesting phenomena were found: the formation of a concentrated sucrose layer on the surface of sucrose solution droplets, the redistribution of fat globules towards the droplet surface at low freezing rates and the partitioning of whey proteins at the droplet surface, were all observed and characterised.