Rate controlling mechanisms in atmospheric freeze drying : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Bioprocess Engineering at Massey University, Manawatū, New Zealand
Atmospheric freeze drying (AFD) can be considered a cost-effective alternative to vacuum freeze drying (VFD) but the very slow drying rate associated with it limits industrial scale adoption. Nevertheless, there are applications, particularly with thin sections such as sliced fruit or leaves, for which hops is an example, where the rate of AFD is relatively high. Therefore, it is important to understand the rate-controlling mechanisms in AFD and how it is affected by the structure of the fruit or leaf. Such understanding will help identify the bottlenecks of the process and thus the steps that may be taken to overcome them. This project has developed a mathematical model for AFD by considering the intrinsic material properties and the theoretical principles of heat and mass transfer. This could help the end-users to run simulations of AFD for different products and arrive at the best drying strategies to achieve faster drying rates before doing time-consuming and expensive experiments. In this work, hops are considered as the model system for the model development. The study needed high-quality experimental data for continuous in-situ weight loss measurements during AFD and there was a lack of such data in the literature reviewed. To deliver this, part of the project has developed a new experimental apparatus. The experimental apparatus developed is capable of continuous weight-loss measurement and data logging temperature and RH of the air in the drying chamber. Temporal weight-loss trial data are used to fit parameters and predict the drying rate and product weight loss. During the study it was found that AFD of hops is a mass transfer limited process. The drying rate was found to increase with process temperature and the adsorbent to hops ratio. Air circulation also helped in increasing the drying rate. The one-dimensional model developed to simulate AFD of hops was able to predict the drying behaviour based on the process parameters and the fitting factors for the hops. Ice sublimation was also studied in this project, based on the hypothesis that the AFD of hops is a type of pure ice sublimation with an additional layer of resistance to mass transfer. This hypothesis was found to be true for the present case and the model was development based on this.
Freeze-drying, Hops, Drying, Mathematical models
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