Abstract:
The aim of this thesis was to study the fundamental principles of the single droplet drying of milk. This involved the investigation of the external conviction of heat and mass transfer, milk droplet diameter changes, milk desorption isotherm and drying kinetics.
An improved glass-filament method for continuous monitoring of the single droplet drying process was developed through a series of modifications. It was used to obtain experimental data on the external heat and mass transfer for pure water droplets with large evaporation rates, on the milk droplet diameter changes, the droplet drying kinetics and the droplet temperature changes.
The influence of the external heat and mass transfer correlation on the drying process has been discussed. The heat balance analysis yields a relationship of hlhm, which can be shown to be different from that calculated using the conventional heat and mass transfer analogy. The above has been demonstrated and its potential impact illustrated in the simulations of drying of the thin skim milk layer with known physical properties.
Extensive experimental results on the effects of drying conditions on drop size changes were obtained. The experiments were conducted using the improved glass-filament technique, complimented by imaging analysis software. Under the constant drying air conditions (temperature and humidity) and in the range tested, a simple relationship between the drop/particle size and the water content has been established.
In order to obtain the desorption isotherm of the milk, a high capacity laboratory humidity generator and control device was developed. It was shown to be highly effective for food research applications. Using this device, the desorption isotherms of milks at elevated temperatures (up to 90°C) and wide humidity range (0-100%RH) were obtained. The experimental approach has allowed us to avoid the problems associated with the amorphous lactose, which is known to undergo glass-transition if the equilibration process takes too long. The data points obtained have been fitted adequately using the GAB model for both the water content and the temperature dependence. Finally, an accurate yet simple model for drying of single droplet, using a reaction engineering approach, was discussed, which does not require solving of partial differential equation for spatial distribution of temperature and concentration. For the first time, two models (the conventional characteristic drying rate curve approach and the reaction engineering approach) are described, validated and compared against a very wide range of the experimental results. Here, both constant drying and time-dependent drying condition are considered. It is the first in literature that time-varying drying conditions are used in the glass-filament type drying experiment.