Single droplet drying of food and bacterium containing liquids and particle engineering

Droplet drying is an important phenomenon that widely presents in industrial powder manufacturing processes. Droplet drying involves simultaneous heat and mass transfer. When a droplet being dried contains dissolved solids, the properties of the solids could lead to interesting behaviours in drying, such as shell formation, crystallization and denaturation of bioactive materials. Such phenomena could make droplet drying a very complex process to understand. Meanwhile, these would also affect the functionality of the final dried particles. The study of droplet drying, the associated functionality changes and the mechanisms behind these changes require careful and sometimes new experimental approaches. Firstly, this study has extended the glass filament single droplet drying (SDD) experiment for studying the three phenomena mentioned above. The glass filament SDD technique was initially optimized to realize accurate measurements of droplet kinetics parameters during drying, including droplet diameter, droplet temperature and droplet mass. Using this technique, the effect of initial droplet properties on the drying behaviour was investigated. It was found that the initial droplet sizes tested did not affect drying behaviour. The drying kinetics of lactose droplets with three initial sizes could collapse to a general trend when correlated using the Reaction Engineering Approach (REA). This was the first time that this has been investigated. The so-called material-specific master activation-energy curve provides a useful quantitative description of the drying history for all the initial sizes tested. The drying kinetics of high solids skim milk droplets with an initial concentration of 50 wt% was also experimentally determined for the first time in literature and correlated with the REA modelling. Once again, a master activation energy curve was established, which is useful for predicting the drying history of the high solids content skim milk droplets at varied drying air conditions. It is noted that industrial operation in skim milk drying plants, 50 wt% or thereabout is a common practice. By comparing the drying behaviour of 50 wt% skim milk droplets to that of lower initial concentrations previously reported, it was found that the effect of drying air temperature is solids content dependent at high solids levels. Both droplet shrinkage kinetics and shrinkage ratio during the high solids milk drying were more significantly affected by the drying air temperature used. On the contrary, the drying rate appeared to be less affected by the increase in the drying air temperature. Perhaps this indicates that the mass transfer limiting regimes become real. The shrinkage coefficient of milk droplets with initial concentrations between 10 and 50 wt%, when correlated to the corresponding initial concentration, formed an consistent trend. This trend was used to develop a master equation for predicting the shrinkage kinetics of milk droplets within the stated range of initial concentration. The newly developed in situ analysis of particle surface formation allows the changes in the dissolution behaviour of the semi-dried particle to be monitored while droplet drying is in progress. By comparing the changes in dissolution behaviour of four dairy materials during drying, the possible material migration behaviour during milk droplet drying was examined. Fat was considered to start to accumulate on the surface of the droplet being dried as soon as drying started. Such accumulation appeared to take some time to complete. By the middle stage of drying, the fat was most probably concentrated on the surface layer leading to the dominant fat coverage on the spray dried milk powders, which provided evidence to the claims made in previous studies using XPS. Lactose was used as a model sugar to study its crystallization behaviour during droplet drying, which was achieved by both the improved SDD experiments incorporating dissolution tests and post-drying instrumental analysis including SEM and XRD. Dried lactose particles could possess two-layer morphology with the interior showing higher crystallinity. This finding demonstrated the importance of the moisture content on the crystallization during drying. It is believed that as drying progresses there is a critical crystallization period where the crystallization rate was maximal. Drying conditions that enhance the lactose crystallization during drying include a higher drying air temperature, a higher initial α-lactose level, and a longer exposure time. The study of the retention of microbial viability was carried out with a model bacterium, Lactococcus lactis ssp. cremoris. By correlating the inactivation kinetics explicitly to the drying process parameters, it was found that the inactivation constant kd obtained at six drying conditions can be correlated to the droplet temperature Td with a general trend. The results indicated that the heat-induced inactivation remained to be the main cause of cell death during a convective droplet drying process at an elevated temperature. The effects of other droplet drying parameters on cell survival were also discussed. Environmental parameters influence the cell inactivation kinetics by either stabilizing cellular components or by altering the droplet drying kinetics. The prominent protective effect provided by reconstituted skim milk was found to have at least two mechanisms: (1) particle temperature was kept lower at the later stage of drying; (2) there were fewer cells presented on the surface of dried particle. The trend of kd plotted against Td showed that a transition temperature exists at around 62-65oC, coinciding with the peak of denaturation of 30S ribosomal subunit on a thermogram as measured by DSC in previous works.