Experimental and computational fluid dynamics studies on spray-freeze-drying and spray-drying of proteins

This thesis presents an experimental and computational fluid dynamics (CFD) study of spray-drying, spray-freezing and spray-freeze-drying of whey proteins. The effects of varying feed concentration (20 - 40% w/v) and outlet temperature (600 - 120 degrees Celcius) on whey protein denaturation (determined by DSC) and solubility at pH 4.6 (by Kjeldhal and RP-HPLC methods) have been investigated in a pilot-scale cocurrent spray dryer. The study confirms that low outlet gas temperatures (60 and 80 degrees Celcius) produce the lowest amount of denaturation, with almost complete denaturation observed at 120 degrees Celcius. Slightly more denaturation was found with a 40% feed concentration. A reversed phase HPLC technique has been applied to measure the loss of solubility of a-lactalbumin and P-lactoglobulin. Significantly higher losses in solubility were observed for P-lactoglobulin compared to alactalburnin. Increasing the feed concentratioq at higher outlet temperatures also caused noticeable increases in insolubility. The reversed phase HPLC results were consistent with those obtained from total protein nitrogen content (Kjeldhal) analyses. This comparative study suggests that the protein solubility can also be calculated from RP-HPLC technique. Spray-freeze drying is an alternative approach to spray drying, which is less likely to cause protein denaturation and loss of solubility. Conventional freeze-drying involves high capital and operating costs, due to the low temperatures, high vacuum and very long drying times. One solution to this problem is to reduce the dimensions of the material being dried. This is the basis of the spray-freeze-drying technique, involving atomisation of a liquid to form droplets, freezing the droplets and subliming off the ice at low temperature and atmospheric pressure in a fluidised bed. However, the quantities of gas required for atmospheric freeze-drying are prohibitively expensive. A pilot-scale spray-freeze-drying process was investigated, in which fluidisation was performed at sub-atmospheric pressures, allowing rapid freezedrying (in about one hour) but using much less gas. This was demonstrated using whey protein which yields a product with a highly porous structure, with little loss of protein solubility. This process has potential to produce high-value-added food and pharmaceutical products more quickly and cheaply than is currently possible by commercial vacuum freeze-drying processes. CFD simulations were developed for short and tall form spray-dryers to study the particle velocity, temperature and residence time during drying. These simulation results agreed well with the published experimental data. The tall-form spray dryer model predictions showed that more particles impact on the cylindrical wall position and this may affect the protein denaturation and solubility. This study suggests that a short form dryer with a bottom outlet is more suitable for drying of proteins. Similarly, a CFD simulation for the spray-freezing operation was developed to study the gas flow pattern and particle trajectories and histories. This CFD model also includes the latent heat effects during the phase change. The simulation predictions agreed reasonably well with the experimental results. A comparison of simulations for a solid and a hollow cone spray suggested that the latter yields lower particle temperatures with low particle collection efficiency. A modified chamber geometry was proposed and the simulation showed that the new design could achieve higher particle collection efficiencies.