Mechanisms of Particle Formation and Size Prediction in Whole Fat Milk Droplet Drying: From Single Droplet Drying Experiments to Spray Drying

Accurate prediction of particle size distribution in spray-dried dairy powders remains a critical unsolved challenge in industrial drying processes, leading to inconsistent product quality and energy inefficiencies. This thesis addresses this gap by systematically investigating the microphysical mechanisms governing single droplet drying and particle formation, with a focus on whole milk drying process. Through a comparative analysis of three single droplet drying methodologies—sessile, filament-suspended, and acoustic levitation. This research identifies filament suspension as the most representative experimental method for replicating spray drying conditions.
Experimental results reveal that droplet drying behaviour transitions through three distinct regimes including shrinkage, plateau and expansion which depending on solid content (20–50% w/w) and temperature (20–127°C). A critical expansion temperature was identified, decreasing linearly from 99°C for 20% solids to 86°C for 50% solids, driven by colligative boiling point depression and crust mechanical properties. Imaging and synchronized thermocouple measurements showed that low-solid droplets undergo ductile expansion, while high-solid droplets experience brittle fragmentation due to rapid shell formation and internal vapor pressure buildup. A mechanistic model was developed to predict expansion onset and final particle size by coupling liquid bridge strength which derived from interfacial tension and viscosity with internal vapor pressure calculated via ideal gas law approximations. The model successfully correlates single droplet expansion ratios with pilot-scale spray dryer particle size data, achieving predictive errors less than 7% for D50 values under different drying conditions.