Microstructure of spray dried particles at different scales of drying
Abstract: Spray drying is a preferred method to produce dry powder of thermally sensitive materials such as food and pharmaceuticals. Drying conditions and formulation determine primary properties such as morphology and composition that finally provide functionality such as dissolution, encapsulation and flowability. Even though spray drying is a well-established method to produce dry powder numerous challenges are still encountered before we have a full understanding of the process. Indeed, the droplets undergo fast water evaporation, dimensional changes and temperature changes. It has previously been shown that adsorption of macromolecules at the droplet surface and phase segregation can occur while drying which will influence the particle properties. The first objective of this work was to provide a deeper understanding on how the surface morphology of powder particles depends on the surface properties of the components. The second objective was to explore how the internal microstructural depends on key properties of the components. The third objective was to investigate how well the particle morphology, surface and internal microstructure in single particle drying can predict the corresponding properties in full scale spray drying. Three lactose-macromolecule (protein, poloxamer, modified cellulose) feeds were dried in a single particle dryer and in a laboratory spray dryer. The impact of the concentration of the polymer on the surface rheology of the feed was observed. The morphology, surface composition and internal composition of the dried particle were observed by scanning electron microscopy (SEM), X-ray electron microscopy (XPS), and confocal Raman microscopy. (Paper I and II). To further investigate the internal microstructure and phase separation a two polymer feed (modified cellulose and maltodextrin) was spray dried at various dryer scales and at different solids content and ratios. The internal morphology was studied by confocal Raman microscopy. (Paper III). To evaluate the influence of the droplet breakup on the microstructure different atomization speed and oil droplet size in an acacia gum, maltodextrin and sunflower oil emulsion was spray dried. The characteristics of the powder were analyzed by LV-SEM and confocal Raman microscopy. (Paper IV). The aim of the last study was to reveal the morphology and internal microstructure correlation of whole milk powder spray dried in a single particle dryer, laboratory dryer, pilot plant dryer and a full scale dryer. LV-SEM imaging and confocal Raman microscopy were the techniques used to characterize these particles. (Paper V). The results show that interfacial properties of the components in the formulation can explain the dried particle morphology, although other factors such as the drying conditions and feed concentration also play a role. Particle surface composition is found to be influenced by the adsorption rate of macromolecules in the liquid feed. Even though spray drying is a fast drying process we found that ingredients can phase separate while drying. Further, although the particle size and drying time differs, analogies in individually dried particles, laboratory dried particles, pilot plant dried particles and full scale dried particles were found in terms of powder morphology, surface composition and internal composition.
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