23. Processing of Functional Capsule Powder Particles Based on Multiple Emulsions Using a Prilling Process

Present studies show the ability of cold spray processing (prilling) to tailor the morphology of simple or double emulsion-based fluid systems as investigated for two types of air-assisted nozzle geometries under various processing conditions. The spray process parameters varied were: (i) gas to liquid flow rate ratio (GLR), (ii) spraying pressure and (iii) total mass flow rate. The results depicted that the emulsion flow inside the nozzle (liquid cap) as well as in the spray (outside nozzle) have distinct impact on the resulting product structure due to the respective flow stresses acting. Increasing the flow stresses either lead to an additional dispersing impact or to separation and coalescence of the disperse fluid phase(s). Besides the process parameters, the material characteristics of the emulsion systems such as viscosity ratio λ of dispersed to continuous phase and the interfacial tension γ were varied in a wider range. The results demonstrated a systematic increase in structure stability for higher λ values within a range of 0.32–30. As representative dimensionless numbers, (a) a critical liquid Weber number We _l,Drop,cr/λ and (b) a critical gas Weber number We _g,Drop,cr/λ were defined to describe the effect of liquid cap-tip and air-assisted spraying, respectively, with respect to preserving the disperse microstructure of the treated emulsions. Above these critical We numbers, the dispersed emulsion phase drops were broken up and drop mean sizes were exponentially decreased due to the flow stresses acting either in the liquid flow inside the nozzle or in the spray filament outside the nozzle. Dynamic viscosity η and dynamic moduli (G′, G″) of treated emulsions increased with decreasing droplet size of the dispersed phase(s) thus altering the spraying performance as well as the properties of the liquid product systems reconstituted from resulting spray-chilled powders. A third critical Weber number We _g,Nozzle,cr was derived for the spray droplet (tertiary droplet) generation by the spray filament breakup providing information of the smallest spray droplet that could be attained, while keeping the dispersed emulsion (secondary) droplets unchanged in size. The impact of We _g,Nozzle on the resulting spray (tertiary) mean drop size was systematically explored for internal (INMIX) and external (EXMIX) liquid-gas mixing air-assisted nozzles. High-speed videography and laser shadowgraphy were applied to visualize liquid spray filament stretching and breakup, as well as the velocity distribution in the sprays. Sufficiently gentle spray conditions for complete preservation of the disperse emulsion structure were only achieved in the Rayleigh filament breakup regime.

Accordingly, a pressure controlled rotary “Rayleigh atomizer” was developed to study emulsion spraying by filament stretching and gently spray drop formation, preserving the emulsion (secondary) droplet structure. At the same time pressure adjustment enabled higher throughput rates compared to conventional rotary spraying nozzles for which only centrifugal forces determine filament stretch and throughput rate simultaneously. Filament length and drop size decreased with increasing rotational speed at a given total pressure (centrifugal pressure + static liquid pressure at the nozzle inlet) or flow rate, and the filament length and drop size increased with higher liquid pressures and related throughput rates at a given rotational speed. Chilling solidification of the spray drops was superimposed in selected cases. Prilling (spraying + chilling) was carried out for various emulsion systems in a prilling tower applying average air temperatures of ca. −10 °C for higher melting fat-continuous emulsions down to −50 °C for low melting oil- or water-continuous emulsions, in order to produce solid powder particles. The micro-structure of the solid particles was analyzed in further detail by cryo-scanning electron microscopy (Cryo-SEM). Concerning emulsion structure preservation in the sprayed products, the results clearly demonstrated that the disperse structure can differ significantly from the initial emulsion structure if critical flow stress conditions are exceeded. Respective process-structure functions were also quantified.

For emulsion-based prilled powders, the applicability and adjustability as functional component carriers for controlled release applications is of big interest in industries such as food, pharmaceutical and cosmetics. For related functional component release experiments we designed an in vitro gastric/duodenal setup. With this, the release kinetics of functional components encapsulated/embedded in dispersed secondary or primary emulsion drop phase(s) were quantified under simulated gastric or duodenal digestion conditions. Accordingly, an iron compound (micronutrient) was encapsulated into the primary and/or secondary dispersed emulsion droplets of simple or double emulsions, and related solid emulsion powder particles were produced through prilling applying our selected air-assist  atomizers. In a first testing step, the iron release kinetics for selected products were systematically investigated in the in vitro gastric system at pH ≈ 2.0, and quantified as a function of prill powder particle size, secondary emulsion drop size and prill powder storage time under ambient conditions.