Flow processing and gel formation for producing biopolymer drops with functional shapes and diffusion properties
This thesis focuses on the development of novel processes which combine flow-induced drop deformation and simultaneous temperature-induced gel formation in order to create shaped biopolymer drops. The drops are functional in terms of rheology and diffusion properties, factors strongly influencing the consistency of food products and flavour or drug release.
The concept of drop-shaping was developed in a discontinuous flow process in a 4-roll mill with millimetre-sized gelatine drops in high-viscous oil. A variety of gelled drop shapes were produced, such as ellipsoids, long threads, crosses, and more complex shapes. The impact of flow type, flow strength, drop size, and gel kinetics on drop-shaping was established, and the interplay between the parameters gave great freedom in combining them to achieve a specific drop shape. The mechanism behind drop-shaping was explained in terms of elongation, relaxation, and pinching.
This knowledge was implemented in the development of a flow channel for fast continuous drop-shaping of micrometer-sized and highly monodisperse κ-carrageenan drops in sunflower oil. Flow obstacles could be inserted into the flow channel for shaping under different flow types. A narrow parallel shaping channel made it possible to create drops of triangular or parabolic shape by carefully varying process parameters. This process also gave the opportunity to study gel and surfactant kinetics on short time scales and at high cooling gradients which can not be achieved in ordinary equipment. Rheological measurements proved that emulsion viscosity can be increased by 20% by replacing 17% of spherical micrometer-sized κ-carrageenan drops in sunflower oil with the same amount of parabolically shaped drops of the same volume.
Diffusion properties were established by correlating the self-diffusion behaviour of poly(ethyleneglycol) (PEG) of different molecular weights in relevant κ-carrageenan bulk gels for drop-shaping with the gel's microstructure. This was achieved by the synergistic combination of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) diffusometry. Different κ-carrageenan microstructures were produced by varying the cooling gradient and salts for gelation, since these parameters are also decisive for drop shaping. It was shown that microstructures with small voids gave highly reduced PEG self-diffusion coefficients, and that the self-diffusion coefficient was dependent on the diffusion observation time. These effects decreased with increasing void size. However, if large-scale gel heterogeneities were present, diffusion was not strongly hindered anymore. The diffusing probe resided mainly in the large openings and avoided the areas of gel networks. PEG diffusion was measured in spherical gelled κ-carrageenan drops with an average diameter of about 2.5 μm. The results were similar to those obtained in the bulk phase studies. This suggests that spheres and bulk phases have the same network structure.
Many of the results here were only obtained by combining techniques that are already well established in their own right. Few have made use of this synergistic potential up till now.