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Structure engineering of emulsions by micro-machined elongational flow processing

Deliverables

The simultaneous superposition of flow and gelation can be used for the synthetic of non-spherical gel beads and microcapsules. The successful formation of these particles depends on a well defined choice of all parameters (e.g. flow velocity, concentration, temperature). Various oil-soluble emulsifiers were used in order to decrease the interfacial tension between the continuous oil-phase and the aqueous biopolymer droplet. An increased surfactant concentration decreased the interfacial tension, which finally induced enhanced droplet deformation. These studies contain information on interfacial kinetics and colloid science and they can be used to teach students in advanced topics of interfacial and food science. Results of our research program will be part of the lecture "colloid chemistry" which is held in regular intervals. This applied research provides an excellent opportunity for students to connect the somewhat abstract concepts to real systems. A variety of techniques will be used to present the scientific problems. The visualisation of adsorbed food emulsifiers at interfaces by means of Brewster-Angle-Microscopy is a good example for such projects. Interfacial pressure versus mean molecular area curves which were measured by a film balance illustrate the role that different molecule components may play in stabilising the interface. The kinetics of the adsorption process will be characterised through measurements of the interfacial tension as a function of time.
By combining structure kinetics with elongation flow at well-defined rates and residence times a fixation of the drop deformation within a narrow range can be obtained. This allows for a more uniform flow-induced structuring at reduced energy input than has been possible up to now. The concept opens up possibilities to create new structure elements of significance for the texture and the final acceptance of the consumer. Shapes such as rods, ellipsoids, spheres etc can be generated. Shapes can trigger interactions of high phase volume emulsions and lead to different shape-dependent rheological and sensorial properties. By flow-induced structuring, products with a range of properties can be generated from the same mixture of ingredients. This means that raw materials can be used in a more efficient way to increase stability with a lesser need of unwanted stabilising agents etc. The basic knowledge to generate droplets with preferred characteristics, for instance small droplets with a narrow size distribution, can also be used for the production of active particles, such as micro-capsules carrying functional components. This can improve the over all quality of the food and the micro-capsules can carry functional components such as vitamins, antioxidants, flavours. The outcome of this project will be of importance not only for the emulsions dealt with here, but also for other structured food products. The double-capillary-process principally generates two concentric droplets and produces core-shell-capsules with an elastic biopolymer skin and an inner phase that always remains in a liquid state. Threadlike particles with typical sizes of 1.8mm were formed. Production with thinner capillaries leads to smaller capsules. The active agent solved in the aqueous inner core can be chosen independently from the biopolymer shell material, unless it weakens the stability of the enclosing membrane.
The deformation of the droplets suspended to shear forces generally increases by lowering the interfacial tensions of the reacting liquids. Different types of oil-soluble emulsifiers were used in order to increase surface areas. This concept can also be used for the synthesis of smaller, micron-sized particles. In order to get more insight into basic mechanisms of interfacial processes, surface forces and surfactant aggregation processes under static and dynamic conditions were investigated. The kinetics of the membrane formation and the surface viscosity have been studied by observing adsorption behaviour using a pendant drop, the DuNouy ring method and a deep channel viscometer. The basic structures of these surfactant membranes were also investigated using Brewster-Angle-Microscopy. The results indicate the formation of structured surfactant monolayers and food-emulsifier-polysaccharide complexes. There are significant differences between the introduced emulsifiers. But all investigated surfactants are suitable to improve the droplet deformation. The data, thus obtained, were used to investigate deformation and bursting of droplets, skin formation and fixation of flow induced droplet structures. Non-spherical capsules are characterised by relatively large surface areas and this might improve accumulation times in the stomach or intestine walls. These processes can also lead to more efficient drug incorporation. Further investigations are required to develop functional microcapsules in which the permeability is controlled by changes of temperature, pH, or chemical compounds. Such smart microcapsules can be synthesized by using non-spherical particles as a kind of precursor, which is modified and coated in a second step by a multifunctional shell.
The flow cell for particle generating and structuring of such particles in non-equilibrium shapes consists out of three major unit-operations: Sizing, Deformation and Gelling of the dispersed phase that is injected into a co-flowing continuous phase. Due to optimised tuning of flow, gelation, and interfacial kinetics the flow cell can produce dispersed phases with narrow size distribution and, as a consequence, equally shape and fixated particles. The use of non-equilibrium shape particles incorporated into consumer products offers a wide range of different textures, ingredient releases and other physical properties. Since in particulated food and other suspensions not all dispersed particles have to be shaped, a minor amount of shaped particles are sufficient to produce new products. The flow cell therefore aims to generate not pure mass of gelled particles but fewer well-characterised shape particles. Such well-defined particle will then be added to existing or newly designed products to enhance amongst other properties the chemical, physical, sensory, and release properties. From a commercial point of view, the flow cell is a new apparatus to produce well-defined particulated dispersions of gelled food-grade or non food-grade aggregates. Beside the invented apparatus, down-stream devices, such as mixers, packing machinery have to adapt to the new technique. Here an input to machine manufactures is expected. A second commercial spin-off is the mentioned generation of shaped particles and the input on enhanced or newly developed food or other consumer produces. The aspect of generating microcapsules is an additional bonus track in terms of new products and techniques. Form food and non-food products social benefits in terms of low fat products, products where critical ingredients such as gelatin is replaced by shaped particles, etc. is expected. New and cheap methods to produce microcapsules have an important impact on health aspect of the society. Scientifically the flow cell links an enormous body of work related to fluid dynamics of co-flowing fluids, gelation kinetics on short times, influence of surfactants on deformation and gelling of biopolymers, as well as incorporation into an industrial apparatus. All mentioned aspects covering new areas in science and technology and are therefore first but important steps.
The flow-induced synthesis of non-spherical gel beads and microcapsules succeeds, when these particles are orientated and stretched due to the action of velocity gradients and are chemically fixed at the same time by polymerisation reactions. The simultaneous superposition of flow, gelation and relaxation leads to the formation of unusual shapes, such as rod-like particles, pearl strings or parachute shaped particles. The synthesis of these capsules can be performed in a double capillary apparatus. In this instrument, the inner capillary is filled with an aqueous solution of calcium lactate and the outer tube contains a dilute solution of sodium alginate. As soon as these two liquids get in contact, a surface gelation occurs and a stable gel phase forms within a few milliseconds. The double capillary generates two concentric droplets, and the inner phase, containing the calcium ions, always remains in a liquid state. The outer droplet finally forms the elastic biopolymer shell (skin). The inner phase can also contain additive chemical compounds like dyes or vitamins, unless it does not weaken the mechanical stability of the surrounding shell. The double capillary is inserted in a larger tube, where sunflower oil is flowing. As soon as the concentric droplets are formed at the tip of the double capillary, they are suspended to shear forces, which lead to large deformations. Due to the action of flow different types of non-spherical capsules and gel beads were synthesised.
The prototype bench scale flow cell, was equipped with a double capillary system for controlled droplet sizing, and the geometry of the cell was improved to optimise the flow profile to yield structured particles. The effects of process conditions like oil and water phase temperatures and flows were investigated. Optimal emulsifiers and gelling agents were selected from the fundamental investigations on membrane formation kinetics and gelling kinetics. In a non-gelling system, strong elongated particles were shown (by in-line registration with a high speed camera) to be formed in the shaping zone of the cell. In order to capture these particles for application in food products, gelling kinetics must be such that these structures are fixed when leaving the flow cell. In the optimised gelling system the results are less clear, but deformation seems to take place indeed. However, after collection of the particles and characterisation by means of microscopy, only particles with very limited aspect ratio could be detected in low concentration. These results confirm the process principles to achieve structured particles by shaping in the designed flow cell. However further insight in the kinetics of biopolymer gelling in small droplets must be gathered to select the proper gelation system for particle fixing.

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