Phase Separation and Gelation of Biopolymers in Confined Geometries
Doktorsavhandling, 2012

Many biopolymer mixtures exhibit segregative phase separation that generates regions enriched in one polymer and depleted in the other. Today, much is known of how phase separating biopolymer systems behaves in bulk phase, and the final morphology can be controlled and tailored with high precision. However, few experimental studies have examined such systems in restricted geometries, where the system might be affected and limited by surrounding surfaces. Restricted geometries may be of importance for the properties of multi-phase materials, such as foods and composite materials. In this thesis the effects of confinements on phase separating and gelling biopolymer mixtures has been investigated, with emphasis on the phase separation kinetics and the final morphology. Two biopolymer systems were used; gelatin–maltodextrin and whey protein isolate–gellan gum mixtures. Various types of restricted geometries such as microdroplets, parallel cover glasses, and networks of cellulose fibres were used to evaluate the effects of confinements. The structural evolution and the final microstructures were characterized by confocal laser scanning microscopy and image analysis. The findings showed that the size of a confinement had a marked impact on the resulting microstructure in both systems. The morphology observed within the studied confinements differed from those seen in the bulk phase and included core-shell, Janus-like microstructures in microdroplets and columnar structures in confinements with solid surfaces. In the whey protein isolate–gellan gum system, the structural evolution was similar inside large microdroplets and the bulk phase. It was also found that when the characteristic wavelength of the spinodal decomposition was comparable to the size of the confinement, a structure transition took place and core-shell structures were obtained. Furthermore, the confinement size influenced the initiation of the phase separation in the gelatin–maltodextrin system. Use of conventional emulsification and a microfluidic technique to produce microdroplets was also evaluated. The former method allowed examination of microstructures in emulsion droplets of different sizes within a single sample, ensuring the same composition and thermal history. Monodisperse microdroplets with highly reproducible internal morphology were generated using microfluidics. The internal microstructure was designed using different cooling protocols to control the phase separation and gelation kinetics, and the biopolymer concentration. Homogeneous (no phase separation), discontinuous, and bicontinuous microstructures were observed. The effects of confinement on the internal morphology were investigated by performing elastic Lennard-Jones simulations, which showed good correlation with the experimental structures. Initial studies of the relationship between the microstructure and the diffusion properties of phase separated gels provided promising results, which opens new possibilities to control such properties in microdroplets through careful design of the internal morphology. To summarize, the work underlying this thesis has demonstrated that exploitation of phase separated systems within confined geometries offers great potential to tailor materials with new functionalities.

microstructure

gelation

phase separation

spinodal decomposition

confinement

biopolymer mixture

image analysis.

structure evolution

gelatin–maltodextrin

confocal laser scanning microscopy

whey protein isolate–gellan gum

Room HA2, Hörsalsvägen 4, Chalmers
Opponent: Prof. Ian Norton, University of Birmingham, UK

Författare

Sophia Fransson

Chalmers, Kemi- och bioteknik, Teknisk ytkemi

SuMo Biomaterials

Livsmedel består av många olika komponenter såsom proteiner, kolhydrater, fett, vitaminer och vatten. Blandningar av proteiner och polysackarider (biopolymerer) delar gärna upp sig i separata faser, vilka är berikade av den ena komponenten och utarmade på den andra, ett fenomen som kallas för fasseparation. Fasseparation av biopolymerblandningar i kombination med gelbildning används idag i livsmedelsindustrin i produkter såsom lågfettprodukter. Fasseparationsprocessen och hur denna kan styras är till stor del känt i bulk. Däremot vet man mindre om hur denna påverkas inuti begränsande geometrier - vilka påträffas i komplexa strukturer som livsmedel. Forskningen i denna avhandling har fokuserat på att undersöka hur blandningar av biopolymerer påverkas då de stängs in i väldefinierade geometrier, vilka till största delen har utgjorts av mikrometerstora droppar. Resultaten visar tydligt att begränsande geometrier har stor påverkan på strukturutvecklingen och den slutliga mikrostrukturen samt att det är möjligt att kontrollera den med hög precision. Vidare har ny metodik för att producera droppar med reproducerbar struktur utvecklats. De erhållna resultaten skulle kunna användas för att skapa material med funktionella egenskaper. Genom att utnyttja hur biopolymererna organiserar sig själva skulle man kunna skapa kapslar inifrån, där strukturen kan skräddarsys, vilka skulle i framtiden kunna användas för kontrollerad frisättning av aktiva ämnen såsom vitaminer eller läkemedel.

Food consists of several components such as proteins, carbohydrates, fats, vitamins and water. Blends of proteins and polysaccharides (biopolymers) often separate into different phases, which are enriched in one component and poor in the other, a phenomenon that is called phase separation. In the food industry, phase separation of biopolymer mixtures is used in low fat products among others. The mechanisms for phase separation and how to control it are to large extent known in bulk. However, little is known about the effects on it within small, restricted geometries – which is found in complex structures such as food. The research in this thesis have focused on exploring the effects on biopolymer mixtures confined in defined geometries, which to large extent were constituted by microdroplets. The results presented show that the volume had a marked impact on the both structural evolution and the final microstructure of the mixtures and that it was possible to control these processes with high precision. Furthermore, a new methodology to produce reproducible microstructures within droplets of micrometer sizes was developed. The results from this research could be used to create materials with functional properties. By utilizing the biopolymer’s ability to self-organize could be used within capsules. In the future, such capsules could potentially be used in applications for controlled release of active substances such as vitamins or pharmaceuticals.

Ämneskategorier

Materialteknik

Annan teknik

Livsmedelsteknik

Kemi

Styrkeområden

Materialvetenskap

ISBN

978-91-7385-748-2

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie

Room HA2, Hörsalsvägen 4, Chalmers

Opponent: Prof. Ian Norton, University of Birmingham, UK