Ultrasonic emulsification in biopolymer complex emulsion systems

An emulsion is a colloidal mixture of two immiscible liquids, in which one is dispersed into another with the help of shear forces and the presence of surface-active compounds. The range of applications for emulsions is enormous, with emulsions found in industries such as food, cosmetics, medicine and petrochemicals. Understanding how to control the physical properties of emulsions has been a long-standing research challenge. In particular, the ability to achieve long-term stability of emulsions is of great significance in many practical applications. The physical stability of emulsions can be improved with a reduction in the droplet size of the dispersed phase, which is determined by the extent and intensity of the emulsification process and the associated balance between droplet breakup and coalescence. High-intensity low-frequency ultrasound (US) is a promising high-shear emulsification method that leverages the physical effects created by acoustic cavitation. Biopolymers are naturally occurring polymers that have many applications in food and personal care products. In particular, biopolymer emulsions are found in many daily products. The ability to tailor the physical properties of such emulsions, including texture, stability and flow behaviour, is a focus of much research and development. Emulsions are commonly fluid suspension, however they can also undergo a solution-gel transition, greatly altering the macroscopic texture into a semi-solid material. Emulsion gel formation has been reported in the literature, however, little is understood about the role of the emulsification process, in particular, associated turbulent flow, on the mechanisms on emulsion gel transition. In this thesis, ultrasonic emulsification involving complex biopolymer systems has been studied. The thesis presents new fundamental understandings of this process, in particular the effect of the turbulent environment on biopolymer emulsion gel formation. This understanding has been used to develop a novel type of emulsion gel, for which the formation mechanism has been investigated and described. A novel demulsification technology has been also developed for enhanced oil separation from biomass systems. The basic concepts underlying emulsions, emulsification and biopolymers are introduced first introduced in Chapter 1 along with fundamentals of US and sonochemistry. In Chapter 2, a detailed review of the literature is presented describing the current understanding of emulsification in turbulent flows, the mechanism and control of ultrasonic emulsification, the mechanism of colloidal sol-gel transition in biopolymer emulsion systems, and flow-induced phase inversion. Additional attention is given to literature on casein micelles and dairy emulsion, as this was the system explored in most detail experimentally in the thesis. Knowledge gaps in the literature are identified and the research aims and thesis scopes placed into this context. The experimental details, including materials, methodologies and analytical techniques are presented in Chapter 3. The experimental research results are presented and discussed in Chapter 4 to 6. In Chapter 4, three essential experimental parameters in the ultrasonic emulsification process, namely sonication time, acoustic amplitude and processing volume, are individually investigated, theoretically and experimentally, and correlated to the emulsion droplet sizes produced. The results show that with a decrease in droplet size, two kinetic regions can be separately correlated prior to reaching a steady-state droplet size: a fast size reduction region and a steady-state transition region. In the fast size reduction region, the power input and sonication time can be correlated to the volume-mean diameter by a power-law relationship, with separate power-law indices of -1.4 and -1.1, respectively. A proportional relationship is found between droplet size and processing volume. The effectiveness and energy efficiency of droplet size reduction has been compared between US and high-pressure homogenisation (HPH) based on both the effective power delivered to the emulsion and the total electric power consumed. Sonication could produce emulsions across a broad range of sizes, while high-pressure homogenisation is able to produce emulsions at the smaller end of the range. For ultrasonication, the energy efficiency is higher at increased power inputs due to more effective droplet breakage at high US intensities. For HPH the consumed energy efficiency is improved by operating at higher pressures for fewer passes. At the laboratory scale, the US system requires less electrical power than HPH to produce an emulsion of comparable droplet size. The energy efficiency of HPH is greatly improved at large scale, which may also be true for larger-scale ultrasonic reactors. In Chapter 5, shear-induced emulsion gel formation has been demonstrated for the first time in a micellar casein emulsion system subjected to micro-turbulent environments created by ultrasonication and high-pressure homogenisation. Importantly, the emulsion and gel-like properties are stabilised solely by the casein micelles in combination with the droplet packing structure, circumventing the usual requirement for chemical surfactants and stabilisers. The mechanism of shear-induced emulsion gel formation has been investigated experimentally in relation to the roles of casein micelles, oil fraction and shear environments, and discussed in relation to existing theories. Based on this, the mechanism of gel formation has been proposed as a novel colloidal packing phenomenon, triggered by the formation of droplet breakup under micro-turbulent environments, which results in fractal packing of droplets and casein micelles over three orders of magnitude from 10-1 to 101 micron. In Chapter 6, the proposed mechanism underlying the shear-induced formation of casein micelle emulsion gels has been extended to other systems by providing a more general explanation on a fundamental level. The effect of micellar casein concentration on the sol-gel transition is investigated to reveal a shear- and concentration-dependency and non-monotonic rheological behaviour. A successful imaging protocol is developed to examine the in-situ interfacial rearrangement of micellar casein. The method involved substituting food oils with a volatile solvent, so that both the oil phase and water can be removed and the micellar bridging network inspected in detail by scanning electron microscopy. A large degree of interfacial deformation of micellar casein has been observed, from spheres to discs, which were further inter-connected to form cellular protein 3D networks. The combination of emulsion microstructure and the rheological profiles could be simply explained by the established sol-gel transition models in colloidal suspension systems. Rather, these experimental results are explained using a combination of two theories: random Apollonian packing and droplet size distribution scaling under turbulent flows. The existence of a sol-gel transition could be related to power-law correlations in the number size distribution of the droplets within a certain size range. The results confirm that the casein micelles and emulsion droplets could be unified as ‘spheres’ participating collectively in random close packing. The mechanism of sol-gel transition under turbulent flows has been proposed more generally as a result of turbulence-driven dynamic changes in the number size distribution of ‘spheres’. The formation of shear-induced emulsion gels has been then further extended with key criteria proposed, based on the new multi-theory mechanism framework. The understandings gained in this thesis are summarised and concluded in Chapter 7. Future research directions are also discussed based on these new findings. Highlights of two commercialisable extensions of the work of the thesis are also provided as supplementary material in Appendices A and B.