Mechanistic understanding of emulsion formation during processing

In the light of the recent research interest in emulsion formation, this PhD thesis seeks to expand the current body of knowledge regarding the role of various emulsifiers on emulsion formation mechanisms. This study is particularly relevant to the food industries, where formulated emulsions often result in complex microstructures. Therefore, controlling the production processes requires knowledge regarding the effect of individual constituents on the final product. The experimental investigation presented here examines emulsion behaviour during processing. Attempts have been initially focused on the development of a technique (reflectance technique), and subsequently a methodology, to investigate droplet size evolution during processing. This technique is based on the relationship between reflected light from the emulsion and the droplet size, at any given dispersed phase volume fraction, and emulsifier type. Consequently, measurements of the ‘light reflectance’ during the process can be used to determine the droplet size evolution in real-time. This technique was applied to the study of emulsification in batch mixing systems. The developed methodology was used to investigate the effect of various operating parameters and formulations on the droplet size evolution during processing. These parameters include: the emulsifier type (surfactants, proteins, solid particles and mixed emulsifier systems) and concentration; hydrodynamic condition of the process; and the dispersed phase volume fraction. It was shown that using emulsifiers results in a higher droplet break-up frequency at the early stages of the process. The break-up frequencies remained the same in the presence of a range of emulsifier concentrations. Out of all the emulsifiers used, silica particles showed II the lowest droplet break-up frequency. This is because the interfacial tension is not affected when silica particles are adsorbed on the interface of droplets. It was further shown that the final droplet size is mostly affected by the extent of droplet break-up, whilst droplet coalescence has minimal influence. Therefore, using a higher concentration of emulsifier results in a lower final droplet size, as a consequence of higher adsorption rate of remaining emulsifiers in the aqueous phase, which in turn increases the droplet break-up frequency through decreasing the interfacial tension. This hypothesis was demonstrated by using solid particles in emulsification which showed similar final droplet size to the experiment in the absence of added emulsifier, since interfacial tension is not affected by their adsorption. It is demonstrated that the droplet coalescence cannot be completely suppressed by the presence of surfactants (Tween 20) or proteins (sodium caseinate or WPI), due to the desorption of these emulsifiers from the interface. On the other hand, droplet coalescence was arrested in the presence of solid particles, since their adsorption can be considered an irreversible phenomenon. When the dispersed phase volume fraction was increased up to 50 %, a minimum was observed in the droplet break-up frequency at the dispersed phase volume fraction of 20 %. This was caused by the influence of two opposing factors; the increase in the dispersed phase volume fraction dampens the energy dissipation in the system, which tends to decrease the droplet break-up. In contrast, larger droplets are involved in break-up phenomena at higher dispersed phase volume fractions, which promote the droplet breakup. Increasing the dispersed phase volume fraction, on the other hand, resulted in a decrease in droplet coalescence, caused by an increase in the dampening effect of the dispersed phase, which decreases the energy dissipation in the system.