The effect of interfacial compositions on the dispersed phase-induced gelation and controlled digestion of mono and bilayer nanoemulsions

Abstract

This thesis examines the role of the interfacial thickness (δ) in controlling the gelation and digestion behaviour of oil-in-water emulsions (oil volume fraction, φ = 0.2 to 0.4)-stabilized by food-grade emulsifiers and polysaccharides. Importance was given to addressing the increase in effective volume fraction (φeff) of oil droplets beyond maximum random jamming (φMRJ) by reducing droplet size, removing excess emulsifier and changing the interfacial composition. In the first study, the gelation in 40 wt% canola oil-in-water nanoemulsions was investigated as a function of excess emulsifier Citrem (citric acid esters of mono and di-glycerides) removal from the aqueous phase. The removal of excess Citrem increased the viscosity, yield stress and storage moduli of nanoemulsions, more significantly at smaller droplet sizes. It was attributed to a change in inter-droplet interaction from non-DLVO oscillatory structural forces to DLVO dominated repulsive forces after removing excess Citrem. This also increased the δ and φeff beyond φMRJ, leading to a self-standing repulsively jammed nanoemulsion gel. Next, the droplet velocity and packing behaviour of Citrem-stabilized nanoemulsions were tracked using an analytical photo-centrifuge to predict their stability and shelf-life. The reduction of droplet size and removal of excess micelles improved the accelerated stability and shelf-life of the nanoemulsions. The droplets’ packing density (φp) was decreased under the applied RCF after removing excess micelles, which we related to strong repulsive forces between nanodroplets. To further increase the δ, a second layer of polysaccharide (chitosan and pectin) with different magnitude of charge was deposited on Citrem and whey protein isolate (WPI)-stabilized nanodroplets, respectively. Two different layer-by-layer (LbL) electrostatic deposition techniques, namely one-step versus two-step, were utilized for Citrem-chitosan and WPI-pectin systems, respectively. It was found that the rheology of bilayer emulsions was affected by the droplet size, presence and absence of excess emulsifier, polysaccharide concentration and charge, and the type of LbL method used. In the one-step LbL method, a liquid-like behaviour of Citrem-stabilized monolayer emulsions transformed into repulsive bilayer weak emulsions gel above a critical chitosan concentration, where electrostatic and steric repulsive forces had a significant contribution in elevating δ and φeff. However, the two-step LbL method and removal of excess emulsifier were more effective in creating well-distributed bilayer nanodroplets with increased interfacial thickness leading to an increase in gel strength compared to the monolayer emulsions at a lower φ. The deposition of the second layer also controlled the lipase action during in vitro digestion leading to lowering of lipid digestibility. Overall, the study showed that the random jamming amongst the nanodroplets could be induced by increasing δand φeff beyond φMRJ where emulsions behave like a viscoelastic gel. The fundamental knowledge developed from this research can be used to develop food-grade low-fat emulsion gels with controlled digestion.