The influence of phospholipids and food proteins on the size and stability of model sub-micron emulsions

Abstract

The influence of both the nature of the surfactant and surfactant concentration on the processes of droplet breakup and coalescence in the formation of decane in water sub-micron emulsions in a high-pressure homogenizer were investigated. Emulsions were produced using a Microfluidics inc. M110-S microfluidizer with an impinging jet high-shear chamber. For all the food grade emulsifiers studied, the droplet size decreased with increasing concentration (weight %) reaching a limiting droplet size between 0.5 and 1% for the proteins and ∼1.5% for the Phospholipids. A hydrophobic fluorescent dye (1-undecylpyrene) was used to establish the extent of competition between droplet breakup and coalescence in the emulsification process. For the food proteins and phosphatidylglycerol, droplet coalescence in the process reduced as the amount of emulsifier increased, becoming zero at concentrations of about 0.5–1% i.e. the same concentration as that required to produce the limiting minimum droplet size. For phosphatidylcholine some coalescence in the process was observed up to the highest concentration studied (2%) which is indicative of the fact that it normally stabilises water in oil emulsions so favouring coalescence in the process. The data collected in this study show that in the emulsification process droplet size is determined by both breakup and coalescence events, and that the final droplet size is probably a consequence of multiple breakup events until at higher emulsifier concentrations the droplet size is reached which is limited by the breakup capabilities of the homogenizer. Emulsion stability over 400 h was investigated by measuring changes in the droplet size using dynamic light scattering. For the proteins the increase in droplet volume was shown to be linear with respect to time, indicating an Ostwald ripening process. Although there was coalescence on storage at the lowest concentrations of phospholipid used, there was no observed ripening at any emulsifier concentration showing that phospholipid interfaces are structured in such a way as to resist ripening even though decane has a solubility in water. The ripening rate for whey and β-lactoglobulin were observed to be approximately 10 times higher than the ripening rate calculated using the Lifshitz–Slesov–Wagner (LSW) theory ∼10–20 nm 3 s −1. Ripening rates are explained in terms of the nature of the interface formed.