Milk Protein Adsorption at Oil/Water Interfaces: Molecular to Bulk Behaviour

Abstract

Protein adsorption at liquid-liquid interfaces is a major element of many biological processes and dairy-based functional foods. Due to experimental limitations, however, there is still a remarkable lack of understanding of the adsorption mechanism, particularly at a molecular level. Several approaches have been utilized to explain this phenomenon, directly and indirectly.

Atomistic molecular dynamics simulations were used to elucidate the approach and adsorption mechanism of β-lactoglobulin (β-LG) at the oil/water interface. β-LG is one of the main classes of milk protein and is among the most common proteins used in the food industry due to its excellent natural amphiphilicity, abundance, low cost and nutritional importance. β-LG adsorption at three different oil-water interfaces, decane, octanol and triolein in order of increased hydrophilicity was investigated.

Through multiple independent simulations of β-LG relative to the three surfaces the rate at which β-LG approaches the oil/water interface was determined. The adsorption process dynamics, energetic and structural changes upon adsorption to the different surfaces were investigated.

The viscoelastic properties and average conformations of adsorbed β-LG at MCT-water interface were investigated using interfacial shear rheometry. Here the effects of pH and ionic strength (salt concentration) were investigated to better probe and understand the interfacial behaviour and conformational transitions that occur at the oil–water interface. In particular the kinetics of adsorption of the protein at the interface were explored. The possibility of multilayer film formation β-LG/alginate/chitosan, based on the electrostatic deposition principle at the interface was also investigated.

This detailed characterisation of the adsorption of β-LG at oil/water interfaces informed us in the design and development of novel food products. In particular the focus here is on protein-stabilized emulsions prepared with β-LG, soybean oil and water. The effects of pH and ionic strength variation on for example, the emulsion droplet size distribution, the charge on the emulsion droplets were investigated. Emulsion stability was investigated using zeta potential, light scattering and macroscopic phase separation measurements. Additionally, confocal microscopy and cryo-electron microscopy were used to monitor the microstructural changes of the emulsions and probe the structure of the emulsifier thin film. The bulk rheological behaviour (viscosity and viscoelastic properties) of the emulsions was investigated.

The stability analysis and microstructural studies confirmed that emulsions with pH values above and below the isoelectric point of β-LG and at low ionic strength have electrostatically charged droplets and are stable. Droplet size and size distribution of these emulsions ranged from a few hundreds of nanometres to a few micrometres. The high viscosity of the samples at low shear rate was used as a good indication of the samples stability towards creaming in storage conditions. The viscoelastic properties of these emulsions confirmed the 2D network formation of the proteins coated droplets.

The collective results are considered with respect to ascertaining the correlation and causation relationships between the structure of the interfacial thin film and the emulsion macroscopic behaviour. The physical stability of emulsions can be improved by engineering their interfacial properties when designing food products with the correlation provided in this study between structure and properties of food emulsions. This study will establish a fundamental understanding of protein-stabilized emulsions and this information could be used by food and pharmaceutical industries to use protein-stabilized emulsions as vehicles for carrying nutrients/drugs and flavour release.