Orogenic Displacement of Proteins

A cause of instability in food foams and emulsions is competition between proteins and surfactants for control of the air-water or oil-water interface. By using AFM to visualise the structural changes as surfactants invade a protein-stabilised interface we have discovered a new generic mechanism for competitive displacement of proteins.

When proteins adsorb at interfaces they partially unfold and interact to form elastic networks. Although surfactants are more surface-active than proteins the formation of a network frustrates displacement of individual proteins. If present in sufficient quantity, and given sufficient time, the surfactants will eventually displace the protein. The use of AFM explains how they do it.

AFM data showing displacement of a Β-lactoglobulin protein film (green) from an air-water interface by progressive addition of surfactant Tween 20 (black). Image sizes (a) 1.0 x 1.0 µm, (b) 1.6 x 1.6 µm, (c) 3.2 x 3.2 µm and (d) 10.0 x 10.0 µm. A schematic model of the displacement is illustrated below the images.

The surfactants enter defects in the protein network. They consolidate and expand these areas. The area occupied by protein decreases, but this is not by erosion of proteins at the boundary between the domains. Measurements of the height of the protein network show that the volume remains constant with increasing surface concentration of surfactant (surface pressure) until the protein network breaks. At this point the volume (surface concentration) of protein decreases. The network is broken in order to displace protein

Initially, as the protein network is compressed individual proteins refold, then the network folds and buckles, and finally the network snaps. Because of the folding and failure of the network the process has been called ‘orogenic displacement’. The mechanism is generic.

A generic model suggests generic solutions. Interfaces can be stabilised by strengthening the protein interactions within the network or by trying to prevent surfactants adsorbing at the interface. Such solutions have been used to solve problems in brewing, baking and dairy applications.

• Modelling studies support the orogenic mechanism
• The orogenic model has been extended to mixtures of proteins and to model protein isolates
• The orogenic model can explain novel synergistic interactions between surfactants

Further Reading:

Gunning AP, Wilde PJ, Clark DC, Morris VJ, Parker ML & Gunning PA
Atomic force microscopy of interfacial protein films. J. Colloid & Interface Sci. 183 (1996) 600-602

Mackie AR, Gunning AP, Wilde PJ & Morris VJ
The orogenic displacement of protein from the air/water interface by competitive adsorption. J Colloid & Interface Sci. 210 (1999) 157-166

Gunning AP, Mackie AR, Wilde PJ & Morris VJ
Bursting the bubble; how surfactants destabilise protein foams revealed by atomic force microscopy. Surface & Interface Analysis. 27 (1999) 433-436

Gunning AP, Mackie AR, Wilde PJ & Morris VJ.
In-situ observation of surfactant-induced displacement of protein by atomic force microscopy. Langmuir 15 (1999) 4636-4640

Mackie AR, Gunning AP, Wilde PJ & Morris VJ
Orogenic displacement of protein from the oil-water interface. Langmuir 16 (2000) 2242-2247

Mackie AR, Gunning AP, Wilde PJ & Morris VJ
The competitive displacement of Β-lactoglobulin from the air-water interface by SDS. Langmuir 16 (2000) 8176-8181

Gunning AP, Mackie AR, Kirby AR & Morris VJ
Scanning near-field optical microscopy of phase separated regions in a mixed interfacial protein (BSA) surfactant (Tween 20) film. Langmuir 17 (2001) 2013-2018

Mackie AR, Gunning AP, Ridout MJ, Wilde PJ & Morris VJ
Orogenic displacement in mixed Β-lactoglobulin /Β-casein films at the air/water interface. Langmuir 17 (2001) 6593-6598.

Wilde PJ, Mackie AR, Husband FH, Gunning AP & Morris VJ
Proteins and Emulsifiers at Liquid Interfaces. Advances in Colloid and Polymer Science 108-9 (2004) 63-71

Gunning PA, Mackie AR, Gunning AP, Wilde PJ, Woodward NC & Morris VJ
The effect of surfactant type on protein displacement from the air-water interface. Food Hydrocolloids 18 (2004) 509-515

Mackie AR, Gunning AP, Wilde PJ, Morris VJ, Pugnaloni LA & Dickinson E
The growth of surfactant domains in protein films. Langmuir 19 (2003) 6032-6038

Woodward NC, Wilde PJ, Mackie AR, Gunning AP, Gunning PA & Morris VJ
The effect of processing on the displacement of whey protein; applying the orogenic model to a real system. J. Agric Food Chem 52 (2004) 1287-1292

Gunning PA, Mackie, AR. Gunning AP, Woodward NC, Wilde PJ & Morris VJ
The Effect of Surfactant Type on Surfactant-Protein Interactions at the Air-Water Interface. Biomacromolecules 5 (2004) 984-991

Gunning PA, Mackie AR, Gunning AP, Wilde PJ & Morris VJ
Molecular interactions in mixed protein-ionic surfactant interfaces Food Colloids: Interactions, Microstructures and Processing (ed. Dickinson E) RSC Special publication No. 298, 143-151 (2005).

Morris V. J. Studies on molecular organisation at the water interface
Water Properties of Food, Pharmaceutical, and Biological Materials: Food Preservation Series: ISOPOW 2004 Conference Proceedings. 273-288 2006.

Lucero, A., Nino, MRR., Gunning, AP., Morris, VJ., Wilde, PJ., Patino, JMR. Effect of hydrocarbon chain and pH on structural and topographical characteristics of phospholipid monolayers.
JOURNAL OF PHYSICAL CHEMISTRY B 112 25 7651-7661 2008.

Maldonado-Valderrama, J., Woodward, NC., Gunning, AP., Ridout, MJ., Husband, FA., Mackie, AR., Morris VJ., Wilde, PJ. Interfacial characterization of beta-lactoglobulin networks: Displacement by bile salts.
LANGMUIR. 24 13 6759-6767 2008.