Micro-mechanical testing of interfacially adsorbed protein networks
Authors
Jones, Daniel Brian
Date
2002-07-16Awarding Institution
University of Cambridge
Author Affiliation
Department of Chemical Engineering
Qualification
Doctor of Philosophy (PhD)
Type
Thesis
Metadata
Show full item recordCitation
Jones, D. B. (2002). Micro-mechanical testing of interfacially adsorbed protein networks (Doctoral thesis). https://doi.org/10.17863/CAM.16081
Description
Droplet breakup is a key step in emulsion processing and manufacture. An ability to
predict droplet dismption in a known flow field is essential to allow emulsion product
formulation with the optimal droplet size, minimise energy input during manufacture
and to reduce shear damage of the surface-active species. Conventional theories for
predicting droplet dismption are based upon interfacial energy alone and fail when a
protein emulsifier is present at moderate concentration. This is because the protein
forms an interfacial network having mechanical strength.
New equipment, the Cambridge Interfacial Tensiometer (CIT), was designed and
constmcted to directly determine the mechanical properties of protein films adsorbed
at the air-water and oil-water interfaces. The technique is an interfacial twodimensional
analogy of conventional materials testing methodology, but is conducted
with high spatial resolution and requires the measurement of micro-Newton forces.
Interfacial elasticity modulus values were of order 200 mN/m for P-lactoglobulin
networks at the air-water interface, consistent with a calculated ensemble average
estimated using Atomic Force Microscopy derived data on the unfolding of individual
protein molecules~ Interfacial elasticity modulus increased with protein concentration,
although a 31% enhancement in the maximum stress transmitted through the protein
film resulted when sub-interfacial P-lactoglobulin concentration was reduced from
1.0 mg/mL to 0.01 mg/mL. Reduced competition for interfacial space at lower protein
concentrations is believed to result in greater conformational change, and hence
entanglement, on adsorption to the interface from dilute solution. Importantly, this
study suggests that network mechanical response is determined by residual protein
tertiary stmcture, and that adsorbed networks should therefore be analysed as nanostmctured
biomaterials. The mechanical properties of adsorbed layers of de novo
peptides were determined with the CIT. The peptides were shown to exhibit the
extremes of protein behaviour previously observed, demonstrating the possibility of
controlling product form and functionality through combined hydrodynamic and
molecular specification.
This is the first study to establish the mechanical properties of an adsorbed protein
film using conventional stress-strain approaches to high material defmmations. The
work demonstrates that droplets surrounded by interfacially adsorbed protein should
be viewed as deformable capsules or cells enclosed within a stress-transmitting
network. Deformation and dismption could then be predicted by existing theories for
such systems, using the constitutive data provided by the CIT stress-strain tests. Such
an approach is expected to be superior to existing methods based solely on interfacial energy.
t Data from Carrion-Vazquez et al. (1999) and Best et al. (200 1 ).