Chemistry Reference
In-Depth Information
Chapter 16
Theoretical Study of Phase Transition
Behaviour in Mixed Biopolymer +
Surfactant Interfacial Layers Using the
Self-Consistent-Field Approach
Rammile Ettelaie,
1
Eric Dickinson,
1
Lei Cao
1
and Luis A.
Pugnaloni
2
1
PROCTER DEPARTMENT OF FOOD SCIENCE, UNIVERSITY OF
LEEDS, LEEDS LS2 9JT, UK
2
INSTITUTE OF PHYSICS OF LIQUIDS AND BIOLOGICAL
SYSTEMS, CC565, CALLE 59, NO 789,
1900, LA PLATA,
ARGENTINA
16.1 Introduction
The simultaneous presence of large amphiphilic biopolymers and low-molec-
ular-weight surfactant-like molecules is a common feature of many food
emulsions and foams. While both groups of molecules deliver their required
functionality through their strong tendency for adsorption at interfaces, they
play quite different roles in the formation and stability of emulsion drops or
bubbles.
1
Low-molecular-weight surfactants can greatly reduce the interfacial tension,
thus promoting the creation of new interfaces and therefore aiding the emul-
sification process. They also have fast adsorption kinetics, with bulk-interface
equilibration times rarely above a few seconds.
2
Thus, they can quickly adsorb
at newly created interfaces to provide short-term stability through mechanisms
such as the Marangoni effect.
3
In contrast, adsorption/desorption times asso-
ciated with proteins and other amphiphilic biopolymers are far longer - they
can be of the order of hours or even days.
4
Furthermore, globular proteins
undergo unfolding and substantial structural changes during adsorption. This
often leads to the formation of intermolecular bonds as the reactive segments of
the chains become exposed. The resulting viscoelastic interfacial films are
crucial in providing much of the required long-term stability in many food
emulsions and foam systems.
3
A different stabilization mechanism, more
245
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