Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 305

Chapter 20
Particle Tracking as a Probe of
Microrheology in Food Colloids
Eric Dickinson, Brent S. Murray and Thomas Moschakis
PROCTER DEPARTMENT OF FOOD SCIENCE, UNIVERSITY OF
LEEDS, LEEDS LS2 9JT, UK
20.1 Introduction
Food colloids are typically heterogeneous, and the degree of heterogeneity is
susceptible to change during processing and long-term storage. For a thorough
understanding of the relationship between bulk properties and colloidal inter-
actions, we require detailed information about structure and mechanical
properties on various length scales. Confocal microscopy is emerging as a
powerful technique for monitoring and quantifying the evolution of structural
change on the microscopic scale as a function of variables such as time,
temperature and shear deformation. In addition, the dynamical analysis of
sequences of images from confocal microscopy has the potential to provide new
insight into local rheological properties within heterogeneous systems.
Microrheology is the study of viscous and viscoelastic properties of regions
within a material via the tracking of the motion of microscopic tracer parti-
cles. 1,2 There are two classes of microrheology techniques: those involving
active manipulation of probe particles within the sample, and those involving
passive observation of thermal fluctuations of probe particles. The active
manipulation of micrometre-sized magnetic particles by magnetic fields was
pioneered more than 80 years ago in connection with the gelation behaviour of
gelatin. 3 Since then, magnetic particles have been used extensively for investi-
gating the microrheology of biological systems, including living cells. 4 Never-
theless, it is the technique of passive particle tracking in confocal microscopy
which probably has the most obvious potential in the field of food colloids. As
an example of this potential, we report here on the passive tracking of inert
colloidal particles to probe the in situ viscoelasticity of a model food emulsion.
The fundamental assumption of tracer microrheology is that the Brownian
motion of the diffusing particles is controlled by the mechanical properties of
the surrounding medium. Typically the experimental information consists of
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