Surface Topography of Heat-Set Whey Protein Gels: Effects of Added Salt and Xanthan Gum - Food Colloids: Self-Assembly and Material Science - page 474

The lack of knowledge about the surface creation of assembled particle
systems is largely due to the lack of suitable techniques for their surface
characterization. Self-assembled particle systems are often mechanically weak,
and so a small surface load may cause significant network deformation and
surface distortion. Additionally, the presence of surface moisture makes some
surface characterization techniques (e.g., scanning electron microscopy) not
feasible for these systems. The contribution of the associated hydrodynamic
flow of surface water also makes it more complicated for such systems in
interpreting results from traditional tribology techniques. Nevertheless, there
have been reported successes of using classical surface friction techniques on
surface investigation of wet polymer gels. For instance, Gong and Osada 2,3
have characterized the surface properties of a number of synthetic polymer gels
by measuring surface friction as a function of surface load and sliding speed.
And Baumberger et al. 4 have successfully used a modified tribometer to assess
the surface slipperiness properties of gelatin gels.
Recently, we developed a simple surface friction device and used it for
surface characterization of wet protein particle gels. 5 The technique gave
effective and reliable differentiation of the surfaces of heat-set whey protein
gels. Complementary confocal laser scanning microscopy (CLSM) was used 6 to
establish characteristic topographical features of the same protein particle gels.
Particle gels differ significantly from polymer gels both in terms of the
gelation mechanisms and in their mechanical properties. 7,8 A particle gel is a
network of particle strings or clusters formed as a result of the interaction and
aggregation of colloidal particles, while a polymer gel arises from the cross-
linking of polymer chains at high enough concentration. A polymer gel typi-
cally has a transparent visual appearance with a relatively large linear regime
(up to several hundred percent strain) and shear-hardening mechanical behav-
iour. 7 A particle gel may be visually opaque or rather translucent depending on
the size of the individual particles and the particle clusters. Mechanically, a
particle gel has a much shorter linear regime, and it often fractures at small
deformations (a few percent). 8 The microstructural features of some protein
particle gels have been thoroughly investigated in relation to their mechanical
and physical properties. 9 The aim of the present work is to understand the
surface characteristics of these particle gels. The model system studied in this
investigation is a heat-set whey protein gel with salt and xanthan gum added to
control the surface texture and microstructure.
33.2 Materials and Methods
Commercial whey protein isolate (WPI) (Lacprodan DI-9224) was provided by
Arla Foods (Videbaek, Denmark). The product contained > 93.5 wt% protein,
o 0.2 wt% fat, o 0.2 wt% lactose, and o 6 wt% moisture. Sodium chloride
and polysaccharide xanthan gum (99% purity) were purchased from Sigma-
Aldrich (UK). Milli-Q (Millipore, Bedford) purified water was used for the
preparation of all solutions.