Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 292

earlier. 5 Semi-thin sections of 1 mm thickness, obtained using an Omu2 Ultra-
microtome (Reichert, Austria), were placed between slide and coverslide using
immersion oil (518c, Zeiss, Germany) as mounting agent. The sections were
then examined by phase-contrast light microscopy (PCLM) using a Zeiss
Axioplan 2 light microscope (Carl Zeiss, Germany) in phase contrast mode
with 40 objective Ph 2 Plan-Neofluar. For each sample, 20-30 micrographs
were taken using a ProgRes 3008 camera (Kontron, Germany). Image analysis
was performed using a mathematical morphology technique for shape charac-
terization and segmentation, 6,7 implemented by means of the SDC morphology
toolbox in the Matlab environment (Version 6.5, MathWorks, USA). The ratio
of the image surface area taken by the aggregates and the total image surface
area, A aggregates /A image , was calculated from the segmented images.

19.3 Manipulation of Particle Interactions
The structural elements present in unheated skim milk are the roughly spherical
colloidal casein particles of average radius R

100 nm as determined by
dynamic light scattering, and the globular whey proteins (a few nanometres in
size), both dispersed in milk serum containing mainly lactose and salts.
Xanthan is an anionic extracellular heteropolysaccharide of remarkably high
chain stiffness, produced by the bacterium Xanthomonas campestris. 8 The
radius of gyration R g of the XG sample used was estimated to be of the order
of 200 nm, based on the values of the radii determined by SEC-MALLS.
The incorporation of the polymer XG, the continuous acidification, and the
skim milk pre-heating are presented in what follows as effective means to manip-
ulate the casein particle interactions in skim milk, to tailor the gel microstructure.

B

19.3.1 Depletion-Induced Phase Separation
In unheated skim milk, the inclusion of Z0.015 wt.% XG resulted in phase
separation at 401C at the natural pH of the mixtures (pH 6.5). The process of
initial microscopic phase separation, investigated by CLSM after mixing, is
illustrated in Figure 1 for an unheated skim milk sample containing 0.02 wt.%
polysaccharide. At this XG level, phase separation was already visible in the
first image taken 3.5 min after the end of mixing, where the appearance of
slightly dark zones indicated the formation of areas depleted of protein.
Ongoing phase separation resulted then in the formation of discrete growing
domains deficient in protein (rich in polysaccharide), as illustrated by the series
of CLSM images taken with time intervals of 5 min.
Phase separation in skim milk containing XG can be explained by a depletion
interaction mechanism, depicted in Figure 2 at the level of a pair of spherical
colloidal particles in a common solvent with non-adsorbing polymers. The
centres of mass of the polymer molecules, depicted here as random coils, are
excluded from a region away from the surface of the particles - the depletion
layer (shown as dashed lines). Owing to the lower polymer segment concentra-
tion, the osmotic pressure due to the polymer is smaller in the depletion layer