Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 297

(a)

(b)

0.25

0.20

H

Top

0.15

0.10

H Col

0.05

0.00

0.000 0.005 0.010 0.015 0.020 0.025

conc. Xanthan [%, w/w]

Figure 5 Macroscopic phase separation observed in the mixtures after incubation at 401C
for two days at pH 6.5. (a) Estimated volume fraction ( F PS ) of the XG-rich
phase in unheated skim milk ( J ) and in pre-heated skim milk ( K ) containing
XG at various levels. The value of F PS is calculated as the ratio of the height of
the top layer (H top ) to the total height of the liquid (H col ), both determined
from the photographs of the tubes as illustrated in (b) for phase separation in
pre-heated skim milk containing 0.025 wt.%XG (no account is taken of concave
tube bottom)
(Copyright Nestec Ltd (2006))

In unheated skim milk, an increase in XG level resulted in an increase in G 0
up to a maximum observed in this series for 0.015 wt.% XG incorporated
(Figure 6(a)). Further increase in XG content (0.02 and 0.025 wt.%) then
caused G 0 to drop even below its value for the pure skim milk gel. The protein
network microstructure of the gel made from unheated skim milk without
added polysaccharide appeared fine and regular (see Figure 7), but more coarse
and irregular in the presence of 0.015 wt.% XG. The maximum gel strength
observed at the latter polysaccharide concentration can be attributed to the
crowding of protein particles in protein-rich domains resulting in more massive
network-constituting entities, with still high connectivity between them. The
impact of polysaccharide inclusion on network microstructure was much more
pronounced at the highest XG level investigated (0.025 wt.%), where the
protein was concentrated in domains appearing as large, rather isolated
regions. Here the poor apparent connectivity between the massive protein
aggregates is the likely reason for the low gel strength observed, since only
material integrated well into the stress-bearing network can be expected to
contribute to gel rigidity.
Pre-heating of skim milk affects substantially both the microstructure and
the storage modulus of gels formed in the absence of polysaccharide addition.
The increase in G 0 by almost one order of magnitude (Figure 6) is in agreement
with the work of Lucey et al. 25 When XG was present, the modulus increased
almost linearly with increasing polysaccharide concentration, similar to the
behaviour of unheated skim milk, but the maximum in G 0 was shifted to a
higher XG level and decreased then only moderately upon further addition of
XG to 0.025 wt.% (see Figure 6(b)). The difference in network microstructure
due to skim milk pre-heating was already visible in the reference gels made from