Chemistry Reference
In-Depth Information
reduction in gelation time and a significant increase in firmness (
G
)of
the gels. According to van Vliet and Keetels (1995), the deformation of
gels is proportional to the number of contact points per cross section
of the network. Cross-linking by heat-denatured whey proteins within
gels from heated milk could be responsible for increased
G
of the
network (Lucey
et al
., 1997a, 1998). Using confocal scanning laser mi-
croscopy, Lucey
et al
. (1998) found that heating milks at
80
◦
C resulted
in a microstructure of GDL-induced acid gels that appeared 'branched'
and had a higher 'apparent interconnectivity' of aggregates compared to
unheated or less severely heated milks, which had irregular clusters and
strands making up the network and less 'apparent interconnectivity'.
The effect of composition on acid milk gels was studied using small
strain rheology. When the native fat globule membrane is intact, fat
globules act as inert fillers, since they do not interact with casein (van
Vliet, 1988). Increasing the volume fraction of fat having intact native
membrane resulted in a decrease in
G
of acid milk gels (van Vliet and
Dentener-Kikkert, 1982). On the contrary, gels made with homogenised
milk had an increase in
G
since the native fat globule membrane was
replaced by casein and whey protein, allowing for interactions between
fat globule membrane and the protein matrix. Native whey proteins have
been found to act as fillers in yoghurt. Lucey
et al
. (1999) showed that
addition of whey protein concentrates to yoghurt milk, after milk had
been heated, resulted in the reduction of
G
and shear stress at yielding.
The influence of stabilisers on yoghurt has been studied (Everett and
McLeod, 2005). With increasing concentration of adsorbing polymer
(pectin and carrageenan), the stabilisation-destabilisation mechanisms
undergo transitions from (i) bridging flocculation (decrease in
G
), to
(ii) steric stabilisation (increased
G
and phase angle), to (iii) depletion
flocculation by unadsorbed polysaccharide in the serum phase (increased
phase angle). For non-adsorbing polysaccharides (guar, locust bean and
xanthan gums), increasing concentration led to a transition from deple-
tion flocculation to colloidal particles trapped in a viscous polymer net-
work as shown by a decrease and then increase in
G
(Syrbe
et al
., 1998).
∼
7.5
EFFECT OF ORAL PROCESSING ON INTERPRETATION
OF RHEOLOGICAL MEASUREMENT
Texture is an essential part of the whole spectrum of sensory properties
of foods. Consumers find texture more important in solid foods than
liquids (Matsumoto and Matsumoto, 1977). The breaking down of solid
foods and the mixing with saliva during chewing lead to changes in
texture perception of solid foods far more than the change in viscosity
