Biomedical Engineering Reference
In-Depth Information
because of the size distribution of the particles and because of the mechanism of
oc
formation. Finally, the whole network is inhomogeneous, as it consists of large conglom-
erates and cavities (each up to 10
m in size). Since the concept of fractal structure implies
self-similarity over a range of length scales, this is an interesting variation of the concept.
de Kruif et al.( 1995 ) tried to correlate the viscoelastic properties and structure of milk
gels, and obtained df f = 2.1 for acid casein gels and df f = 2.3 for rennet casein gels, based on
Bremer
μ
s theory (Bremer et al., 1989 ). Structural observations by CSLM gave a similar
value for df, f , and they concluded that these gels have similar structures, i.e. stretched
stranded gels. However, although all three types of milk protein gels, chymosin treated
casein gels, acid casein gels and whey protein gels ( Section 9.2 ) showed fractal structures
at some level, gel properties could not be described completely by the scaling laws
proposed by Bremer et al.( 1989 ) and Shih et al.( 1990 ).
Finally, van Vliet et al.( 1991 ) examined the relationship between syneresis and the
rheological properties of particle gels of milk, and found that low fracture stresses and high
tan
'
correlated with an increased tendency to show syneresis. The relation between structure
and rheological properties of gels of the protein mix known as whey protein isolate (WPI)
δ
-
a
commercial blend of
was studied by gel permeability,
electron microscopy and dynamic viscoelasticity by Verheul and Roefs ( 1998 ). They
found that gels became coarser (with larger pore size) with increasing [NaCl], as con
β
-Lg, BSA and
α
-lactalbumin
-
rmed
by an increase in gel permeability. Although the modulus continued to increase, no structural
change was detected after gel formation, while only a part of the protein in the dispersion
contributed to the gel network, again indicating that, without adjustment, the fractal concept
cannot be applied to whey protein gels (Verheul and Roefs, 1998 ).
9.4
Fibrillar gels formed from partially denatured proteins
When proteins, particularly
β
-Lg and BSA, are heated below pH 4,
ne-stranded or
fibrillar networks are formed and the strands become shorter and apparently stiffer as the
pH is lowered further (Stading et al., 1993a ; Walkenstrom and Hermansson, 1994 ).
These materials appear to share many features in common with the so-called amyloid
fibrils which occur in a number of pathological disease states (Gosal and Ross-Murphy,
2000 ; Dobson, 2001 ; Hughes and Dunstan, 2009 ).
9.4.1
Amyloid gels
'
is a term that goes back to the nineteenth century and refers to a tendency of
starch polymers to stain iodine
Amyloid
'
hence the starch polysaccharides are known as amylose
and amylopectin. Nowadays, however, it refers almost exclusively to proteinaceous
systems and, in a clinical context, describes an abnormal deposition of dense and
insoluble
-
-'
'-
which damage organs or tissues, including the
brain. Amyloid deposits occur in the so-called prion diseases such as bovine spongiform
encephalopathy (BSE) and new variant Creutzfeldt-Jakob disease (CJD), in dementias
such as Alzheimer
fibrillar material
plaques
'
s and Huntington
'
s chorea and in other pathologies such as primary
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