Biomedical Engineering Reference
In-Depth Information
dried samples, not from aqueous solutions, so junction zone formation in solution cannot
be ruled out.
Gellan
hyaluronan
A mixture of gellan and the important anionic glycosaminoglycan hyaluronan at non-
gelling concentrations was found to form a gel in the presence of calcium ions (Mo et al.,
2000 ). At pH 2.5, G 0 and G 00 for mixtures of calcium hyaluronate and sodium type gellan,
each at non-gelling concentrations, showed a rapid increase after mixing and, after time,
formed a typical high-modulus gel. Although gellan solutions form a phase separated gel
because of lowered solubility, and become turbid at acidic pH (Moritaka et al., 1995 ), a
mixture of calcium hyaluronate and sodium type gellan gave more transparent gels at pH
2.5 than at pH 7.0.
-
Xanthan mixtures
Xanthan
galactomannans
When hot 1:1 solutions of locust bean gum and xanthan gumwere mixed together and then
cooled, the mixture formed what was reported to be a
-
firm, rubbery, thermoreversible gel.
The effect, which cannot be achieved using either hydrocolloid alone, was presumed to be
the result of some form of junction zone formation. This was speci
ed by Dea and co-
workers to be between the galactose-de
cient segments of the locust bean gum (LBG) and
the xanthan gum (Dea and Morrison, 1975 ;Deaet al., 1977 ;Morriset al., 1977 ); see
Figure 10.12 .
Experimental
findings obtained using HPLC (Cheetham et al., 1986 ), NMR (Lazaridou
et al., 2001 ; Vittadini et al., 2002 ), X-ray diffraction (Cairns et al., 1987 ) and rheology
(Shatwell et al., 1990a , 1990b , 1990c) continue to support such an intermolecular binding
process. The most commonly accepted model is that the binding region for galactomannan
is an unsubstituted mannan region, the so-called
, since locust bean gum, a
galactomannan with a lower galactose content, has higher gelling abilities than guar gum, a
galactomannan with higher galactose content. However, as mentioned above, this is still
controversial ( Figure 10.14 ). Indeed, yet another proposal is that galactomannan sections
with all the galactose residues on the same side of the chain would form junction zones by
interacting with xanthan (McCleary, 1979 ).
Shatwell et al.( 1990a , 1990c ) studied the effect of acetyl and pyruvate groups in
xanthan on the interaction with LBG and KGM based on G 0 , tan
'
smooth region
'
and the minimum
gelation concentration. Most xanthan samples formed a relatively good gel with LBG,
but a mutant xanthan sample with high acetyl and low pyruvate content was an exception.
Deacetylation of a xanthan sample resulted in a signi
δ
cant change, while depyruvylation
made little difference. They suggested that the acetyl group has an inhibitory effect upon
gelation but that the pyruvate group plays no role in the interaction. Pai and Khan ( 2002 )
examined the rheology of blends of xanthan and enzymically modi
-
ed guar
EMG, a
-
material structurally similar to LBG
and found that the elastic modulus increases with
increasing extent of enzymatic modi
cation, i.e. with decreasing galactose content
( Figure 10.18 ). This again suggests that galactose side chains inhibit the association
with xanthan.
Search WWH ::




Custom Search