Chemistry Reference
In-Depth Information
0.25
2.5
300
pH2
Total work of failure
pH3
pH4
pH5
Young's modulus
250
0.20
2.0
200
0.15
1.5
150
0.10
1.0
100
0.05
0.5
50
0.00
0.0
0
0
1
2
3
4
5
6
7
0.0
(a)
0.1
0.2
0.3
0.4
0.5
0.6
(b)
pH
True strain
Fig. 10.20
Effect of pH on the structure of a 2% gellan gel. (a) True stress-true strain
curves and (b) Young's modulus and total work of failure given as a function of pH (adapted
from Norton et al., submitted).
10
.
3
.
6
.
2
Gellan
Recently, work has been reported on the acid gelation of gellan as a route
for causing self-structuring of the stomach. The advantage of gellan is
that the acid range required for gelation is very close to those observed in
the human stomach. In addition, and probably much more importantly,
the rate of gelation is slower than that observed for alginate. Some
recent work (Norton
et al.
, submitted) has shown that by slowly adding
hydrochloric acid, gellan will gel at pH 4 and below. A maximum in gel
strength is observed between pH 3 and 4 (Fig. 10.20). As can be seen
from the figure, the pH affects gelation: the stiffness of the structures
(Young's Modulus) and the total energy required for these structures to
'fail' (Total Work of Failure) both gave maximum values between pH
3 and pH 4 (Fig. 10.20b). Acid gels produced at either lower or higher
pH values (pH 2 or pH 5) were considerably weaker (Fig. 10.20a). As
the pH is reduced further, extensive aggregation of the gellan chains
was observed. This was seen as the gels, which at higher pHs were
clear, became very turbid. The aggregation resulted in weaker gels and
gels in which water could be squeezed out (Fig. 10.21) as if they were
sponges.
These authors showed that, as would be expected for hydrocolloid
gels, both the gel strength and the work of failure increase with the
gellan concentration (Fig. 10.20) and with a lowering of pH to 3. These
curves are shown to be linear and have a critical concentration (i.e. there
is a finite concentration to form a gel) with the strongest gel always
occurring at approximately pH 3. What was very clear was that the
material properties had a linear dependency on polymer concentration
for the pHs studied. This indicates that acid gels are behaving in a way
