Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 262

this varied considerably (for these systems, from ~3% to ~12%w/w) depending on pH and
[NaCl]; (b) log G 0 ; (c) log G 00 showed a slight

minimum whose frequency increased with temperature; (d) both G 0 and G 00 fell with
increasing temperature, but this was essentially reversible on cooling; (e) linear viscoelastic
behaviour was found up to quite large values (>10% strain), especially far from pI.
All of these essentially qualitative observations have been con

showed only a slight increase with ω

rmed for other globular
proteins under equivalent conditions. Many such data has been collected, particularly for
β

-lactalbumin (Kavanagh et al., 2000a ,
2000c ), ovalbumin (Koike et al., 1996 ), soy glycinins (Kohyama et al., 1995a , 1995b )
and a number of other systems. Indeed it seems that, at one level, the species and type of
globular protein is not signi

-Lg (Stading et al., 1992 , 1993a) but also for α

cant. On the contrary, by adjusting pH and salt conditions,
essential commonality of behaviour can be seen.
For example, rheological studies performed by Stading and Hermansson (Stading et al.,
1993b ) determined the critical gelation concentration of -Lg. The samples were pH

adjusted over a range of pH, but the effect of additional salt was not pursued. Samples
were heated to 95°C and held there for 1 h, and critical gelation concentration was de

β

ned
as the concentration at which G 0 exceeded the noise level after heating. At intermediate pH
levels (4.5

5.6) close to the protein isoelectric point, the critical concentration was as low
as 1%w/w, while at lower pH values (2.5 and 3) the critical concentration increased to 5%
w/w, and increased further to 10% w/w when the pH was 7.
Subsequently, Renard and Lefebvre (Renard and Lefebvre, 1992 ) examined the effects
of pH and ionic strength. Various concentrations of

-

β -Lg were prepared at a variety of pH

values (2, 5, 6, 7 and 9) and ionic strengths (0 0.15 MNaCl). The samples were heated in

sealed tubes and c 0 determined, using the simple method of ensuring the meniscus did not
deform upon tilting the tube. At extreme pH values (2 and 9), high critical concentrations
were found to be c.8% w/w. Increasing the salt content reduced c 0 , while at pH values
near the isoelectric point (pH 5, 6, 7) values were close to 1% w/w, regardless of the salt
content. Because of differences in the samples and heating regimes employed, only semi-
quantitative agreement would be expected between the results obtained by these two
groups.
As mentioned above,

-

fibrillar in nature.
One interesting observation in gelation kinetics measurements is that, somewhat unex-
pectedly, G 0 is already higher than G 00 , even from the beginning of the measurement, as
pointed out by several workers (Matsumoto and Inoue, 1993 ; Inoue, 1994 ; Ikeda and
Nishinari, 2000 , 2001 ; Ikeda et al., 2000 ). This tendency has been recognized for other
protein concentrations and temperatures, and also at various frequencies and strains. One
possibility is that the sample is already structured before the gelation point, in a way
analogous to a charged colloid system (Nicolai and Durand, 2007 ).
Figure 9.3 shows typical results for the gelation kinetics of

the low pH systems ( Section 9.5 ) are

-Lg, heated at pH 7 to
80°C. As can be seen just by inspection of this data, the critical concentration must be
below 13% w/w: modelling the results gives a value around 12.1%. The corresponding
frequency sweep follows the behaviour suggested above, and the minimum in G 00

β

is

centred around 0.3 rad s − 1 .