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mathematical analysis as described elsewhere.
33
A modified version of the
Foamscan apparatus (ITConcept, Longessaigne, France) has been used to
perform the experiments. A square glass column was used to minimize optical
artifacts associated with air bubble imaging. The temperature within the
column and the cuvette was set to 27
21C. Foaming of the protein+cosolute
solutions was measured by sparging with N
2
at 80 mL min
1
in 50 mL of the
heated protein+cosolute solution. This flow-rate was found to allow efficient
foam formation prior to strong gravitational drainage. The porosity of the
glass frit allowed the formation of air bubbles with diameters between 10 and
16 mm. Sparging with N
2
was stopped after a foam volume of 180 cm
3
was
reached. At the end of bubbling, the mean air-bubble diameter was followed
with time. In addition, evolution of the liquid fraction in the foam was followed
at two different positions - at the top and bottom of the foam - by means of
electrical conductimetry.
12.3 Results and Discussion
12.3.1 Protein Denaturation and Aggregation in Presence of
Cosolutes
The picture of the sample tubes in Figure 1 shows the appearance of the b-LG
dispersions at pH 7.0 after heating at 801C for 10 min in presence of arginine
HCl. With increasing cosolute concentration, turbidity increased gradually
because of the formation of protein aggregates, but the samples remained
homogeneous with no sign of sedimentation. At a critical cosolute concentra-
tion (C
cs
*), which was different for the three cosolutes (results for NaCl and
GdnHCl are not shown), precipitation occurred and large cloudy aggregates
were formed,
leading to a macroscopic phase separation. Overall,
the
Figure 1 Appearance of glass tubes containing solutions of
b
-LG (10 g L
1
)+cosolute
after heat treatment (801C, 10 min, pH 7.0). The arginine HCl concentrations
increase from left to right: 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150 and
200 mM
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