b-Lactoglobulin Aggregates from Heating with Charged Cosolutes: Formation, Characterization and Foaming - Food Colloids: Self-Assembly and Material Science - page 186

7e -4

6e -4

5e -4

4e -4

3e -4

2e -4

1e -4

0

0 20 40 60 80 100 120

Concentration in cosolute [mM]

Figure 4 Second virial coefficient A 2 of b -LG solutions heated in presence of cosolutes
(pH 7.0, 801C, 10 min): K , arginine HCl; J , NaCl. Vertical error bars
represent the standard deviation

the denatured proteins was reached (Figure 3). Particle surface charges are
sufficiently neutralized and heteroaggregation of oppositely charged particles is
favoured. Under these conditions, aggregation is purely diffusive (DLCA) and
it depends only on solvent viscosity, which explains why the aggregation
proceeds very fast (k ¼ k max ). It is important to note that, under these reduced
electrostatic repulsion conditions between denatured protein monomers, chem-
ical aggregation is also driven by covalent interaction (an increase in the
activation of thiol groups of more than 35% was measured compared to b-
LG alone) beside physical interaction (attractive forces and hydrophobic
interaction). 18 At higher cosolute concentrations, the value of W increases
again for all three cosolutes, leading to sticking probability values of a ¼ 0.74
for A100, a ¼ 0.48 for A200 or a ¼ 0.69 for N200 (Figure 3). This behaviour
can be explained by an 'overcharging effect', resulting in slower protein
aggregation due to repulsive forces and the necessity for more protein/particle
contacts until aggregation occurs (RLCA). Several studies on colloidal aggre-
gation have reported similar behaviour 25,26 with a shown to depend on the
surface charges and on overcharging effects at higher electrolyte concentra-
tions.
The apparent surface properties of the aggregates were investigated by means
of electrophoretic mobility measurements. Figure 5 shows the averaged elect-
rophoretic mobility of aggregate populations formed during heat treatment of
b-LG at 801C for 10 min at pH 7.0 in the presence of arginine HCl, NaCl and
GdnHCl. The mobility was found to decrease with increasing cosolute con-
centration and to level off at concentrations higher than 60 mM of cosolute.
The change in mobility distribution with increasing cosolute concentration
(results not shown) demonstrates overall charge neutralization and a significant
broadening of the particle distributions. The presence of various populations
with different surface charges is consistent with the hypothesis of heteroaggre-
gation between particles.