Casein Interactions: Does the Chemistry Really Matter? - Food Colloids: Self-Assembly and Material Science - page 156

10.2 Caseins as Polymers
Many of the properties of polymers can be described by considering each
molecule as a wriggly piece of string in constant Brownian motion. The mean-
square end-to-end distance of the polymer molecule is a good descriptor of its
size in solution, and size variation with solvent quality can be captured by the
Flory Huggins w -parameter. In effect, this energy term expresses the prefer-
ences of the monomers making up the polymer for locating themselves ran-
domly within the solvent or close to other polymer segments. In the ideal
solvent, there is no clear preference, and so the polymer component remains
completely dispersed. In a non-ideal solvent, the polymer string collapses upon
itself, and polymer aggregation and precipitation may follow.
Proteins, including the caseins, are polyelectrolytes. The solvent quality in a
protein solution can be altered by adjusting the pH or ionic strength. In
particular, we know that the caseins can be precipitated by lowering the pH to
their isoelectric point, and that the heavily phosphorylated members of the
family (a s1 -, a s2 - and b-caseins) can be precipitated by calcium addition. So, are
these simply manifestations of the caseins being transferred into a poor solvent
environment? Attractive as this polymer physics representation may appear as
a unifying approach, it is clear that, in the case of calcium addition, at least,
additional mechanisms are at work. It has been readily demonstrated 1 by
simple equilibrium dialysis experiments that calcium ions bind weakly to the
calcium-sensitive caseins. Though speculation still exists over the precise iden-
tity of the binding sites, the number of calcium ions bound as a function of the
free ion concentration can be readily quantified.
By way of contrast, the completely different approach of applying the
classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid sta-
bility 2 to the calcium-induced precipitation of the caseins has been more
successful. 3,4 Colloid stabilization according to the DLVO theory is achieved
by the domination of repulsive terms over attractive components in the pair
potential existing between charged colloidal particles, which has the conse-
quence of generating an energy barrier to aggregation. 2 The electrostatic
repulsive contribution to the pair potential of mean force is given by:

U E ¼ e a C 0
2

ln 1 þ exp k r

ð Þ ð 1 Þ

f g ;

where a is the particle radius, r the inter-particle separation distance, e the
dielectric constant, and k the Debye-Hu ¨ ckel parameter. The electrical potential
C 0 can be further related to the net protein charge Q by:

Q
a e 1 þ k a

C 0 ¼

Þ :

ð 2 Þ

ð

Colloid stability is thus controlled by the magnitude of the charge carried by
the particle (protein), and so anything which modifies that charge impacts on
the predicted stability. Since the repulsion energy is proportional to Q 2 ,itis