Physical Chemistry of Foods

In foods, the DLVO theory is often insufficient to calculate the total
interaction. In many casessteric interactionis involved, caused by adsorbed
or grafted polymers, which provide a layer of protruding polymer chains
around a particle. If such layers overlap, their mixing generally causes the
osmotic pressure to increase, and repulsion occurs. However, if the solvent
quality for the polymer is poor, attraction may prevail, although this is
rather exceptional. On still closer approach of the particles, the polymer
chains lose conformational freedom by volume restriction, and this leads to
very strong repulsion. If the thickness of the polymer layer is not very small
compared to the particle size, steric repulsion is generally greater than van
der Waals attraction, and stability against aggregation is assured.
If the polymer involved is also charged, the situation is more
complicated, and the ionic strength is an important variable. If the latter
is neither very small nor very large, the resulting ‘‘electrosteric’’ repulsion
tends to be strong.
On the other hand, dissolved polymers may causedepletion interaction.
This is because the polymer molecules cannot come very close to the particle
surface, which amounts to polymer being depleted from part of the solvent.
Hence, polymer concentration, and thereby osmotic pressure, is increased. If
particles aggregate, the depletion layer decreases in volume due to
overlapping, and the polymer concentration decreases, hence the osmotic
pressure decreases. This means that an attractive force acts between the
particles. It increases with concentration and radius of gyration of the
polymer.
Table 12.3 summarizes the interactions mentioned and—in a
qualitative sense—the main factors affecting the strength of the interactions.
Some other interactions can act, especiallybridging; this means that a
polymer molecule becomes adsorbed onto two particles at the same time. It
can especially happen if insufficient polymer is present to cover the particles.
Moreover, protruding polymer chains may show cross-linking when the
layers of two particles overlap. A case in point is cross-linking of negatively
charged groups by divalent cations. Several other interactions can occur.
Most of those are fairly rare, or act only at very close distance, like solvation
forces.
Finally, after particles are aggregated, slowstrengtheningof bonds
often occurs. Actually, most particle pairs are kept aggregated by a
‘‘junction’’ that involves a great number of separate bonds, and that number
may increase with ageing.