Physical Chemistry of Foods

globules. It will then be wetted by oil from the other globule, and oil–oil
contact is made, without full coalescence occurring. Irregular clumps are
formed that will fully coalesce upon melting of the fat crystals. Partial
coalescence is very much enhanced in a shear field and can then readily be a
million times faster than the coalescence of the same droplets but without
crystals. Several variables affect the rate of the process, and the results
mostly cannot be predicted, although the trends are clear. If the globules are
quite small and are coated with protein, they tend to be stable. Addition of a
small-molecule surfactant, which tends to displace (part of) the adsorbed
protein, will then induce instability.
Agitationor, more precisely, the presence of a velocity gradient thus
greatly enhances the partial coalescence rate. It can also speed up
aggregation. On the other hand, particle aggregates can be disrupted, and
the formation of a ‘‘fractal’’ gel can be prevented. Agitation strongly
disturbs sedimentation.
According to theKelvin equation, the solubility of the material in a
small particle is enhanced over the equilibrium value because of the particle
curvature. The smaller the particle, the higher the solubility, leading to
diffusion of material from small to large particles, which is calledOstwald
ripening. The result is that small particles disappear and large ones grow.
Prerequisite is that the material of the particles be at least slightly soluble in
the surrounding liquid. Ostwald ripening can occur in suspensions,
emulsions, and foams.
It is often the most important instability infoams, because gases are
relatively soluble in water. Small CO 2 bubbles may disappear within a
minute, air bubbles within an hour. Ostwald ripening will stop if the surface
dilational modulus of the bubble surface is larger than the surface tension.
Proteins are suitable to stabilize against Ostwald ripening because they
generally give a high modulus. However, the modulus tends to decrease
slowly in time, so that Ostwald ripening is only retarded, not prevented. A
highly insoluble protein layer, as is formed during whipping of egg white,
can virtually stop the process.
Triglyceride oil-in-wateremulsionsdo not show Ostwald ripening: the
oil is insoluble in water. Water-in-oil emulsions can readily show it, but then
it can be stopped by providing the aqueous phase with some salt. Shrinking
of a bubble then increases its salt content, and hence its osmotic pressure,
which counteracts the pressure increase due to the increase in curvature.