of the hairs is close to zero, and the brush collapses as predicted. This leads
to rapid aggregation. A third cause is the addition of ethanol. This will
significantly diminish the solvent quality for the hairs, which would also
cause collapse of the brush. However, as long as the chains are charged,
their solvent quality cannot be diminished greatly, unless the ethanol
concentration is so high as to decrease markedly the dielectric constant and
thereby the ionization of ionogenic groups. It is indeed observed that the
concentration of ethanol needed to cause aggregation of the casein particles
decreases with decreasing pH.
Another example is given by O–W emulsions made with pureNa-
caseinatein water. The ionic strength is then quite small, and the pH would
be, e.g., 6.6, two units above the isoelectric pH of casein. The emulsion
droplets then show weak aggregation, which can be undone by adding a
little NaCl. In all probability, the caseinate at the surface of the droplets
forms an osmotic brush, and at the prevailing low ionic strength many of the
carboxyl groups on the casein would be undissociated, implying that the
brush is more or less collapsed. Adding salt then increases dissociation and
thereby the negative charge. Hence the adsorbed layer will swell and the
repulsion will be greatly enhanced. The effect of salt is to some extent
comparable to the salting in of proteins discussed in Section 7.3.
There are few proteins like the caseins, which behave more or less like
random coils. The prime example for steric repulsion isb-casein, which has a
strongly amphiphilic character (see Figure 7.1) and which provides a brush
upon adsorption (see Figure 10.12d).Gelatincan adsorb in a similar manner
(at least at temperatures above about 30 8 C), and possibly some denatured
globular proteins. All proteins adsorb at apolar surfaces, including O–W and
A–W interfaces, but the repulsion that they provide will generally be for the
most part electrostatic; which means that a pH near the isoelectric point and
a very high ionic strength may cause aggregation. As a rule of thumb, if
conditions are such that the protein used is poorly soluble, it will probably
not provide sufficient repulsion to, say, oil droplets to prevent their
aggregation. Close to the isoelectric pH of the protein there will even be
electrostatic attraction at close distance, because the protein will then have
both positive and negative groups.
Other Surfactants. Most polysaccharides do not adsorb onto
apolar surfaces. But those that do adsorb can provide strong steric
repulsion. See Section 10.2.2 for adsorbing polysaccharides.
An important class of small-molecule surfactants containspolyox-
yethylene chains. The prime example used in foods is the Tweens (see Table
10.2). They adsorb strongly at apolar surfaces and the POE chains, if long
enough, provide strong steric repulsion, sinces is relatively large. The