Physical Chemistry of Foods

increases with increasing ionic strength (see Section 7.3). Two polyelec-
trolytes of equal charge density may show segregative phase separation
irrespective of ionic strength, since separation would not cause separation of
counterions.
The incompatibility may have various consequences. It may be a
nuisance when a homogeneous liquid is desired, since it may lead to slow
separation into layers. If one of the phases is high in protein, any
aggregation of the protein during heating will proceed faster. Moreover, at
higher temperature the incompatibility of proteins and polysaccharides is
generally greater, and the lower viscosity allows faster phase separation to
occur. On the other hand, the phenomenon can be made useful in
concentrating one of the polymers; it can be seen as a kind of membraneless
ultrafiltration. Careful optimization of conditions can yield a practicable
process. Water-in-water emulsions can be useful stages in manufacturing
foods, for instance when either of the phases is made to gel. The droplets can
be very easily deformed, because of their very small interfacial tension, and
this can be employed in making threadlike particles that can then be made to
gel (‘‘spinneretless spinning’’).
Complex Coacervationoccurs if the two types of polymers have side
groups that are mutually attractive. A prime example is a mixture of a
protein below its isoelectric pH (positive groups) and an acid polyelectrolyte
(negative groups), the first known system being an acid solution of gelatin
with gum arabic. Figure 6.19b gives an idealized phase diagram. As in
Figure 6.19a, the tie lines give the composition and the volumes of the two
phases obtained.
Coacervation of two polyelectrolytes occurs especially at low ionic
strengthI, for instance below 0.2 molar. At higherI, the charges are sensed
at very small distances only; see Figure 6.8. Some proteins may also exhibit
complex coacervation with a polyacid at a pH above their isoelectric point.
An example is given by caseinate withk-carrageenan, where some positive
groups on the caseinate are responsible for the attractive interaction,
although the protein has more negative groups. In such a case, coacervation
only occurs at intermediateI:ifIis very small, the negative and positive
groups on the protein molecule cannot be sensed separately, whereas this
can occur at higherI(cf. Figure 6.9).
Actually, attractive interactions between two types of polymers can
become manifest in various ways. Besides coacervates, small soluble
complexes can be formed. If the interactions are weak, a homogeneous
weak gel may form. If the interactions are strong, coprecipation of both
polymers may occur; this can be applied to separate proteins from solutions,
for instance with carboxymethyl cellulose. The relations governing which of
these phenomena will occur are not fully understood.