Physical Chemistry of Foods

the phases is very small, 10^7 to 10^4 N?m^1 , being higher for greater

incompatibility.

An important case of incompatibility is formed by mixtures ofproteins

and neutral polysaccharides. Examples are in Figure 6.20. It is seen that the

phase diagrams may be very asymmetric, the protein concentrations needed

for phase separation being much larger than the polysaccharide concentra-

tion. The asymmetry is stronger for globular proteins than for more or less

unfolded molecules like gelatin or casein. Proteins are polyelectrolytes, and

if the pH is not close to the isoelectric point and the ionic strength is low,

phase separation does not occur. This is due to the Donnan effect (Section

6.3.3). The presence of counterions implies that phase separation would go

along with separation of salt ions, causing considerable loss of mixing

entropy. This, of course, counteracts any decrease in enthalpy due to phase

separation. As seen in Table 6.2, the relative difference in salt concentration

between the ‘‘compartments’’ decreases with ionic strength, and it almost

vanishes at 0.1 molar. It is indeed observed that phase separation only

occurs at ionic strengths of 0.1 molar or higher, unless the pH is near the

isoelectric point of the protein. In the latter case a low salt content promotes

phase separation, presumably because the solubility of the protein strongly

FIGURE6.20 Examples of incompatibility of proteins and polysaccharides. (After
results by V. B. Tolstoguzov.)