Physical Chemistry of Foods

conformation changes of globular proteins upon adsorption may be to some
extent irreversible. Consequently, g, G, and surface composition may
markedly depend on the history of the adsorbed layer.

Desorption of Proteins. If a spread layer of protein on water is
applied between barriers (cf. Figure 10.2b) and the layer is then compressed
to a given value ofPthat is kept constant, desorption of protein occurs. The
protein can readily diffuse away, since the subsurface concentration will be
(far) below the value that would be in equilibrium with the surface load.
Some results for globular proteins are given in Table 10.3. The times
involved are quite long. This is primarily due to the large activation free
energy for desorption. Its value equals at leastg/GJ?mol
^1 , assumingGto
be expressed in mol?m
^2. For a an average protein, the value would be
about 250 kJ?mol
^1 or about 100kBT per molecule, which is quite
substantial.
It is further seen that for a higher value ofP, which means a greater
difference in chemical potential of the protein between interface and
‘‘solution,’’ desorption is faster. In practice, unlike the situation to which the
table refers, the surface pressure will generally decrease when protein is
desorbed; this means that the rate of desorption will become ever slower.
The variation in desorption rate among proteins (at the same value ofP)
must primarily be due to differences in the activation free energy for
desorption. As expected, desorption tends to be slower for a higher molar
mass, but the correlation is not perfect. Besides its molar mass, the
conformation of a protein at the interface will affect the activation free
energy for desorption.
It may finally be mentioned that the desorbed protein may be in a
denatured state (see Section 7.2.2).

TABLE10.3 Characteristic Times (h) needed for Desorption of

Some Proteins of Various Molar Mass (M) from the A–W Interface

Surface pressure/mN.m
^1

Protein M/kDa 15 20 25 30 35 40 45

Insulin 6 3 0.5

b-Lactoglobulin 17 8 3 2

Myoglobulin 17 5 2.5 1

g-Globulin 160 18 8 4

Catalase 230 5 2 1.5

Source:From results by F. MacRitchie.J. Colloid Interf. Sci.105 (1985) 119.