8.3 ‘‘WATER BINDING’’
The quotation marks in the heading of this section signify that water
binding is a questionable term. In the author’s view, the concept has caused
more confusion than understanding. We all know that many solid foods
hold water. It is also observed that fairly dry foods often are quite stable to
deterioration, although these foods may contain a significant amount of
water. This has led to the notion that at least part of the water is bound. It
is, however, uncertain what that means.
In Section 3.2, solvation, includinghydration, is briefly discussed, and
it is clear that solute molecules and ions, or chemical groups on
macromolecules and surfaces, can be hydrated. This means that one or a
few water molecules reside longer (e.g., for 10^10 s) at a given site than they
would in pure water (about 10^13 s). Especially charged groups, and to a
lesser extent groups with a dipole, are hydrated, but not most hydroxyl
groups. One may now speak of bound water, but it is often uncertain what
the amount would be. Another case of water binding is water of hydration
in crystals or in polymer crystallites; here the residence time of the water
molecules may be much longer, up to years. Globular protein molecules
contain some buried ‘‘structural’’ water molecules, and their residence time
may be long, say several minutes; however, it only concerns very little water,
some grams per 100 g protein.
There are several relations between water content and some
macroscopic property that have been interpreted byassumingthat part of
the water is of a special category, often equated with ‘‘bound water.’’ For
instance,
- Nonreactive water. At low water content, the reactivity of water is
decreased, but reactivity is precisely given by the water activity. In other
words, all the water has a smaller reactivity. Of greater importance, almost
all reactions are slower in a concentrated system than in a dilute solution,
including reactions not involving water. As is discussed in Section 8.4, this
has other causes, especially small diffusivity. The term ‘‘nonreactive water’’
thus makes no sense. - Nonsolvent water. When a solution can be separated from a food,
for instance by centrifugation or by ultrafiltration, it is often observed that
the mass ratio of a given solute S to water in that solution is larger than the
ratio in the whole food. This can be interpreted as part of the water that is
left behind (in the pellet or in the retentate) being not available as a solvent.
This may be bound water, but the greater part of it may not. It is generally
observed that the amount of nonsolvent water depends on the nature of the
solute, and it generally increases with increasing molar mass of solute. This
phenomenon can be ascribed to negative adsorptionof the solute, for