1 : 8? 10 ^5 =RT& 7? 10 ^9 per Pa at room temperature. If we apply an
external pressure of 1 bar (10^5 Pa), it would be equivalent to a decrease inaw
by 0.0007. Such a small change cannot even be detected by most methods.
In a situation where the food is a porous water-filled matrix in contact
with air, a contact line matrix-water-air exists. Assuming the pores to be
cylindrical, a capillary pressure can be calculated (see Section 10.5.2), and it
is higher for a smaller pore diameter. For rigid pores with a radius of 1mmin
a matrix that is completely wetted by water, a pressure of 1.4 bar would be
obtained, corresponding to a lowering ofaw by 0.001. In practice, the
lowering ofawwould be less, because the conditions mentioned would not
be completely fulfilled.
The quantity of imbibed water in a gellike system would vary with all
factors that affect the equilibrium state of swelling of the gel, such as
concentration of cross-links and solvent quality and, in the case of
polyelectrolytes (proteins), pH and ionic strength. Lowering of solvent
quality may be seen as a decrease in hydration of the polymer. Since it
would also cause shrinkage of the gel, it is tempting to explain the decrease
in the amount of water held as a decrease in the amount of water ‘‘bound.’’
However, it concerns very different amounts of water. This is illustrated in
Figure 8.8. It is seen that the amount of water held by the protein varies
between 2 and 7 g/g, whereas the water associated with polar groups
(predominantly the peptide bonds) of a protein rarely is>0.2 g/g and does
not change significantly with pH.
Methods. The various cases discussed above would all provide a
means of determination of what has been considered to be bound water.
Moreover, other methods have been applied, such as deriving the amount of
‘‘monolayer water’’ from an assumed relation between water content andaw
(Section 8.2); or from decreased ‘‘water mobility’’ as deduced from NMR
spectroscopy. The methods give widely varying results. Gelatin, for instance,
yields values ranging from 0.2 to 100 g per g protein, the latter value
representing the amount of water held in the gel. Even when excluding
methods determining held water, the results obtained may vary by more
than an order of magnitude.
It is therefore advisable not to use the concept of bound water. It
would be much better to be specific and speak of held water, nonsolvent
water, or nonfreezing water, according to the method of determination or
according to the relevance for the effect considered. After all, the
phenomena discussed are real and often of importance.
Note The presence of very narrow and rigid, but noncylindrical,
pores in a material could in principle explain hysteresis between