Physical Chemistry of Foods

other organic solvents. The presence of other polymers may lead to phase
separation (Section 6.5.2), which might be interpreted as a decrease of
solubility.
The dependence of solubility ontemperaturevaries. We will consider
here the range of zero to about 50 8 C; at still higher temperature unfolding
may occur. For hydrophilic proteins, the solubility may increase with
temperature, by up to 4%per K. For more hydrophobic proteins, solubility
decreases with increasing temperature, by up to 10%per K. This is in
accordance with the strong temperature dependence of hydrophobic bonds
in the range considered (Fig. 3.4). Low temperature may also cause
dissociation of quaternary structures.
When considering the effect of denaturation on the solubility of
globular proteins, two cases should be distinguished. The first is denatura-
tion by such agents as detergents, urea, or guanidinium salts. As long as
these compounds are present, the solubility is enhanced. This stands to
reason, since the denaturation (unfolding) occurs because the free energy is
lower for an increased solvent–solute contact. The second is irreversible
denaturation, especially as caused by heat treatment. The increased
exposure of apolar groups now allows many intermolecular hydrophobic
bonds to be formed, i.e., cause aggregation. Some denatured proteins are
virtually insoluble. This will greatly depend, however, on pH and ionic
strength. Figure 7.14a gives an example for whey protein. This is a mixture,
for the most part consisting of globular proteins, isoelectric pHs around 5,
that are subject to irreversible heat denaturation. Here turbidity was used as
a measure for aggregation, hence for ‘‘insolubility.’’
This brings us to a final remark about the solubility ofprotein
preparations, i.e., the more or less crude mixtures as applied in the food
industry. Solubility is an essential criterion for most functional applications.
The tests applied to assess this quality generally involve mixing of a given
amount of the material with a given amount of a specified solvent, usually a
buffer. Vigorous mixing then is followed by centrifuging at specified
conditions. The amount of protein or nitrogen in the supernatant is
determined and compared to the total amount present. The result is so many
‘‘percent soluble.’’ Some results are shown in Figure 7.14b.
Some remarks should be made about these tests. First, the meaning of
the word solubility is fundamentally different from the definition used by
physical chemists, given at the beginning of this section. Suppose that a
‘‘solubility’’ of 50%is observed. If this were a true solubility, doubling of the
amount of solvent would lead to 100%. For the protein preparation,
doubling the amount of solvent may well leave the result at 50%; in other
words, half of the material would be well soluble, and the other half not at
all. In most cases, however, the situation will be somewhere in between. This