that very deep freezing is needed to decrease the water activity greatly.
According to Eq. (8.3), the osmotic pressure is given by
P¼ 46? 104 TlnaW.
Generally, freeze-concentration leads to anincrease in ionic strength.
The effect is quite strong if salts are the main solutes (cf. Figure 16.14). An
important consequence is that it causes salting out of proteins (Section 7.3),
which may subsequently cause them to become insoluble (denatured?). This
has been observed, e.g., for egg white and muscle protein, and it is illustrated
for the latter in Figure 16.11b. Upon further freeze concentration, the ion
activity coefficients will decrease and the ions will associate. Eventually, the
ionic strength will decrease again.
Freeze-concentration may lead to crystallization of some solutes.
Solutes that are poorly soluble can reach their solubility before the viscosity
of the liquid becomes so high as to impede crystallization strongly. If the
crystals make up only a small fraction of the material, they become
FIGURE16.11 Effects of freezing and temperature on reaction rates in foods. (a)
The reaction rate constantkfor mutarotation of glucose in a solution of 100 g
glucose per liter in 0.1 molar HCl.Tf¼freezing point. (Results of T. E. Kiovsky, R.
E. Pincock. J. Am. Chem. Soc. 88 (1966) 4704.) (b) The percentage of the protein in
the moisture expressed from red meat that has become insoluble, after 3 months
storage of the beef at various temperatures. (Results of D. B. Finn. Proc. Roy. Soc.,
Ser. B, 111 (1963) 715.)