440 DAIRY CHEMISTRY AND BIOCHEMISTRY
product are at unit activity), n is the number of electrons transferred per
molecule, R is the universal gas constant (8.314JK-'mol-'), T is tempera-
ture (in Kelvin), F is the Faraday constant (96.5 kJ V- ' mol-') and ured and
uox are activities of the reduced and oxidized forms, respectively. For dilute
solutions, it is normal to approximate activity by molar concentration.
Equation 11.7 can be simplified, assuming a temperature of 25"C, a transfer
of one electron and that activity
E, = E, + 0.059 log [Ox]/[Red]. (11.8)
Thus, E, becomes more positive as the concentration of the oxidized form
of the compound increases. E, is influenced by pH since pH affects the
standard potential of a number of half-cells. The above equation becomes:
E, = E, + 0.059 log [Ox]/[Red] - 0.059 pH. (11.9)
The E, of milk is usually in the range + 0.25 to + 0.35 V at 25"C, at pH
6.6 to 6.7 and in equilibrium with air (Singh, McCarthy and Lucey, 1997).
The influence of pH on the redox potential of a number of systems is shown
in Figure 11.1.
The concentration of dissolved oxygen is the principal factor affecting the
redox potential of milk. Milk is essentially free of 0, when secreted but in
equilibrium with air, its 0, content is about 0.3 mM. The redox potential of
anaerobically drawn milk or milk which has been depleted of dissolved
oxygen by microbial growth or by displacement of 0, by other gases is
more negative than that of milk containing dissolved 0,.
The concentration of ascorbic acid in milk (1 1.2- 17.2 mgl- ') is sufficient
to influence its redox potential. In freshly drawn milk, all ascorbic acid is in
the reduced form but can be oxidized reversibly to dehydroascorbate, which
is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the
lactone ring of dehydroascorbate, which results in the formation of 2,3-
diketogulonic acid, is irreversible (Figure 11.2).
The oxidation of ascorbate to dehydroascorbate is influenced by 0,
partial pressure, pH and temperature and is catalysed by metal ions
(particularly Cu2 +, but also Fe3 +). The ascorbate/dehydroascorbate system
in milk stabilizes the redox potential of oxygen-free milk at c. 0.0 V and that
of oxygen-containing milk at + 0.20 to + 0.30 V (Sherbon, 1988). Riboflavin
can also be oxidized reversibly but its concentration in milk (c. 4pM) is
thought to be too low to have a significant influence on redox potenial. The
lactate-pyruvate system (which is not reversible unless enzyme-catalysed) is
thought not to be significant in influencing the redox potential of milk since
it, too, is present at very low concentations. At the concentrations at which
they occur in milk, low molecular mass thiols (e.g. free cysteine) have an
insignificant influence on the redox potential of milk. Thiol-disulphide
interactions between cysteine residues of proteins influence the redox
properties of heated milks in which the proteins are denatured. The free
concentration: