Chemistry Reference
In-Depth Information
The redox potential of certain half-cells is affected by a variation in the pH,
which can be represented as follows when a one-electron transfer is involved:
½
Ox
½
Red
E
h
¼
E
0
þ
0
:
059 log
0
:
059pH
(4)
The E
h
of individual milk samples in equilibrium with air falls within the
range +0.25 to +0.35 V at 258C and at milk's normal pH (Jackson, 1936;
Eilers et al., 1947; Harland et al., 1952; Bhandari and Singh, 2003). Milk is
essentially oxygen-free when secreted but about 0.3 mmol L
1
O
2
is present
after equilibrium with air is established. The removal of oxygen by nitrogen
sweeping lowers the redox potential to about
0.12 V.
The major components of milk other than water, i.e., fat, lactose and
protein, have no effect on its redox potential. The redox systems in milk
involve lactate-pyruvate, ascorbate and riboflavin. The ascorbic acid content
of fresh milk is about 11.2-17.2 mg L
1
. As milk is drawn from the udder, all
ascorbate is in the reduced form, but reversible oxidation to dehydroascor-
bate occurs at a rate dependent on temperature and on the concentrations of
Cu, Fe and O
2
(Walstra and Jenness, 1984).
The ratio of ascorbate to dehydroascorbate remains high until the system
disappears from the milk, and this system stabilizes the E
h
of oxygen-free milk
at
0.0 V. Ascorbate is preserved in milk by preventing contamination with Cu
and by deaeration. The riboflavin system in milk is active, but its concentration
in milk is too low to influence the E
h
value of milk significantly.
The E
h
of milk is strongly influenced by heat treatment, bacterial activity,
contamination with metal ions such as Cu
2+
, concentration of O
2
and exposure
to light. The effects of heat treatment of milk on oxidation-reduction have been
studied (Josephson and Doan, 1939; Eilers et al., 1947; Greenbank and Wright,
1951; Harland et al., 1952; Higginbottom and Taylor, 1960). The E
h
of milk
decreases during heating; this is largely related to the liberation of the active
sulphydryl group of
-lactoglobulin, which is oxidized by atmospheric oxygen.
Reducing substances produced in milk as a result of the Maillard reaction
between lactose and protein also influence the redox potential of milk, parti-
cularly concentrated and dried milks (Harland et al., 1952). High-temperature
short-time heating of milk results in lower E
h
values, less oxidation of ascorbic
acid and greater retention of disulphide-reducing substances in liquid milk
and milk powders. The removal of oxygen from the system before heat treat-
ment results in lower E
h
values and better retention of various reducing sub-
stances than when the heating is done in air or in equilibrium with oxygen
(Higginbottom and Taylor, 1960).
Bacterial activity reduces the E
h
, largely through the consumption of
available oxygen in the medium in the course of bacterial metabolism
