Biomedical Engineering Reference
In-Depth Information
with ligands such as EDTA which normally inhibit oxidation.
78
Sulfite radicals can
also be formed by the reaction of sulfite ion with radicals derived from other
reactions.
Much of the interest in radicals derived from S(IV) stems from the fact that they
can react with oxygen (
k
= 1.5
10
9
mol
-1
l s
-1
) to form a peroxyl radical (
•
SO
5
-
)
which is a more powerful oxidizing agent
79
than
•
SO
3
-
. Species such as
•
OH,
•
O
-
,
and
•
SO
4
-
are formed by the reduction of
•
SO
5
-
by metal ions, e.g., Fe
2+
, by S(IV),
or as a result of the decomposition of
•
SO
5
-
. It is observed that S(IV) in food is
converted partly to sulfate and oxidation of S(IV) is, therefore, taking place. Hence,
it is clear that the oxidation of S(IV) in food can lead potentially to oxidative spoilage.
An impressive demonstration of this fact is an experiment whereby a solution
(in ethanol-water-chloroform) of
×
-carotene is decolorized in a few seconds upon
the addition of a solution of S(IV). This reaction results in as many as 17 atoms of
oxygen becoming incorporated for every molecule of
β
-carotene oxidized.
80
Other
demonstrations (carried out by the author but unpublished) include the accelerated
development of rancidity in emulsified unsaturated triglyceride mixtures in the
presence of S(IV). Such observations have led to the term “sulfite-mediated” oxi-
dation being used to describe the process. A most interesting example of such an
oxidation is the “oxidative depolymerization” of starch first identified by Hill and
S(IV) dramatically reduce the swelling volume and viscosity of starch, and there is
β
FIGURE 8.8
Effect of sodium sulfite concentration on the swelling volume (——) and
solubility (------) of cassava starch (1% w/v) and held at 95°C for 1 h. The solubility is the
total weight of carbohydrate found in the supernatant expressed as a percentage of the dry
weight of starch in the whole system. These graphs have been plotted using the data shown
in Mat Hashim, D. B., Moorthy, S. N., Mitchell, J. R., Hill, S.E., Linfoot, K. J., and Blanshard,
J. M. V.,
Stärke,
471, 1992, and kindly provided by Dr. S.E. Hill.
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