Effects of Sulfur Dioxide on Food Quality - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 277

The p K a of HSO 3 - is similar to that of H 2 PO 4 - , and it is reasonable to expect
that the S(IV) species could act as general acid-base catalysts in chemical reactions,
as do phosphate species. Usually, this occurs when proton transfer takes place in
the rate-determining step of the reaction and the catalyst is either the proton donor
or acceptor. It is significant that S(IV) catalyze the mutarotation of glucose, 76 pre-
sumably by speeding up the rate of ring opening, although there is no evidence that
it increases per se the concentration of the acyclic structure. Insight into the mech-
anism of the catalysis of the hydrolysis of gluconolactone by S(IV) comes from an
investigation of the hydrolysis of p -nitrophenyl acetate. 77 In this case, SO 3 2- is 16,000
times more effective a catalyst than HPO 4 2- ; this is attributed to the ability of SO 3 2-
to initiate the hydrolysis by nucleophilic attack at the carboxyl-carbon atom, as
follows:
The implications of this reactivity have not yet been explored fully, but it is possible
that esters which are components of natural or additive food flavors could be “desta-
bilized” by S(IV).
This discussion of the effects of S(IV) on the stability of ascorbic acid appears
to contradict the normally accepted role of S(IV) as a protector of the vitamins in
food. However, the chief cause of its degradation is oxidation and S(IV) is capable
of preventing this from taking place by scavenging oxygen, reducing the monode-
hydroascorbic acid intermediate, or stabilizing dehydroascorbic acid. It is suggested
that the rate of the S(IV)-mediated degradation of ascorbic acid is slow in comparison
with the other reactions that lead to its loss from food, and the overall effect is,
therefore, that S(IV) stabilizes the vitamin.
H OMOLYTIC R EACTIONS
It is well known that sulfite ion is a good reducing agent which reacts with oxygen
and numerous inorganic and organic oxidizing agents. The oxidation of S(IV) takes
place by a one- or two-electron transfer mechanism. Here we will focus on the one-
electron route because this leads to free radical intermediates and has the greater
potential impact on food quality.
The primary step in the one-electron oxidation of S(IV) is the formation of the
sulfite radical • SO 3 - as a result of the transfer of one electron from the sulfite ion to
a suitable electron acceptor, such as a transition metal ion in one of its higher
oxidation states. 1 Whereas the oxidation of S(IV) is inhibited by low concentrations
of organic compounds such as alcohols, it is striking that high concentrations of
ethanol appear to increase the rate of autoxidation. There is evidence that this
increase is due to the participation of transition metal complexes, including those