Biomedical Engineering Reference
In-Depth Information
explain the success and versatility of sulfites as food additives, and discuss the use
of these species as probes in a new approach to the study of Maillard browning.
A general review, to 1980, of the chemistry of sulfite relevant to its enzymology,
microbiology, and food applications has been published by Wedzicha.
1
More
recently, Taylor et al.,
2
Rose and Pilkington,
3
Wedzicha, Bellion, and Goddard,
4
and
Wedzicha
5,6
have discussed the subject with various emphasis. The review by Taylor
et al. is of particular interest as this is one of the most detailed accounts of safety
aspects of the use of sulfites in foods. On the other hand, Rose and Pilkington mainly
consider the mechanism of the antimicrobial action of SO
2
and is a key contribution
to this field of study.
CHEMICAL NATURE OF THE SPECIES
The terms
sulfur dioxide
or
sulfite(s)
refer to oxospecies of sulfur in oxidation state
(IV); they are all derived by the dissociation of the so-called sulfurous acid H
2
SO
3
.
Despite the wide use of the term
sulfurous acid
in chemical literature, it is
acknowledged
7,8
that this species does not exist as such, or is present at very low
concentrations in equilibrium with aqueous (dissolved) SO
2
. Spectroscopic data
show the interaction between SO
2
and H
2
O to be immeasurably weak. Thus, it has
become conventional to represent the dibasic acid as SO
2
.H
2
O, while recognizing
that it has the properties of H
2
SO
3
. The following equilibria need to be taken into
account in the discussion of the reactivity of sulfites in food:
SO
2
(g) + H
2
O(1)
SO
2
·H
2
O(1)
K
H
= 1.2
×
10
-3
mol kg
-1
bar
-1
(8.1)
SO
2
·H
2
O
HSO
3
-
+ H
+
p
K
1
= 1.89
(8.2)
SO
2
·H
2
O + X
-
SO
2
X
-
(H
2
O)
(8.3)
HSO
3
-
SO
3
2-
+ H
+
p
K
2
= 7.18
(8.4)
HSO
3
-
+ M
+
MHSO
3
(8.5)
SO
3
2-
+ M
+
HSO
3
-
(8.6)
2HSO
3
-
2
O
5
2-
+ H
2
O
K
= 0.05 mol
-1
l
(8.7)
S
2
O
5
2-
+ M
n+
MS
2
O
5
(2-n)-
(8.8)
where the values of all equilibrium constants given for the specific reactions are at
25°C and infinite dilution,
K
H
is Henry's constant, and M
n+
represents a metal ion.
The dissolution of SO
2
in water obeys Henry's law, but the apparent value,
(
K
H
)
app
, of Henry's constant (i.e., the total concentration of S(IV) species in solution
divided by the pressure of SO
2
in equilibrium with this solution) depends on tem-
perature,
9
and it is easy to show that the value depends on pH as follows:
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