Biomedical Engineering Reference
In-Depth Information
The effects of pH on k f and k r are almost in balance over the pH range 3 to 6 such
that the value of K remains almost constant. However, the adducts become progres-
sively labile (formed and decomposed more rapidly) as pH is increased. Outside this
range of pH, hydroxysulfonates are less stable; as pH is reduced to below 3, the
increasing conversion of S(IV) to SO 2 results in the apparent value of K passing through
a minimum at around pH 2, although the rates of formation and decomposition of the
adducts are very slow. On the other hand, they decompose rapidly at pH > 7.
The classification of S(IV) in foods into free and bound S(IV) is well known 25
and referred to often in the food industry, but there are many instances where its
significance is poorly understood. Bound S(IV) is sometimes referred to as reversibly
bound . Free S(IV) is the term used to describe the additive present in the form of
SO 2 , or any S(IV) species (e.g., SO 3 2- , S 2 O 5 2- ) which are converted rapidly to SO 2
upon acidifying. This term is, therefore, synonymous with the more accurate (in the
chemical sense) use of the term S(IV) in this chapter. Bound S(IV), which is now
regarded as mostly in the form of hydroxysulfonates, was defined originally in terms
of the different stability and the rates of formation and decomposition of these
products. Thus, bound S(IV) is the amount of the additive that is converted to the
free form by raising the pH of a sample to at least pH 10, whereas free S(IV) is
usually analyzed at pH
2, under which conditions hydroxysulfonates are most
stable. The standard method of analysis of S(IV) in food, based on that devised by
Monier-Williams, 32 involves prolonged boiling of the sulfited sample in a strongly
acidic solution. Under these conditions the very low pH and the high temperature
assist in the decomposition of any hydroxysulfonates present in the sample and allow
all the S(IV) to be desorbed from solution as gaseous SO 2 .
Hydroxysulfonates are usually decomposed in the human gastrointestinal tract
and their toxicity is equivalent to that of free S(IV); 33 thus legislation stipulates the
total (i.e., free + bound) S(IV) present in food at the time of sale. The reason for
taking an interest in the relative amounts of free and bound S(IV) in food is that
bound S(IV) does not exhibit the antimicrobial properties of the free additive. The
author has often observed that foods (particularly fruit juices) containing acceptable
amounts of total S(IV) spoil because the level of free S(IV) is too low; often the
quality assurance protocol adopted during food manufacture does not include the
monitoring of free S(IV) levels. It is important, therefore, to understand the factors
that determine the S(IV)-binding capacity of different types of foods.
Hydroxysulfonate dissociation constants for a wide range of carbonyl com-
pounds, representing the extremes of stability normally encountered for food com-
ponents, are given in Table 8.3 . 34-3 6 The value for a simple aldehyde, e.g., acetalde-
hyde, is of the order 10 -5 to 10 -6 mol l -1 representing a very stable adduct. On the
other hand, the carbonyl group of reducing sugars exists in equilibrium with cyclic
structures, and the values of their hydroxysulfonate dissociation constants depend
on the proportion of acyclic form present. Thus, the values 0.9 and 15 mol l -1 for
glucose and fructose hydroxysulfonates, respectively, indicate unstable adducts;
fructose is regarded as not forming such an adduct to a significant extent, in practice.
Problems stemming from S(IV) binding are seen most commonly in fruit products,
particularly beverages, and most of our understanding of the underlying reasons has
come from investigations on fermented beverages. 36-38 Acetaldehyde, 2-ketoglutaric
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