Biomedical Engineering Reference
In-Depth Information
dehydrates further to hydroxymethylfurfural HMF, which has a low potential for
browning.
In general, the major products of the irreversible combination of S(IV) with food
components are organic sulfonates
1
most often formed as a result of the inhibition
of nonenzymic browning reactions. It is the only permitted additive for this purpose
and, consequently, this property is of considerable technological importance. In food
dehydration, the additive is used to prevent spoilage in the intermediate moisture
phase of the process, at which stage the rate of browning is at a maximum, and to
protect the dehydrated food from browning while in storage. The effect of S(IV) is
to delay the onset of browning
51
but, once it commences, browning continues at the
same rate in sulfited and unsulfited systems. In most dehydrated foods, the additive
continues to react after dehydration and the shelf-life of the products is determined
by the time required for the level of the additive to fall to such an extent that it no
longer inhibits browning. This is a complex function of temperature and moisture
content for a given food product.
The principal mechanism of inhibition of browning involves the nucleophilic
addition of SO
3
2-
to the
-unsaturated carbonyl moiety of DDH (
Figure 8.6
,
reaction II), leading to 3,4-dideoxy-4-sulfohexosulose (DSH) as follows:
51
α
,
β
where the reaction could equally involve the acyclic
cis-
or
trans
-isomers of DDH,
i.e.,
The formation of DSH results in the binding of S(IV) in such a way that it
cannot be recovered by conventional analytical procedures, and is usually referred
to as
irreversibly
bound S(IV). However, a proportion of S(IV) present in reducing
sugar-amino acid-S(IV) mixtures is also bound reversibly as hydroxysulfonates.
Initially this binding is to glucose, and in a typical model reaction mixture where
[glucose}>>[S(IV)], pH 5.5, the concentration of hydroxysulfonate is given by
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