Biomedical Engineering Reference
In-Depth Information
It is often said that S(IV) is unique in its ability to control browning in food.
The mechanism of inhibition of enzymic browning is the reaction of sulfite ion with
the
o
-quinones which are formed by the enzymatic oxidation of
o
-diphenols. Essen-
tially the quinones are reduced to the sulfonated phenols (
Figure 8.6
,
reaction VI).
In the case of catechol oxidation, it has been shown that the 4-sulfocatechol which
is formed upon reduction of the quinone is unreactive towards polyphenol oxidase
and so represents a relatively stable product.
92
In general, inhibitors are expected to
work in two ways. Either they inhibit the enzyme or, in the same way as S(IV), they
react with the quinone intermediates. Ascorbic acid is probably the best known
reagent that acts by the latter of these mechanisms. However, there has been a long-
standing interest in specific inhibitors of the enzyme which work at much lower
concentrations than typical concentrations of S(IV) added to food. Hydroxycinnamic
and benzoic acids,
93
kojic acid (5-hydroxy-2-(hydroxymethyl)-
-pyrone),
94
4-hexyl-
resorcinol,
95
ficin,
96
C
3
-C
5
aliphatic primary alcohols,
97
and even honey
98
are exam-
ples of potentially useful inhibitors of varying degrees of effectiveness. Of course,
where applicable, a very simple solution to prevent enzymic browning when foods
are stored is the action of heat (e.g., by blanching), since polyphenol oxidase is
relatively heat labile. When foods are treated in this way, only nonenzymic browning
is said to occur.
The search for alternative antibrowning agents against nonenzymic browning
has proven to be particularly difficult. It has long been known
99
that thiol compounds
inhibit the Maillard reaction in a way similar to S(IV), but the practical use of such
additives was not advocated seriously until Friedman conducted a series of studies
involving N-acetyl cysteine and glutathione.
100-102
It is believed that thiols react with
intermediates such as DH in much the same way as sulfite ion
103,104
because the thiol
group is similarly nucleophilic. Studies in the author's group suggest that N-acetyl
cysteine is not sufficiently stable in acid solution and is converted slowly to cysteine
which reacts with Maillard intermediates to form characteristic “meaty” odors. On
the other hand, dipeptides of cysteine (N-substituted cysteine) with another amino
browning of a glucose-glycine mixture in the presence of S(IV), mercaptoethanol,
the dipeptides, and glutathione. These results indicate that the dipeptides are gener-
ally as effective as S(IV) except that S(IV) is better at controlling the formation of
low levels of color in the early stages of the reaction. Thiols are seen to be highly
reactive towards cabbage on blanching and during dehydration,
106
but unpublished
evidence suggests that the mixture of products is much more complicated than that
obtained when S(IV) is used as the antibrowning agent. There is as yet no evidence
regarding the nature of the products formed when thiols inhibit such browning
reactions.
γ
ACKNOWLEDGMENTS
Much of the work described in this chapter was originally inspired by David
McWeeny in the early 1970s, when the author was affiliated with the Ministry of
Agriculture, Fisheries and Food. He is indebted to Dr. McWeeny for his continued
interest and valuable discussion. The work has been made possible by generous
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