Biomedical Engineering Reference
In-Depth Information
While there is much interest in acetaldehyde, important food flavors arise from a
wide range of volatile aldehydes, e.g., through enzymic oxidation of unsaturated fatty
acids in plant products such as tomato, cucumber, and peas. The known antioxidant
properties of this additive are often confused between its ability to react with oxygen
or by inhibiting enzymes in one of many ways, with the reaction of S(IV) with the
final carbonyl products of oxidation. In many situations it is difficult to distinguish
between these possibilities. A particular example where current thinking suggests that
S(IV) inhibit the sensory impact of an oxidation product is through their reaction with
the carbonyl moiety of
trans
-2-nonenal to form involatile hydroxysulfonate salts. The
aldehyde is alleged to be the cause of a “cardboard”-like taint (staleness) when beer
undergoes oxidative spoilage, e.g., after beer is allowed to stand in air or when beer
containers are defective in so far as they allow ingress of oxygen.
42
Evidence for the
important role played by nonenal in eliciting the sensation of staleness is demonstrated
by spiking beer with the aldehyde to obtain a stale product. When acetaldehyde is
added to sulfited stale beer, which has no taint, the taint promoting substance is
released.
43
It is found that nonenal is released in preference to other aldehydes as a
result of competition between it and acetaldehyde for the S(IV) in solution.
44
The defect may be avoided or removed by the addition of S(IV); levels as low
as 2 ppm SO
2
are found to be sufficient with concentrations of nonenal at the ppb
level. While low levels of acetaldehyde appear to reverse the anti-staling effect of
added S(IV), there are no reliable values of nonenal-S(IV) hydroxysulfonate disso-
ciation constants. One reason is that the very low solubility of this aldehyde in water
means that equilibrium measurements require correspondingly low concentrations
of S(IV) which are difficult to measure accurately. However, since in this situation,
hydroxysulfonate formation is the conversion of a relatively non-polar species into
a species with significant polarity, i.e., because of the sulfonate group, it is likely
that the hydroxysulfonate of nonenal is at least as stable as that of acetaldehyde.
The addition of an excess of acetaldehyde to a solution of nonenal hydroxysulfonate
should compete successfully for the S(IV). The same considerations could apply
also to hexanal and other hydrophobic aldehydes derived from the oxidation of
unsaturated triglycerides. Sulfite ion can also react with nonenal by nucleophilic
attack on the
-unsaturated carbonyl moiety (as in
Figure 8.6
,
reaction II). There
is no evidence of this reaction in beer; the reason it apparently does not take place
is important because such a reaction would obviate the need for residual S(IV) in
beer. While any explanation can only be speculative, it is reasonable to suggest that
the highly efficient formation of nonenal hydroxysulfonate (which is not attacked
by SO
3
2-
at the C
α
,
β
-unsaturated
carbonyl moiety to such an extent that the overall rate of its biomolecular reaction
with S(IV) is too slow to be significant. The synthesis of this hydroxysulfonate is
carried out usually at high concentration in water-non-aqueous solvent mixtures.
C bond) reduces the effective concentration of the
α
,
β
Inhibition of Nonenzymic Browning
To most individuals, the term nonenzymic browning is synonymous with the Mail-
lard reaction, i.e., the reaction between reducing sugars and amino acids, peptides,
and proteins. Current understanding of its mechanism stems from that proposed by
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