2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 211
Table 3.34. Occurrence and properties of various hydroperoxide-lyases
Occurrence Substrate Products of the
catalyses
Apple, tomato, 13(S)-hydroperoxy-9-cis,11- hexanal + 12-oxocucumber,
tea leaf trans-octadecadienoic 9-cis-dodecenoic
(chloroplasts), soy acid (13-LOOH) acid
beans, grape
Apple, tomato, 13(S)-hydroperoxy-9-cis,11- (Z)-3-hexenal +
cucumber, tea leaf trans, 15-cis-octadecatrienoic 12-oxo-9-cis-
(chloroplasts), soy acid (13-LnOOH) dodecenoic acid
beans, grape
Cucumber, pear 9(S)-hydroperoxy-10-trans, 12-cis- (Z)-3-nonenal +
octadecadienoic acid (9-LOOH) 9-oxo-nonanoicacid
Cucumber, pear 9(S)-hydroperoxy-8-trans, 12-cis, (Z,Z)-3,6-nonadienal +
15-cis-octadecatrienoic acid 9-oxononanoic acid
(9-LnOOH)
Champignon 10(S)-hydroperoxy-10-trans, 12-cis- 1-octen-3(R)-ol +
octadecadienoic acid (10-LOOH) 10-oxo-8-trans-
Champignon 10(S)-hydroperoxy-8-trans,12- dcenoic acid (Z)-1,5-
cis-15-cis-octadecatrienoic
acid (10-LnOOH)
octadien-3(R)-ol+
10-oxo-8-transdecenoicacid
acid. Since hydroxy but not hydroperoxy acids
taste bitter, this reaction should contribute to the
bitter taste generated during the storage of oats
(cf. 15.2.2.3).
3.7.2.4 Hydroperoxide–Protein Interactions
3.7.2.4.1 Products Formed
from Hydroperoxides
Hydroperoxides formed enzymatically in food
are usually degraded further. This degradation
can also be of a nonenzymatic nature. In
nonspecific reactions involving heavy metal
ions, heme(in) compounds or proteins, hydroperoxides
are transformed into oxo, expoxy,
mono-, di- and trihydroxy carboxylic acids
(Table 3.35). Unlike hydroperoxides, i. e. the
primary products of autoxidation, some of
these derivatives are characterized as having
a bitter taste (Table 3.35). Such compounds
are detected in legumes and cereals. They may
play a role in other foods rich in unsaturated
fatty acids and proteins, such as fish and fish
products.
In order to clarify the formation of the compounds
presented in Table 3.35, the reaction
sequences given in Fig. 3.32 have been assumed
to occur. The start of the reaction is from the
alkoxydiene radical generated from the 9- or
13-hydroperoxide by the catalytic action of heavy
metal ions or heme(in) compounds (cf. 3.7.2.1.7).
The alkoxydiene radical may disproportionate
into a hydroxydiene and an oxodiene fatty acid.
Frequently this reaction is only of secondary
importance since the alkoxydiene radical rearranges
immediately to an epoxyallylic radical
which is susceptible to a variety of radical combination
reactions. Under aerobic conditions the
epoxyallylic radical combines preferentially with
molecular oxygen. The epoxyhydroperoxides
formed are, in turn, subject to homolysis via an
oxyradical. A disproportionation reaction leads
to epoxyoxo and epoxyhydroxy compounds.
Under anaerobic conditions the epoxyallylic
radical combines with other radicals, e. g. hydroxy
radicals (Fig. 3.32) or thiyl radicals
(Fig. 3.33).
Of the epoxides produced, the allylic epoxides
are known to be particularly susceptible
to hydrolysis in the presence of protons.
As shown in Fig. 3.32 trihydroxy
fatty acids may result from the hydrolysis
of an allylic epoxyhydroxy compound.