2009_Book_FoodChemistry

210 3 Lipids
hydrolysis, the allene oxide originating from the
13-hydroperoxide of linoleic acid gives rise to
α-orγ-ketol fatty acids depending on whether the
OH-ion attacks at C-13 or C-9 (Formula 3.75).
The allene oxide can also react with other nucleophiles,
e. g., ROH, RSH, as well as the anion
of linoleic acid. Allene oxide cyclase (AOC)
competes with these non-enzymatic reactions,
cyclizing the allene oxide to 15,16-dihydro-
12oxophyto-dienoic acid (Formula 3.75). AOS
and AOC convert the 13-hydroperoxide of
linolenic acid to 12-oxophytadienoic acid.
Fig. 3.31. Mechanism of the cleavage of hydroperoxides
by lyases (according to Wurzenberger and
Grosch, 1986)
a in plants, b in mushrooms
with the OOH-group and the C-atom with the
adjacent methylene group. The (Z)-3-aldehydes
in plants formed by the splitting reaction can
transform themselves into the respective (E)-2-
aldehydes. Isomerases that catalyze this reaction
were identified in cucumbers, apples and tea
chloroplasts.
The widespread presence of the C6- and
C9-aldehydes in fruits and vegetables as well
as the C8-alcohols in mushrooms (Table 3.34)
permits the conclusion to be drawn that
enzymatic-oxidative cleavage of linoleic and
linolenic acid with the enzymes lipoxygenase,
hydroperoxide-lyase and, if necessary, an
aldehyde-isomerase generally contributes to the
formation of aroma in these food items. This
process is intensified when oxygen can permeate
the cells freely by destruction of tissue (during
the chopping of fruits and vegetables).
Allene oxide synthase (AOS) is a cytochrome
P 450 enzyme which was first found in flaxseed.
It catalyzes the degradation of hydroperoxides to
very instable allene oxides (t 1/2 at 0 ◦ C: 33 s). On
(3.75)
Linoleic acid 9-hydroperoxide formed by
lipoxygenase in potato is changed enzymatically
by elimination of water into a fatty acid with
a dienyl-ether structure:
(3.76)
In addition to lipoxygenase, lipoperoxidase
activity has been observed in oats. The
9-hydroperoxide formed initially is reduced
to 9-hydroxy-trans-10,cis-12-octadecadienoic