2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 209
Non-specific LOX occur in legumes, e. g., LOX 1
in peas (Table 3.33) and LOX III in soybeans
(pH optimum: 6.5). These enzymes
oxidize linoleic acid to mixtures of 9- and
13-hydroperoxides, which approach racemic
proportions. In addition, oxo fatty acids and
volatile compounds are formed, i. e., the product
spectrum resembles that formed by the autoxidation
of linoleic acid. Moreover, they also react
with esterified substrate fatty acids. In contrast to
specific LOX, they do not require prior release
of fatty acids by a lipase enzyme for activity in
food.
The non-specific lipoxygenases can cooxidize
carotenoids and chlorophyll and thus can degrade
these pigments to colorless products.
This property is utilized in flour “bleaching”
(cf. 15.4.1.4.3). The involvement of LOX in
cooxidation reactions can be explained by the
possibility that the peroxy radicals are not as
rapidly and fully converted to their hydroperoxides
as in the case of specifically reacting
enzymes. Thus, a fraction of the free peroxy
radicals are released by the enzyme. It can
abstract an H-atom either from the unsaturated
fatty acid present (pathway 2a in Fig. 3.30) or
from a polyene (pathway 2b in Fig. 3.30).
The non-specific lipoxygenases present in
legumes produces a wide spectrum of volatile
aldehydes from lipid substrates. These aldehydes,
identical to those of a noncatalyzed autoxidation,
can be further reduced to their alcohols,
depending on the status of NADH-NAD ⊕ .
3.7.2.3 Enzymatic Degradation
of Hydroperoxides
Animals and plants degrade fatty acid hydroperoxides
differently. In animal tissue, the enzyme
glutathione peroxidase (cf. 7.3.2.8) catalyzes
a reduction of the fatty acid hydroperoxides to
the corresponding hydroxy acids, while in plants
and mushrooms, hydroperoxide lyase (HPL),
hydroperoxide isomerase, allene oxide synthase
(AOS) and allene oxide cyclase (AOC) are
active. The HPL reaction is highly interesting
with regard to food chemistry since the hydroperoxides,
which are formed by lipoxygenase
catalysis of linoleic and linolenic acid, are
precursors of odorants. Those are important
for fruits, vegetables and mushrooms, like the
green-grassy or cucumberlike smelling aldehydes
hexanal, (Z)-3-hexenal (“leafy aldehyde”),
(Z,Z)-3,6-nonadienal and the mushroomlike
(R)-l-octen-3-ol (Table 3.34). The suggested
mechanism is a β-cleavage of the hydroperoxide
(Fig. 3.31).
The difference in volatile products in plants
(aldehydes) and mushrooms (allyl alcohols) is
due to the different substrate and reaction specificity
of HPL. In the first case, in hydroperoxides
with conjugated diene systems (Fig. 3.31a), the
bond between the C-atom bearing the HOOgroup
and the C-atom of the diene system is
cleaved. In the second case (Fig. 3.31b), cleavage
of hydroperoxides with isolated double bonds occurs
in the opposite direction between the C-atom
Fig. 3.30. Reactions of non-specific lipoxygenase (according to Weber and Grosch, 1976). (1) Main catalysis
pathway; (2a) and (2b) cooxidation pathways. LH: linoleic acid; Car-H: carotenoid; LOOH: linoleic acid
hydroperoxide