Chemistry Reference
In-Depth Information
affect lipid oxidation by changing the balance of concentration and activity
between heme iron and free iron.
Although lipid oxidation is considered a deteriorative process responsible for
generating off-flavors, specific oxidation products are desirable flavor com-
pounds particularly when formed in more precise reactions, such as the genera-
tion of fresh fish flavor (German et al., 1992). The specificity and stability of
endogenous LOX and the oxidation volatiles generated by the enzymes in the
gills of marine and freshwater fish were important in the generation of fresh fish
flavors. Understanding these enzyme systems may facilitate the development of
methods for the generation of desirable flavors in seafood products.
5.3.5 In dairy products
Many enzymes in milk are known to initiate lipid oxidation. Xanthine oxidase is
a flavin enzyme found in milk fat globule membranes that generate superoxide
radical (O
2
ÿ·
), a reaction by an O
2
taking up one electron. Superoxide dismutase
catalyzes the dismutation of this radical to produce hydrogen peroxide, thus
superoxide dismutase serves as an antioxidant in the presence of catalase which
can remove hydrogen peroxide from the system. In the presence of Fe
2+
, H
2
O
2
produces very reactive HO
·
by the Fenton reaction. This active free radical can
react non-selectively with all organic molecules. Milk neutral lipid has little
polyunsaturated fatty acid and inherently stable to oxidation. Copper and iron
exposure and photo-oxidation have been the major causes of oxidation. Ribo-
flavin generated singlet oxygen oxidation may play important role in milk lipid
oxidation. Cupric catalyzed oxidation of milk fat seems to give rise to 1-octene-
3-one or possibly 1,5-octadiene-3-one from n-3 fatty acids. These compounds
play a minor role in the oxidation of vegetable oil but they are the dominant
flavor in milk fat.
5.3.6 In fruits and vegetables
Lipid derived flavors are produced in tomatoes during ripening via the action of
endogenous enzymes such as LOX. They may be further influenced by non-
enzymatic oxidative decomposition reactions which occur during processing and
storage (Karmas et al., 1994). The generation of lipid oxidation compounds in
fresh tomatoes during ripening by LOX activity is considered a desirable
reaction.
Undesirable LOX catalyzed oxidation is observed during potato processing.
The impact of processing on lipid oxidation in potato flakes was investigated by
Gosset and co-workers (2008). LOX activity was mostly related to lipid oxida-
tion and flavor generation. However, non-enzymatic lipid oxidation products
were also found during processing, and these auto-oxidative processes cannot be
inactivated by the main endogenous non-enzymatic antioxidants in potato
tubers, such as ascorbic acid, phenolic compounds and carotenoids, because
these antioxidants are degraded during processing. Therefore, efficient
exogenous antioxidants as well as adequate storage conditions should be used.
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