The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 292

cis,cis-1,4-pentadiene units to first produce free radicals and subsequently after
oxygenation conjugated unsaturated hydroperoxy acids. The occurrence and the
mode of action of lipoxygenases have been reviewed recently. 1,7-9
Lipoxygenases have been found in plants, animal tissues including marine prod-
ucts, and more recently in mushrooms and other fungi. In plants the enzyme has
been found in various organs and a comprehensive list of plant sources where
lipoxygenase has been identified has been compiled. 8 Lipoxygenase activity has
been reported to be higher in leaves used for making high quality black tea than in
those used for lower quality products. It is not known whether lipoxygenase is
essential for the survival of plants, although in animals lipoxygenases and similar
oxidizing enzymes such as cyclooxygenases give rise to important physiologically
active compounds. It is not known whether a lipoxygenase-free plant or seed will
be viable, although the technology to develop such products is available.
O XIDATION OF P OLYUNSATURATED F ATTY A CIDS
A very large number of different types of compounds, volatile and non-volatile,
contribute to the aroma of fruit and vegetables. Some aroma compounds are present
in the intact product, being produced during the normal metabolism of the plant.
However, many of the volatile aroma compounds are produced only when the raw
fruit or vegetable is subjected to chewing, cutting, or processing which causes mixing
of enzymes and substrates that are normally compartmentalized. Lipoxygenases are
of interest to the food scientist both due to their role in the genesis of flavor and
aroma compounds in plant products and their ability to form free radicals that can
attack other constituents such as vitamins, colors, phenolics, and proteins. Lipoxy-
genase-produced aroma compounds are desirable in many foods but may also give
rise to off-flavors, particularly in soybean products. Fujimaki et al. 10 showed that
hexanal is primarily responsible for the green-bean-like flavor of defatted soy-flour
owing to its extremely low flavor threshold. Lipoxygenase-2 (LOX-2) is believed to
be the isoenzyme mainly responsible for the generation of n-hexanal in soybeans. 11
Enzyme systems degrading linoleic acid and linolenic acid to hexanal and cis-3-
hexenal, respectively, plus isomerization of the latter to trans-2-hexenal, have been
reported in tea leaf chloroplasts. 12,13 Off-flavors in sweet corn have been attributed
to lipoxygenase activity and it has been claimed that the enzyme is responsible for
off-flavor development in cottonseed oil. 14 Furthermore, addition of purified lipoxy-
genase to blanched peas produced similar off-flavors. 15 Although heat treatment can
be used to inactivate lipoxygenase activity, the functional properties of other protein
constituents may be affected. In unblanched stored vegetables, lipoxygenases might
also be responsible for the bleaching of chlorophyll. However, during pasta manu-
facture, the bleaching action on carotenoids can be beneficial .
Different lipoxygenases from various plant and animal species oxidize polyun-
saturated fatty acids stereospecifically. The insertion of molecular oxygen is chiral
and positionally specific. 9 Following oxidation of linoleic acid by many plant lipoxy-
genases, the hydroperoxide group may be located at carbon-9 or carbon-13, depend-
ing on the type of lipoxygenase. The hydroperoxides are then degraded further by