Lipoxygenase and lipid oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 115

5.3.8 In tree nuts
Almond seeds age rapidly during storage at high relative humidity (80%) and
moderate temperature (20 ëC). The content of unsaturated fatty acids (18:2 and
18:3) decreased during accelerated aging, and the aged seeds contain high levels
of malondialdehyde (Zacheo et al., 1998). LOX was believed to cause this
oxidation because increased activity of this enzyme was observed during the
aging experiment. In hazelnut, significant genotypic variability was found in
total fat, fatty acid and alpha-tocopherol contents, as well as LOX activity in
various varieties of the nuts (Pershern et al., 1995). Relationships were found
between shelf-lives for hazlenuts and their polyunsaturated fatty acids, -
tocopherol and LOX activity levels.
5.4 Prevention of lipoxygenase (LOX)-initiated reactions
5.4.1 Enzyme inhibitors
A natural LOX inhibitor, sclerotiorin, purified from the fermented broth of
Penicillium frequentans, can be used as a potent, reversible, uncompetitive
inhibitor against soybean LOX-1 (Chidananda and Sattur, 2007). The inhibitor
also showed an antioxidant property by scavenging free radical. Therefore,
sclerotiorin may inhibit LOX in two ways: by interacting with the enzyme-
substrate complex, and as an antioxidant by quenching or trapping the free
radical intermediates formed in the enzyme reaction. Sclerotiorin compared well
with other known natural and synthetic LOX inhibitors.
Esculetin, a known inhibitor of LOX, inhibited the production of 12-
hydroxyeicosatetraenoic acid from the reaction of LOX with AA, and the
hydroperoxide level also was further reduced by the presence of antioxidants,
namely BHT, ascorbic acid and tocopherols (Saeed and Howell, 2001).
LOXs can be inactivated by LOX substrate analogs. The substrate analogs
include 18:1 fatty acid, and `suicide' inhibitors such as 5, 8, 11, 14 eicosa-
tetraynoic acid and colneleic acid (Zhuang et al., 2002). Some nonsteroidal anti-
inflammatory drugs also can be inhibitory to LOXs.
5.4.2 Heating
Heating can effectively inactivate LOX enzymes and prevent undesirable reac-
tions to some degree. For heat denaturation of soybean LOX, steam treatment
for 2 min can inactivate LOX-1 and LOX-2, and the defatted flour thus will have
a better flavor score than the unheated beans. Toasting of soybeans at 100±
110 ëC also inactivates these isozymes to produce good quality soy protein
products. In the conventional solvent extraction of soybean oil, the slight heating
conditioning (65±75 ëC) of the cracked seeds before flaking is not sufficient to
inactivate these enzymes. Once deactivated, LOX may also act as non-
enzymatic catalysts for lipid oxidation, as discussed earlier.
Lipid nutrients can be preserved by heat deactivation of the LOXs. For
example, less than 10% of the vitamin E in wholemeal and white wheat flours