The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 297

bond to give two carbonyl fragments. Such processes, if continued, may lead to the
formation of odorous molecules causing off-flavor and loss of color. Some work
with tomato LOX has indicated that cleavage can occur at a number of the double
bonds in lycopene and that 6-methyl 2-heptanone may be one of the products. For
the apparent associated co-oxidation of other substances, including thiol groups and
inhibitors, it is possible that free radicals are first dissociated from the enzyme.
Concentrations of

-carotene higher than 14 µM inhibited oxidation by chickpea
lipoxygenases, 49 which may be due to the formation of an irreversible enzyme —
β

β

-carotene complex. 55 Endogenous inhibitors of lipoxygenase in plant sources
include chlorophyll, -tocopherol, and phenolic compounds. These substances could

act as scavengers for released, or possibly enzyme-bound radicals, and it has been
suggested that the mechanism for bleaching of chlorophyll differs from that for
carotenoids. 54

α

M ANIPULATION OF F ACTORS TO E NHANCE S HELF L IFE

The first and principal approach to the reduction of suspected enzymic off-flavor
development in foods is always inactivation or inhibition of the suspected offending
enzymes. The alternative approaches that are becoming increasingly feasible are the
development of cultivars deficient in the undesirable isoenzymes. As many fruits
and vegetables contain different types of lipoxygenases, and a range of lyases and
isomerases capable of further degrading hydroperoxides, the manipulation of flavor
volatiles in foods should be possible. Aromas derived from polyunsaturated fatty
acids arise from the concerted action of this group of enzymes. For lipoxygenases
there are a number of examples to be found in the literature where lipoxygenases
have been found to be absent. In some cases, as for soy flour and soy milk, a lack
of LOX-2 has resulted in less rancid products, 56 whereas, for soybean varieties
lacking LOX-1, no significant changes in the flavor or stability of the soybean oil
have been found. 57
Homogenates prepared from LOX-1-deficient, LOX-3-deficient, and LOX-1 plus
-3-deficient soybeans generated significant amounts of n-hexanal, whereas only the
LOX-2-deficient line had reduced amounts of hexanal and removal of LOX-3
increased n-hexanal amounts. 58,59 Lines lacking all three isoforms, although not in
isogenic backgrounds, still had 70% of the normal amounts of n-hexanal, 59,60 indi-
cating that there are other pathways to hexanal. However, for soybean lipoxygenase
LOX-2 Hildebrand et al. 58 have claimed this isoenzyme to be more effective for
hexanal formation, although these authors also claimed that LOX-1 was involved.
Soybean varieties lacking some or all of the lipoxygenase isoenzymes should theo-
retically be less susceptible to lipoxygenase-mediated oxidation and the production
of undesirable off-flavors. Soybean LOX-1 and -3 are inherited independently of
each other 61 and soybean LOX-2 is closely linked to the LOX-1 gene. 62 Naturally
occurring soybean varieties lacking more than one isoenzyme have not been iden-
tified. 63 However, using appropriate crosses, near-isogenic soybean seeds have been
developed that lack either isoenzymes-1 and -3 or isoenzymes-2 and -3. Plants grown
from seeds lacking two lipoxygenase isoenzymes have shown no obvious deleterious