Biomedical Engineering Reference
In-Depth Information
of enzymes in many instances is thermostable and generates free radicals. Neverthe-
less, both polyphenol oxidases and peroxidases are extensively and beneficially
involved during the manufacture of tea, coffee, and cocoa. Another group of enzymes,
the lipoxygenases, catalyze the oxidation of polyunsaturated fatty acids to produce
hydroperoxides which can be broken down by other enzymes to form desirable and
characteristic aroma compounds. Alternatively, the latter group of enzymes may also
be responsible for off-flavors, especially in seed products. A large number of com-
pounds contribute to the aroma of fruits and vegetables and are mainly produced
when the cellular structure of the foodstuff is damaged during preparation in the
kitchen or during processing. Hexanal is believed to be one of the major compounds
responsible for off-flavor in soybeans. It is also possible that lipoxygenases might
have a role in the formation of oxidative rancidity of animal food products. Likewise,
lipoxygenase may also catalyze the cooxidation of carotenoids including
-carotene
resulting both in the loss of essential nutrients and the development of off-flavors.
β
O XYGEN R EACTIVITY
Unlike the numerous enzymes involved in metabolism, the oxidative enzymes that
are of prime importance to the food scientist are those able to incorporate either
both or one atom of molecular oxygen into susceptible substrates. Molecular or
triplet O 2 has two unpaired electrons, each located in a different outer orbital. These
two electrons have the parallel spins, so if O 2 attempts to oxidize another atom or
molecule by accepting a pair of electrons from it, both of the new electrons must
be of anti-parallel spin so as to fit the vacant spaces in the orbitals. Thus, a spin
restriction is an impedance on oxidation, which tends to make O 2 accept electrons
one at a time in order to allow time for spin inversion. Thus, O 2 is poorly reactive
towards non-radical species, and thus permits the existence of many organic sub-
stances in aerobic environments. Transition metals are found at the active sites of
many oxidases and oxygenases, including polyphenol oxidases, peroxidases, lipoxy-
genases, and ascorbic acid oxidase; their ability to accept and donate single electrons
overcomes the spin restriction. In common with other enzymes, oxygenases also
exist as families of isoenzymes with small structural variations and slightly different
catalytic properties. Enzyme systems may initiate free radical mediated oxidation in
foods, resulting in changes in the sensory quality of the food; for example, peroxi-
dases, lipoxygenases, and microsomal enzymes may be involved not only in lipid
peroxidation but more so in the generation of free radicals capable of reacting with
a wide range of other substances. However, within fresh foods, indigenous antioxi-
dants may inactivate free radicals, although our current knowledge of free radical
mediated biochemical changes within foods and their inhibition is incomplete. 1 The
potential to preserve quality by limiting the manifestations of free radical activity
during storage and processing has not been fully exploited by the food industry.
E FFECTS ON S HELF L IFE
Unprocessed foods are biological systems often composed of aerobically respiring
cells. The dissociation of organelles and the breakdown of cellular structure during
 
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