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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 17 Mass spectrum of 2-pentenylfuran [52].<br />

XII. SINGLET OXYGEN QUENCHING MECHANISMS<br />

Other than exclusion of light <strong>and</strong> reduction of oxygen present, the use of quenching<br />

agents is perhaps the best way to reduce singlet oxygen oxidation <strong>and</strong> may be the<br />

only protective measure if the other two alternatives are not feasible. Natural food<br />

components, such as tocopherols, carotenoids, <strong>and</strong> ascorbic acid, can act as effective<br />

quenching agents [53]. The quenching agent may interfere with the development or<br />

activity of singlet oxygen at several stages of development. Figure 18 shows the<br />

development of singlet oxygen <strong>and</strong> its subsequent reaction with substrate (A) to form<br />

the oxidized product (AO 2).<br />

At every stage in this reaction there is at least one alternate route, which, if<br />

taken, would minimize the oxidation of the substrate (A). The first step represents<br />

the return of the excited singlet sensitizer ( 1 Sen*) to ground state ( 1 Sen) without ISC<br />

to form the excited triplet sensitizer ( 3 Sen*). The second represents interaction with<br />

a quenching agent (Q) at a rate represented as k Q, returning the excited triplet sensitizer<br />

( 3 Sen*) to ground state ( 1 Sen) prior to interaction with triplet oxygen. The<br />

excited triplet sensitizer ( 3 Sen*) may interact with triplet oxygen (O 2) to form singlet<br />

oxygen ( 1 O 2). Following its creation, there are three fates for singlet oxygen in foods.<br />

(a) It may naturally decay to the ground state at a rate represented as k d. (b) It may<br />

Table 7 2-Pentenylfuran Peak Areas<br />

Light exposure (days)<br />

1<br />

2<br />

3<br />

4<br />

Added chlorophyll a<br />

0 ppm 1 ppm 5 ppm<br />

0<br />

0<br />

0<br />

0<br />

a<br />

Electronic counts of gas chromatogram.<br />

Source: Ref. 50.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.<br />

0<br />

0<br />

0<br />

1534<br />

0<br />

0<br />

1502<br />

3018

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