09.12.2012 Views

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Figure 3 Singlet oxygen formation by chemical, photochemical, <strong>and</strong> biological methods<br />

[23].<br />

may also undergo an intersystem crossing in which the excited singlet state molecule<br />

becomes an excited triplet state molecule. The excited triplet state sensitizer may<br />

then react with triplet state atmospheric oxygen to form singlet oxygen <strong>and</strong> singlet<br />

sensitizer via triplet–triplet annihilation mechanism.<br />

IV. TYPE I AND TYPE II PATHWAYS OF EXCITED<br />

TRIPLET SENSITIZER<br />

The excited triplet sensitizer may interact directly with substrate such as linoleic acid<br />

or phenol compounds by donating an electron or accepting hydrogen, resulting in<br />

the production of free radicals or free radical ions as shown in Figure 5. This mechanism<br />

is known as type 1 pathway [25,26]. The photosensitizer acts as a photochemically<br />

activated free radical initiator. The product radicals have a variety of possible<br />

reactions, such as reaction with, or electron transfer to, oxygen <strong>and</strong> electron or hydrogen<br />

abstraction from other substrates. After initiation of the free radicals (R�),<br />

the radical compound may react with triplet oxygen via free radical oxidation mechanisms.<br />

The rate of the type 1 pathway is mostly dependent on the type <strong>and</strong> concentration<br />

of the sensitizer <strong>and</strong> substrate. Compounds that are readily oxidizable<br />

compounds, such as phenols, <strong>and</strong> easily reducible compounds, such as quinones,<br />

tend to favor type 1 pathway [14].<br />

The excited triplet sensitizer is capable of interacting with triplet oxygen in<br />

one of two manners, known as the type 2 pathway. The first, <strong>and</strong> most significant,<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!