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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Table 8 Singlet Oxygen Quenchers <strong>and</strong> Their Quenching Rates<br />

Quenching compound Quenching rate (M �1 s �1 )<br />

�-Apo-8�-carotenal<br />

�-Carotene<br />

Lutein<br />

Zeaxanthin<br />

Lycopene<br />

Isozeqaxanthin<br />

Astaxanthin<br />

Canthaxanthin<br />

�-Tocopherol<br />

1,4-Diazabicyclo-(2,2,2)-octane<br />

Dimethylfuran<br />

BTC a<br />

TPB b<br />

3.1 � 10 9<br />

4.6 � 10 9<br />

5.7 � 10 9<br />

6.8 � 10 9<br />

6.9 � 10 9<br />

7.4 � 10 9<br />

9.9 � 10 9<br />

11.2 � 10 9<br />

2.7 � 10 7<br />

1.5 � 10 7<br />

2.6 � 10 7<br />

1.2 � 10 9<br />

3.7 � 10 7<br />

a<br />

Bis(di-n-butyldithiocarbamato)nickel chelate.<br />

b<br />

{2,2�-Thiobis(4-1,1,3,3-tetramethylbutyl)phenalto)]-n-butylamine)<br />

nickel chelate.<br />

Source: Ref. 1.<br />

The quenching ability of �-carotene is the best studied energy-transfer quenching<br />

mechanism; however, other compounds with similar properties may be effective<br />

singlet oxygen quenchers if their singlet energy level is below 22 kcal. Lee <strong>and</strong> Min<br />

[42] tested the effect of various amounts of �-carotene added to soybean oil that<br />

contained 4 ppm chlorophyll <strong>and</strong> was exposed to light. Figure 19 shows the ability<br />

of increasing amounts of �-carotene to reduce the head space oxygen depletion.<br />

XIV. TOCOPHEROLS<br />

Tocopherols are well-studied free radical scavengers. They are the most abundant<br />

antioxidants in nature <strong>and</strong> are the primary antioxidant in vegetable oils. When present<br />

in systems that are vulnerable to singlet oxygen oxidation, tocopherols have demonstrated<br />

the ability to inhibit lipid peroxidation. Tocopherols were identified in<br />

soybean oil averaging about 1100 ppm <strong>and</strong> exist in �-, �-, �-, <strong>and</strong> �-tocopherol<br />

forms at approximately 4%, 1%, 67%, <strong>and</strong> 29%, respectively. Jung <strong>and</strong> others [54]<br />

studied the effectiveness of �-, �-, <strong>and</strong> �-tocopherol in quenching the photooxidized<br />

singlet oxygen oxidation of soybean oil <strong>and</strong> determined that �-tocopherol had the<br />

greatest quenching rate. Foote <strong>and</strong> others [55] found that the mechanism by which<br />

tocopherols act in quenching singlet oxygen involves charge transfer. The reaction<br />

involves an electron donation from tocopherol to singlet oxygen, forming a charge<br />

transfer complex. The transfer complex undergoes an ISC to ultimately form triplet<br />

oxygen <strong>and</strong> the starting tocopherol. The overall rate is dependent on the ability of<br />

the donor to accept electrons in that the most easily oxidized donors are the most<br />

efficient quenchers.<br />

Singlet oxygen destruction of vitamin D 2 in a model system was reduced by<br />

the presence of �-tocopherol proportional to its concentration in solution [56]. Figure<br />

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

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