Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 18 Formation and reactions of singlet oxygen with substrate [30].
react with a singlet state substrate (A) at a state represented as k r forming the oxidized
product AO 2. (c) It may be destroyed by a quenching agent by either combining with
the quencher, at a rate represented as k ox�Q, to form the product QO 2 or by passing
its energy to the quenching agent and returning to free triplet oxygen, at a rate
represented as k q.
As represented by Figure 18, there are three points at which a quenching agent
may act; one is quenching of the excited triplet sensitizer, and the other two are
quenching of singlet oxygen by chemical or physical means. Chemical quenching
involves reaction of singlet oxygen with the quenching agent to produce an oxidized
product (QO 2). Physical quenching results in the return of singlet oxygen to triplet
oxygen without the consumption of oxygen or product formation achieved by either
energy transfer or charge transfer. Therefore, triplet oxygen quenchers must either
be able to donate electrons or to accept energy 22.4 kcal above ground state. An
example of the latter is �-carotene, which has a low singlet energy state and can
therefore accept the energy from singlet oxygen [7]. Ascorbic acid is an example of
a chemical that can quench the excited sensitizer. Table 8 lists quenching rates of
several quenching agents.
XIII. CAROTENOIDS
�-Carotene is considered to be the most powerful physical quenching agent in foods.
Foote [25] found that one molecule of �-carotene can quench 250–1000 molecules
of singlet oxygen at a rate of 1.3 � 10 10 M �1 s �1 . Energy transfer from singlet oxygen
to a quencher results in the formation of triplet oxygen and an excited triplet state
quencher. The triplet state quencher may also be formed in the energy transfer reaction
between singlet state quencher and an excited triplet state sensitizer in what
is referred to as a triplet sensitizer quenching reaction. Investigations into the ability
of �-carotene to act as a quencher in food systems has been prominent in recent
years.
The rate of singlet oxygen quenching by carotene is dependent on the number
of conjugate double bonds in the carotenoid. Lee and Min [7] evaluated the effectiveness
of five carotenoids in quenching chlorophyll-sensitized photooxidation of
soybean oil and reported that the effectiveness increased with the number of double
bonds in the carotenoid and the amount of carotenoid added. Those carotenoids with
seven or less double bonds are ineffective as quenchers, being unable to accept the
energy from singlet oxygen.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.