Food Lipids: Chemistry, Nutrition, and Biotechnology

esulting in drastically increased rates of oxidation even at very low temperatures,
lowering the quality of foods during processing and storage [8]. (b) Singlet oxygen
oxidation can produce compounds absent in triplet oxygen oxidation due to the
different reaction mechanisms of singlet oxygen from the triplet oxygen [6,13].
This chapter reviews the important chemical mechanisms involved in the oxidation
of edible oils by singlet and triplet oxygen for the formation of volatile compounds
and the effects of the process on the flavor quality of edible oils. Singlet
oxygen oxidation will be emphasized in that its importance to lipid oxidation has
received increasing attention recently.
II. TRIPLET OXYGEN AND SINGLET OXYGEN CHEMISTRY
Differences in the chemical properties of triplet and singlet oxygen are best illustrated
by their molecular orbitals. The molecular orbital of triplet oxygen is shown in Figure
1. The spin multiplicity used to define spin states of molecules is defined as 2S �
1, where S is the total spin quantum number. The total spin quantum number (S) of
triplet oxygen is 1. Triplet state oxygen has three closely grouped energy states by
two unpaired electrons under a magnetic field. Therefore, the triplet state oxygen has
paramagnetic and diradical properties and gives spin multiplicity of 3. Triplet oxygen
reacts with radical compounds in foods. Most compounds are in the singlet state,
but the most abundant and stable atmospheric oxygen is in triplet state. The molecular
orbital of singlet oxygen differs from that of triplet oxygen in that electrons in
the � antibonding orbital are paired, as shown in Figure 2. The molecule is singlet
if the resultant spin (S) is zero, dictating the multiplicity of the state, 2S � 1, to
be 1.
Singlet oxygen is an energetic molecule that is in violation of Hund’s rule and
the resulting electronic repulsion can produce five excited state conformations. The
1
� state of singlet oxygen is responsible for most singlet oxygen oxidation in foods
and is generally referred to as singlet oxygen [13]. The most energetic electrons of
the activated 1 � state have opposite spins and lie in one single orbital, as shown in
Figure 2. Its energy is 22.4 kcal above the ground state of triplet oxygen and exists
long enough to react with other singlet state molecules [14]. Singlet oxygen is not
a radical compound and can only react with nonradical, singlet state, double-bonded
compounds. The lifetime of singlet oxygen is from 50 to 700 �s, depending on the
solvent system of foods [15]. The reaction temperature has little effect on the oxidation
rate of singlet oxygen with foods due to the low activation of 0–6 kcal/mol
[15]. A summary of chemical properties of singlet and triplet oxygen is shown in
Table 1.
III. SINGLET OXYGEN FORMATION
Singlet oxygen can be formed chemically [16], enzymatically [17,18], photochemically
[19], and by decomposition of hydroperoxides [20]. Photosensitizers such as
chlorophyll, pheophytins, riboflavin, and myoglobin in foods can absorb energy from
light and transfer it to triplet oxygen to form singlet oxygen [19,21,22]. Figure 3
shows several pathways for the formation of singlet oxygen in biological systems
[23].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.