Food Lipids: Chemistry, Nutrition, and Biotechnology

otene is a polyene, synthesized from eight isoprene units. It has an intense orangered
color and, as a food additive, is used primarily as a colorant for oils and fats.
Carotenoids can act as primary antioxidants by trapping free radicals or as secondary
antioxidants by quenching singlet oxygen. Carotenoids are typically secondary antioxidants
in foods. However, in the absence of singlet oxygen (low oxygen partial
pressure), carotenoids may also prevent oxidation by trapping free radicals and acting
as chain-breaking antioxidants.
Singlet oxygen is unstable and can react with lipids to produce free radicals.
In the presence of �-carotene, singlet oxygen will preferentially transfer energy to
�-carotene to produce triplet state �-carotene [Eq. (20)]. The transfer of energy from
singlet oxygen to carotenoid takes place through an exchange electron transfer mechanism
[22].
Triplet state �-carotene releases energy in the form of heat, and the carotenoid
is returned to its normal energy state [Eq. (21)]. Carotenoids are very effective
quenchers: one carotenoid molecule is able to interact with numerous singlet oxygens.
�-Carotene, for example, can quench up to 1000 molecules of singlet oxygen
[23].
1 3 3
O2 � �-carotene → �-carotene* � O 2
(20)
3 �-Carotene* → �-carotene � heat (21)
The ability of carotenoids to quench singlet oxygen is directly related to the number
of carbon double bonds in the compound. Carotenoids with nine or more conjugated
double bonds are very effective antioxidants. �-Carotene, isozeaxanthin, and lutein
are all effective singlet oxygen quenchers. Xanthophylls are not as efficient at scavenging
singlet oxygen because of the addition of functional groups to the hydrocarbon
structure.
Carotenoids are known to scavenge free radicals at low oxygen pressures (