Thermal and Photochemical Degradation of Carotenoids - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 239

process, the predominant cis - isomer of -cryptoxanthin, formed

in processed red, yellow, and orange fruits and vegetables, is the 13- cis - form (and 13

β -carotene, lutein, zeaxanthin, and β

- cis - for asym-
metric carotenoids) followed by the smaller quantities of 9- cis - and 15- cis - isomers. However, in
processed green vegetables, the 9- cis - isomers of

′

- cis -)
are predominantly formed. The degradation rates of different carotenoids depend not only on the
carotenoid structure but also on their localization within the cell organelles, e.g., the slower deg-
radation rates of lycopene were observed in food systems as compared to those of

β -carotene and lutein (in this case also the 9 ′

β -carotene, in

contrast to what happens in model systems.

12.3 DIRECT AND SENSITIZED LIGHT-INDUCED DEGRADATIONS
In addition to food systems, carotenoid pigments are ubiquitous in the photosynthetic membranes
of plants and bacteria where they are involved in the collection of sunlight for photosynthetic work,
the dissipation of excited triplet-state energy, and the regulation of singlet energy l ow to the photo-
synthetic reaction centers (Cogdell and Frank 1987). Most of the photobiological properties of caro-
tenoids, such as antenna and photoprotector pigments, are related with their peculiar photophysical
behavior and the characteristics of their excited states, which in turn are strongly determined by the
extension of the one-dimensional

-electron conjugated system (Gust et al. 1993). The character-
istic intense absorption band in the visible region of carotenoids (400-550 nm,

π

ε max ~10 5 M −1 cm −1 ),
produced by the strong electric dipole, allowed transition from the ground state ( S 0 ) to the second
excited singlet state S 2 . The electronic transition to the lowest excited singlet state, S 1 , is forbidden
by symmetry and thus its absorption band is undetected under normal conditions. As expected for
upper excited states, the lifetime of the S 2 state is very short (<200 fs) and therefore the l uorescence
and intersystem crossing quantum yields are extremely low (<10 −4 ) (Gust et al. 1993). However,
despite their very fast unimolecular deactivation pathways, carotenoids show photochemical activ-
ity either by intra- (or direct photolysis) or intermolecular (photosensitized) populations of their
singlet and triplet excited state manifolds.

12.3.1 P HOTOLYSIS IN M ODEL AND F OOD S YSTEMS
Direct photolysis or steady state photolysis of carotenoids in model systems, e.g., homogenous
solvents and microheterogeneous solutions (micelles, liposomes, etc.), has been studied under dif-
ferent conditions by several groups. In general, irradiation with UV or visible light produces the
bleaching of the characteristic intense absorption band of the carotenoid at 400-550 nm together
with a progressive blueshifting and absorbance increment in the region of 300-400 nm due to the
formation of products with shorter chromophores, as depicted for the photodegradation of lycopene
in Triton X-100 aqueous micelles, Figure 12.4.
The extent of the photodegradation reaction is measured by the photodegradation quantum yield,
Φ pd , which is dei ned as the fraction of molecules degraded in relation to how many photons were
absorbed, and quantii es the light sensitivity of the molecule (Turro 1978). Usually, 1 but

values higher than unity can indicate more complex processes, such as radical chain reactions.
Skibsted and coworkers have determined the dependence of solvent, oxygen partial pressure,
temperature, and irradiation wavelength on the

Φ pd ≤

Φ pd of several C 40 carotenoids, such as astaxanthin,

canthaxanthin, zeaxanthin, lutein, and -carotene (Jørgensen and Skibsted 1990, Christophersen et

al. 1991, Nielsen et al. 1996, Mortensen and Skibsted 1999, Hansen and Skibsted 2000). In general, it
was observed that shorter irradiation wavelength, higher solvent polarity, and higher oxygen concen-
tration all give rise to higher

β

Φ pd values. For instance, the Φ pd value for β -carotene in air-saturated

10 −6 at 436 nm, whereas the
Φ pd values of canthaxanthin were almost one order of magnitude higher in chloroform and acetone
than in toluene and vegetable oil (Christophersen et al. 1991, Nielsen et al. 1996). In addition, the

toluene solution was strongly reduced from 2.1 × 10 −3 at 313 nm to 1.7 ×

Φ pd
increased between two and three times by changing the oxygen partial pressure from zero to unity