Thermal and Photochemical Degradation of Carotenoids - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 232

CHART 12.1
Observed Rate Constants ( k obs ) for the Equilibration of
All- trans - b -Carotene in the Dark at 45°C

Reactions k obs (min −1 )

k 2 k 1

13- cis all- trans → 13- cis 1.6 × 10 −4

9- cis All- trans

k -1

k -2

13- cis → all- trans 4.2 × 10 −4

All- trans → 9- cis 6.9 × 10 −6

9- cis → all- trans 7.6 × 10 −5

Source: Data from Pesek, C.A. and Warthesen, J.J., J. Agric. Food Chem ., 38, 1313, 1990.

under heating: (a) all- trans - isomerized into 13- cis - > 15- cis - ≈ 9- cis -; (b) 7- cis - isomerized into
7,13 -di- cis - > 9,15-di- cis - >

all- trans - > 9,13-di- cis -; (d) 13- cis - isomerized into all- trans - >> 15- cis -; and (e) 15- cis - isomer-
ized into all- trans - > 13- cis -. Major differences in the isomerization patterns found for thermal
isomerization when compared to direct photoisomerization and sensitized photoisomerization (see
Section 12.3) were (a) starting from 7- cis -, only di- cis - isomers were formed and (b) starting from
13- cis - and 15- cis - mutual isomerization between the central- cis isomers took place (Kuki et al.
1991). Moreover, calculated

′ -di- cis - > 7,15-di- cis - > 7,13-di- cis -; (c) 9- cis - isomerized into 9,13 ′

-carotene, in the S 0
state showed that the C-C bond order decreases from both ends (0.922) toward the center (0.833)
indicating that thermal isomerization ( S 0 -state) can take place more easily in the central part (Kuki
et al. 1991).
The isomerization and degradation of dried

π bond orders for docosaundecaene, a model for β

-carotene were evaluated in an oven heated at
temperatures between 50°C and 150°C up to 30 min, as well as by rel ux heating at 70°C during
140 min, using i rst-order kinetic decay (Chen and Huang 1998). Although the degradation of all-
trans -

β

-carotene became signii cant after heating at 50°C and 100°C for 25 and 10 min, respectively,
no signii cant changes were found in the amounts of 9- cis -, 13- cis -, or 15- cis -

β

β -carotenes (Table

12.2). When all- trans - -carotene was heated under rel ux in hexane, its concentration decreased

with increasing heating time; however, after 70 min levels remained constant indicating that the
whole system approached an equilibrium ( trans : cis ≈ 49:51). The major isomers formed during
heating were 13- cis -

β

-carotene was
only found at temperatures higher than 120°C (Chen and Huang 1998). These results also indicated
that rel ux heating is more likely to induce

β -carotene, both under oven and rel ux, while the 13,15-di- cis - β

β -carotene isomerization, while oven-heating is more

likely to cause -carotene degradation. This phenomenon can be attributed either to differences in

degradation mechanisms as affected by the temperature, the oxygen access, and the physical state
of the reaction system or by the highest activation energy required for the formation of di- cis - as
compared to mono- cis - carotenoid (Zechmeister 1944).
The degradation of

β

-carotene crystals upon heating at 150°C i tted a reversible i rst-order
model, trans - to cis - conversion occurred two- to threefold slower than that observed for the back-
ward reaction; in other words, the equilibrium toward the all- trans - isomer was favored (Chen et al.
1994). Four cis - isomers of

α - and β

β -carotene (13,15-di- cis -, 15- cis -, 13- cis -, and 9- cis -) and three isomers

of -carotene (15- cis -, 13- cis -, and 9- cis -) were formed during the heating of their respective all-

trans - carotene crystals. The 13- cis isomer of both carotenes was found in greater amounts (Chen
et al. 1994). In this system,

α

-carotene (Table 12.2).
A dry, thin lycopene layer heated at 50°C, 100°C, and 150°C showed i rst-order kinetic decay
(Lee and Chen 2002). At 50°C, isomerization dominated in the i rst 9 h; however, degradation was
favored afterward. On the other hand, at 100°C and 150°C degradation proceeded faster than isomer-
ization. Although cis isomer identii cation was not coni rmed by standards, the mono- cis lycopene
isomers, 5- cis -, 9- cis -, 13- cis -, and 15- cis -, degraded at the same rate as did all- trans -lycopene,

α -carotene degraded faster than β