Thermal and Photochemical Degradation of Carotenoids - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 231

carotenoid content increased from 10.9 μ g/g in unblanched pumpkin puree to 12.5 μ g/g after 2 min

blanching and to 14.1 μ g/g after further treatment at 60°C for 2 h (Dutta et al. 2006).

12.2.1 T HERMAL D EGRADATION IN M ODEL S YSTEMS
The spontaneous isomerization of all- trans - carotenoids at room temperature is a slow process,
and its rate depends on the solvent and the pigment structure. For example, the initial solutions of
β

-carotene in a mixture of tetrahydrofuran (THF), methanol, and acetonitrile containing ca . 95%
of all- trans - and 5% of 9- cis - plus 13- cis -isomers was transformed to 90% all- trans - -carotene

and 9% of 9- cis - plus 13- cis -carotene after 24 h of spontaneous isomerization at 25°C (Pesek et al.
1990). However, in chloroform, the amounts of 13- cis - and 9- cis -

β

β -carotene, respectively, increased

to 15.6% and 13.6% of the total β -carotene content (Pesek et al. 1990). Moreover, thermal isomer-

ization of -carotene in hexane was reported to be <1.3% for 1 h at room temperature (Kuki et al.

1991). In addition, as can be seen in Table 12.1, increased temperature accelerated this process, and
equilibrium was reached within 15-60 min under rel ux (Zechmeister 1944).
Based on high performance liquid chromatography (HPLC) studies regarding the equilibration
of isomeric fractions of

β

-carotene isomers at 45°C, a model consisting of two reversible concurrent
isomerization reactions was developed by Pesek et al. 1990. Under dark storage conditions at 45°C,
a

β

-carotene solution reached an equilibrium after 4-6 days yielding approximately 66% all- trans -,
8% of 9- cis -, and 25% of 13- cis -

β

-carotene. The observed rate constant ( k ) for the formation of
the 13- cis - isomer was faster than that of the 9- cis -

β

-carotene isomer, and the back rate constants
toward the all- trans - isomer were intrinsically faster as compared to the formation of cis -isomers of
β

β

-carotene (Chart 12.1).
A more complex isomerization pattern induced by heating was proposed for -carotene isomers

(all- trans -, 7- cis -, 9- cis -, 13- cis -, and 15- cis -) in n -hexane solution heated at 80°C (Kuki et al.
1991). Starting from each

β

β -carotene isomer, the following isomerization products were observed

TABLE 12.1
Percentage of b -Carotene
Stereoisomers Formed at Different
Temperatures in Petroleum
Ether-Benzene Solution

Experimental
Time (h) T (°C) cis -Isomers (%)

20 24 1.0

168 5.5

1176 11.1

40 1 4.0

3 5.4

24 11.2

60 1 7.5

3 9.7

80 1 8.5

3 31.9

24 34.1

Sources: Data from Carter, G.P. and Gillam, A.E.,
Biochem. J., 33, 1325, 1939; Zechmeister,
L., Chem. Rev ., 34, 267, 1944.