Chemistry Reference
In-Depth Information
with l uorescent light (20 W, 1500 lux), (Chen et al. 1996). The isomerization and the degradation
of carotenoids were monitored by HPLC with diode-array detection and the results showed that
the amounts of lutein,
-carotene in carrot juice decreased with increasing storage
temperature and that 9-
cis
- isomers were the major types of isomers formed under light storage
(Chen et al. 1996).
The photostability of carotenoids in freeze-dried powder from carrot pulp waste under light at
25°C was analyzed by HPLC with photodiode-array detection upon illumination with l uorescence
light (1500 lux) (Tang and Chen 2000). Results showed that the amounts of all-
trans
- forms of
main components,
α
-carotene, and
β
-carotene, and lutein, decreased with increasing illumination time
with the formation of 9-
cis
derivatives as main isomers. The degradation rates of the total amount
of all-
trans
plus
cis
forms of each pigment at 25°C were 5.4
α
-carotene,
β
×
10
−4
, 1.6
×
10
−3
, and 1.8
×
10
−3
h
−1
for
-carotene was identical to
that observed in homogeneous solvents (Pesek and Warthesen 1990), Table 12.4. Additionally the
Hunter
L
and
b
values of the powder decreased with increasing storage time and temperature, while
the
a
(red) value showed an insignii cant change (
p
> 0.05).
The stability of carotenoids in tomato juice during storage under l uorescent light (2500 lux)
at 4°C, 25°C, and 35°C for 12 weeks was studied by Lin and Chen (2005). Light enhanced the
degradation and the isomerization of all-
trans
-lutein; with more formation of 13-
cis
-lutein than
9-
cis
-lutein. Similar trends were observed for
lutein,
α
-carotene, and
β
-carotene, respectively. The degradation rate of
β
-carotene but also the formation of di-
cis
isomers
was observed. For lycopene, 15-
cis
-lycopene was the major isomer formed during dark storage at
4°C, while 9-
cis
- and 13-
cis
-lycopene were favored at 25°C and 5-
cis
- as well as 13-
cis
-lycopene
dominated at 35°C. Under light storage, both 9-
cis
and di-
cis
-lycopene were the main isomers gen-
erated at 35°C, whereas 13-
cis
- and 15-
cis
-lycopene were the most abundant at 4°C and 25°C.
Therefore, by increasing the storage temperature larger losses of the all-
trans
- and
cis
- forms of
lutein,
β
-carotene, and lycopene occurred during illumination. All-
trans
-lycopene showed the high-
est degradation efi ciency, followed by all-
trans
-
β
β
-carotene and all-
trans
-lutein. More
cis
isomers
of lycopene than lutein or
-carotene were generated during storage. However, the major type of
isomers formed may vary, depending on storage conditions (Lin and Chen 2005).
β
12.3.2 P
HOTOSENSITIZED
D
EGRADATION
IN
M
ODEL
AND
F
OOD
S
YSTEMS
The photosensitized transformation of carotenoids has been studied using several sensitizer mol-
ecules, such as chlorophylls, iodine, rose bengal (RB), and methylene blue (MB) and in general
terms isomerization is the major pathway of reaction.
The products of dye-sensitized photoisomerization (excitation at 337 nm of anthracene as
sensitizer) and direct photoisomerization (excitation at 488 and 337 nm) of all-
trans
-, 7-
cis
-, 9-
cis
-,
13-
cis
-, and 15-
cis
- isomers of
-carotene in deaerated (by N
2
bubbling)
n
-hexane were analyzed by
HPLC (Kuki et al. 1991). The following isomerization patterns were found for each starting isomer:
(a) all-
trans
β
13-
cis
> 9-
cis
> 15-
cis
, (b) 7-
cis
all-
trans
> 9-
cis
> 7,15-di-
cis
≈7,13
′
-di-
cis
,
(c) 9-
cis
all-
trans
>> 9,15-di-
cis
> 9,13
′
-di-
cis
> 9,13-di-
cis
> 13-
cis
, (d) 13-
cis
all-
trans
>> 9-
cis
, and (e) 15-
cis
Φ
iso
) in the photosensi-
tized reaction is between two and four orders of magnitude higher when compared with the direct
photolysis at 488 and 337 nm photolysis, respectively (Chart 12.3).
In addition, the results indicated that the efi ciency of
cis
all-
trans
. The isomerization quantum yields (
trans
increased as the initial
cis
double bond coni guration is shifted from the center of the polyenic chain, consistent with the
T
1
,
triplet excited state potential curve that has a very shallow minimum at the 15-
cis
position compared
to the deep minima at the all-
trans
position. The results strongly suggest that isomerization takes
place via the
T
1
state of the carotenoid even in the case of direct photoexcitation, with their
→
Φ
iso
much lower than in photosensitized process because of the very low intersystem crossing quantum
yield,
Φ
isc
(<10
−3
) (Nielsen et al. 1996).
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