Changes of Lemon Flavor Components in an Aqueous Solution during UV Irradiation

Article abstract of DOI:10.1021/jf960100h

A lemon flavor composed of lemon oil, water (pH 6 phosphate buffer), and ethanol, and the lemon-flavored drink were irradiated with UV light. Citral (1 and 2), which is one of the most important components expressing the typical lemon-like odor, significantly decreased with Z-E isomerization and there appeared 2-(3-methyl-2-cyclopenten-1-yl)-2-methylpropionaldehyde (6), trans-1,3,3-trimethylbicyclo[3.1.0]hexane-1-carboxaldehyde (7), cis-1,3,3-trimethylbicyclo[3.1.0]hexane-1-carboxaldehyde (8), (1,2,2-trimethyl-3-cyclopenten-1-yl)acetaldehyde (9), α-campholenealdehyde (10), photocitral A (8), epiphotocitral A (4), and photocitral B (5). New compounds of aldehyde 9, 6 and 10, were newly identified as photoreaction products of citral. Limonene, terpinolene, and nonanal decreased, while p-cymene increased after UV irradiation. Other components, such as sesquiterpene hydrocarbons, citronellal, linalool, and terpineols, were only slightly changed. These results suggested that citral is a more UV-unstable component in lemon flavor and the photolysis of citral could affect other components in lemon flavor during UV irradiation.

Full text of DOI:10.1021/jf960100h

J. Agric. Food Chem. 1997, 45, 463
−466
463
Ch a n ges of Lem on F la vor Com p on en ts in a n Aqu eou s Solu tion
d u r in g UV Ir r a d ia tion
Yuko Iwanami, Hideki Tateba,* Nobuko Kodama, and Katsumi Kishino
Ogawa and Company, Ltd., 6-32-9 Akabanenishi, Kita-ku, Tokyo 115, J apan
A lemon flavor composed of lemon oil, water (pH 6 phosphate buffer), and ethanol, and the lemon-
flavored drink were irradiated with UV light. Citral (1 and 2), which is one of the most important
components expressing the typical lemon-like odor, significantly decreased with Z-E isomerization
and there appeared 2-(3-methyl-2-cyclopenten-1-yl)-2-methylpropionaldehyde (6), trans-1,3,3-
trimethylbicyclo[3.1.0]hexane-1-carboxaldehyde (7), cis-1,3,3-trimethylbicyclo[3.1.0]hexane-1-car-
boxaldehyde (8), (1,2,2-trimethyl-3-cyclopenten-1-yl)acetaldehyde (9), R-campholenealdehyde (10),
photocitral A (3), epiphotocitral A (4), and photocitral B (5). New compounds of aldehyde 9, 6 and
10, were newly identified as photoreaction products of citral. Limonene, terpinolene, and nonanal
decreased, while p-cymene increased after UV irradiation. Other components, such as sesquiterpene
hydrocarbons, citronellal, linalool, and terpineols, were only slightly changed. These results
suggested that citral is a more UV-unstable component in lemon flavor and the photolysis of citral
could affect other components in lemon flavor during UV irradiation.
Keyw or d s: Changes of lemon flavor components; photoreaction of citral
INTRODUCTION
EXPERIMENTAL PROCEDURES
Ma ter ia ls. Cold-pressed lemon oil (Citrus limon) from
California (FCC grade) was used. The distilled lemon oil was
prepared by steam distillation of the same lemon oil for the
comparison test as mentioned later. Citral (Quest, The
Netherlands) composed of geranial (1) and neral (2) (2:1) was
purified up to 99% (GC) by column chromatography. R-Cam-
pholenealdehyde (10) was obtained from Hoechst AG. Gera-
nial diethyl acetal (11) and neral diethyl acetal (12) were
prepared according to the usual method (Liu et al., 1984). 2-(3-
Methyl-2-cyclopenten-1-yl)-2-methylpropionaldehyde (6) was
prepared from 1R(-)-fenchone obtained from Aldrich Chemical
Co. in three steps according to the procedures described
previously (Erman et al., 1986; Varech and J acques, 1969;
Delepine, 1924). The total yield of 6 was 4%: MS data are
shown in Table 2; IR (gas phase) cm-1 3050, 2968, 2803, 2693,
1735, 1464, 1381; NMR δH (CDCl3) 1.00 (6H, s), 1.59 (1H, dddd,
J ) 5.9, 8.0, 8.0, 13.7 Hz), 1.73 (3H, br s), 1.97 (1H, dddd, J )
6.7, 6.7, 9.3, 13.7 Hz), 2.20 (2H, m), 2.89 (1H, m), 5.22 (1H, br
s), 9.50 (1H, s).
UV Ir r a d ia tion of Lem on F la vor a n d Its Dr in k . After
a lemon oil (0.1 g) was mixed in a 100 mL of 0.05 M phosphate
buffer (pH 6)/ethanol (35/65), by stirring vigorously, the upper
oil layer was removed. The residual solution, containing 90%
of essential oil, was purged with nitrogen gas, followed by
sealing the solution into 10 separate 10 mL clear glass bottles.
Half of these bottles were irradiated with UV light (e400 nm;
Toshiba FL20BLB×2 +FL20SE×2) from a 50 cm height for 4
days at 30 °C, while the other five bottles were wrapped with
aluminum film to cut the light and allowed to stand at 30 °C
for the same number of days. The unreacted citral content
was determined by HPLC. The lemon flavor components were
recovered from each solution by Porapak Q column chroma-
tography (Shimoda et al., 1987). The recoveries of citral were
68-80%. Each component was identified by comparing the
Kovats index and mass spectrum with those of the relevant
standard sample. The amount of each component was calcu-
lated by computing the gas chromatography (GC) area against
an internal standard (2-octanol, 50 µg). The typical compo-
nents were injected under the same analytical conditions.
Their response factors to the flame ionization detector (FID)
were calculated as shown in Table 1. Quantitative data of
other components were estimated by using the same response
factors in Table 1. The lemon-flavored drink was provided as
follows: After a lemon oil (4.2 g) was mixed in 100 mL of water/
ethanol (35:65), by shaking vigorously for 15 min, it was stored
The lemon flavor composed of lemon oil, water, and
ethanol is one of the most useful flavoring materials for
various foods, such as drinks, candies, and cakes.
However, it is well-known that the flavor often deterio-
rates with age because of various factors. Among those
many factors, there are considered to be two major
factors, heat and light. The effects of heat on the
deterioration of lemon flavor have been reported in
many papers. Terpene hydrocarbons are mainly changed
to terpene alcohols by acid-catalyzed hydroxylation
(Clark and Chamblee, 1992), and citral having typical
lemon-like odor degrades due to cyclization and oxida-
tion reactions (Kimura et al., 1983). However, few
reports about the effect of light on lemon flavor, even
on other citrus flavors, were found except reports on
photo-oxidation (Schieberle and Grosch, 1989; Ziegler
et al., 1991). As various drinks packaged in clear bottles
containing less oxygen were generally manufactured
and commercialized, it is very important to understand
the effect light, especially UV-light, has on lemon flavor.
The experimental condition was carefully chosen to
clearly understand the UV effects on flavor deteriora-
tion. Lemon oil is mainly composed of terpene hydro-
carbons, such as pinenes, limonene, and terpinenes, and
oxygen-containing compounds, such as nonanal, cit-
ronellal, linalool, and citral (1 and 2), many of which
are main contributors to the odor of lemon flavor as
recognized by aroma extract dilution analysis (AEDA)
(Schieberle et al., 1988). The odorous change of the
lemon flavor can be evaluated by quantitative analyses
of these aroma components. Because most of these
components in lemon flavor are easily degraded by heat
in the presence of acid, the heat effects should be
severely controlled.
The aim of this study is to clarify how the lemon flavor
deteriorates in a drink system by UV irradiation under
strictly controlled conditions.
* Author to whom correspondence should be ad-
dressed.
S0021-8561(96)00100-8 CCC: $14.00
© 1997 American Chemical Society

464 J. Agric. Food Chem., Vol. 45, No. 2, 1997
Iwanami et al.
Ta ble 1. Resp on se F a ctor s of Ma in Ar om a Com p on en ts
(F ID)
analysis. The helium gas flow rate was 1 mL/s, with an
injection splitter having a split ratio of 50:1.
HP LC. HPLC analyses of the photodeterioration of lemon
flavor and citral were performed by using a HP1090M instru-
ment equipped with a photodiode array detector. The pho-
tolysis of citral was monitored by reversed-phase HPLC using
a Capcell Pak C18 SG120 (S-5µm) column (Shiseido, 4.6 mm
diameter × 250 mm) with a constant elution gradient from
10% (v/v) acetonitrile in water to acetonitrile with a constant
flow rate of 1 mL/min.
response
factora
response
factora
component
limonene
nonanal
citronellal
linalool
component
neral
R-terpineol
geranial
geranyl acetate
geraniol
0.8
1.1
1.4
0.9
0.8
1.0
0.9
1.0
1.5
1.4
â-caryophyllene
a
GC area of internal standard (2-octanol)/GC area of each
GC/F ou r ier Tr a n sfor m In fr a r ed Sp ectr oscop y (GC/
F TIR). GC/FTIR data were collected on a Hewlett-Packard
5965B system interfaced to a Hewlett-Packard 5890 Series II
gas chromatograph. Separation was achieved by using a fused
silica capillary column, 60 m × 0.25 mm i.d., 0.25 µm film
thickness (DB-Wax, J &W). The oven and injection tempera-
tures were the same as described for the GC analysis. The
transfer line was held at 270 °C. The helium gas flow rate
was 1 mL/s, with an injection splitter having a split ratio of
50:1. Vapor-phase FTIR spectra were recorded from 750 to
component.
Ta ble 2. MS Sp ectr a l Da ta of P h otor ea ction P r od u cts
(3-10)
compd
no.
RIa
MS spectral data
3
1515 152 (2), 137 (10), 123 (100), 109 (20), 95 (40),
81 (70), 79 (20), 67 (36), 55 (22), 41 (38), 39 (30)
1492 152 (2), 137 (4), 123 (3), 109 (35), 95 (20), 82 (36),
81 (87), 70 (100), 69 (35), 67 (42), 55 (16),
41 (33), 39 (19)
1434 152 (5), 137 (75), 123 (85), 109 (70), 95 (45),
84 (40), 83 (65), 81 (100), 79 (40), 69 (80),
67 (40), 55 (50), 41 (70), 39 (41)
1442 152 (2), 137 (4), 123 (1), 109 (2), 91 (5), 81 (100),
79 (20), 65 (2), 53 (7), 41 (7), 39 (5)
1393 152 (7), 137 (15), 124 (10), 123 (100), 109 (10),
95 (20), 82 (60), 81 (90), 79 (25), 67 (30),
55 (15), 41 (24)
4
5
4000 cm-1 with a resolution of 8 cm-1
.
Oth er Sp ectr om etr ies. High-resolution MS was deter-
mined at 20 eV by using a Hitachi M80B spectrometer. NMR
spectra were obtained in CDCl3 with TMS as an internal
standard on a Bruker AM400 instrument.
6
7
RESULTS AND DISCUSSION
UV Ir r a d ia tion of Lem on F la vor a n d Its Dr in k .
Even though lemon flavors are often used in soft drinks
together with lemon or other fruit juices that contain
natural or added acids, the test of UV irradiation effect
to lemon flavor was carried out in a higher pH test
solution, pH 6 phosphate buffer/ethanol solution, to
prevent the acid-catalyzed reactions. Actually, few acid-
catalyzed reactions of lemon oil components were ob-
served at pH 6 and room temperature.
8
9
1402 152 (3), 137 (10), 124 (13), 123 (100), 109 (12),
94 (20), 82 (12), 81 (75), 79 (20),
67 (25), 55 (10), 41 (16)
1472 137 (2), 119 (4), 109 (35), 108 (100), 95 (30),
93 (57), 91 (17), 81 (22), 67 (30), 55 (10),
41 (15), 39 (13)
1497 152 (3), 137 (1), 119 (4), 109 (14), 108 (100),
95 (28), 93 (64), 91 (16), 81 (8), 67 (16), 55 (7),
41 (10), 39 (6)
10
The change of components in the lemon flavor was
consistent with that in lemon-flavored drink. This
change for lemon flavor corresponded with that for the
lemon drink stored for 7 days at a distance of 10 cm
from fluorescent light (12 000 lx). Figure 1a shows the
results obtained by UV irradiation experiments of the
lemon flavor. The component analyses were repeated
five times for the lemon flavor. Quantitative data are
expressed as mean values of the five results. The
coefficient of variation (CV) was <23% for any data.
Statistically significant data were calculated. Citral
decreased rapidly with Z-E isomerization, and new
peaks, such as I, II, and III, appeared. Monoterpene
hydrocarbons, such as limonene and terpinolene, and
nonanal decreased, while p-cymene increased. Other
components, such as sesquiterpene hydrocarbons, cit-
ronellal, linalool, and terpineols, changed only slightly.
The fresh, sweet, and typical lemon-like odor disap-
peared, while a dusty odor increased.
The original cold-pressed citrus oil has some of low-
volatile or nonvolatile components, such as tocopherols
(Ifuku and Maeda, 1978; Piironen et al., 1986), cou-
marins (Cieri, 1969) and other (Angelo et al., 1970).
Those substances were confirmed to be extractable by
water/ethanol solution from the cold-pressed oil. There-
fore, those components were also considered to be the
important factors for deterioration of lemon flavor. To
confirm the effect of those components, the distilled
lemon oil, which did not contain low-volatile or non-
volatile components, was prepared from the original
cold-pressed lemon oil by steam distillation, and its
flavor was irradiated with UV light by using the same
procedure as described previously. The terpene hydro-
carbons and nonanal in the distilled oil solution tended
a
RI, Kovat’s retention index on DB-Wax column.
at -15 °C for 30 min and centrifuged at 3000 rpm for 5 min.
The upper oil layer was removed. The extract (0.4 g) from
lemon oil was dissolved in water (400 mL). After filtration,
the solution was packed in two separate 190 mL clear glass
bottles, purged with nitrogen gas, and sealed with aluminum
caps. The experiment of UV irradiation for the drink was
carried out in the same condition as for the lemon flavor.
P h otor ea ction of Citr a l in Eth a n ol. Citral (1 g) was
dissolved in 300 mL of ethanol, which was then irradiated with
a 400 W high-pressure mercury lamp through a Pyrex tube
under nitrogen. The reaction mixture was analyzed using GC
and high-performance liquid chromatography (HPLC). Com-
pounds 3-9 were isolated from a concentrate of the reaction
mixture by column chromatography (n-hexane/ethyl acetate)
and preparative GC.
GC. GC was carried out by using a Hewlett-Packard 5890
Series II gas chromatograph equipped with FID. Separation
was achieved on a fused silica capillary column, 30 m × 0.25
mm i.d., 0.25 µm film thickness (DB-Wax, J &W Scientific Inc.).
The oven temperature was programmed from 80 to 200 °C at
2 °C/min. The temperatures of both injection and the detector
were 250 °C. The nitrogen gas flow rate was 50 mL/s, with
an injection splitter having a split ratio of 200:1.
Preparative GC was carried out by using a Hewlett-Packard
5890A gas chromatograph equipped with a fused silica capil-
lary column, 30 m × 0.53 mm i.d., 1 µm film thickness
(Supelcowax TM10, Supelco Inc.), and thermal conductive
detector (TCD).
GC/Ma ss Sp ectr om etr y (GC/MS). Electron impact mass
spectrometric data were collected by using a Hewlett-Packard
5971A mass spectrometer interfaced to a Hewlett-Packard
5890 Series II gas chromatograph. Separation was achieved
on a fused silica capillary column, 60 m × 0.25 mm i.d., 0.25
µm film thickness (Supelcowax TM10). The oven and injection
temperatures were the same as those described for the GC

UV Changes of Lemon Flavor Components
J. Agric. Food Chem., Vol. 45, No. 2, 1997 465
F igu r e 2. GC chromatogram of photoreaction products of
citral in ethanol on DB-Wax.
F igu r e 1. Changes of lemon flavor components: (a) lemon
flavor made from original cold-pressed lemon oil; (b) lemon
flavor made from distilled lemon oil. UVS, UV-irradiated
flavor.
F igu r e 3. Structure of product 9. Lines show NOEs. Dotted
lines show J H(HZ)s.
to similarly decrease as shown in Figure 1b. The
differences in the amounts of the products were checked
by using Mann-Whitney’s U-test. A clear difference
was observed in citral (P < 0.05). The subtle differences
were indicated in octanal and terpinolene. Therefore,
the low-volatile or nonvolatile components in lemon
flavor have less effect on the volatile components except
citral. Citral was recovered more from the reaction
mixture of original cold-pressed lemon oil solution than
from the reaction mixture of freshly distilled lemon oil
solution. These results indicated that the volatile
components except citral of original cold-pressed lemon
oil solution should be only slightly affected with light
due to the UV-sensitive components which were sug-
gested to stabilize citral oppositely.
P h otor ea ction of Citr a l (1 a n d 2) in Eth a n ol.
Because citral, which is composed of 1 and 2 isomerizing
rapidly, disappeared during UV irradiation on lemon
flavor, we tried to investigate the photoreaction of citral.
It was already reported that citral was converted to
photocitral A (3), epiphotocitral A (4), and photocitral
B (5) at room temperature (Cookson et al., 1963) and to
the 1,2-migration products (7 and 8) of the formyl group
at higher temperature (g80 °C) (Wolff et al., 1980) in
an aprotic solvent under nitrogen. The photoreaction
of citral in a protic solvent except for 3-5 (Bu¨chi and
Wu¨est, 1965) has not been reported.
literature (Kaiser and Lamparsky, 1976; Wolff et al.,
1980). The structure of 6 was confirmed by comparing
the spectral data of an authentic sample prepared by
using the procedure mentioned under Experimental
Procedures. In the photoreaction of citral, 6 was a new
product, the formation of which requires the 1,3-
migration of the formyl group. Small amounts of 7 and
8 were obtained at room temperature, and the yields
increased at 80 °C. Diethyl acetals (11 and 12) might
be obtained from the hydrogen abstraction reaction of
excited citral because 11 and 12 were not obtained in
the dark under this condition. The structure of com-
pound 9 was presumed on the basis of the various
spectral data as shown in Figure 3. The high-resolution
mass spectrum of 9 showed that the molecular formula
of M+ - 15 was C9H13O (m/ z calcd for C9H13O, 137.0966;
found, 137.0944). GC/FTIR suggested the presence of
a carbonyl group (1734 cm-1) and methine (3050 cm-1).
1H NMR showed an aldehyde proton (9.82, CHO)
correlated to methylene protons (2.30 and 2.50, 6-CH2),
three methyl groups (0.94, 6H, 2-CH3, and 1.06, 3H,
1-CH3) and methylene protons (2.26 and 2.55, 5-H)
correlated to an olefin (5.57, 4-H), which were connected
to another olefin methine proton (5.50, 3-H). A long-
range correlation between the 5-H and 3-H was ob-
served. NOE was also observed in the 3-H when the
signal of 2-CH3 was irradiated. 13C NMR data, which
were obtained in CDCl3, showed 10 signals at 22.4 (q),
22.8 (q), 23.2 (q), 44.5 (t), 45.8 (s), 48.9 (s), 51.3 (t), 126.2
(d), 141.6 (d), and 191.9 (d) ppm. These data also
supported the structure. Aldehyde 9, which was a new
compound, and 10 might be derived from 5. The Kovats
indices on DB-Wax and MS spectral data of 3-10 are
summarized in Table 2.
Citral in ethanol was irradiated by UV light under
nitrogen, and the reaction products were analyzed.
From the photolysis of citral, aldehydes of 7, 8, and
R-campholenealdehyde (10) and also neral diethyl acetal
(12) and geranial diethyl acetal (11) were produced in
addition to the reported components of 3-5, as shown
in Figure 2. 10-12 were identified by comparing the
GC/MS data with those of standard samples. Photoci-
trals 3-5 and aldehydes 7 and 8 were identified by
comparing the spectral data with those reported in the
The new peaks I, II, and III (Figure 4b) in the
photoproducts of lemon flavor were assigned to com-

466 J. Agric. Food Chem., Vol. 45, No. 2, 1997
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should be one of the main factors which allows deterio-
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