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Analytical, Nutritional and Clinical Methods
Analysis and formation of trans fatty acids in hydrogenated soybean
oil during heating
W.H. Liu, B. Stephen Inbaraj, B.H. Chen
*
Department of Nutrition and Food Science, Fu Jen University, Taipei 242, Taiwan
Received 4 January 2006; received in revised form 25 July 2006; accepted 27 October 2006
Abstract
Hydrogenated oil has been widely used for production of shortenings or margarine, however, the presence of trans fatty acids may be
detrimental to human health. The objectives of this study were to develop an improved method for analysis of trans fatty acids and eval-
uate their formation in both unhydrogenated and hydrogenated soybean oil during heating at 160, 180 and 200 °C for varied length of
time. Results showed that among the four columns tested, an Agilent HP-88 column (100 0.25 mm I.D., 0.2-lm film thickness) could
resolve eight trans fatty acids and nine cis fatty acids simultaneously within 31 min with injector temperature 240 °C, detector temper-
ature 250 °C, and column temperature 170 °C in the beginning, maintained for 24 min, increased to 220 °C at 7.5 °C/min, 230 °Cat
10 °C/min, and maintained for 5 min. The contents of both cis and trans fatty acids showed a decreased trend for the increase of heating
time or temperature. No trans fatty acid formation was observed even after extensive heating of unhydrogenated and hydrogenated soy-
bean oil for 24 h. This phenomenon demonstrated that trans fatty acids can only be formed under severe conditions.
Ó2006 Elsevier Ltd. All rights reserved.
Keywords: Trans fatty acid; Hydrogenated soybean oil; GC–MS; Heating
1. Introduction
Fats and oils are one of the major nutrients in the diet to
maintain human health by providing body energy and
essential fatty acids such as linoleic acid (Frankel, 1998).
Because of presence of two isolated double bonds, linoleic
acid is susceptible to oxidation or degradation during heat-
ing (Chen, Tai, Chen, & Chen, 2001). In an attempt to
enhance the stability of unsaturated fatty acids in edible
oils, the hydrogenation process has been often employed
for production of shortenings or margarine (Kris-Etherton,
1995). However, the isomerization of cis to trans fatty acids
can occur during hydrogenation and result in a wide distri-
bution of trans fatty acids in bakery and fried products
(Aro et al., 1998; Romero, Cuesta, & Sa
´nchez-Muniz,
2000). Although reported data on trans fatty acid contents
in food products can be varied from one country to
another, the food made with hydrogenated fats such as
cookies and other bakery products, have been shown to
be the main source of trans fatty acid in the diet (Vicario,
Griguol, & Leon-Camacho, 2003).
Epidemiological studies have revealed that the intake of
trans fatty acids in excess may raise the cholesterol level in
blood (Mensink & Katan, 1990, 1993), and the concentra-
tion of low density lipoprotein in the plasma could be ele-
vated following the consumption of hydrogenated fat
containing high levels of trans fatty acids (Han et al.,
2002). Several authors also reported that trans fatty acids
may adversely affect the inflammatory process in athero-
sclerosis by increasing the peripheral blood mononuclear
cell production of inflammatory cytokines (Libbey, Rid,
& Maseri, 2002; Taubes, 2002). More recently, Kummerow
et al. (2004) demonstrated that trans fats inhibit the meta-
bolic conversion of linoleic acid to arachidonic acid and to
other polyunsaturated fatty acids, a risk factor in the devel-
opment of coronary heart disease.
0308-8146/$ - see front matter Ó2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodchem.2006.10.069
*
Corresponding author. Tel.: +886 2 29053626; fax: +886 2 29021215.
E-mail address: nutr1007@mails.fju.edu.tw (B.H. Chen).
www.elsevier.com/locate/foodchem
Food Chemistry 104 (2007) 1740–1749
Food
Chemistry
In view of the impact of trans fatty acids on human health,
the quantitation of trans fatty acids in heated oil and food
products is extremely important. The analysis of trans fatty
acids has been previously achieved by high-performance
liquid chromatography (HPLC) (Adlof, 1994; Adlof, Copes,
& Emken, 1995; Christie & Breckenridge, 1989). However,
the resolution of trans fatty acids remains poor. Juane
´da
(2002) used two C-18 columns to separate trans isomers of
oleic acid in milk by HPLC, but this method fails to resolve
trans forms of linoleic acid and linolenic acid. To remedy the
problem, several authors have used gas chromatography
(GC) to analyze trans fatty acids instead (American Oil
Chemists’ Society, 1990; Juane
´da, 2002). Based on a GC
method developed by American Oil Chemists’ Society
(1990), a total of 21 fatty acids, including 15 cis fatty acids,
1trans oleic acid, 3 trans linoleic acids and 2 trans linolenic
acids were separated within 37 min by using a SP-2340 col-
umn (60 m 0.25 mm I.D., 0.2-lm film thickness) contain-
ing 100% polybiscyanopropyl siloxane as stationary phase.
Nevertheless, the resolution is inadequate since several peaks
of trans fatty acids are overlapped. Juane
´da (2002) devel-
oped a GC method to separate 18 fatty acids in milk, includ-
ing 10 trans fatty acids, and the major drawback is that the
separation time is lengthy (50 min) and only 4 trans fatty
acids are adequately resolved. Since many published reports
still encounter difficulties in separating cis and trans fatty
acids simultaneously, it is imperative to develop a precise
method to determine cis and trans fatty acids in food prod-
ucts. Moreover, starting January 1, 2006, the US Food and
Drug Administration issued a rule that the trans fatty acid
content should be declared in the nutrition label of conven-
tional foods and dietary supplements (Food & Drug Admin-
istration, 2003). The objectives of this study were to develop
a GC method for analysis of trans fatty acids in unhydroge-
nated and hydrogenated soybean oil during heating.
2. Materials and methods
2.1. Materials
All fatty acid standards, including lauric acid methyl
ester (C12:0), myristic acid methyl ester (C14:0), palmitic
acid methyl ester (C16:0), stearic acid methyl ester
(C18:0), arachidic acid methyl ester (C20:0), palmitoleic
acid methyl ester (C16:1, D9cis), oleic acid methyl ester
(C18:1, D9cis), linoleic acid methyl ester (C18:2, D9cis,
D12 cis), linolenic acid methyl ester (C18:3, D9cis,D12
cis,D15 cis), internal standard heptadecanoic acid methyl
ester (C17:0), 9-trans-hexadecenoic acid methyl ester
(C16:1, D9trans), 6-trans-octadecenoic acid methyl ester
(C18:1, D6trans), 9-trans-octadecenoic acid methyl
ester (C18:1, D9trans), 11-trans-octadecenoic acid methyl
ester (C18:1, D11 trans), 9-trans-12-trans-octadecadienoic
acid methyl ester (C18:2, D9trans,D12 trans), 9-cis-11-
trans-octadecadienoic acid methyl ester (C18:2, D9cis,
D11 trans) and 10-cis-12-trans-octadecadienoic acid methyl
ester (C18:2, D10 cis,D12 trans), were from Nu-Chek-Prep
Inc. (Elysian, MN, USA), and 9,12,15-trans-octadecatrie-
noic methyl ester (C18:3, D9trans,D12 trans,D15 trans)
was from Sigma (St. Louis, MO, USA).
The analytical-grade solvents such as n-hexane, metha-
nol and chloroform were from Merck Co. (Darmstadt,
Germany). Chemicals, including potassium hydroxide,
anhydrous sodium sulfate, sodium chloride and boron
fluoride (in methanol) were from Riedel-de Ha
¨en Co. (See-
lze, Germany). Deionized water was obtained using a Milli-
Q water purification system from Millipore Co. (Bedford,
MA, USA). Unhydrogenated soybean oil was from Chia-
Hsin Chemical Co. (Taichung, Taiwan), while hydroge-
nated soybean oil was from Nan-Chiao Chemical Co.
(Taoyuan, Taiwan).
2.2. Heating of oil
A 5-l unhydrogenated or hydrogenated soybean oil was
poured into an oil tank separately, which was preheated to
160, 180 and 200 °C, and the heating time started to count
for 4, 8, 12, 16, 20 and 24 h. The temperature-controlled oil
tank (model B503) was from I-Seng Scientific Co. (Taipei,
Taiwan). After the desired heating time was reached, the oil
tank was cooled immediately to room temperature and a
10-ml oil sample was collected and poured into a 40-ml
brown vial for storage at 20 °C. Both fresh unhydroge-
nated and hydrogenated soybean oil were used as control
samples to compare with heated samples. Duplicate exper-
iments were carried out for each temperature and heating
time, and a total of 42 treatments were used.
2.3. Preparation of fatty acid methyl esters
A modified method based on Vicario et al. (2003) was
used. A 0.5-g oil sample was mixed with 10-ml methanolic
potassium hydroxide solution (0.5 N), and the mixture was
saponified at 90 °C in a water bath for 10 min. After cool-
ing to room temperature, a 8-ml BF
3
-CH
3
OH solution was
added and the mixture was standing in a water bath at
90 °C for 5 min to promote formation of methyl ester.
Again, the mixture was cooled to room temperature, then
8-ml hexane was added and the solution was heated in a
water bath at 90 °C for 3 min to allow complete esterifica-
tion of fatty acids. After cooling to room temperature, the
saturated saline solution was added to terminate the reac-
tion. The solution was allowed to settle until two layers
were formed, and the supernatant was collected, followed
by addition of 0.2-g anhydrous sodium sulfate to remove
excessive moisture and evaporation of the solution to dry-
ness. The residue was dissolved in 10-ml hexane and 1-ll
was injected into GC.
2.4. GC analysis of fatty acid methyl esters
Initially, four GC capillary columns were compared with
respect to the separation efficiency of standards of five sat-
urated fatty acid methyl esters, four unsaturated fatty acid
W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749 1741
methyl esters, eight trans fatty acid methyl esters and inter-
nal standard (C17:0). In addition, the various injector, col-
umn and detector temperatures, as well as flow rate and
split ratio were also evaluated. The GC instrument (model
6890) equipped with flame ionization detector (FID) and
mass spectrophotometer (model 5973) was from Agilent
Technologies (Palo Alto, CA, USA). All the standards were
dissolved in hexane to a concentration of 100 lg/ml each
and stored at 20 °C until use. The characteristics of each
GC column are listed below: (1) the DB-1 column
(60 m 0.32 mm I.D., 0.25-lm film thickness, coated with
100% dimethylpolysiloxane) was from J & W Scientific
Co. (Folsom, CA, USA); (2) the INNOWAX column
(30 m 0.32mm I.D., 0.25-lm film thickness, coated with
100% polyethylene glycol) was from Agilent Technologies
(Palo Alto, CA, USA); (3) the INNOWAX column (60 m
0.32 mm I.D., 0.25-lm film thickness, coated with 100%
polyethylene glycol); (4) the HP-88 column (100 m
0.25 mm I.D., 0.2-lm film thickness, coated with 88%
cyanopropyl-methylaryl polysiloxane) was also from Agi-
lent Technologies.
The various cis and trans fatty acids in the oil were iden-
tified by comparing retention time and mass spectra of
unknown peaks with reference standards and cochroma-
tography with added standards. For GC–MS, the interface
temperature was 270 °C with an electron multipler voltage
70 eV and ion voltage 1360 V, and detection was per-
formed by total ion mode with a scanning range of 35–
500 and rate at 2.94 scans/s. Eight concentrations of each
fatty acid standard (0.5, 0.8, 1.2, 1.5, 2.0, 3.0, 4.0 and
5.0 ppm) was prepared in hexane and the detection limit
was calculated based on S/NP3, whereas the quantitation
limit was based on S/NP10. For quantitation, eight con-
centrations (5, 10, 20, 40, 60, 100, 150 and 250 ppm) of
oleic acid methyl ester (C18:1, D9cis) and linoleic acid
methyl ester (C18:2, D9cis,D12 cis) were prepared and
mixed with internal standard (C17:0) for a final concentra-
tion of 91 ppm. Likewise, eight concentrations of the other
fatty acid standards (5, 10, 20, 40, 60, 80, 100 and 150 ppm)
were prepared and mixed with internal standard for a con-
centration of 91 ppm. Then the standard curves were
obtained by plotting concentration ratio against area ratio,
and the correlation coefficient (r
2
) was calculated with the
linear equations used for quantitation. The amount of each
fatty acid in the oil was calculated based on the following
formula:
W¼
A=RRF
Ai
Wi
Ws
recovery
where relative response factor (RRF) = (A/Ai)(W
i
/W);
Wis the concentration (mg/g) of each fatty acid in the oil
sample; Ais the peak area of each fatty acid standard; A
i
is
the peak area of internal standard; W
i
is the concentration
of internal standard; and W
s
is the weight of the sample.
The recovery was accomplished by adding two concen-
trations (10,000 and 20,000 ppm) of 1-ml of each fatty acid
standard to 0.5 g oil for extraction, with the exception of
C12:0, C18:2 (D9cis,D12 trans) and C18:2 (D9cis,D11
trans), because these three fatty acids were found not pres-
ent in heated oil. The recovery of each fatty acid was calcu-
lated based on the ratio of the amount of each standard
obtained after and before GC.
2.5. Statistical analysis
All the experiments were performed in duplicate and the
data were subjected to analysis of variance using ANOVA
and Duncan’s multiple range test for comparison of signif-
icant difference (P< 0.05) using SAS (2003).
3. Results and discussion
3.1. GC analysis of cis and trans fatty acid standards
Initially, the official method published by the American
Oil Chemists’ Society (American Oil Chemists’ Society,
1990) was adopted for separation of cis and trans fatty
acid standards. However, the resolution of trans fatty
acids remains poor, and thus a modified method was
developed. Four GC capillary columns differing in length
and polarity of stationary phase as described in Section 2
were evaluated, and the various GC separation conditions
were also compared. After numerous studies, an Agilent
HP-88 column was found to be the most appropriate
for simultaneous separation of trans and cis fatty acids.
For the other three columns, only 9 cis fatty acid stan-
dards, including C12:0, C14:0, C16:1 (D9c), C16:0,
C18:2 (D9cD12c), C18:3 (D9cD12cD15c), C18:1 (D9c),
C18:0 and C20:0 were separated by using a DB-1 column.
Likewise, a total of 15 fatty acids, including 6 more trans
fatty acids were separated using an INNOWAX column
(30 or 60 m). The difference is that a 60-m INNOWAX
column resulted in a much longer retention time than a
30-m INNOWAX column, and several trans fatty acids
were overlapped for both columns. The GC chromato-
gram of fatty acid methyl ester standards using a HP-88
column is shown in Fig. 1. A total of 18 peaks, including
5cis saturated fatty acids, 4 cis unsaturated fatty acids, 8
trans fatty acids and internal standard (C17:0) were
resolved within 31 min, with helium as carrier gas and
flow rate at 3 ml/min, injector temperature at 240 °C
and detector temperature at 250 °C. The split ratio was
10:1 and the column temperature was programmed as fol-
lows: 170 °C in the beginning, maintained for 24 min,
raised to 220 °C at 7.5 °C/min, 230 °Cat10°C/min and
maintained for 5 min. With the exception of trans isomers
of oleic acid (peaks 7, 8 and 9), all the other fatty acids
were adequately resolved. We have to point out here that
the partial overlap of trans oleic acid should not affect
quantitation because, for food labeling, these three iso-
mers can be regarded as those representing the total
amount of trans oleic acid. Nevertheless, when compared
to some other previous reports (American Oil Chemists’
Society, 1990; Juane
´da, 2002), this method is much better
1742 W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749
in terms of retention time and separation number of trans
fatty acids.
Table 1 shows the quality control data of 14 fatty acid
standards. Three fatty acid standards, namely, C12:0,
C18:2 (D9cis,D12 trans) and C18:2 (D9cis,D11 trans) were
excluded because they were not detected in heated soybean
oil. Both intra- and inter-day tests are routinely used for
evaluation of precision of the developed analytical method,
which are often carried out by comparing the concentra-
tion difference of multiple analyses within one day and
between days, and the concentration difference should be
as minimal as possible to attain a high reproducibility for
this method (International Conference on Harmonization,
1996). The coefficient of variation (CV) (%) of the intra-day
variability based on the mean concentration of five injec-
tions within one day ranged from 0.2% to 2.7% while the
CV of the inter-day variability based on the mean concen-
tration of five injections every week for a total of 5 weeks
ranged from 1.0% to 3.6%. This result clearly indicated a
high reproducibility was achieved by this method. Table
2shows the detection limit (DL) and quantitation limit
(QL) of 14 fatty acid standards. The DL based on S/
NP3 ranged from 0.8 to 1.2 ppm, whereas the QL based
on S/NP10 ranged from 2.6 to 3.9 ppm. These values
were lower than a report by Ruiz-Jimenez, Priego-Capote,
and Luque de Castro (2004), who determined the amount
of trans fatty acids in bread and found the DL ranged from
0.98 to 3.93 ppm and the QL ranged from 3.23 to
12.98 ppm. The r
2
of all the linear regression equations of
14 standard curves were higher than 0.99. Table 3 shows
the recovery data of each fatty acid standard added to
heated soybean oil. A high recovery of 94.4–102.7% was
attained for all the 14 fatty acid standards, which was
higher than a previous study by Indarti, Majid, Hashim,
and Chong (2005), who analyzed the fatty acid content in
fish oil and reported the recovery to be approximately
80%. This difference may be due to variation in extraction
min
5 10 15 20 25 30
pA
10
12
14
16
18
20
22
24
solvent
1
2
345IS
6
7
8910
11
12
13
14
15
16
17
Fig. 1. GC chromatogram of FAMEs standards using a HP-88 column.
Helium was used as carrier gas. The oven temperature was programmed as
follows: 170 °C in the beginning, maintained for 24 min, increased to
220 °C at 7.5 °C/min, to 230 °Cat10°C/min, maintained for 5 min.
Peaks: (1) lauric acid methyl ester, (2) myristic acid methyl ester, (3)
palmitic acid methyl ester, (4) 9-trans-hexadecenoic acid methyl ester, (5)
palmitoleic acid methyl ester, (6) stearic acid methyl ester, (7) 6-trans-
octadecenoic acid methyl ester, (8) 9-trans-octadecenoic acid methyl ester,
(9) 11-trans-octadecenoic acid methyl ester, (10) oleic acid methyl ester,
(11) 9-trans-12-trans-octadecadienoic acid methyl ester, (12) linoleic acid
methyl ester, (13) 9,12,15-trans octadecatrienoic acid methyl ester, (14)
arachidic acid methyl ester, (15) linolenic acid methyl ester, (16) 9-cis, 11-
trans-octadecadienoic acid methyl ester, (17) 10-cis, 12-trans-octadecadi-
enoic acid methyl ester. IS = internal standard.
Table 1
Quality control data of 14 fatty acid methyl esters standards analyzed by
GC
Fatty acid methyl ester standard Intra-day
a
variability
Inter-day
b
variability
CV (%) CV (%)
Myristic acid methyl ester (C14:0) 1.7 2.4
Palmitic acid methyl ester (C16:0) 0.2 1.0
9-trans-Hexadecenoic acid methyl ester
(C16:1,9t)
2.1 3.2
Palmitoleic acid methyl ester (C16:1,9c) 2.7 3.6
Stearic acid methyl ester (C18:0) 0.2 1.8
6-trans-Octadecenoic acid methyl ester
(C18:1,6t)
0.5 2.3
9-trans-Octadecenoic acid methyl ester
(C18:1,9t)
0.7 1.7
11-trans-Octadecenoic acid methyl ester
(C18:1,11t)
0.8 1.8
Oleic acid methyl ester (C18:1,9c) 0.7 1.4
9-trans-12-trans-Octadecadienoic acid
methyl ester (C18:2,9t12t)
1.6 2.0
Linoleic acid methyl ester (C18:2,9c12c) 1.5 2.2
9,12,15-trans-Octadecatrienoic acid
methyl ester (C18:3,9t12t15t)
0.4 2.0
Arachidic acid methyl ester (C20:0) 0.7 2.5
Linolenic acid methyl ester
(C18:3,9c12c15c)
0.7 1.6
a
Mean concentration of five injections within one day.
b
Mean concentration of five injections every week for a total of five
weeks.
Table 2
Detection and quantitation limits of 14 fatty acid methyl esters standards
analyzed by GC
Fatty acid methyl ester standard DL
(ppm)
a
QL
(ppm)
b
Myristic acid methyl ester (C14:0) 0.8 2.6
Palmitic acid methyl ester (C16:0) 0.8 2.6
9-trans-Hexadecenoic acid methyl ester (C16:1,9t) 1.2 3.9
Palmitoleic acid methyl ester (C16:1,9c) 0.8 2.6
Stearic acid methyl ester (C18:0) 0.8 2.6
6-trans-Octadecenoic acid methyl ester (C18:1,6t) 0.8 2.6
9-trans-Octadecenoic acid methyl ester (C18:1,9t) 0.8 2.6
11-trans-Octadecenoic acid methyl ester (C18:1,11t) 0.8 2.6
Oleic acid methyl ester (C18:1,9c) 0.8 2.6
9-trans-12-trans-Octadecadienoic acid methyl ester
(C18:2,9t12t)
0.8 2.6
Linoleic acid methyl ester (C18:2,9c12c) 1.2 3.9
9,12,15-trans Octadecatrienoic acid methyl ester
(C18:3,9t12t15t)
1.2 3.9
Arachidic acid methyl ester (C20:0) 1.2 3.9
Linolenic acid methyl ester (C18:3,9c12c15c) 1.2 3.9
a
DL: limit of detection based on S/N=3.
b
QL: limit of quantitation based on S/N=10.
W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749 1743
procedure. It is also possible that the methyl esterification
method (the boron-trifluoride method) used in our experi-
ment may enhance the recovery substantially. Lee, Wang,
and Ming (1990) compared the effect of four methyl ester-
ification methods on the recovery of fatty acids in salad oil
and depicted that a high recovery (>96%) could be
achieved by either boron-trifluoride, sulfuric acid-reflux
or tetramethyl-ammonium-salt method. Conversely, a low
recovery (85.7%) was yielded by the sodium methoxide
method, probably because of moisture absorption and
decomposition into sodium hydroxide, resulting in an
incomplete esterification.
3.2. Fatty acid composition change in soybean oil during
heating
Tables 4–9 show the fatty acid composition in unhydro-
genated and hydrogenated soybean oil. Fresh unhydroge-
nated soybean oil was found to contain three saturated
fatty acids (C16:0, C18:0 and C20:0), 3 cis unsaturated
fatty acids (C18:1, C18:2 and C18:3), of which linoleic acid
constituted the largest portion (407.8 mg/g), followed by
oleic acid (198.9 mg/g). However, in fresh hydrogenated
soybean oil, three saturated fatty acids (C16:0, C18:0 and
C20:0), 2 cis unsaturated fatty acids (C18:1 and C18:2)
and four trans fatty acids (C18:1, D6t; C18:1, D9t;
C18:1, D11tand C18:2, D9tD12t) were present, with cis
oleic acid (199.5 mg/g) dominating, followed by trans oleic
acid (194.7 mg/g). No linolenic acid (C18:3) was detected,
mainly because of conversion into linoleic acid (C18:2) or
oleic acid (C18:1) or stearic acid (C18:0) during hydrogena-
tion. By comparing the various trans forms of fatty acids,
oleic acid was the most susceptible to formation in hydro-
genated soybean oil. Karabulut, Kayahan, and Yaprak
(2003) and Schmidt (2000) studied the formation of trans
fatty acids during oil hydrogenation and the contents of
trans forms of both oleic acid and linoleic acid followed
an increased trend for the increase of reaction time.
3.2.1. Unhydrogenated soybean oil
Table 4 shows the fatty acid composition change of
unhydrogenated soybean oil during heating at 160 °C
for 4, 8, 12, 16, 20 and 24 h. Compared to fresh soybean
oil, the levels of five fatty acids, namely, C16:0, C18:0,
C18:1 (D9c), C18:2 (D9cD12c) and C18:3 (D9cD12cD15c)
decreased along with increasing heating time, probably
because of degradation during extensive heating. After
prolonged heating for 24 h, a sharp decline by 20.2
(22.2%), 6.9 (22.9%), 46.6 (23.4%), 83.6 (25.5%) and
7.3 mg/g (20.4%) occurred for C16:0, C18:0, C18:1
(D9c), C18:2 (D9cD12c) and C18:3(D9cD12cD15c), respec-
tively. However, no trans fatty acid was formed under this
heating condition. The total amounts of fatty acids for
24 h samples were not the same as control samples, which
could be accounted for by the instability of cis fatty acid
under drastic condition for the former (Chen et al., 2001).
In a study dealing with oxidative stability of methyl lino-
leate, methyl oleate and methyl stearate during heating,
Chen et al. (2001) reported that the degradation could
proceed faster than the peroxide formation at elevated
temperature (200 °C). Table 5 shows the fatty acid com-
position change in unhydrogenated soybean oil during
heating at 180 °C. Likewise, the contents of all the fatty
acids exhibited a decreased tendency for the increase in
heating time. After 24-h heating, a marked decline of
21.8 (24.0%), 10.6 (35.2%), 51.9 (26.1%), 11.7 (27.4%),
3.8 (35.0%) and 13.8 mg/g (38.7%) was observed for
C16:0, C18:0, C18:1 (D9c), C18:2 (D9cD12c), C20:0 and
C18:3 (D9cD12cD15c), respectively. Also, no trans fatty
acid was formed in soybean oil heated at 180 °C. Simi-
larly, the levels of all the fatty acids dropped pro-
nouncedly during heating of soybean oil at 200 °C
(Table 6), and a greater loss by 24.3 (26.7%), 12.0
(39.9%), 85.2 (42.8%), 144.1 (35.3%), 4.3 (39.8%) and
19.7 mg/g (55.2%) occurred for C16:0, C18:0, C18:1
(D9c), C18:2 (D9cD12c), C20:0 and C18:3 (D9cD12cD15c),
respectively,after extensive heating for 24 h. Again, no
Table 3
Recovery data of fatty acid methyl esters standards when added to heated soybean oil
Fatty acids Recovery (%) Total average
5000 ppm 10,000 ppm
First Second Average First Second Average
C14:0 100.1 102.9 101.5 103.1 104.4 103.8 102.7 ± 1.6
C16:0 96.1 97.0 96.6 97.2 98.3 97.8 97.2 ± 0.9
C16:1,9t94.0 96.2 95.1 95.2 97.6 96.4 95.8 ± 0.9
C16:1,9c96.1 97.4 96.8 97.1 98.0 97.6 97.2 ± 0.6
C18:0 97.2 98.5 97.9 97.7 99.1 98.4 98.2 ± 0.4
C18:1,6t94.8 96.3 95.5 96.3 98.3 97.3 96.4 ± 1.3
C18:1,9t95.5 94.3 94.9 94.8 96.8 95.8 95.4 ± 0.7
C18:1,11t93.2 94.6 93.9 95.7 97.2 96.5 95.2 ± 1.8
C18:1,9c96.1 97.5 96.8 96.7 98.6 97.7 97.2 ± 0.6
C18:2,9t12t94.9 96.8 95.8 96.0 98.4 97.2 96.5 ± 1.0
C18:2,9c12c93.7 95.8 94.7 96.3 97.1 96.7 95.7 ± 1.4
C18:3,9t12t15t95.4 93.3 94.3 95.2 96.4 95.8 95.0 ± 1.0
C20:0 92.3 93.1 92.7 96.5 95.7 96.1 94.4 ± 2.4
C18:3,9c12c15c95.9 97.1 96.5 96.4 98.4 97.4 96.9 ± 0.6
1744 W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749
trans fatty acid was formed in heated soybean oil at
200 °C. This result demonstrated that the higher the tem-
perature, the faster the degradation of cis fatty acids
(Frankel, 1998). Moreover, a drastic heating condition
(>200 °C and >24 h) should be required to generate trans
fatty acid formation in the oil. Theoretically, cis fatty acid
should be more susceptible to heat loss than trans fatty
acid (Frankel, 1998). Our result did prove that cis fatty
Table 4
Fatty acid composition change during heating of soybean oil at 160 °C for varied length of time
Fatty acid (mg/g)
C
Heating (h)
Control
A
4 8 12 16 20 24
C14:0 ND
B
ND ND ND ND ND ND
C16:0 91.0 ± 1.9
a
77.9 ± 1.1
bc
75.7 ± 1.4
cd
74.1 ± 1.0
de
73.2 ± 1.5
de
71.8 ± 1.0
e
70.8 ± 1.5
e
C16:1,9tND ND ND ND ND ND ND
C16:1,9cND ND ND ND ND ND ND
C18:0 30.1 ± 1.5
a
27.0 ± 0.8
bc
26.2 ± 0.8
bcd
25.6 ± 1.2
cde
24.9 ± 0.2
cde
24.1 ± 0.4
de
23.2 ± 1.1
e
C18:1,6tND ND ND ND ND ND ND
C18:1,9tND ND ND ND ND ND ND
C18:1,11tND ND ND ND ND ND ND
C18:1,9c198.9 ± 2.0
a
173.5 ± 1.6
b
170.6 ± 1.3
b
164.6 ± 1.6
c
160.6 ± 1.3
cd
157.4 ± 1.1
d
152.3 ± 3.1
e
C18:2,9t12tND ND ND ND ND ND ND
C18:2,9c12c407.8 ± 1.8
a
357.1 ± 3.1
b
352.5 ± 1.6
b
345.0 ± 1.7
c
340.6 ± 1.8
cd
336.9 ± 1.8
d
324.2 ± 2.0
e
C18:3,9t12t15tND ND ND ND ND ND ND
C20:0 10.8 ± 0.9
a
10.1 ± 1.5
ab
9.6 ± 0.7
ab
9.1 ± 0.7
ab
8.7 ± 0.4
ab
8.6 ± 0.1
b
8.1 ± 1.0
b
C18:3,9c12c15c35.7 ± 1.0
a
33.3 ± 0.6
bc
32.1 ± 0.9
bc
31.1 ± 1.2
cd
29.3 ± 0.8
de
28.4 ± 0.6
e
28.4 ± 1.2
e
Others 17.4 13.9 13.5 13.1 12.6 12.2 11.3
Subtotal (trans)NDNDND ND ND NDND
Subtotal (cis) 642.4 563.8 555.1 540.7 530.4 522.7 504.8
Subtotal (sat)
D
131.9 115.0 111.5 108.8 106.7 104.5 102.1
trans/cis nil nil nil nil nil nil nil
A
Control: fresh unhydrogenated soybean oil.
B
ND: not detected.
C
Means of duplicate analyses ± standard deviation.
D
Sat: saturated fatty acid.
a–e
Symbols bearing different letters in the same row are significantly different (P< 0.05).
Table 5
Fatty acid composition change during heating of soybean oil at 180 °C for varied length of time
Fatty acid (mg/g)
C
Heating (h)
Control
A
4 8 12 16 20 24
C14:0 ND
B
ND ND ND ND ND ND
C16:0 91.0 ± 1.9
a
73.2 ± 2.9
b
71.4 ± 2.8
b
71.5 ± 1.6
b
71.3 ± 1.1
b
70.5 ± 1.6
b
69.2 ± 1.3
b
C16:1,9tND ND ND ND ND ND ND
C16:1,9cND ND ND ND ND ND ND
C18:0 30.1 ± 1.5
a
23.4 ± 1.4
b
22.0 ± 1.1
bc
23.1 ± 1.5
b
21.5 ± 0.8
bc
21.0 ± 1.7
bc
19.5 ± 1.3
bc
C18:1,6tND ND ND ND ND ND ND
C18:1,9tND ND ND ND ND ND ND
C18:1,11tND ND ND ND ND ND ND
C18:1,9c198.9 ± 2.0
a
169.8 ± 1.7
b
167.9 ± 2.3
bc
163.5 ± 2.0
cd
159.2 ± 3.3
d
151.7 ± 1.0
e
147.0 ± 2.3
e
C18:2,9t12tND ND ND ND ND ND ND
C18:2,9c12c407.8 ± 1.8
a
342.7 ± 3.6
b
340.2 ± 2.2
bc
334.6 ± 2.0
c
324.8 ± 4.9
d
305.8 ± 4.0
e
296.1 ± 2.2
f
C18:3,9t12t15tND ND ND ND ND ND ND
C20:0 10.8 ± 0.9
a
8.3 ± 1.3
b
9.3 ± 1.3
ab
8.4 ± 1.5
ab
7.6 ± 0.2
b
7.5 ± 1.1
b
7.0 ± 0.8
b
C18:3,9c12c15c35.7 ± 1.0
a
29.0 ± 2.1
b
27.4 ± 1.3
bc
24.9 ± 1.3
cd
23.3 ± 1.1
d
22.6 ± 1.3
d
21.9 ± 2.2
d
Others 17.4 13.2 12.9 12.4 11.3 10.3 9.5
Subtotal (trans)NDNDND ND NDNDND
Subtotal (cis) 642.4 541.5 535.5 523.0 507.3 480.0 464.9
Subtotal (sat)
D
131.9 105.0 102.7 102.9 100.5 99.0 95.6
trans/cis nil nil nil nil nil nil nil
A
Control: fresh unhydrogenated soybean oil.
B
ND: not detected.
C
Means of duplicate analyses ± standard deviation.
D
Sat: saturated fatty acid.
a–f
Symbols bearing different letters in the same row are significantly different (P< 0.05).
W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749 1745
acid could undergo degradation under severe heating con-
ditions, and the degraded products could be aldehyde,
alcohol, ketone or hydrocarbon compounds, depending
on heating temperature and time (Frankel, 1998). In a
similar study dealing with heating of sunflower oil at
220, 240 and 270 °C for 5 h alone, Kamel and Kakuda
(1994) reported no trans fatty acid formation at 220 °C.
However, at 240 and 270 °C, the levels of trans fatty acids
Table 6
Fatty acid composition change during heating of soybean oil at 200 °C for varied length of time
Fatty acid (mg/g)
C
Heating (h)
Control
A
4 8 12 16 20 24
C14:0 ND
B
ND ND ND ND ND ND
C16:0 91.0 ± 1.9
a
70.0 ± 1.2
b
70.2 ± 0.5
b
69.4 ± 1.6
bc
68.3 ± 0.1
bc
67.4 ± 0.8
bc
66.7 ± 0.6
c
C16:1,9tND ND ND ND ND ND ND
C16:1,9cND ND ND ND ND ND ND
C18:0 30.1 ± 1.5
a
21.2 ± 1.6
b
21.7 ± 0.7
b
20.0 ± 1.3
bc
20.3 ± 1.1
bc
19.4 ± 1.3
bc
18.1 ± 0.8
c
C18:1,6tND ND ND ND ND ND ND
C18:1,9tND ND ND ND ND ND ND
C18:1,11tND ND ND ND ND ND ND
C18:1,9c198.9 ± 2.0
a
167.4 ± 3.5
b
158.7 ± 2.8
c
150.0 ± 2.4
d
139.1 ± 1.8
e
126.0 ± 2.0
f
113.7 ± 2.1
g
C18:2,9t12tND ND ND ND ND ND ND
C18:2,9c12c407.8 ± 1.8
a
340.4 ± 3.3
b
329.2 ± 0.3
c
315.9 ± 2.3
d
299.5 ± 2.7
e
280.5 ± 2.6
f
263.7 ± 2.3
g
C18:3,9t12t15tND ND ND ND ND ND ND
C20:0 10.8 ± 0.9
a
7.6 ± 1.0
bc
7.9 ± 0.5
bc
7.7 ± 0.6
bc
7.4 ± 0.4
bc
7.0 ± 0.4
c
6.5 ± 0.5
c
C18:3,9c12c15c35.7 ± 1.0
a
27.5 ± 2.1
b
26.8 ± 0.0
b
23.3 ± 1.1
c
21.1 ± 1.9
cd
18.4 ± 1.1
de
16.0 ± 1.0
e
Others 17.4 12.9 11.8 10.2 9.3 8.1 7.2
Subtotal (trans)NDNDNDNDNDNDND
Subtotal (cis) 642.4 535.2 514.6 489.2 459.6 424.9 393.4
Subtotal (sat)
D
131.9 98.8 99.8 97.1 96.1 93.7 91.3
trans/cis nil nil nil nil nil nil nil
A
Control: fresh unhydrogenated soybean oil.
B
ND: not detected.
C
Means of duplicate analyses ± standard deviation.
D
Sat: saturated fatty acid.
a–g
Symbols bearing different letters in the same row are significantly different (P< 0.05).
Table 7
Fatty acid composition change during heating of hydrogenated soybean oil at 160 °C for varied length of time
Fatty acid (mg/g)
C
Heating (h)
Control
A
4 8 12 16 20 24
C14:0 ND
B
ND ND ND ND ND ND
C16:0 106.0 ± 0.8
a
97.8 ± 0.8
b
95.3 ± 0.8
c
94.2 ± 0.8
cd
93.7 ± 0.6
cd
92.7 ± 0.6
de
91.0 ± 0.9
e
C16:1,9tND ND ND ND ND ND ND
C16:1,9cND ND ND ND ND ND ND
C18:0 81.0 ± 0.8
a
77.2 ± 1.0
b
74.6 ± 1.1
c
73.7 ± 1.0
cd
72.6 ± 0.8
cde
71.6 ± 1.0
de
70.9 ± 0.8
e
C18:1,6t50.5 ± 1.0
a
48.0 ± 1.1
ab
46.7 ± 1.4
b
46.0 ± 1.2
bc
45.7 ± 0.8
bc
43.5 ± 1.3
cd
42.1 ± 1.0
d
C18:1,9t70.4 ± 1.1
a
66.5 ± 1.0
b
65.0 ± 1.0
bc
63.2 ± 0.6
cd
61.5 ± 1.0
d
58.9 ± 1.1
e
56.1 ± 1.2
f
C18:1,11t73.8 ± 0.9
a
68.4 ± 0.7
bc
66.7 ± 1.0
c
64.2 ± 0.9
d
62.7 ± 1.1
de
60.4 ± 1.1
e
57.4 ± 1.3
f
C18:1,9c199.5 ± 1.1
a
186.5 ± 1.1
b
182.9 ± 0.8
c
180.0 ± 1.6
c
174.9 ± 1.1
d
166.8 ± 1.4
e
158.6 ± 1.1
f
C18:2,9t12t9.5 ± 0.5
a
9.1 ± 0.5
ab
8.8 ± 0.4
abc
8.7 ± 0.4
abc
8.3 ± 0.4
bc
8.2± 0.4
bc
7.8 ± 0.6
c
C18:2,9c12c8.0 ± 0.5
a
8.0 ± 0.4
a
7.6 ± 0.6
ab
7.3 ± 0.6
ab
6.4 ± 0.6
bc
6.0 ± 0.4
c
5.6 ± 0.5
c
C18:3,9t12t15tND ND ND ND ND ND ND
C20:0 6.2 ± 0.3
a
6.1 ± 0.3
a
6.1 ± 0.4
a
5.9 ± 0.6
a
6.0 ± 0.4
a
6.1 ± 0.6
a
5.9 ± 0.4
a
C18:3,9c12c15cND ND ND ND ND ND ND
Others 80.7 72.3 70.1 68.9 67.8 66.1 64.4
Subtotal (trans) 204.2 192.0 187.2 182.0 178.1 171.0 163.4
Subtotal (cis) 207.5 194.4 190.5 187.3 181.3 172.8 164.2
Subtotal (sat)
D
193.3 181.1 175.9 173.8 172.3 170.4 167.7
trans/cis 1.0 1.0 1.0 1.0 1.0 1.0 1.0
A
Control: fresh unhydrogenated soybean oil.
B
ND: not detected.
C
Means of duplicate analyses ± standard deviation.
D
Sat: saturated fatty acid.
a–f
Symbols bearing different letters in the same row are significantly different (P< 0.05).
1746 W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749
rose by 3% and 11%, respectively. Also, no trans fatty
acid formation was observed in several vegetable oils
when heated at 170 and 350 °C for 30 min or 200 and
220 °C for 16 h, and thus Mo
¨llenken (1998) concluded
that trans fatty acids would be difficult to form unless a
severe cooking condition was used. This phenomenon fur-
ther proved that the heating conditions in our experiment
are inadequate to induce formation of trans fatty acids.
Table 8
Fatty acid composition change during heating of hydrogenated soybean oil at 180 °C for varied length of time
Fatty acid (mg/g)
C
Heating (h)
Control
A
4 8 12 16 20 24
C14:0 ND
B
ND ND ND ND ND ND
C16:0 106.0 ± 0.8
a
95.5 ± 1.1
b
94.4 ± 1.1
b
93.2 ± 1.1
bc
91.7 ± 0.8
cd
89.9 ± 0.6
de
87.7 ± 0.8
e
C16:1,9tND ND ND ND ND ND ND
C16:1,9cND ND ND ND ND ND ND
C18:0 81.0 ± 0.8
a
74.9 ± 1.1
bc
73.6 ± 0.9
cd
73.1 ± 1.0
cd
71.7 ± 0.8
de
70.1 ± 1.0
ef
68.9 ± 1.1
f
C18:1,6t50.5 ± 1.0
a
46.7 ± 1.3
b
46.2 ± 1.3
bc
45.3 ± 1.2
bc
43.3 ± 1.6
cd
41.5 ± 1.5
de
38.7 ± 1.6
e
C18:1,9t70.4 ± 1.1
a
63.7 ± 1.3
bc
62.2 ± 1.3
c
61.3 ± 1.2
c
58.6 ± 1.2
d
55.4 ± 1.0
e
50.7 ± 0.6
f
C18:1,11t73.8 ± 0.9
a
65.2 ± 1.6
bc
63.7 ± 1.6
c
63.7 ± 1.6
c
60.4 ± 1.1
d
57.4 ± 1.0
e
51.5 ± 0.8
f
C18:1,9c199.5 ± 1.1
a
183.3 ± 1.3
bc
181.6 ± 1.1
c
177.7 ± 1.1
d
169.3 ± 1.2
e
159.1 ± 1.8
f
144.0 ± 2.4
g
C18:2,9t12t9.5 ± 0.5
a
8.9 ± 0.5
ab
8.9 ± 0.5
ab
8.8 ± 0.4
ab
8.6 ± 0.5
abc
7.8 ± 0.6
bc
7.5 ± 0.4
c
C18:2,9c12c8.0 ± 0.5
a
7.8 ± 0.5
ab
7.4 ± 0.5
ab
7.1 ± 0.6
ab
6.6 ± 0.5
bc
5.7 ± 0.4
c
5.5 ± 0.6
c
C18:3,9t12t15tND ND ND ND ND ND ND
C20:0 6.2 ± 0.3
a
5.9 ± 0.6
a
5.9 ± 0.5
a
6.1 ± 0.4
a
6.0 ± 0.4
a
5.8 ± 0.1
a
5.8 ± 0.5
a
C18:3,9c12c15cND ND ND ND ND ND ND
Others 80.7 70.1 69.1 67.5 65.2 63.0 60.4
Subtotal (trans) 204.2 184.5 180.9 179.0 170.8 162.1 148.2
Subtotal (cis) 207.5 191.0 189.0 184.7 175.8 164.7 149.5
Subtotal (sat)
D
193.3 176.2 173.8 172.4 169.4 165.7 162.4
trans/cis 1.0 1.0 1.0 1.0 1.0 1.0 1.0
A
Control: fresh unhydrogenated soybean oil.
B
ND: not detected.
C
Means of duplicate analyses ± standard deviation.
D
Sat: saturated fatty acid.
a–g
Symbols bearing different letters in the same row are significantly different (P< 0.05).
Table 9
Fatty acid composition change during heating of hydrogenated soybean oil at 200 °C for varied length of time
Fatty acid (mg/g)
C
Heating (h)
Control
A
4 8 12 16 20 24
C14:0 ND
B
ND ND ND ND ND ND
C16:0 106.0 ± 0.8
3a
93.8 ± 0.5
b
93.0 ± 1.3
bc
91.6 ± 0.9
bc
90.9 ± 1.3
c
87.8 ± 1.0
d
84.1 ± 1.2
e
C16:1,9tND ND ND ND ND ND ND
C16:1,9cND ND ND ND ND ND ND
C18:0 81.0 ± 0.8
a
72.6 ± 0.7
bc
71.6 ± 1.1
cd
73.1 ± 0.9
bc
69.7 ± 0.8
de
68.0 ± 0.8
ef
66.1 ± 1.1
f
C18:1,6t50.5 ± 1.0
a
43.7 ± 1.7
b
44.6 ± 1.4
b
44.8 ± 1.0
b
40.4 ± 1.1
c
37.2 ± 1.5
d
35.2 ± 1.3
d
C18:1,9t70.4 ± 1.1
a
61.7 ± 1.2
bc
59.0 ± 1.9
cd
59.0 ± 1.9
cd
56.3 ± 2.1
d
52.2 ± 1.6
e
45.8 ± 1.4
f
C18:1,11t73.8 ± 0.9
a
63.3 ± 1.1
b
63.0 ± 1.2
b
61.2 ± 1.5
b
56.9 ± 1.3
c
52.7 ± 0.8
d
46.8 ± 1.3
e
C18:1,9c199.5 ± 1.1
a
181.4 ± 1.1
bc
179.3 ± 1.3
c
172.4 ± 1.6
d
154.7 ± 1.2
e
143.9 ± 1.7
f
130.7 ± 1.6
g
C18:2,9t12t9.5 ± 0.5
a
8.7 ± 0.1
abc
8.6 ± 0.4
abc
8.5 ± 0.6
abc
8.2 ± 0.6
bcd
7.6 ± 0.5
cd
7.2 ± 0.5
d
C18:2,9c12c8.0 ± 0.5
a
7.2 ± 0.6
ab
6.7 ± 0.6
abc
6.4 ± 0.4
bc
6.2 ± 0.8
bc
5.6 ± 0.4
c
5.2 ± 0.4
c
C18:3,9t12t15tND ND ND ND ND ND ND
C20:0 6.2 ± 0.3
a
5.9 ±0.5
a
5.9 ± 0.3
a
5.9 ± 0.3
a
5.7 ± 0.6
a
5.5 ± 0.4
a
5.7 ± 0.2
a
C18:3,9c12c15cND ND ND ND ND ND ND
Others 80.7 67.8 67.5 65.0 64.3 60.2 55.6
Subtotal (trans) 204.2 177.4 175.2 173.6 161.8 149.6 134.9
Subtotal (cis) 207.5 188.6 186.0 178.8 160.8 149.5 135.9
Subtotal (sat)
D
193.3 172.2 170.4 170.5 166.3 161.3 155.8
trans/cis 1.0 0.9 0.9 1.0 1.0 1.0 1.0
A
Control: fresh unhydrogenated soybean oil.
B
ND: not detected.
C
Means of duplicate analyses ± standard deviation.
D
Sat: saturated fatty acid.
a–g
Symbols bearing different letters in the same row are significantly different (P< 0.05).
W.H. Liu et al. / Food Chemistry 104 (2007) 1740–1749 1747
3.2.2. Hydrogenated soybean oil
Table 7 shows the fatty acid composition change in
hydrogenated soybean oil during heating at 160 °C for 4,
8, 12, 16, 20 and 24 h. A loss of 8.2, 3.8, 3.9, 5.4 and
13.0 mg/g was reached 4 h after heating for C16:0, C18:0,
C18:1 (D9t), C18:1 (D11t) and C18:1 (D9c), respectively.
In comparison with fresh hydrogenated soybean oil, a dis-
tinct decrease by 15.0 (14.2%), 10.1 (12.5%), 8.4 (16.6%),
14.3 (20.3%), 16.4 (22.2%), 40.9 (20.5%), 1.7 (17.9%) and
2.4 mg/g (30.0%) was shown for C16:0, C18:0, C18:1
(D6t), C18:1 (D9t), C18:1 (D11t), C18:1 (D9c), C18:2
(D9tD12t) and C18:2 (D9cD12c), respectively, after 24-h
heating. Likewise, both cis and trans fatty acids can
undergo degradation simultaneously after extensive heat-
ing. Nevertheless, no trans fatty acid was formed under this
condition. A similar trend was observed for the fatty acid
composition change during heating of hydrogenated soy-
bean oil at 180 °C(Table 8). A large decline by 18.3
(17.3%), 12.1 (15.0%), 11.8 (23.4%), 19.7 (28.0%), 22.3
(30.2%), 55.5 (27.8%), 2.0 (21.1%) and 2.5 mg/g (31.3%)
was attained 24 h after heating for C16:0, C18:0, C18:1
(D6t), C18:1 (D9t), C18:1 (D11t), C18:1 (D9c), C18:2
(D9tD12t) and C18:2 (D9cD12c), respectively. Also, no
trans fatty acid was formed. The same tendency also
applied to hydrogenated soybean oil when heated alone
at 200 °C for 24 h (Table 9), i.e., the contents of C16:0,
C18:0, C18:1 (D6t), C18:1 (D9t), C18:1 (D11t), C18:1
(D9c), C18:2 (D9tD12t) and C18:2 (D9cD12c) showed a
greater decrease by 21.9 (20.7%), 14.9 (18.4%), 15.3
(30.3%), 24.6 (34.9%), 27.0 (36.6%), 68.8 (34.5%), 2.3
(24.2%) and 2.8 mg/g (35.0%), respectively, while no trans
fatty acid formation was observed.
By comparison of the results shown above, it may be
concluded that an Agilent HP-88 column could provide
effective separation of eight trans fatty acids and nine
cis fatty acids within 31 min. Both the degradation of
cis and trans fatty acids could proceed fast at elevated
temperature. No trans fatty acid was formed in unhydro-
genated and hydrogenated soybean oil during heating at
160, 180 or 200 °C for 24 h, implying that trans fatty acid
can only be formed under drastic heating condition. The
technique developed in this study may be adopted as a
reference method for routine analysis of trans fatty acids
in commercial food products. As mentioned before, the
nutrition labeling of trans fatty acids has become an
urgent issue to solve, and application of this method
can provide valuable information to assist consumers in
maintaining healthy dietary practices. Further research is
necessary to study the formation of trans fatty acids in
bakery and fried products with hydrogenated oil as heat-
ing medium.
References
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methyl esters by silver ion high performance liquid chromatography.
Journal of Chromatography, 659, 95–99.
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