Comparison of the Volatiles Formed by Oxidation of

Article pubs.acs.org/JAFC

Comparison of the Volatiles Formed by Oxidation of Phosphatidylcholine to Triglyceride in Model Systems Li Zhou,*,†,‡ Minjie Zhao,‡ Françoise Bindler,‡ and Eric Marchioni‡ †

College of Food Science and Technology, Nanjing Agricultural University, Nanjing, Jiangsu 210095, People’s Republic of China Equipe de Chimie Analytique des Molécules Bioactives (IPHC-LC4, UMR 7178), Université de Strasbourg, 74 route du Rhin, 67400 Illkirch, France

‡

S Supporting Information *

ABSTRACT: The oxidative stability of oleoyl and linoleoyl residues esterified in the form of triglyceride (TAG) and phosphatidylcholine (PC) during thermal treatment was investigated. Headspace solid-phase microextraction (HS-SPME) followed by gas chromatography−mass spectrometry (GC−MS) analysis was used to determine the volatile compounds from oxidized PL and TAG molecular species. The results showed that aldehydes were the major volatile oxidized compounds (VOCs) of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC), and 1,3-distearoyl-2-linoleoyl-glycerol (SLS), while ketones, especially saturated methyl ketones, were the major VOCs of 1,3-distearoyl-2-oleoyl-glycerol (SOS). The monitoring of the oxidative degradation using liquid chromatography− electrospray ionization−mass spectrometry (LC−ESI−MS) showed that either monounsaturated or diunsaturated fatty acyl groups were less oxidized when in the form of PCs than when in the form of TAGs. This finding demonstrated that the choline group in the form of PCs could increase the stability of fatty acyl groups to oxidation in comparison to TAGs. KEYWORDS: phosphatidylcholine, triglycerides, oxidative stability, volatile oxidized compound, oleic acid, linoleic acid

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from oxidized phosphatidylcholine (PC) and figured out their origin.22 However, to our knowledge, no studies have been carried out in detail on the comparison of volatiles by oxidation of PC to TAG. Several methods are available to measure the progress of lipid oxidation. Conventional measurements for the assessment of lipid oxidation are usually determined by simple spectroscopic techniques, such as thiobarbituric acid reactive substances (TBARS), carbonyl group, and diene conjugate methods. However, these photometric assays could only roughly reflect the level of lipid oxidation, and their sensitivities are limited, while chromatographic techniques (gas chromatography and/ or high-performance liquid chromatography together with different detectors, including mass spectroscopy) are useful for a more detailed description of the reaction products. The aim of this study was to compare the oxidative stability of PC to TAG. As a model system, oleyol and linoleoyl residues in the form of PC and TAG were chosen, because they exist widely in food matrices, such as soybean, egg yolk, and ox liver, which were identified in our previous study. 4,19 The comparison of the oxidative stability of PC and TAG was made in terms of the kinetics of the formation of volatile oxidized compounds (VOCs), which was monitored by headspace solid-phase microextraction and gas chromatography−mass spectrometry (HS-SPME−GC−MS), and the kinetics of degradation of the starting material, which was

INTRODUCTION Polyunsaturated fatty acids (PUFAs) play very important roles in human health, in particular, in protecting the liver,1,2 in improving memory and learning,3,4 and in protecting the cardiovascular system.5−7 Phospholipids (PLs) are rich in PUFAs and are an important source of PUFAs.8 Supplementation of food products with PUFA-rich PLs has recently emerged as an interesting way of increasing the daily intake of PUFA and, thereby, received more and more attention. Another important source of PUFA is triglycerides (TAGs), which are the major carriers of fatty acids (FAs), with 50−100 g/day for an adult, followed by PLs, with 2−10 g/day.9,10 However, according to the bioavailability of FAs, PLs are much more efficient than TAGs.11−13 Moreover, cellular permeability to PUFA and their intracellular level were indeed much higher when linked to PL than to TAG,10,14 which suggests that food supplementation with PUFA-rich PL could enhance essential FA assimilation. The bioavailability of PUFA in the form of PL or TAG is an important factor that should be considered, but their stabilities in food sources are also as important. The information on the evaluation of relative stability of TAG and PL is limited. Most of the studies were contributed on the comparison of the stability of oil-enriched PLs to that of oils containing few or no PLs at all.13,15,16 Lipid oxidation is a major cause of quality deterioration of many natural and processed foods.17,18 It leads to the development of undesirable off-flavors and potentially toxic compounds. Our previous studies have investigated the formation mechanism of non-volatiles of PLs.19,20 As for volatile compounds, there are also major decomposition products of lipids.21 We have identified the volatile compounds © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8295

April 26, 2014 July 27, 2014 August 3, 2014 August 3, 2014 dx.doi.org/10.1021/jf501934w | J. Agric. Food Chem. 2014, 62, 8295−8301

Journal of Agricultural and Food Chemistry

Article

addition, retention indices (RIs) were calculated on the basis of a series of n-alkanes with a chain length from C5 to C20. They were compared to literature values to support tentative identification.23−25 LC−ESI−MS Analysis. After oxidative treatment of the samples, the remaining PCs and TAGs were evaluated using a Varian Prostar HPLC system made of two 210 solvent delivery modules, a 410 autosampler, and a 1200 L triple quadrupole mass spectrometer with ESI operated in positive-ion mode (Agilent, Les Ulis, France). Highpurity nitrogen (99.99%, Domnik Hunter) was used as the nebulizing gas, set at 317 kPa, and as the drying gas, set at 300 °C. Separation was performed on a Zorbax column C8 (250 × 4.6 mm, 5 μm, Agilent) using an isocratic elution of 100% CH3OH containing 5 mM ammonium formate at a flow rate of 1 mL min−1. A total of 20 μL of each diluted oxidized sample was injected. A split system allowed for the HPLC effluent to enter the mass spectrometer at a flow rate of 0.2 mL min−1. The injection volume was performed in positive mode in a mass range between m/z 500 and 1000 and in selected ion monitoring (SIM) mode by extracting ion [M + H]+ m/z 788.6 for SOPC, m/z 786.6 for SLPC, m/z 889.8 for SOS, and m/z 887.8 for SLS. The percentage of the remaining precursors was determined as follows:

monitored using liquid chromatography−electrospray ionization−mass spectrometry (LC−ESI−MS).

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MATERIALS AND METHODS

Materials. The following standards (>95%) were used to identify the volatile compounds, including pentanal, hexanal, heptanal, octanal, nonanal, (E)-2-hexenal, (E)-2-heptenal, (E)-2-octenal, (E)-2-decenal, (E,E)-2,4-decadienal, 1-pentanol, 1-hexanol, 1-octen-3-ol, 1-heptanol, 1-octanol, 2-decanone, 2-undecanone, 1-octen-3-one, 2-pentyl-furan, (E)-2-undecenal, (E)-3-octen-2-one, 5-ethyldihydro-2(3H)-furanone, and dihydro-5-propyl-2(3H)-furanone. They were purchased from Sigma-Aldrich (St. Louis, MO). 1-Stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) (99.9%) and 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) (99.9%) were purchased from Avanti Polar Lipids (Alabaster, AL). 1,3-Distearoyl-2-oleoyl-glycerol (SOS) (99.9%) and 1,3-distearoyl-2linoleoyl-glycerol (SLS) (99.9%) were obtained from Larodan (Malmö , Sweden). High-performance liquid chromatography (HPLC)-grade methanol and ammonium formate were purchased from VWR (Strasbourg, France). Chloroform of analytical grade was purchased from Riedel de Haën (Sigma-Aldrich, Seelze, Germany). The SPME fiber tested in this study was 65 μm of polydimethylsiloxane/divinylbenzene (PDMS/DVB), purchased from Sigma-Aldrich (St. Louis, MO). Sample Preparation. Stock solutions of 10 mg mL−1 were prepared by weighing standards (SOPC, SLPC, SOS, and SLS) in a 5 mL amber vial and dissolving them in chloroform. A total of 100 μL of these solutions (1.0 mg of sample) were placed in a 20 mL dark glass injection vial previously desorbed, uncapped by a heating treatment at 200 °C during 2 h in an oven. After evaporation of chloroform at room temperature in the fume chamber, each injection vial was sealed with an aluminum crimp cap provided with a polytetrafluoroethylene/ silicone septum (Agilent Technologies, Massy, France) and kept at −20 °C until oxidative treatment. Oxidative Treatments. The samples were heated in an oven at a specified temperature comprised between 100 and 200 °C for a duration ranging from 0 to 180 min. After oxidation, samples were cooled quickly in ice for 5 min, then left at room temperature for 5 min, and finally injected in HS-SPME−GC−MS for analysis. A total of 1.0 mL of methanol was added in the vial; then a 1/50 dilution was performed to obtain 0.02 mg/mL; and 10 μL was injected in the LC− ESI−MS system. All samples were analyzed in triplicate. HS-SPME. Prior to analysis, the PDMS/DVB fiber was preconditioned (at 250 °C for 30 min) in the bakeout oven of the CombiPAL injector with the temperature and duration suggested by the manufacturers. The extraction was carried out for 25 min at 50 °C,22 and each measurement was repeated 3 times. GC−MS Analysis of Volatiles. GC−MS analysis was performed in a 450 Varian gas chromatograph fitted with a CombiPAL (CTC Analytics AG, Switzerland) automatic HS-SPME injector. Helium was used as the carrier gas at a constant flow rate of 1 mL min−1. The injector operated in the splitless mode, and its temperature was set at 250 °C. The separation of VOCs was performed on a DB-wax column (60 m × 0.25 mm, 0.15 μm, Agilent, Santa Clara, CA). The oven temperature program started at 40 °C, was raised at a rate of 4 °C min−1 to 220 °C (held for 5 min), and then was raised at a rate of 4 °C min−1 to 240 °C (held for 17 min). The detection was performed by an Agilent 240 mass selective detector (ion trap) set in positive electron impact (+EI) mode with 70 eV of electron energy. The electron multiplier was set by the autotune procedure. MS data were collected with the daughter ion scan mode over the m/z range from 50 to 315. Transfer line temperature, trap temperature, and ion source temperature were 110, 150, and 150 °C, respectively. VOCs were tentatively identified by matching the experimental mass spectra with the reference mass spectra in the National Institute of Standards and Technology (NIST) mass spectral library. Most VOCs identified in this way were then confirmed by comparing their mass spectra and retention times to those obtained from authentic reference compound (STD) under the same analytical conditions. In

%=

(area)t × 100 (area)t◦

where (area)t is the area of the peak corresponding to the ion m/z of the starting material (PC or TAG) after oxidation and (area)t° is the area of the same ion m/z without oxidative treatment. All samples were analyzed in triplicate, and results are given as the mean ± standard deviation (SD).

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RESULTS Determination of Optimal Oxidation Temperatures. To determine the optimal oxidation conditions of SOPC and Scheme 1. Homolytic−Heterolytic Mechanism of the Decomposition of Hydroperoxides28

SLPC, the total areas of VOCs generated at several temperatures was determined. The temperatures allowing for the highest yield of total VOCs were different for thermally oxidized SOPC and SLPC. According to our previous study,22 the highest production of total VOCs was 175 and 125 °C for SOPC and SLPC, respectively, in the same oxidation time. This was in accordance with a previous report showing that free linoleic acid was more sensitive to oxidation than free oleic 8296

dx.doi.org/10.1021/jf501934w | J. Agric. Food Chem. 2014, 62, 8295−8301


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Table 1. Volatile Compounds Identified from Oxidized SOPC and SOSa area (×106) VOCs

b

RI

hexanal heptanal octanal nonanal 2-nonenalf (E)-2-decenal (E)-2-undecenal

1080 1184 1278 1380 1535 1645 1750

1-hexanol 1-heptanol 3-decen-1-olf 1-octanol

1357 1460 1507 1562

2-octanonef 2-nonanonef 2-decanone 2-undecanone 2-dodecanonef dihydro-5-propyl-2(3H)-furanone 5-butyldihydro-2(3H)-furanonef dihydro-5-pentyl-2(3H)-furanonef

1262 1375 1493 1598 1678 1793 1853 1898

SOPC

c

Aldehyde 1.06 ± 0.03 7.46 ± 0.06 21.3 ± 0.0 27.7 ± 0.3 3.18 ± 0.10 29.4 ± 0.0 16.5 ± 0.2 Alcohol 1.36 ± 0.03 6.56 ± 0.13 3.06 ± 0.06 11.8 ± 0.1 Ketone 5.86 ± 0.34 5.77 ± 0.12 7.17 ± 0.03 4.35 ± 0.06 4.41 ± 0.13 4.74 ± 0.03 3.62 ± 0.10 1.4 ± 0.1

SOSc

origin22

identificationd

6.57 19.9 28.4 51.0 8.95 28.8 11.6

± ± ± ± ± ± ±

0.19 0.2 0.3 0.1 0.04 0.1 0.1

Oe O O O O O O

MS, MS, MS, MS, MS, MS, MS,

STD, STD, STD, STD, RI STD, STD,

2.47 3.26 4.6 9.37

± ± ± ±

0.03 0.08 0.2 0.10

O O O O

MS, MS, MS, MS,

STD, RI STD, RI RI STD, RI

7.01 15.2 37.2 40.1 45.9 42.4 12.3 15.2

± ± ± ± ± ± ± ±

0.15 0.1 0.3 0.1 0.2 0.8 0.1 0.1

Sg S S S S O O O

MS, MS, MS, MS, MS, MS, MS, MS,

RI RI STD, RI STD, RI RI STD, RI RI RI

RI RI RI RI RI RI

Results (n = 3) were expressed as the mean ± SD. Extraction conditions: PDMS/DVB, 50 °C, and 25 min. Oxidation conditions: 175 °C and 120 min. bRetention indices (RIs) were determined on DB-wax using the homologous series of n-alkanes. cData were expressed in terms of peak areas. d Identification: MS, by comparison of the reference mass spectra in the NIST mass spectral library; STD, by comparison of authentic reference compounds; and RI, by comparion of RIs to those from the literature. eO: VOCs formed from the oleic acid chain of SOPC or SOS. fCompound tentatively identified according to MS and by comparison of the RI to the literature. gS: VOCs formed from the stearic acid chain of SOPC or SOS. a

Table 2. Volatile Compounds Identified from SLPC and SLSa area (×106) VOCs

RIb

SLPCc

pentanal hexanal (E)-2-hexenal (E)-2-heptenal (E)-2-octenal 2-nonenalf 2,4-nonadienalf (E,Z)-2,4-decadienalf (E,E)-2,4-decadienal 4-oxononanalf

984 1080 1216 1320 1425 1535 1667 1788 1810 1832

3.57 30.1 1.38 31.6 36.2 4.08 6.89 3.53 3.35 4.53

1-pentanol 1-octen-3-ol

1250 1455

6.25 6.59

2-heptanonef 1-octen-3-one (E)-3-octen-2-one 3-nonen-2-onef 5-ethyldihydro-2(3H)-furanone

1180 1280 1385 1512 1678

5.1 7.16 28.9 5.0 3.67

2-pentyl-furan

1240

9.67

Aldehyde ± 0.04 ± 0.2 ± 0.07 ± 0.3 ± 0.2 ± 0.11 ± 0.03 ± 0.07 ± 0.02 ± 0.04 Alcohol ± 0.21 ± 0.01 Ketone ± 0.2 ± 0.03 ± 0.3 ± 0.0 ± 0.04 Furan ± 0.02

SLSc

origin22

± ± ± ± ± ± ± ± ± ±

Le L L L L L L L L L

MS, MS, MS, MS, MS, MS, MS, MS, MS, MS,

1.99 ± 0.02 2.18 ± 0.03

L L

MS, STD, RI MS, STD, RI

± ± ± ± ±

Sg L L L L

MS, MS, MS, MS, MS,

L

MS, STD, RI

5.35 40.5 0.22 5.37 2.88 0.15 0.48 0.35 0.47 0.11

2.52 0.15 2.0 2.03 7.28

0.21 3.0 0.01 0.09 0.14 0.01 0.03 0.06 0.06 0.04

0.06 0.03 0.1 0.13 0.03

5.56 ± 0.12

identificationd STD, STD, STD, STD, STD, RI RI RI STD, RI

RI RI RI RI RI

RI

RI STD, RI STD, RI RI STD, RI

Results (n = 3) were expressed as the mean ± SD. Extraction conditions: PDMS/DVB, 50 °C, and 25 min. Oxidation conditions: 125 °C and 90 min. bRetention indices (RIs) were determined on DB-wax using the homologous series of n-alkanes. cData were expressed in terms of peak areas. d Identification: MS, by comparison of the reference mass spectra in the NIST mass spectral library; STD, by comparison of authentic reference compounds; and RI, by comparion of RIs to those from the literature. eL: VOCs formed from the linoleic acid chain of SLPC or SLS. fCompound tentatively identified according to MS and by comparison of the RI to the literature. gS: VOCs formed from the stearic acid chain of SLPC or SLS. a

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Figure 1. Kinetics of VOCs produced from oxidized SOPC during heat treatment at 175 °C monitored from 0 to 180 min.

acid.9 To compare to the oxidative stability of TAG, 175 and 125 °C were chosen for SOS and SLS, respectively. Identification of VOCs. A variety of volatile and nonvolatile secondary products are formed from hydroperoxides when lipid oxidation is carried out at elevated temperatures. The mechanism suggested for the formation of oxidized products involves the homolytic cleavage of the hydroperoxide group to yield an alkoxyl radical and a hydroxyl radical.26 The further decomposition of an alkoxyl radical involving carbon− carbon bond scission is a major pathway for the formation of volatile products.27 The α-scission and β-scission of an alkoxyl radical lead to different decomposition products (Scheme 1). Oxidation was performed in the solid state because chloroform was evaporated before oxidation. Because of the selectivity of the HS-SPME procedure, identified compounds were those that were present in the sample and were extracted by the PDMS/DVB fiber. Besides, there might be other VOCs that were present in the sample but not extracted by the fiber. Table 1 gives the VOCs of SOPC and SOS at 175 °C with 120 min. A total of 19 VOCs were detected from oxidized SOPC. The major VOCs of SOPC were aldehydes, such as nonanal, (E)-2-decenal, octanal, and (E)-2-undecenal. However, ketones were the predominant group of VOCs formed from oxidized SOS. Among them, saturated methyl ketones, such as 2-octanone, 2-nonanone, 2-decanone, 2-undecanone, and 2-dodecanone, accounted for a large proportion of total VOCs. It was well-known from our previous study that these saturated methyl ketones were formed from the stearic acid chain of SOPC or SOS.22 Three reactions, degradation, βoxidation, and decarboxylation, occurring simultaneously were proposed to account for the homologous series of methyl ketones.29 Furthermore, the results indicated that the total peak areas of saturated methyl ketones from oxidized SOS were

Figure 2. Kinetics of VOCs produced from oxidized SOS during heat treatment at 175 °C monitored from 0 to 180 min: (A) VOCs increased gradually until 90 min and then stabilized or increased slightly, and (B) VOCs increased in a sharp way before 30 min and decreased rapidly until 90 or 120 min.

obviously higher than that from oxidized SOPC. That was in agreement with the fact that SOS contained two stearic acid chains, while SOPC contained only one stearic acid chain. A total of 18 VOCs were detected from oxidized SLPC and SLS (Table 2). Among them, aldehydes were also the major VOCs. In contrast to SOPC/SOS, oxidation of SLPC and SLS produced rather unsaturated ketones, such as (E)-3-octen-2one, formed from linoleic acid chains, with saturated methyl ketones being only in trace amounts because only one (2heptanone) was detected. Up to now, it is quite difficult to explain why SOPC/SOS and SLPC/SLS did not produce the same kind of ketones because they contain both the same C18:0 saturated FA. Perhaps the competitive oxidation equilibrium between sn-1 and sn-2 FA chains of PC/TAG may give information. The competition between stearoyl residues and oleoyl residues for VOC formation is relatively 8298



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Figure 3. Kinetics of VOCs produced from oxidized SLPC during heat treatment at 125 °C monitored from 0 to 180 min: (A) VOCs increased until 90 min and then decreased, and (B) VOCs increased continuously until 150 °C and then decreased.

weak by comparison to stearoyl residues and linoleoyl residues. In other words, the sn-1 FA chains of SOPC/SOS and SLPC/ SLS are stearic acids, but sn-2 FA chains of SOPC/SOS and SLPC/SLS are oleic and linoleic acids, respectively. Because linoleic acid is more sensitive to oxidation than oleic acid, it is easier to be oxidized. That is probably why more ketones tend to be formed from the linoleic acid chains of SLPC/SLS and less from the stearic acid chains. In comparison, more ketones, especially methyl ketones, were derived from the stearic acid chain of SOPC/SOS. It thereby showed that a higher amount of total saturated methyl ketones was formed from SOPC/SOS than that from SLPC/SLS. Oxidative Kinetics. The oxidative kinetics of PLs and TAGs were investigated by analyzing HS-SPME−GC−MS peak areas of all VOCs produced as a function of the oxidation time. Some VOCs had relatively high occurrences in one chemical group. To let the low-content VOCs appear clearly, the curves of relatively high-content and low-content VOCs were divided into two separate figures. The major VOCs of oxidized SOPC were (E)-2-decenal, nonanal, octanal, 2-undecenal, and 1-octanol (Figure 1). Almost all of the VOCs increased rapidly during 0−60 min and reached their maximum at 120 min, then were kept stable or decreased slowly until 150 min, but decreased rapidly after 150 min. As for SOS (Figure 2), the VOCs can be separated into two groups: one group included the products that increased gradually until 90 min and then stabilized or increased slightly (Figure 2A), and the other group included (E)-2-decenal and (E)-2-undecenal, which increased in a sharp way before 30 min, decreased rapidly until 120 and 90 min, respectively, and then reached equilibrium or decreased slightly (Figure 2B). Figure 3 shows kinetics of production of VOCs from oxidized SLPC. Two groups were obtained: one group (Figure 3A) reached their maximum rapidly at 90 min of oxidation,

Figure 4. Kinetics of VOCs produced from oxidized SLS during heat treatment at 125 °C monitored from 0 to 180 min: (A) VOCs increased gradually with the increase of the oxidation time or increased to reach a maximum and then were kept stable, and (B) VOCs increased in a sharp way in the first 30 min and then decreased.

then were kept stable or decreased slowly until 150 min, and decreased sharply after 150 min, and the other group (Figure 3B) increased gradually until 150 min and then decreased rapidly. As far as the VOCs of SLS were concerned, 18 VOCs were detected (Table 2). Hexanal was most abundant among the VOCs. It increased in a sharp way in the first 30 min, reached its maximum at 90 min, then decreased, and stabilized (Figure 4A). The amount of (E)-2-octenal, 2,4-nonadienal, (E,Z)-2,4decadienal, and (E,E)-2,4-decadienal reached a maximum after 30 min of oxidation and then decreased (Figure 4B). Other VOCs increased gradually with the increase of the oxidation time (Figure 4A). SOS is more sensitive to oxidation than SOPC. Two major VOCs of SOS identified, (E)-decenal and (E)-2-undecenal, reached their maximum after only 30 min of oxidative treatment and then decreased (Figure 2B), while most VOCs of SOPC reached their maximum after 120 min (Figure 1). Similarly, four VOCs, which were (E)-2-octenal, 2,4-non8299



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TAG species. Our results were consistent with the results by Song et al.,16 who demonstrated that docosahexaenoic acid (DHA) in the form of PLs was more resistant to oxidative degradation than DHA in the form of TAGs or ethyl esters. Results obtained by Lyberg et al.30 followed the same trend that DHA was protected against hydroperoxide formation when it was incorporated at sn-1 and sn-2 positions of PC, while incorporation of DHA at any positions of a TAG was not an appropriate solution for protecting DHA. Therefore, the result suggests that the polar moiety of PC has a role to play in their stability to oxidation. This hypothesis was proposed by Yoshimoto et al.,31 who supposed that PC can lead to the polarization of the oxygen−oxygen bond in the hydrogen peroxide, which allowed for a nucleophilic attack of an oxygen atom of another peroxide group. That can reduce the proparation phase of the oxidation process. In this study, the results showed that PC species are more resistant to thermal oxidation than their corresponding TAG species, which supported a hypothesis that polar head groups affect oxidative kinetics of glycerol-based lipids. Therefore, concerning FA bioavailability and oxidative stability, supplementation with PUFA PLs was a more potential and interesting way than PUFA TAGs for food development.

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ASSOCIATED CONTENT

S Supporting Information *

Mass spectra of (A) SOPC m/z 788.6 and (B) SLPC m/z 786.6 (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. (A) Oxidative kinetics of SOPC and SOS was monitored over time of thermal treatment at 175 °C, and (B) oxidative kinetics of SLPC and SLS was monitored over time of thermal treatment at 125 °C.

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-25-84396791. E-mail: [email protected]. cn. Funding

adienal, (E,E)-2,4-decadienal, and (E,Z)-2,4-decadienal, reached a maximum at 30 min and then decreased (Figure 4B). While all of the VOCs of SLPC reached their maximum after 90 min, even some VOCs reached their maximum after 150 min (Figure 3B). That shows that SLS is also more sensitive to oxidation than SLPC. Degradation of Starting Material. The quantities of the starting materials SOPC and SOS (SLPC and SLS), expressed as recovery percentages, were monitored by LC−ESI−MS as a function of the oxidation duration. An example of mass spectra of SOPC (A) and SLPC (B) were shown in Figure S1 of the Supporting Information. The percentage of SOPC decreased gradually over time of oxidation; SOPC was not completely decomposed at the end of 180 min; and the remaining percentage of SOPC was 39.6%. While in the case of SOS, the remaining percentage decreased rapidly (by 81%) during the first 30 min and then decreased slightly until an almost total loss of the precursor during 90 min of oxidation (Figure 5A). Figure 5B shows the evolution of the relative amounts of SLPC and SLS over time at 125 °C. The trend of curves was similar to Figure 5A. The amounts of SLS decreased immediately and were significantly sharper than SLPC (p < 0.05). It also showed that SLS disappeared within 90 min of oxidation, while SLPC decreased gently down to 21.3% after 180 min of oxidation. The kinetics of the loss of the precursors of SLS and SLPC followed the same trend as that of SOS and SOPC. PC species thereby appeared more resistant to thermal oxidation than

This work was supported by “Equipe de Chimie Analytique des Molécules Bioactives (IPHC-LC4, UMR 7178), Université de Strasbourg” and a grant from the Natural Science Foundation of Jiangsu Province (BK20140701). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED GC−MS, gas chromatography−mass spectrometry; HS-SPME, headspace solid-phase microextraction; PC, phosphatidylcholine; PDMS/DVB, polydimethylsiloxane/divinylbenzene; PL, phospholipid; SLPC, 1-stearoyl-2-linoleoyl-sn-glycero-3-phosposphocholine; SOPC, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine; TAG, triglyceride; VOC, volatile oxidized compound; SOS, 1,3-distearoyl-2-oleoyl-glycerol; SLS, 1,3-distearoyl-2-linoleoyl-glycerol; PUFA, polyunsaturated fatty acid; FA, fatty acid

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REFERENCES

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Comparison of the Volatiles Formed by Oxidation of

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