Article pubs.acs.org/JAFC
Mitigation of 3‑Monochloro-1,2-propanediol Ester Formation by Radical Scavengers Hai Zhang,† Pengwei Jin,† Min Zhang,† Ling-Zhi Cheong,‡ Peng Hu,† Yue Zhao,§ Liangli Yu,∥ Yong Wang,† Yuanrong Jiang,*,† and Xuebing Xu† †
Wilmar Biotechnology Research and Development Center (Shanghai) Company, Limited, 118 Gaodong Road, Pudong New District, Shanghai 200137, People’s Republic of China ‡ Department of Food Science, School of Marine Science, Ningbo University, Ningbo, Zhejiang 315211, People’s Republic of China § Institute of Food and Nutraceutical Science, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ∥ Department of Nutrition and Food Science, University of Maryland, College Park, Maryland 20742, United States ABSTRACT: The present study investigated the possible mechanism of free radical scavengers on mitigation of 3-monochloro1,2-propanediol (3-MCPD) fatty acid ester formation in vegetable oils. The electron spin resonance investigation showed that the concentration of free radicals could be clearly decreased in 1,2-distearoyl-sn-glycerol (DSG) samples by all four antioxidants (L-ascorbyl palmitate, α-tocopherol, lipophilic tea polyphenols, and rosemary extract) at 120 °C for 20 min under a N2 atmosphere. Moreover, the rosemary extract exhibited the highest inhibition efficiency. The Fourier transform infrared spectroscopy examination of DSG with α-tocopherol at 25 and 120 °C revealed that α-tocopherol could prevent the involvement of an ester carbonyl group of DSG in forming the cyclic acyloxonium free radical intermediate. Furthermore, the ultraperformance liquid chromatography−quadrupole−time-of-flight mass spectrometry analysis showed that α-tocopherol could suppress the formation of 3-MCPD di- and monoesters. Finally, the four antioxidants could decrease 3-MCPD esters in the palm oil during deodorization. Particularly, the rosemary extract also showed the highest efficiency in 3-MCPD ester mitigation. KEYWORDS: 3-MCPD esters, antioxidants, inhibitor, free radical mechanism, ESR, FTIR
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INTRODUCTION
Many studies performed on 3-MCPD ester suppression involved the optimization of the thermodynamic and kinetic factors of the edible oil refining process, such as temperature, time, chlorine sources, etc.17−25 Thus far, limited work has been performed to investigate the 3-MCPD ester formation mechanism at the molecular level as a method to reduce 3MCPD esters. Recently, Li et al. examined the capabilities of antioxidants to inhibit the formation of 3-MCPD esters in the rapeseed oil model by heating treatment without vacuum at different temperatures or over different heating times.26 However, the mechanism of inhibition of 3-MCPD esters by antioxidants was still necessary to be investigated further. The antioxidants are strong electron donors and can act as free radical scavengers.15,16 In the present study, we proposed that the antioxidants could eliminate the precursors of 3MCPD esters, such as CAFR intermediates and chlorine radicals, probably in the following manner: (1) neutralization of carbon-centered free radical (CCFR) by H• radicals to prevent generation of CAFR intermediates and (2) reaction between the Cl• and H• radicals of the antioxidants to form HCl, which can be easily removed under vacuum conditions (Figure 1B). As a consequence, the antioxidants are expected to be able to neutralize the free radicals at the high-temperature deodor-
3-Monochloro-1,2-propanediol (3-MCPD) esters are a group of process-induced contaminants mainly formed during the deodorization process, which have been reported to have nephrotoxic and carcinogenic effects.1−6 A comprehensive review on 3-MCPD esters, including their formation routes, occurrence in various food, detection method, and toxicity, was recently published.7 There are five proposed mechanisms for 3MCPD ester formation from glyceride.8−12 Two of them involved a chlorine anion directly substituted to either a hydroxyl group or a fatty acid ester group at a glycerol carbon atom.8 Another involved the formation of an epoxide ring along with nucleophilic attack by a chloride anion to open the ring.10 The other postulated the formation of an intermediate acyloxonium cation,11,12 followed by cation ring opening by the nucleophilic attack by a chlorine anion. Recently, our group reported the mechanism involving the formation of a cyclic acyloxonium free radical (CAFR) intermediate, followed by its reaction with a chlorine radical or chlorinated compound.13 CAFR was detected using electron spin resonance (ESR) spectrometry and ultraperformance liquid chromatography− quadrupole−time-of-flight mass spectrometry (UPLC−Q− TOF MS). Further, in our previous work,14 we also found that the CAFR intermediate could be generated from either diacylglycerol (DAG) or triacylglycerol (TAG) (Figure 1A), which could react with chlorine radicals to form 3-MCPD esters and/or glycidyl esters (GEs). © 2016 American Chemical Society
Received: Revised: Accepted: Published: 5887
May 4, 2016 July 8, 2016 July 10, 2016 July 10, 2016 DOI: 10.1021/acs.jafc.6b02016 J. Agric. Food Chem. 2016, 64, 5887−5892
Article
Journal of Agricultural and Food Chemistry
Figure 1. Proposed mechanism for the (A) formation and (B) inhibition of 3-MCPD mono- and diesters and GE from DAG or TAG.
Figure 2. Chemical structures of antioxidants: (1) α-tocopherol, (2) L-ascorbyl palmitate, (3) carnosic acid, (4) rosmarinic acid as constituents of rosemary extract, and (5) (−)-epigallocatechin gallate as a constituent of lipophilic tea polyphenol (here, its palmitates were used).
ization stage of the edible oil refining process. Here, the four antioxidants, including L-ascorbyl palmitate, α-tocopherol, lipophilic tea polyphenols, and rosemary extract, were selected as representatives in the mechanism study (Figure 2), which were approved as food additives and widely investigated in food. The effects of antioxidants and their concentration on suppression of free radical formation were monitored using an ESR spectrometer. 1,2-Distearoyl-sn-glycerol (DSG) was selected as a model compound to induce the formation of free radicals during high-temperature treatment. In addition, the present study also investigated the effects of α-tocopherol
on changes of the ester carbonyl groups of DSG during the formation of the CAFR intermediate by Fourier transform infrared spectroscopy (FTIR) because α-tocopherol does not contain any carbonyl group and can prevent possible interference for the analysis. Finally, the effects of antioxidants on mitigation of 3-MCPD ester formation during deodorization of bleached palm oils were also elucidated.
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MATERIALS AND METHODS
Materials. The degummed, neutralized, and bleached palm oil was obtained from Yihai Kerry (Shanghai, China). n-Pentadecane was 5888
DOI: 10.1021/acs.jafc.6b02016 J. Agric. Food Chem. 2016, 64, 5887−5892
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Figure 3. ESR spectra of DSG heated at 120 °C for 20 min with 15% (w/w) (A−E) no antioxidant, L-ascorbyl palmitate, α-tocopherol, lipophilic tea polyphenols, and rosemary extract, respectively. (F) ESR spectrum of DSG with 10% (w/w) rosemary extract at 25 °C. ESR spectra of DSG heated at 120 °C for 20 min with (G−J) 10, 15, and 20% (w/w) rosemary extract, respectively. DMPO was added before ESR measurement. 4.1 software. All peak areas were determined using the total ion counts in the extracted ion chromatogram. Monitoring of the Formation of 3-MCPD Esters during the Deodorization of Bleached Palm Oil with Antioxidants. Bleached palm oils (200 mL) with added L-ascorbyl palmitate, αtocopherol, lipophilic tea polyphenols, and rosemary extract (6%, w/ w) were deodorized at 240 °C for 60 min under vacuum (0.5 kPa) with nitrogen bubbling into oil. The formation of 3-MCPD was monitored using ISQ gas chromatography−mass spectrometry (GC− MS, Thermo Fisher, Waltham, MA) according to American Oil Chemists’ Society (AOCS) Official Method Cd 29c-13.27 The limit of detection (LOD) was 0.025 mg/kg, and the limit of quantitation (LOQ) was 0.10 mg/kg.
purchased from Aladdin Reagent (Shanghai, China). DSG and 5,5dimethylpyrroline-N-oxide (DMPO) were purchased from SigmaAldrich (St. Louis, MO). α-Tocopherol was obtained from DSM (Shanghai, China). Rosemary extract was obtained from Kalsec (Shanghai, China). NaCl, L-ascorbyl palmitate, and lipophilic tea polyphenols, namely, a mixture of (−)-epigallocatechin gallate palmitates, were purchased from Sinopharm Chemical Reagent (Shanghai, China). 2-Propanol and methanol [liquid chromatography−mass spectrometry (LC−MS) grade] were obtained from Merck (Darmstadt, Germany). Monitoring of the Formation of Free Radicals in DSG Systems with or without the Addition of Antioxidants Using ESR. The ability of antioxidants to scavenge free radicals was monitored using ESR. A total of 0.1 mL of DSG (5 mg/mL in npentadecane) with a 0.075 mL solution of each antioxidant (1 mg/mL in n-pentadecane), including L-ascorbyl palmitate, α-tocopherol, lipophilic tea polyphenols, and rosemary extract, were added to nuclear magnetic resonance (NMR) tubes (5 mm diameter, Wilmad, Vineland, NJ) under a N2 atmosphere. The control experiment was performed without the addition of antioxidant. The freshly prepared radical scavenger, 25 μL of DMPO (5 mg/mL in n-pentadecane), was added to the mixture before the heating experiment to prevent selfdecomposition of DMPO into free radicals at a high temperature. The NMR tubes were connected to a vacuum gas manifold using latex tubing (Schlenk system) to create water- and oxygen-free conditions. The mixture was heated to 120 °C using an electric sand bath and held for 20 min. Free radical formation during the heating experiment was monitored using an EMX-8/2.7C ESR spectrometer (Bruker Optics, Karlsruhe, Germany). Monitoring of the Changes in Ester Carbonyl Groups of DSG Using FTIR. The changes in ester carbonyl groups of DSG was monitored using a Nicolet 6700 FTIR spectrometer (Thermo Fisher, Waltham, MA) equipped with a LinkPad temperature-programming accessory (Linkam Scientific, Surrey, U.K.) by a classic transmission method. A mixture of α-tocopherol (10 μL, 1 mg/mL in n-hexane solution) and DSG (10 μL, 5 mg/mL in n-hexane solution) were thoroughly mixed and heated by bulb light to remove n-hexane before KBr was added to prepare a thin-film sample for FTIR measurements. Monitoring of the Formation of 3-MCPD Di- and Monoesters Using UPLC−Q−TOF MS. To evaluate the inhibition effect of α-tocopherol on 3-MCPD ester formation, 2 mL of DSG (0.5 mg/mL in n-pentadecane) was reacted with 2 mg of NaCl (as the chlorine source) at 180 °C in the presence of 50 μL of α-tocopherol (0.5 mg/ mL in n-pentadecane). The control experiment was conducted without α-tocopherol. The reaction mixture was stirred for 2 h and cooled to room temperature. A total of 20 μL of the reaction mixture was diluted 50-fold in methanol/isopropanol (1:1, v:v) and then centrifuged at 10 000 rpm for 10 min. The formation of 3-MCPD esters, including the di and monoesters, were analyzed by a Xevo G2 UPLC−Q−TOF mass spectrometer (Waters, Milford, MA) in electrospray ionization (ESI)-positive mode. Analysis was conducted according to a previously reported method.13 The data analysis was carried out with MassLynx
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RESULTS AND DISCUSSION Monitoring of Free Radical Formation in DSG Systems with or without the Addition of Antioxidants Using ESR. Spectra A−E in Figure 3 show the ESR spectra of DSG heated at 120 °C for 20 min without antioxidant and with L-ascorbyl palmitate, α-tocopherol, lipophilic tea polyphenols, and rosemary extract, respectively. In the control experiment without added antioxidant (spectrum A in Figure 3), heat treatment resulted in a significant increase in ESR signals. In the systems with added antioxidants (spectra B−E in Figures 3), heat treatment led to the decrease in ESR signals, which was related to the ability of antioxidants to scavenge the free radicals. In particular, the rosemary extract exhibited the highest free radical scavenging activity (spectrum E in Figure 3). The dosage of antioxidant also played an important role in the suppression of free radical formation (spectra G−J in Figures 3). The higher the dosage of antioxidants, the better the suppression efficiency became. In contract, DSG with rosemary extract at room temperature led to no signal change (spectrum F in Figure 3). These findings indicated that the antioxidants might be able to inhibit the formation of 3-MCPD esters by preventing the formation of the CAFR intermediate at a high temperature. Monitoring of the Changes in Ester Carbonyl Groups of DSG Using FTIR. Two different ester carbonyl absorption peaks centered at 1735 and 1717 cm−1 could be observed in the FTIR spectra of DSG at 25 °C (Figure 4A). It could be assumed that the ester carbonyl group at the C-1 position might absorb at 1735 cm−1 and the ester group at the C-2 position might absorb at 1717 cm−1.9 When DSG was heated to 120 °C, only a single absorption peak centered at 1746 cm−1 was observed. This hypsochromic shift of 11 cm−1 indicated the possible formation of the CAFR intermediates involving the ester carbonyl groups of DSG.13 Figure 4B shows the FTIR spectra of DSG with α-tocopherol at 25 and 120 °C. Similar to 5889
DOI: 10.1021/acs.jafc.6b02016 J. Agric. Food Chem. 2016, 64, 5887−5892
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α-tocopherol could suppress the formation of 3-MCPD diesters at a high temperature. Similarly, a strong peak of [3-MCPD monoester + Na]+ (m/ z 399.2642) was observed at 4.05 min in the control sample, which became hardly detectable when α-tocopherol was added (spectra C and D in Figure 5). It was indicated that αtocopherol could also inhibit the formation of 3-MCPD monoesters at a high temperature. In both cases (spectrum A versus spectrum C and spectrum B versus spectrum D in Figure 5), the level of 3-MCPD diester was much lower than that of 3MCPD monoester, regardless of the addition of α-tocopherol. This was probably because DAG was used rather than TAG and 3-MCPD di- and monoesters may have different LC−MS responses, which was similar to the findings of Zhang et al.14 Monitoring of the Formation of 3-MCPD Esters during the Deodorization of Bleached Palm Oil with Antioxidants. All of the ESR, FTIR, and UPLC−Q−TOF MS data showed that antioxidants have a positive effect on the inhibition of 3-MCPD ester formation. Subsequently, to assess the practical application of various antioxidants, the four antioxidants, including L-ascorbyl palmitate, α-tocopherol, lipophilic tea polyphenols, and rosemary extract, were selected to evaluate the inhibition efficiency of 3-MCPD ester formation at high deodorization temperatures (Figure 6). 3-MCPD esters
Figure 4. FTIR spectra of (A) DSG and (B) DSG with α-tocopherol (blue lines for α-tocopherol alone) at 25 and 120 °C, respectively.
the FTIR spectra of the single DSG at 25 °C, there were also two ester carbonyl absorption peaks centered at 1734 and 1716 cm−1. However, it is important to note that the FTIR spectra of DSG with added α-tocopherol at 120 °C still presented two absorption peaks with a hypsochromic shift to 1740 and 1723 cm−1, respectively. Because α-tocopherol does not possess any carbonyl groups, it has little effect on the characteristic of the absorption peaks in the region within 1800−1700 cm−1 at 25 or 120 °C (Figure 4B). Therefore, it was demonstrated that αtocopherol could inhibit the formation of CAFR intermediates. In agreement with the proposed inhibition mechanism of 3MCPD esters, both the FTIR and ESR data showed that the antioxidants could suppress the formation of CAFR intermediates likely as a result of the free radical scavenging ability. Monitoring of the Formation of 3-MCPD Di- and Monoesters Using UPLC−Q−TOF MS. To evaluate the αtocopherol inhibition effect on the formation of 3-MCPD esters, α-tocopherol was added to the reaction system, including DSG and NaCl, as the chlorine source at 180 °C, while the control experiment was conducted without αtocopherol. It has been proven that NaCl could lead to the formation of 3-MCPD esters at high temperatures.14 3-MCPD esters, including di- and monoesters, were analyzed by the UPLC−Q−TOF MS method. A strong peak of [3-MCPD diester + Na]+ (m/z 665.5252) at 8.67 min can be observed in the control sample (spectra A and B in Figure 5). The peak intensity of the [3-MCPD diester + Na] + decreased significantly in the system with α-tocopherol, indicating that
Figure 6. 3-MCPD mitigation effect of various antioxidants (6%, w/w) in the deodorization of bleached palm oil process with (A) no antioxidant, (B) α-tocopherol, (C) rosemary extract, (D) lipophilic tea polyphenols, and (E) L-ascorbyl palmitate.
are formed in higher abundance in refined palm oil;28 thus, the palm oil was selected as the target for the application experiments. All of the deodorized samples with added
Figure 5. 3-MCPD (A and B) diester and (C and D) monoester determined with UPLC−Q−TOF MS extracted ion chromatograms in DSG systems with/without α-tocopherol, respectively. 5890
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(5) Buhrke, T.; Weißhaar, R.; Lampen, A. Absorption and metabolism of the food contaminant 3-chloro-1,2-propanediol (3MCPD) and its fatty acid esters by human intestinal Caco-2 cells. Arch. Toxicol. 2011, 85, 1201−1208. (6) El Ramy, R.; Ould Elhkim, M.; Lezmi, S.; Poul, J. M. Evaluation of the genotoxic potential of 3-monochloropropane-1,2-diol (3MCPD) and its metabolites, glycidol and β-chlorolactic acid, using the single cell gel/comet assay. Food Chem. Toxicol. 2007, 45, 41−48. (7) Jedrkiewicz, R.; Kupska, M.; Glowacz, A.; Gromadzka, J.; Namiesnik, J. 3-MCPD: A worldwide problem of food chemistry. Crit. Rev. Food Sci. Nutr. 2015, 00. (8) Rahn, A. K. K.; Yaylayan, V. A. What do we know about the molecular mechanism of 3-MCPD ester formation? Eur. J. Lipid Sci. Technol. 2011, 113, 323−329. (9) Rahn, A. K. K.; Yaylayan, V. A. Monitoring cyclic acyloxonium ion formation in palmitin systems using infrared spectroscopy and isotope labelling technique. Eur. J. Lipid Sci. Technol. 2011, 113, 330− 334. (10) Sonnet, P. E. A short highly regio- and stereoselective synthesis of triacylglycerols. Chem. Phys. Lipids 1991, 58, 35−39. (11) Collier, P. D.; Cromie, D. D. O.; Davies, A. P. Mechanism of formation of chloropropanols present in protein hydrolysates. J. Am. Oil Chem. Soc. 1991, 68, 785−790. (12) Velíšek, J.; Doležal, M.; Crews, C.; Dvořaǩ , T. Optical isomers of chloropropanediols: Mechanisms of their formation and decomposition in protein hydrolysates. Czech J. Food Sci. 2002, 20, 161−170. (13) Zhang, X.; Gao, B.; Qin, F.; Shi, H.; Jiang, Y.; Xu, X.; Yu, L. L. Free radical mediated formation of 3-monochloropropanediol (3MCPD) fatty acid diesters. J. Agric. Food Chem. 2013, 61, 2548−2555. (14) Zhang, Z.; Gao, B.; Zhang, X.; Jiang, Y.; Xu, X.; Yu, L. L. Formation of 3-monochloro-1,2-propanediol (3-MCPD) di- and monoesters from tristearoylglycerol (TSG) and the potential catalytic effect of Fe2+ and Fe3+. J. Agric. Food Chem. 2015, 63, 1839−1848. (15) Shahidi, F.; Zhong, Y. Measurement of antioxidant activity. J. Funct. Foods 2015, 18, 757−781. (16) Molyneux, R. J.; Mahoney, N.; Kim, J. H.; Campbell, B. C.; Hagerman, A. E. Antioxidant constituents in tree nuts: Health implications and aflatoxin inhibition. In Functional Food and Health; Shibamoto, T., Kanazawa, K., Shahidi, F., Ho, C.-T., Eds.; American Chemical Society (ACS): Washington, D.C., 2008; ACS Symposium Series, Vol. 993, Chapter 16, pp 181−191, DOI: 10.1021/bk-20080993.ch016. (17) Ermacora, A.; Hrncirik, K. Study on the thermal degradation of 3-MCPD esters in model systems simulating deodorization of vegetable oils. Food Chem. 2014, 150, 158−163. (18) Ermacora, A.; Hrncirik, K. Influence of oil composition on the formation of fatty acid esters of 2-chloropropane-1,3-diol (2-MCPD) and 3-chloropropane-1,2-diol (3-MCPD) under conditions simulating oil refining. Food Chem. 2014, 161, 383−389. (19) Shimizu, M.; Weitkamp, P.; Vosmann, K.; Matthäus, B. Influence of chloride and glycidyl-ester on the generation of 3MCPD- and glycidyl-esters. Eur. J. Lipid Sci. Technol. 2013, 115, 735− 739. (20) Shimizu, M.; Weitkamp, P.; Vosmann, K.; Matthäus, B. Temperature dependency when generating glycidyl and 3-MCPD esters from diolein. J. Am. Oil Chem. Soc. 2013, 90, 1449−1454. (21) Hamlet, C. G.; Sadd, P. A. Kinetics of 3-chloropropane-1,2-diol (3-MCPD) degradation in high temperature model systems. Eur. Food Res. Technol. 2002, 215, 46−50. (22) Zulkurnain, M.; Lai, O. M.; Tan, S. C.; Abdul Latip, R.; Tan, C. P. Optimization of palm oil physical refining process for reduction of 3-monochloropropane-1,2-diol (3-MCPD) ester formation. J. Agric. Food Chem. 2013, 61, 3341−3349. (23) Freudenstein, A.; Weking, J.; Matthäus, B. Influence of precursors on the formation of 3-MCPD and glycidyl esters in a model oil under simulated deodorization conditions. Eur. J. Lipid Sci. Technol. 2013, 115, 286−294. (24) Matthäus, B. Organic or not organic - that is the question: How the knowledge about the origin of chlorinated compounds can help to
antioxidants showed lower 3-MCPD esters (Figure 6). In comparison to the control sample, the rosemary extract presented the highest inhibition efficiency, which reduced 3MCPD esters by 82.4%, followed by the lipophilic tea polyphenols, L-ascorbyl palmitate, and α-tocopherol. The generation of glycidyl esters share the similar mechanism and variation trend with 3-MCPD (data not shown). The differences in the inhibition efficiency of the four antioxidants were very likely associated with the release activities of their H• free radicals, which could serve as the scavengers for CAFR and Cl•. Molecular structures of antioxidants had been previously reported to affect their radical scavenging properties.29,30 It is worthwhile to mention that there was no obvious taste difference when the antioxidants were used, which may be cost-effective after the process optimization in industry. For the future work, we will try to determine the relationship between the antioxidative strength and the inhibition effect on the formation of 3-MCPD esters, optimize the process, and evaluate the quality influence upon refined fats and oils. In summary, the present study at the molecular level demonstrated that the antioxidants could suppress the formation of the CAFR intermediate, which resulted in mitigation of 3-MCPD ester formation at the high deodorization temperature. The addition of antioxidants may be a potential alternative method in practical application.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Funding
This research was supported by an Arawana Nutrition and Safety Research Grant and also partially supported by a Special Fund for Agro-scientific Research in the Public Interest (201203069) and Ningbo University (421600240). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Zhongfei Zhang from Shanghai Jiao Tong University for his UPLC−Q-TOF assistance. ABBREVIATIONS USED 3-MCPD, 3-monochloro-1,2-propanediol; CCFR, carbon-centered free radical; CAFR, cyclic acyloxonium free radical; DAG, diacylglycerol; TAG, triacylglycerol; DSG, 1,2-distearoyl-snglycerol; DMPO, 5,5-dimethylpyrroline-N-oxide; GE, glycidyl ester
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REFERENCES
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