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Effects of Caffeic Acid Phenethyl Ester and 4‑Vinylcatechol on the Stabilities of Oil-in-Water Emulsions of Stripped Soybean Oil Cai-Hua Jia, Jung-Ah Shin, and Ki-Teak Lee* Department of Food Science and Technology, Chungnam National University, Daejeon 305-764, Republic of Korea ABSTRACT: Caffeic acid phenethyl ester (CAPE) and 4-vinylcatechol (4-VC) were prepared for studying their antioxidative activities in emulsion. Oil-in-water emulsions of stripped soybean oil containing 200 ppm of CAPE, 4-VC, or α-tocopherol were stored at 40 °C in the dark for 50 days, and proton nuclear magnetic resonance (1H NMR) was used to identify and quantify the oxidation products. Emulsion droplet sizes, peroxide values, and levels of primary oxidation products (i.e., hydroperoxides) and secondary oxidation products (i.e., aldehydes) were determined. The results showed that CAPE (200 ppm) and 4-VC (200 ppm) had significantly greater antioxidant activities on the oxidation of stripped soybean oil-in-water emulsions than α-tocopherol (200 ppm). The peroxide values of CAPE (8.4 mequiv/L emulsion) and 4-VC (15.0 mequiv/L emulsion) were significantly lower than that of α-tocopherol (33.4 mequiv/L emulsion) (p < 0.05) on 36 days. In addition, the combinations of CAPE + α-tocopherol (100 + 100 ppm) or 4-VC + α-tocopherol (100 + 100 ppm) had better antioxidant activities than α-tocopherol (200 ppm). For CAPE + α-tocopherol, 4-VC + α-tocopherol, and α-tocopherol, the amounts of conjugated diene forms were 16.67, 13.72, and 16.32 mmol/L emulsion, and the concentrations of aldehydes were 2.15, 1.13, and 4.26 mmol/L emulsion, respectively, after 50 days of storage. KEYWORDS: 1H NMR, caffeic acid phenethyl ester, 4-vinylcatechol, emulsion, antioxidant

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INTRODUCTION Vegetable oils are commonly consumed as food emulsions, which are either water-in-oil (W/O) emulsions (such as butter, etc.) or oil-in-water (O/W) emulsions (such as milk, etc.). However, the susceptibility to oxidation of lipids containing a high amount of polyunsaturated fatty acids limits their applications for many food and cosmetic products because lipid oxidation can lead to the loss of quality and possibly the generation of toxicants.1 One of the effective ways of retarding lipid oxidation is to add antioxidants and many compounds, both natural and artificial, known to inhibit the progress of oxidation. However, consumers are demanding the use of more natural antioxidants, and thus, a number of studies have been conducted to identify natural antioxidants.2−4 Antioxidants retard lipid oxidation via different mechanisms, such as by inactivating free radicals, chelating metals, or scavenging oxygen.1 Furthermore, antioxidant polarity and solubility determine its physical location in emulsion systems (i.e., oil or water phase or the interface between oil and water), which, in turn, influences antioxidant efficacy.5 Caffeic acid, one of the phenolic compounds, is receiving more interest because it is found in nature and has potent antioxidant activities and health benefits.6−8 However, the polar carboxyl group of caffeic acid limits its solubility in multiphase systems and adversely influences its antioxidant activity. To reduce the negative effects of the carboxyl group, many studies have been conducted on the decarboxylation and phenethyl esterification of caffeic acid. Terpinc et al. compared the antioxidant potential of 4-vinyl derivatives from hydroxycinnamic acids to their corresponding phenolic acids. In the linoleic acid emulsion system, 4-vinyl derivatives showed high antioxidant activity, among which 4-vinylguaiacol was the most active compound.9 Chen et al. studied the inhibitory effect of caffeic © XXXX American Chemical Society

acid phenethyl ester (CAPE) on the oxidation of a corn O/W emulsion at 60 °C, revealing the decreasing order of BHT > caffeic acid > CAPE > rosmarinic acid > ferulic acid > chlorogenic acid > α-tocopherol > ferulic acid phenethyl ester.2 According to Wu et al., the numbers of hydroxyl groups or catechol moieties in the molecule were positively related to the radical scavenging activity, while the antioxidant ability in the biomembrane systems depended upon both the numbers of hydroxyl groups or catechol rings and their polarity.10 Because the decarboxylation and phenethyl esterification products of caffeic acid show the antioxidant activities under different conditions,2,9,10 systematic assessment of the antioxidant activity under the same oxidation condition is needed for comparative effectiveness. Furthermore, the measurement of peroxide and aldehydes can further elucidate the antioxidant efficiency of compounds in a complex emulsion system because free radical scavenging measurement is insufficient as an index of antioxidant efficiency.11 The objective of this study was to investigate the effects of CAPE and 4-vinylcatechol (4-VC) on the emulsion oxidation, during which the stability of O/W emulsions was observed. The particle size is an important parameter for evaluating emulsion stability, and thus, diameters of the oil droplet were monitored. Meanwhile, proton nuclear magnetic resonance (1H NMR) was used to identify the synthesized compounds and to monitor the amounts of hydroperoxides, conjugated forms, and aldehydes through oxidation periods along with peroxide value (PV) measurement. Received: May 21, 2015 Revised: October 19, 2015 Accepted: October 22, 2015

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DOI: 10.1021/acs.jafc.5b02423 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Article

Journal of Agricultural and Food Chemistry

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Microfluidics, Newton, MA) prepared final emulsions through two passed at 3000 psi. Additionally, sodium azide (0.02 wt %) was added to inhibit the growth of the microbial. Droplet Size Measurement. A laser particle size analyzer (Mastersizer S, Malvern Instrument, Worcestershire, U.K.) was used to determine the droplet sizes of O/W emulsions. The particle size measurements were recorded as the volume-surface average emulsion diameter d32 [d32 (μm) = ∑di3ni/∑di2ni], where ni was the number of particles with diameter di.15,16 The droplet sizes of samples were determined on 0, 19, 42, and 53 days following emulsion formation. Each sample was analyzed twice, and the data were presented as the average and standard deviation (non-flocculation, within 5%; flocculation, within 20%).17 Oxidation of Emulsions. All tested antioxidants were predissolved in methanol to prepare the stock solution. To evaluate the effect of different antioxidants on emulsion oxidation stability, 20 mL aliquots of each emulsion were mixed with different antioxidants at 200 ppm and compared to the blank control (no antioxidants). The prepared emulsions were kept in 25 mL vials with caps and oxidized in an oven at 40 °C in the dark for 50 days. The PV was determined by means of a previous study18 on the designated days (0, 5, 10, 15, 22, 36, 42, and 50 days). Oil was extracted from the O/W emulsions by adding chloroform (1:1, v/v), vortexing for 1 min, and centrifuging for 5 min at 1500 rpm. The extraction time was 3 for each sample. After collection of the down layer and removal of the solvent by nitrogen, the extracted oil was used to determine the oxidation process. Quantification of oxidation compounds was performed by normalization of the peak area of sn-2 hydrogen of glycerol for the concentration of millimoles per mole of oil.19 The molar mass and weight percentage of soybean oil were considered for calculating the amounts of oxidation compounds as millimoles per liter emulsion. The hydroperoxide (primary oxidation product) and aldehyde (secondary oxidation product) levels in the oil were simultaneously determined by 1H NMR,20 which was carried out in a 600 MHz Bruker Avance III 600 spectrometer (Bruker BioSpin Corporation, Billerica, MA). After 50−100 mg of extracted oil was dissolved in 600 μL of CDCl3, the mixture was placed in a NMR tube for measurement. The parameters contained 12 335.5 Hz of spectral width, 16 scans, 2.656 s of acquisition time, and 25 °C of analysis temperature. Chemical shifts were recorded as parts per million based on the reference of TMS in the ACD Laboratories NMR processor (version 10.0). Statistical Analysis. PV and droplet size were given as the mean ± standard deviation (SD) of duplicates. Statistical analyses were conducted with the SAS software package (SAS Institute, Cary, NC). Analyses of variance were performed by ANOVA, and a probability of less than 0.05 was reported as statistical significance.

MATERIALS AND METHODS

Materials. Soybean oil was obtained from a local market (Daejeon, South Korea). Caffeic acid, dicyclohexyl carbodiimide (DCC), deuterated chloroform [99.9 atom % D, containing 0.1%, v/v, tetramethylsilane (TMS)], phenethyl alcohol, tetrahydrofuran (THF), and Supelclean LC-Si solid-phase extraction (SPE) tubes (6 mL) were from Sigma-Aldrich (St. Louis, MO). N,N-Dimethylformamide (DMF), silica gel 60 (0.0630−0.200 mm), anhydrous sodium sulfate, and sodium acetate trihydrate were provided by Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), Merck KGaA (Darmstadt, Germany), Junsei Chemical Co., Ltd. (Tokyo, Japan), and DC Chemical Co., Ltd. (Seoul, South Korea), respectively. All other reagents were purchased from Daejung Chemicals & Metals Co., Ltd. (Shiheung, South Korea). Production of Stripped Soybean Oil. Stripped soybean oil was prepared by column chromatography.12 A lower layer of 20 g of anhydrous sodium sulfate, a middle layer of 6 g of activated charcoal, and a top layer of 20 g of silica gel were used to remove trace water, tocopherols, diacylglycerol, monoacylglycerol, and free fatty acid, respectively. The mixture (20 g of soybean oil diluted with 30 mL of n-hexane) was loaded on the top of the column and eluted with 300 mL of n-hexane later. After the vacuum rotary unit (RE 111, Büchi, Flawil, Swizerland) evaporated the solvent of the eluent at 40 °C, nitrogen was used to delete a trace amount of n-hexane. The amount of total tocopherols was measured according to Lee et al.13 Synthesis of 4-VC. With minor modification, 4-VC was synthesized according to Terpinc et al.9 Caffeic acid (100 mg) was dissolved in 4 mL of DMF. Sodium acetate (20 mg) as a catalyst was added to the decarboxylation reaction at 110−120 °C in the oil bath for 15 min. After cooling to room temperature, 4-VC was extracted with n-hexane (8 mL each time) and saturated NaCl solution (40 mL each time) several times. The solvent was eliminated by flushing nitrogen, and the product (4-VC) was stored at −20 °C for further experiments. Preparation of CAPE. CAPE was synthesized by means of a previous study.14 A total of 54 mg of caffeic acid and 180 mg of phenethyl alcohol were dissolved in 1.2 mL of THF. After 108 mg of dicyclohexylcarbodiimide as a catalyst was added, the esterification reaction was conducted at room temperature for 8 h, followed by the addition of 0.54 μL of distilled water to terminate the reaction. Ethyl acetate and petroleum ether (1:5, v/v) eluted the brown sticky solution through LC-Si SPE. Finally, the light yellow solid from fractions was recrystallized with 0.5 mL of mixture (1:5, v/v, ethyl acetate/hexane) to obtain the white pure CAPE, which was kept in −20 °C for oxidation evaluation. The two synthesized compounds were identified by 1H NMR (Figure 1). Preparation of O/W Emulsion. O/W emulsions were prepared in 20 mM bis[2-hydroxyethyl]amino-tris-[hydroxymethyl]methane (bis-tris) buffer solution (89.7 wt %, pH 7.0) with the obtained stripped soybean oil (10 wt %) and Tween 20 (0.3 wt %) as an emulsifier. A Silverson homogenizer (model L4RT, Silverson Machines, U.K.) was used to homogenize the mixture at the speed of 5000 rpm for 2 min, and a Microfluidics processor (M-110Y,

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RESULTS AND DISCUSSION 4-VC and CAPE Syntheses and Emulsion Droplet Sizes. The 1H NMR spectroscopic data of synthesized 4-VC

Figure 1. Molecular structures of caffeic acid, CAPE, and 4-VC. B


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Abbreviations: s, singlet; d, doublet; t, triplet; m, multiplet; and dd, doublet of doublets. The results of 1H NMR agreed with refs 9, 14, and 21.

δH (600 MHz, CDCl3): 3.02 (2H, t, J = 7.1 Hz, 11-H), 4.42 (2H, t, J = 7.1 Hz, 10-H), 6.26 (1H, d, J = 16.0 Hz, 8-H), 6.88 (1H, d, J = 8.2 Hz, 5-H), 7.01 (1H, dd, J = 2.0 and 8.2 Hz, 6-H), 7.09 (1H, d, J = 2.0 Hz, 2-H), 7.25−7.34 (5H, m, 13−17-H, Ar−H), 7.57 (1H, d, J = 16.0 Hz, 7-H) CAPE

and CAPE are presented in Table 1, and they were consistent with previous studies.9,14,21 Emulsion droplet sizes of stripped soybean oil containing different antioxidants were evaluated to monitor physical stability for 53 days (Figure 2). The initial mean particle diameter (d32) of

Figure 2. Influence of the storage time on the mean particle size (d32, μm) of emulsions containing different antioxidants [4-VC (200 ppm), CAPE (200 ppm), α-tocopherol (200 ppm), CAPE + α-tocopherol (100 + 100 ppm), 4-VC + α-tocopherol (100 + 100 ppm), and the control] during oxidation at 40 °C in the dark. Data points represented mean ± SD.

the stripped soybean oil O/W emulsions was 0.34 ± 0.01 μm. Droplet sizes did not change significantly in any emulsions during storage (40 °C for 53 days). On day 19, particle sizes of emulsions containing 4-VC, CAPE, α-tocopherol, CAPE + α-tocopherol, or 4-VC + α-tocopherol and the control were 0.37 ± 0.01, 0.41 ± 0.00, 0.38 ± 0.03, 0.39 ± 0.01, 0.40 ± 0.02, and 0.39 ± 0.05 μm, respectively. Similarly, relatively constant droplet sizes were observed until 42 days. Furthermore, the particle sizes (d32) of antioxidant-treated emulsions did not differ from the control emulsion on storage day 53 when particle sizes ranged from 0.36 to 0.45 μm, which indicated that the emulsions were stable. Furthermore, all samples remained visibly stable throughout the oxidation period, showing no creaming. Primary Oxidation. In practical food systems, lipids are usually present in emulsified forms. Lipid oxidation in O/W emulsions is a disadvantage from the standpoint of food safety and consumer health.1,22 Thus, in this study, stripped soybean oil emulsion systems were prepared to compare the antioxidant ability of different caffeic acid derivatives to α-tocopherol. It should be noted that the main antioxidants (i.e., tocopherols) in commercial soybean oil (443.0 ppm) were removed before to allow for accurate evaluation of the antioxidant activities of CAPE and 4-VC. The total tocopherol content of the stripped soybean oil was 8.9 ppm. Because hydroperoxides are the primary products of lipid oxidation, PV is generally used to investigate the initial stages of oxidation.23 Figure 3 shows PV development of stripped soybean O/W emulsion containing caffeic acid derivatives or commercial antioxidant (α-tocopherol) stored at 40 °C in the dark. During 50 days of storage, samples containing 4-VC (200 ppm) or CAPE (200 ppm) had significantly (p < 0.05) lower PV values than the control. The treatments containing CAPE or 4-VC could obviously prolong the lag phase of hydroperoxide formation through comparison to emulsion containing 200 ppm of α-tocopherol and the control (no antioxidants). On day 10, the PV of the control and emulsions containing 200 ppm of α-tocopherol (13 and 8 mequiv/L

a

δH (600 MHz, DMSO-d6): 4.94 (1H, dd, J = 1.0 and 10.7 Hz, CH2), 5.41 (1H, dd, J = 1.0 and 17.5 Hz, CH2), 6.45 (1H, dd, J = 10.7 and 17.5 Hz, −CH), 6.63 (1H, dd, J = 2.0 and 8.0 Hz, Ar−H), 6.67 (1H, d, J = 8.0 Hz, Ar−H), 6.83 (1H, d, J = 2.0 Hz, Ar−H), 8.26 (1H, s, −OH), 8.36 (1H, s, −OH) 4-VC

compound

Table 1. 1H NMR Spectroscopic Results of 4-VC and CAPE

spectroscopic dataa


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(E,E) conjugated dienic systems, which could change to secondary oxidation compounds (aldehydes) by further lipoperoxidation. New signals were observed by 1H NMR between 5.45 and 6.60 ppm, and the assignments of the protons responsible are shown in Table 2. They are consistent with previously presented data.20,25−30 The four protons of the (Z,E) conjugated dienic system were centered at 5.55, 6.55, 6.00, and 5.50 ppm, whereas the four multiplets of the (E,E) conjugated dienic system were located near 5.75, 6.25, 6.05, and 5.45 ppm.27 The second proton of the (Z,E) conjugated dienic system (6.55 ppm) and the second proton of the (E,E) conjugated dienic system (6.25 ppm), which were well-defined, were used to obtain the amount of conjugated forms. It was observed that total amounts of the conjugated forms in freshly prepared emulsions were less than 0.71 mmol/L emulsion at the beginning day (0 day). After prolonged storage (15, 36, and 50 days), the amounts of hydroperoxy-(Z,E) and hydroperoxy(E,E) increased (Table 3). On day 15, total amounts of the conjugated forms of the control, α-tocopherol, 4-VC, CAPE + α-tocopherol, 4-VC + α-tocopherol, and CAPE were 9.12, 2.06, 1.07, 0.82, 0.78, and trace mmol/L emulsion. Similarly, the molar concentrations of (Z,E) and (E,E) increased gradually with further oxidation. On day 36, they were 12.97, 7.43, 2.86, 1.77, 2.10, and 1.01 mmol/L emulsion for the control, α-tocopherol (200 ppm), 4-VC (200 ppm), CAPE + α-tocopherol (100 + 100 ppm), 4-VC + α-tocopherol (100 + 100 ppm), and CAPE (200 ppm), respectively, while on the last day (i.e., day 50), they were 15.29, 16.32, 10.07, 16.67, 13.72, and 1.67 mmol/L emulsion, respectively. Furthermore, the magnitudes of the inhibitory effects of the different antioxidants obtained from the hydroperoxy-(Z,E) and hydroperoxy-(E,E) conjugated forms as determined by 1H NMR were in agreement with PV results. In general, the formation of hydroperoxides increased in all treatments, but the rate of formation was particularly slower for CAPE (200 ppm), followed in decreasing order by 4-VC (200 ppm), 4-VC + α-tocopherol (100 ppm each), or CAPE + α-tocopherol (100 ppm each). Secondary Oxidation. The hydroperoxides in oxidized oil are transient compounds that are easily changed to secondary oxidation products.31 In the present study, signals corresponding to secondary oxidation products (such as aldehydes) were also noticed in the spectral region of 9.45−9.80 ppm (Table 2). According to Falch et al.,32 the detection limits of aldehydes measured by 1H NMR of 600 MHz were found to be 4-VC + α-tocopherol (each 100 ppm) or CAPE + α-tocopherol (each 100 ppm). Moreover, the results of conjugated forms and aldehydes with PV illustrated the relatively low synergism between α-tocopherol and CAPE or 4-VC. The present study showed the abilities of CAPE and 4-VC to retard oxidation in stripped soybean O/W emulsion at 40 °C in the dark. Lipid oxidation occurs easier in O/W emulsions as a E


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Table 4. Concentrations of Aldehydes Generated in Emulsions with Different Antioxidants [4-VC (200 ppm), CAPE (200 ppm), α-Tocopherol (200 ppm), CAPE + α-Tocopherol (100 + 100 ppm), 4-VC + α-Tocopherol (100 + 100 ppm), and the Control] during Oxidation at 40 °C in the Dark (in Units of mmol/L Emulsion) 15 days

36 days

50 days

n-alkanals 4-hydroperoxy-(E)-2-alkenals + 4-hydroxy-(E)-2-alkenals + 4,5-epoxy-(E)-2-alkenals (E,E)-2,4-alkadienals (E)-2-alkenals total aldehydes n-alkanals 4-hydroperoxy-(E)-2-alkenals + 4-hydroxy-(E)-2-alkenals + 4,5-epoxy-(E)-2-alkenals (E,E)-2,4-alkadienals (E)-2-alkenals total aldehydes n-alkanals 4-hydroperoxy-(E)-2-alkenals + 4-hydroxy-(E)-2-alkenals + 4,5-epoxy-(E)-2-alkenals (E,E)-2,4-alkadienals (E)-2-alkenals total aldehydes

4-VC

CAPE

α-tocopherol

CAPE + α-tocopherol

4-VC + α-tocopherol

control

0.00 0.00

0.00 0.00

0.18 0.00

0.00 0.00

0.00 0.00

0.98 0.50

0.00 0.00 0.00 0.19 0.00

0.00 0.00 0.00 0.07 0.00

0.00 0.06 0.24 1.24 0.52

0.00 0.00 0.00 0.09 0.00

0.00 0.00 0.00 0.08 0.00

0.18 0.45 2.11 1.15 1.61

0.00 0.00 0.19 0.43 0.26

0.00 0.00 0.07 0.10 0.00

0.15 0.39 2.30 1.39 1.67

0.00 0.00 0.09 0.56 0.95

0.00 0.00 0.08 0.38 0.44

0.51 0.86 4.13 1.44 2.48

0.03 0.12 0.84

0.00 0.04 0.14

0.26 0.94 4.26

0.16 0.48 2.15

0.10 0.21 1.13

0.45 1.24 5.61

Notes

increase antioxidant activities in stripped soybean O/W emulsions. Moreover, because synergism is related to the molecular structure and cooperative effects of antioxidant compounds, the weak synergism from two combinations (i.e., CAPE + α-tocopherol and 4-VC + α-tocopherol) may be due to the low activities of caffeic acid derivatives in regenerating and recycling the tocopheroxyl radical to α-tocopherol through a reduction reaction.24 From a different perspective, 200 ppm of each antioxidant in emulsion corresponded to 1.47 mmol/L emulsion (4-VC), 0.70 mmol/L emulsion (CAPE), and 0.46 mmol/L emulsion (α-tocopherol), respectively. Despite the lower amount than 4-VC, CAPE was clearly proven to have better antioxidant activity in emulsion. Likewise, CAPE appeared to more effectively retard the oxidation than α-tocopherol because CAPE showed much less PV than α-tocopherol through the period of oxidation. In conclusion, the antioxidant effects in the present study show that CAPE was the most effective antioxidant. On 36 days, the PVs of CAPE, 4-VC, 4-VC + α-tocopherol, CAPE + α-tocopherol, and α-tocopherol were 8.4, 15.0, 13.7, 10.3, and 33.4 mequiv/L emulsion, respectively. Furthermore, the levels of conjugated diene forms and aldehydes for CAPE (1.67 and 0.14 mmol/L emulsion) and 4-VC (10.07 and 0.84 mmol/L emulsion) were lower than α-tocopherol (16.32 and 4.26 mmol/L emulsion) until 50 days. On the basis of PV and 1H NMR results, differences between the antioxidant effects of CAPE, 4-VC, and α-tocopherol are partially explained by their different structural characteristics and polarities. In this study, 200 ppm of CAPE could effectively retard oxidation in O/W emulsion, which may be related to hydroxyl groups in the aromatic structure and their affinities to lipid droplet in the O/W emulsion. Further studies will be undertaken to comprehensively elaborate the antioxidant activities of caffeic acid derivatives through investigating physical location properties, such as the distribution coefficient and interfacial tension in O/W emulsions.

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The authors declare no competing financial interest.

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