Influence of Oxidants on the Stability of Tocopherol in Model Nanoemulsions : Role of Interfacial Membrane Organized by Nonionic Emulsifiers

Nanoemulsions were prepared by using emulsifiers with various sizes of hydrophilic and hydrophobic groups to determine the impact of interfacial characteristics on the stability of α-tocopherol incorporated into the nanoemulsions. ,e α-tocopherol concentration remaining after 3 weeks of storage at 25°C depended greatly on the type of oxidative stress, which indicated that the environment surrounding the oil droplets could determine the stability of α-tocopherol in nanoemulsions. α-Tocopherol was gradually degraded by radical-mediated oxidation over storage, and approximately 60% of its initial concentration remained after 3 weeks of storage. However, under acidand iron-mediated oxidation, α-tocopherol concentration steeply decreases for the initial 3-day storage, but the degradation rate of α-tocopherol decreased after 3 days of storage and over 90% of the initial α-tocopherol remained after 3 weeks of storage. Interestingly, and contrary to our expectations, the thickness and/or density of the droplet interfacial membrane rarely affected the stability of α-tocopherol incorporated into nanoemulsions. Although it is difficult to generalize beyond α-tocopherol, we conclude that the properties of oil droplet surfaces had no influence on the storage stability of α-tocopherol encapsulated in the droplets.


Introduction
Lipid oxidation is one of the greatest concerns for oilcontaining food products because of its negative influence on nutritional quality and consumer health [1].In addition to lipid oxidation, lipophilic functional compounds incorporated into emulsion-based delivery systems can be decomposed by various oxidative stresses [2].To inhibit lipid oxidation and to prevent lipophilic compound decomposition, several synthetic and natural antioxidants are generally incorporated into foods containing considerable amounts of lipids [3].However, as a result of consumers' demands for clean food (i.e., food products that do not contain synthetic additives), food manufacturers have been making various attempts to replace synthetic antioxidants with natural ones.Among the natural antioxidants permitted for food use, tocopherols are important because they exist naturally in many vegetable oils [4] and because they have the ability to retard lipid oxidation by reacting with several radicals generated from lipid molecules, thereby protecting functional compounds from oxidative degradation.α-Tocopherol radicals can form nonradical products if they are reduced by other coexisting antioxidants, with regeneration of α-tocopherol.
In food systems, oil-in-water emulsions generally consist of water and oil, with the oil being dispersed as small droplets in the water [5].Emulsifiers have surface activity, so they can create kinetically stable emulsions by absorbing at the surfaces of droplets newly formed during homogenization [6].Emulsifiers absorbed at the oil droplet surfaces create an interfacial membrane comprised of a layer formed of their hydrophobic tails and a layer formed of their hydrophilic heads.Physical destabilization processes, such as coalescence, flocculation, and Ostwald ripening, are greatly affected by the characteristics of interfacial membranes formed with emulsifiers [7].Interfacial membranes also alter the rates of chemical reactions, such as lipid oxidation [8][9][10], between oil-and aqueous-phase compounds.Additionally, when a functional lipophilic compound is incorporated into emulsion droplets, the interfacial properties of the oil droplet surface are the main factors that control the stability of the functional compound incorporated therein [3,11].Because the interfacial membrane is formed with emulsifiers, the structural and physicochemical properties of emulsifiers play important roles in the emulsion stability and in the storage stability of functional compounds incorporated into the oil droplets [12].
Oil-in-water (O/W) emulsions are widely used as delivery systems in a variety of industries because of their abilities to encapsulate functional lipophilic compounds.Generally, O/W emulsions are classified into conventional emulsions (usually called "emulsions") and nanoemulsions, according to the size of the emulsion droplets [13].Because nanoemulsions have much larger specific surface areas than conventional emulsions, chemical degradation reactions at the oil-water interface can occur more quickly in nanoemulsions than in conventional emulsions [14].erefore, when O/W nanoemulsions are used as nanocarriers in delivery systems, the interfacial membrane formed by the emulsifier is an important factor in controlling the ability of the emulsion to protect the encapsulated functional compounds and to inhibit their diffusion from the oil droplets into the aqueous phase.
erefore, in nanoemulsion-based oral delivery systems for functional lipophilic compounds, it is important to know how the interfacial membrane affects the stability of α-tocopherol incorporated into the emulsion for the prevention of lipid oxidation and to understand the effectiveness of α-tocopherol in inhibiting the degradation of the functional lipophilic compounds by oxidative stress from the aqueous phase.Over the past decades, scientists have investigated how the stability and effectiveness of emulsion systems are altered during the incorporation of α-tocopherol [15,16].
e charge of the emulsion droplet surfaces is one of the major factors influencing the oxidative stability of the emulsified oil and also the stability of the α-tocopherol.e chemical stability of α-tocopherol is effectively improved in emulsions droplets with positively charged surfaces.When α-tocopherol is incorporated into an emulsion with negatively charged interfaces, the α-tocopherol stability can be increased by adding a biopolymer layer with a positive charge to the negatively charged droplet surfaces.However, despite this, most studies on the influence of interfacial membrane properties have shown that the charge of the emulsion droplet surfaces is a key factor influencing the stability of α-tocopherol in emulsions and in lipid oxidation.
erefore, the objective of this work was to determine whether the structural properties of the interfacial membrane could be involved in α-tocopherol decomposition in emulsions, particularly emulsion-based delivery systems.
is evaluation was accomplished by using model emulsionbased delivery systems stabilized by emulsifiers with different hydrophilic head sizes, which led to various droplet interfacial thicknesses.

Emulsion Preparation.
e aqueous phase was prepared by dissolving the emulsifiers in the phosphate buffer (10 mM and pH 7) to a predetermined concentration, and the oil phase was prepared by dissolving α-tocopherol in mediumchain triglycerides at a final concentration of 5 mmol/kg.Coarse emulsions were prepared by homogenizing the oil (5%, w/w) and aqueous (95%, w/w) phases (0.25 mmol α-tocopherol/kg emulsion) in a high-speed blender (T18 Basic Ultra-Turrax, Ika, Staufen, Germany) for 2 min at room temperature.
e oil droplet sizes in the coarse emulsions were then reduced with 5 passes through a microfluidizer (MN400BF, Micronox, Seongnam, Korea) at 100 MPa.After adjustment of the pH level of the emulsions to a predetermined value, the emulsions were purged with nitrogen with gentle stirring for 30 min before the subsequent step.To determine the effect of transition metals on α-tocopherol stability in the emulsions, ferrous sulfate or ferric chloride solution was added to the emulsions at a final concentration of 1 mmol/kg emulsion.Furthermore, to evaluate the effect of free radicals on α-tocopherol stability in the emulsions, AAPH solution was added to the emulsions to a final concentration of 1 mmol/kg emulsion.en, 10 g of the emulsion sample were transferred into 12 mL of a glass vial, closed airtight, and were stored in the dark at 25 °C.

Droplet Size Measurement.
e mean emulsion droplet diameters were measured by using static light scattering (laser diffraction).To avoid multiple scattering effects, all emulsion samples were diluted to a droplet concentration of approximately 0.005% (w/w) with a buffer solution of the same pH value as the sample, and samples were stirred continuously throughout the measurements to ensure homogeneity.e refractive index values for MCT and buffer solution were set at 1.47 and 1.33, respectively.e particle 2 Journal of Chemistry size distribution of the emulsions was then measured by using a commercial static light scattering instrument (BT-9300ST; Bettersize Instruments, Dandong, China).e particle size data are reported as the volume-weighted mean diameter, with n i representing the number of particles with diameter d i .

α-Tocopherol Concentration Measurement. α-Tocopherol
concentration was measured according to the method described by Yang et al. [17] with slight modification.α-Tocopherol concentrations in emulsions were determined by first vigorously vortexing 2 g of emulsion with 4 g of dichloromethane for 2 min.e mixture was then centrifuged at 1,842 × g for 10 min at 25 °C, and the solvent layer was collected.α-Tocopherol concentrations were determined with HPLC by using an Agilent 1100 instrument (Palo Alto, CA, USA).A Triart C18 column (250 mm × 4.6 mm × 5 μm, YMC, Tokyo, Japan) was used, with a methanol mobile phase at a rate of 1 mL/min.e wavelength for detection was 295 nm.Concentrations of α-tocopherol were calculated on the basis of a calibration curve generated by using authentic α-tocopherol.
erefore, the decomposition rate (k) of α-tocopherol was calculated, by assuming a first-order reaction: where C 0 is the initial α-tocopherol concentration (mmol/kg emulsion) and C t is the α-tocopherol concentration remaining at time t (day).e k value was calculated by performing a linear regression on the plot ln(C t /C 0 ) versus t. e equality of coefficients of different linear regressions was analyzed by the Chow test [18].If time-dependent changes in the degradation of α-tocopherol in emulsions were observed (the fast degradation of α-tocopherol in the early stage of storage and the slow its degradation in the late storage period in this study), the initial α-tocopherol decomposition rate (mmol/kg emulsion/day) was determined.e initial α-tocopherol decomposition rate could be determined by calculating the tangential slope at t � 0 because the instantaneous rate at time t is determined by calculating the tangential slope at t on α-tocopherol concentration versus time curve.

Statistical Analysis.
All the experiments were performed in triplicate, and the data are expressed as mean ± standard deviation.Analysis of variance (ANOVA) was performed, and the mean separations were performed using Duncan's multiple range test (p < 0.05).e statistical analyses described above were all conducted using SAS (version 9.4.; SAS Institute Inc., Cary, NC, USA).

Results and Discussion
To minimize the negative effect of micelles on the stability of the emulsions and α-tocopherol therein [19], the minimum emulsifier concentrations (MECs) required to prepare highly stable emulsions with mostly small droplets were determined in our previous study [7].e MECs for P10L, P10S, P20S, P23L, and P100S were 2.903, 3.165, 2.926, 1.784, and 0.994 mM, respectively.As all emulsions prepared at the MECs had similar initial droplet diameters (d 43 � 0.29, 0.28, 0.29, 0.32, and 0.28 μm for P10L-, P10S-, P20S-, P23L-, and P100S-stabilized emulsions, respectively) and the droplet sizes rarely changed after 21 days of storage, and any significant difference in the α-tocopherol decomposition rate between emulsions could not stem from an effect of the oil droplet interfacial area.Although the emulsifier concentration in emulsions were different from each emulsion, the facts that all emulsions had a similar oil droplet size indicated that emulsions had the different emulsifier loading (emulsifier concentration per unit droplet surface area) values which could be attributed to droplet interfacial density.
e emulsifier loading values for P10L-, P10S-, P20S-, P23L-, and P100S-stabilized emulsions were calculated as 1.91, 2.90, 1.87, 1.24, and 0.62 μmol/m 2 , respectively, indicating that the interfacial density of emulsions differed.When oils are stabilized by emulsifiers to spherical droplets, the interfacial membrane of oil droplet surfaces are comprised of the inner layer formed with the hydrophobic tails of emulsifiers and the outer layer formed with their hydrophilic heads.Considering the molecular structures of emulsifiers used in this work, it means that the thickness of the outer layer of the interfacial membrane could be mainly attributed by a number of oxyethylene groups of the hydrophilic groups of emulsifiers and that the length of alkyl chains of the hydrophobic tails of emulsifiers could determine the thickness of its inner layer.
Since the partition coefficient of α-tocopherol is approximately 12 [20], it is likely that the concentration of α-tocopherol in the aqueous phase was negligible.erefore, if α-tocopherol degradation is observed after a certain period of storage and prooxidants are present in the aqueous phase, most of α-tocopherol must have decomposed at the emulsion droplet surface, rather than in the aqueous phase.

Influence of the Level on α-Tocopherol Degradation.
Vitamin E compounds, including α-tocopherol, exhibit fairly good stability in the absence of oxygen and lipid peroxides [21].However, with the consideration that commercially available emulsion-based foods are generally acidic [10] and molecular oxygen is never completely removed from them, it is important to understand the influence of the pH level on the chemical stability of α-tocopherol in emulsions.erefore, to examine how the characteristics of emulsion droplet surfaces affect the chemical decomposition of α-tocopherol in acidic environments, the pH level of the emulsions was adjusted to 7 or 3 and the emulsions were then stored.Because mediumchain triglycerides consist of only saturated fatty acids, they are exceptionally stable to oxidation [22].In addition, most of the oxygen molecules were removed by nitrogen purging.erefore, if α-tocopherol degradation is observed to a considerable level, it could be the result of factors other than lipid peroxides derived from the oxidation of the medium-chain triglyceride carrier oil.One possible reason for the reduction of α-tocopherol during storage is the presence of a trace amount of oxygen molecules in the emulsions.In this study, to minimize the effect of oxygen molecules on α-tocopherol decomposition, nitrogen purging was carried out to remove oxygen molecules.However, it seems that the oxygen molecules in the aqueous phase were not completely removed.e effects of the emulsifier and pH level on α-tocopherol stability in emulsions are shown in Figure 2. As indicated in Table 1, regardless of the pH level, P100S-stabilized emulsions showed the highest initial decomposition rate of α-tocopherol among the emulsions.P100S has the largest hydrophilic head size among the emulsifiers used and the P100S-stabilized emulsion contained the smallest amount of the emulsifier among the emulsions prepared in this work, so it appears that the thick and/or loosely-packed interface is disadvantageous for the stability of α-tocopherol encapsulated in the emulsions.Because the P10S-and P20S-stabilized emulsions have droplet surfaces of similar density, it was expected that α-tocopherol in the emulsion stabilized with P20S, which has a hydrophilic head size that is twice as large as that of P10S, would be more stable than that in the P10S-stabilized emulsion; however, there was no significant difference in the initial decomposition rate of α-tocopherol in these two emulsions (p > 0.05).P20S-and P23L-stabilized emulsions have interfacial membranes of similar thickness because the difference in the oxyethylene group number of the hydrophilic heads of P20S and P23L is only three, while the P20Sstabilized emulsion has a denser interfacial membrane than the P23L-stabilized emulsion, as described above.However, both of these emulsions showed very similar initial decomposition rates of α-tocopherol, independent of the pH level.It was apparent that the thickness and/or density of the droplet surfaces did not affect the initial α-tocopherol decomposition rate.In addition, considering the content (>90%) of α-tocopherol remaining after 21 days of storage, the variation in initial α-tocopherol decomposition rates among the emulsions did not have much effect.

Influence of Transition Metals on α-Tocopherol
Degradation.
e previous findings indicate that iron ions could be the direct or indirect reasons for the degradation of the several food components including lipids [23] and the precursors of vitamins such as carotenoids [24] because of their electron transfer reaction.e cation radicals could be formed by the interaction of iron ions with those food components.It means transition metals like iron could act as oxidizing agents.
erefore, in this study, iron ions were chosen as oxidants for studying the degradation of α-tocopherol.e initial decomposition rates of α-tocopherol in emulsions with iron were different from those of α-tocopherol in iron-free emulsions.As shown in Table 1, the emulsions stabilized with different emulsifiers in the absence of iron had different initial α-tocopherol decomposition rates, whereas little difference was observed in the initial α-tocopherol decomposition rates for emulsions stabilized with different emulsifiers in the presence of iron.However, similar to the observation mentioned above, there was no significant difference in the α-tocopherol content remaining in the emulsions after 21 days of storage (p < 0.05), which suggests that iron did not have an influence on the stability of α-tocopherol in the emulsions (Figures 3 and 4).
Irrespective of the oxidative state of the iron, the initial decomposition rates of α-tocopherol in emulsions in the presence of ferrous iron were not significantly different from those in emulsions stored with ferric iron (p < 0.05).In addition, when emulsions contained iron with the same oxidative state, they showed very similar initial decomposition rates of α-tocopherol, regardless of the pH level.Although all of the emulsifiers used in this work were nonionic, the droplet surface charges of the emulsions were slightly negative and their values changed depending on the pH level (−7.3, −6.8, −5.5, −9.4, and −1.7 mV for P10L-, P10S-, P20S-, P23L-, and P100S-stabilized emulsions at pH 7, respectively, and around −1.5 mV at pH 3).Because the droplet surfaces were more negatively charged at pH 7, except those in the P100Sstabilized emulsion and could attract iron molecules to the surface of the emulsion droplets, it was expected that α-tocopherol would be rapidly decomposed at pH 7 because the 4 Journal of Chemistry iron molecules would accumulate around the more negatively charged droplet surfaces at this pH level. is suggests that the iron did not decompose α-tocopherol by direct interaction at the interfacial membrane.erefore, the stability of α-tocopherol in emulsions was not influenced by the thickness and/or density of the droplet interfaces.

Influence of Radicals on α-Tocopherol Degradation.
AAPH may be a suitable material for studying the influence of radicals on the stability of α-tocopherol encapsulated in emulsions because it is a self-generator of free radicals through spontaneous decomposition at room temperature.Quite different from the previous findings, in this case, the     Journal of Chemistry α-tocopherol concentration gradually decreased during the 21-day storage period (Figure 5).e values of k for α-tocopherol in emulsions stored at pH 3 ranged from 0.0293 to 0.0344 day −1 , and the values of k for α-tocopherol in emulsions stored at pH 7 ranged from 0.0218 to 0.0252 day −1 (Table 2).Although α-tocopherol decomposed more quickly in acidic conditions than neutral conditions, the lack of correlation between the k value and the properties (thickness and/or density) of the interfacial membranes suggests that the interfacial characteristics played little or no role in improving the stability of emulsified α-tocopherol against radical-mediated oxidation.
During the design of this experiment, we expected that the properties of the interfacial membranes of oil droplets would affect the storage stability of α-tocopherol incorporated in emulsions.Although there is a lack of information about the influence of the density of interfacial membranes on the oxidative stability of emulsified oils and the storage stability of encapsulated functional lipophilic compounds, according to previous studies, the emulsion interfacial thickness could be one of the important determinants of the oxidative stability of food emulsions [16].Song et al. [11] reported that the storage stability of β-carotene in emulsions varied depending on the droplet interfacial thickness, and they also revealed that the droplet interfacial density may be a factor to consider for improving β-carotene stability.As described above, all of the emulsions analyzed in this study had different densities and thicknesses for their interfacial membranes.For example, the P100S-stabilized emulsion had the thickest interfacial membrane but its density was the lowest among the emulsions, whereas the P10L-and P10S-stabilized emulsions had the opposite properties.
e stability of α-tocopherol in the emulsions greatly depended on the environmental conditions surrounding the emulsion droplets, and the denseness and/or thickness of the interfacial membrane of the oil droplets did not play a crucial role in improving the stability of the encapsulated α-tocopherol.In conclusion, our findings, together with those of previous studies, suggested that the data are still insufficient to generalize the influence of droplet interface characteristics on the oxidative stability of emulsified oils

Table 1 :
e initial decomposition rate (mmol/kg emulsion/day) of α-tocopherol in emulsions stabilized with emulsifiers having various sizes of hydrophilic and hydrophobic groups.100 stearyl ether.e values with different small letter superscripts in the same row are significantly different (p < 0.05) by Duncan's multiple range test.e values with different capital letter superscripts in the same column are significantly different (p < 0.05) by Duncan's multiple range test.

Table 2 :
e decomposition rate (k) of α-tocopherol in emulsions stabilized with emulsifiers having various sizes of hydrophilic and hydrophobic groups in presence of radicals.