Oil-in-Water Emulsions Stabilized by Saponified Epoxidized Soybean

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Oil-in-Water Emulsions Stabilized by Saponified Epoxidized Soybean Oil-Grafted Hydroxyethyl Cellulose Xujuan Huang,† Qiaoguang Li,† He Liu,*,† Shibin Shang,*,†,‡ Minggui Shen,† and Jie Song§ †

Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Key Laboratory of Biomass Energy and Material, National Engineering Laboratory for Biomass Chemical Utilization, Key and Laboratory on Forest Chemical Engineering, State Forestry Administration, Nanjing 210042, Jiangsu Province, China ‡ Institute of New Technology of Forestry, Chinese Academy of Forestry, Beijing 100091, China § Department of Chemistry and Biochemistry, University of MichiganFlint, Flint, Michigan 48502, United States ABSTRACT: An oil-in-water emulsion stabilized by saponified epoxidized soybean oil-grafted hydroxyethyl cellulose (H− ESO−HEC) was investigated. By using an ultrasonic method, oil-in-water emulsions were prepared by blending 50 wt % soybean oil and 50 wt % H−ESO−HEC aqueous suspensions. The influence of H−ESO−HEC concentrations on the properties of oil-inwater emulsions was examined. The H−ESO−HEC concentrations in the aqueous phase varied from 0.02 to 0.40 wt %. When the H−ESO−HEC concentration was 0.4 wt %, the emulsion remained stable for >80 days. The mean droplet sizes of the emulsions decreased by increasing the H−ESO−HEC concentration and extending the ultrasonic time. The adsorption amounts of H−ESO−HEC at the oil−water interface increased when the H−ESO−HEC concentrations in the aqueous phase increased. The rheological property revealed that the apparent viscosity of the H−ESO−HEC-stabilized oil-in-water emulsions increased when the H−ESO−HEC concentrations increased. Steady flow curves indicated an interfacial film formation in the emulsions. The evolution of G′, G″, and tan η indicated the predominantly elastic behaviors of all the emulsions. KEYWORDS: hydroxyethyl cellulose, soybean oil, emulsion, rheology, interfacial film

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INTRODUCTION Oil-in-water emulsions are thermodynamically unstable systems that consist of liquid (oil) droplets dispersed in water. These unstable systems could result in various phenomena, such as creaming, sedimentation, flocculation, coalescence, and phase inversion.1 Sufficient long-term physical and kinetic stability is clearly an important goal in developing a new emulsion formulation. Moreover, the size distribution of the droplets, the state of aggregation, and the spatial arrangement of a stable emulsion showed no obvious changes over the time-scale of observation. Generally, oil-in-water emulsions are always stabilized by surface-active molecules called emulsifiers, which include ionic or nonionic surfactants, polymers, and solid particles.2−4 Polymers, in particular, are among the most significant emulsifiers to apply for stabilizing emulsions. Polymers in oil-in-water emulsions can reduce the interfacial tension between the two phases as well as increase the steric repulsion between the droplets by adsorbing to the surface of freshly formed oil droplets during homogenization.5 The development of polymers as emulsifiers in emulsions has attracted much attention.6,7 Within the family of polymers, modified natural polymers are particularly attractive because of their low cost and their ability to biodegrade, emulsify, and thicken in water.8 Several modified natural polymers (starch granules, spruce galactoglucomannans,9 and regenerated chitin nanofibers10) have been reported for the oil-in-water emulsions. Cellulose, the most abundant natural resource on earth, has attracted extensive interest as a renewable polymer.11,12 Amorphous cellulose,13 regenerated cellulose,14 microfibrillated cellulose,15,16 cellulose nanocrystals (CNCs),17,18 and cellulose ethers19−21 were reported to stabilize oil-in-water emulsions. © 2017 American Chemical Society

Especially, the cellulose ethers, which are commercial products, have been used widely in the food and cosmetics industries for their emulsifying property. Oil-in-water emulsions stabilized by hydrophobically modified hydroxyethyl cellulose (HMHEC; commercial name, Natrosol Plus 330) have shown as much stabilizing ability as hydroxyethylcellulose (HEC) from which it is derived.22 Hydrophobically modified ethyl(hydroxyethyl)cellulose (HM−EHEC) improved the stability of macroemulsions dramatically compared with emulsions stabilized with EHEC. The introduction of hydrophobic groups onto the polymer backbone provides stability by an associative mechanism, which thereby increases the viscosity of the continuous phase.25 Therefore, these modifications of cellulose ethers provide new ideas for the development of novel emulsifiers applied in oil-in-water emulsions. Saponified fatty acid-grafted hydroxyethylcellulose polymer (H−ESO−HEC) was prepared by HEC as a backbone in our previous study.26 The simultaneous introduction of the hydrophobic and hydrophilic units adjusts the amphiphilicity of the H−ESO−HEC and provides an opportunity to form interfacial films in water. This novel polymer can effectively lower the surface tension of water to 26 mN/m. In this work, H−ESO− HEC-stabilized oil-in-water emulsions with different amounts of H−ESO−HEC concentrations were investigated. The oil fraction in oil-in-water emulsions mostly ranged from 10 to 50 wt % to maintain an adequate viscosity and flowability of the oilReceived: Revised: Accepted: Published: 3497

February April 15, April 18, April 18,

13, 2017 2017 2017 2017 DOI: 10.1021/acs.jafc.7b00662 J. Agric. Food Chem. 2017, 65, 3497−3504

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Journal of Agricultural and Food Chemistry

Figure 1. Ideal illustration of adsorption of the H−ESO−HEC polymers at the oil−water interface and the formation of interfacial films that involves emulsion droplets (left) and the structure of the H−ESO−HEC polymer at the oil−water interface (right).

in-water emulsions.13−15,22 Furthermore, the viscosity of the emulsions increased significantly with higher oil fractions. Because of its application, 50 wt % oil fractions were selected to investigate the emulsion stability. The state of H−ESO−HEC polymers in emulsions was divided into three parts. The hydrophilic sodium carboxylate and fatty acid alkane chains of saponified ESO grafted in HEC acted as hydrophilic units and hydrophobic units in emulsions, respectively. The hydrophilic HEC backbone was absorbed at the oil−water interface. Overall, the H−ESO−HEC polymers were arranged in order at the oil− water interface (Figure 1). The effects of H−ESO−HEC concentrations and ultrasonic time on droplet sizes of H− ESO−HEC stabilized emulsions were also characterized. The adsorbed amount of H−ESO−HEC polymer at the oil−water interface was calculated by total organic carbon (TOC) technology. The rheological properties of H−ESO−HECstabilized oil-in-water emulsions were evaluated through steady flow tests and oscillatory shear measurement. H−ESO−HEC displayed excellent emulsifying abilities under low emulsifier content in soybean oil-in-water emulsions. It shows important implications for the design and preparation of biobased polymer emulsifier from agricultural resources.

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(converter part no. CV 334, Sonics Vibra-Cell, Sonics & Materials, Inc., Newton, CT, USA). The mixtures were handled with a dipping titanium probe close to the surface by alternating 1 s of sonication with a 3 s standby. The effective ultrasonic time was 10 s for each emulsion. The samples were stored at room temperature for at least 1 day before being analyzed. Emulsion Characterization. Interfacial Tension. The interfacial tensions of ESO and H−ESO−HEC aqueous suspensions before emulsification were measured at 25 °C using the standard ring (R = 9.58 mm and r = 0.185 mm) on the Sigma 701 automatic surface tensiometer. Before each measurement, the standard ring was flushed with water and then heated over an alcohol blast burner for a few seconds to remove contamination. The instrument was calibrated with water and then checked by measuring the ESO−water interfacial tension with different concentrations of H−ESO−HEC aqueous suspensions. Each concentration of H−ESO−HEC aqueous suspension in water−ESO interfacial tension was performed in three repeat measurements. Emulsion Droplet Size Measurement. Droplet sizes of the freshly prepared (1 day) and stored (80 days) emulsions were analyzed by using a laser-diffraction particle-size analyzer (Mastersizer 2000, Malvern Instruments Ltd., Worcestershire, UK). Before analysis, the emulsions were diluted with distilled water to a concentration of approximately 0.001 wt % to avoid multiple scattering effects. Optical properties of the samples were defined as follows: Refractive indices of the soybean oil and water were 1.47 and 1.33, respectively. Emulsion droplet-size measurement was measured on the basis of light-scattering theory. To minimize errors, measurements were duplicated three times, and the average value was retained for the discussion. The droplet size measured in the dissociating solution was referred to as the “primary” particle size and, in this work, was measured directly in water and referred to as the “effective” particle size.25 Optical Microscopy. A photomicrograph of the H−ESO−HECstabilized emulsion was observed by using a polarized optical microscope (XP-203E, Shanghai Changfang Optical Instrument Co., Ltd., Shanghai, China). A drop of diluted emulsion was placed on a cover glass and viewed by using an optical microscope fit with a digital camera. Emulsion droplet size was reported as the surface-weighted mean particle diameter (d32) and the volume-weighted mean diameter (d43). Adsorption of the H−ESO−HEC Polymer at the Oil−Water Interface. The adsorbed amounts of H−ESO−HEC polymer at the oil−water interface of the emulsions were measured by using a method of Nilsson and Sun.20,24 First, the emulsions were separated by centrifugation at 4000 rpm for 40 min and at 8000 rpm for another 30 min until a clear subnatant was obtained. In addition, there is no oil separated from the emulsion. Then the amount of H−ESO−HEC in the subnatant was determined by total organic carbon (TOC) technology. The adsorbed amount of H−ESO−HEC at the oil−water interface of the emulsions was obtained from the difference between the amount in

MATERIALS AND METHODS

Experimental Procedure. Preparation of H−ESO−HEC Polymer. H−ESO−HEC polymeric derivative was prepared according to a previous paper.22 First, ESO-grafted HEC derivative (ESO−HEC) was prepared by ring-opening polymerization in DMSO through the use of SnCl4 as a catalyst. Then, ESO−HEC was heated to 80 °C in NaOH solution and agitated for 12 h under a condensing condition. After the reaction, the hydrolysis product (H−ESO−HEC) was obtained through a multistep purification process, including the use of ultrafiltration membranes to eliminate the influence of smaller molecules and electrolytes. Finally, the H−ESO−HEC was dried by reduced pressure distillation. Preparation of H−ESO−HEC-Stabilized Oil-in-Water Emulsions. H−ESO−HEC aqueous solutions were prepared at concentrations of 0.02, 0.05, 0.10, 0.20, and 0.40 wt % by dilution of the stock 0.40 wt % H−ESO−HEC solution by using deionized water. H−ESO−HEC aqueous solution was stirred gently for 24 h at room temperature before use. The H−ESO−HEC concentrations reported in this paper refer to the aqueous phase and not to the whole emulsion. H−ESO−HECstabilized oil-in-water emulsions (20 g) were prepared by blending 50 wt % soybean oil and 50 wt % H−ESO−HEC aqueous suspensions through the use of an Ultrasonic Cell disrupter System at 0.26 kW 3498

DOI: 10.1021/acs.jafc.7b00662 J. Agric. Food Chem. 2017, 65, 3497−3504

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Journal of Agricultural and Food Chemistry the referenced sample containing no disperse phase and the amount in the subnatant after separation of the emulsion: Madsorbed = M referenced − Msubnatant

(1)

Here, Madsorbed is the mass of H−ESO−HEC adsorbed at the oil−water interface of the emulsions, Mreferenced is the mass of the H−ESO−HEC in the referenced sample, and Msubnatant is the mass of the H−ESO−HEC in the subnatant. The adsorbed amount (Γ) was obtained by relating the adsorbed amount to the specific surface area of the emulsion: Γ=

4Madsorbedd32 3V

(2)

Here, d32 is the area-weighted droplet diameter, and V is the dispersephase volume. Visual Assessment of Creaming. The stability of emulsions was assessed by monitoring the variation of the emulsion volume with storage time. Immediately after preparation, the emulsions (20 g) were poured into glass tubes (27 mm diameter and 80 mm height) and sealed with plastic caps to prevent evaporation. The sample tubes were kept at room temperature, and the creaming was tracked with time for 80 days. Both the total emulsion height (HT) and the serum-layer height (HS) were measured. The extent of creaming was reported as creaming index (CI), defined by the following equation: CI =

HS × 100% HT

(3)

Rheology Measurements. Rheological properties of the H−ESO− HEC (0.02, 0.05, 0.10, 0.20, and 0.40 wt %) stabilized oil-in-water emulsions were measured at 25 °C by using a rotational rheometer (Haake Mars II, Thermo Electron GmbH, Karlsruhe, Germany), equipped with a circulating water bath (DC5, Haake) and a Peltier (TC 81, Haake) temperature-control unit for maintaining the desired temperatures during analysis. The measurement was performed with a measuring geometry (PP60 Ti, gap size of 0.105 mm) for dynamic viscoelastic measurements and a cone plate sensor (C60/2° Ti, gap size of 0.998 mm) for steady flow tests. After the samples were loaded into the rheometer, they remained unperturbed for 2 min before the following tests were performed. For dynamic viscoelastic measurements, the linear viscoelastic range was determined with a strain sweep (0.01−100%) at a fixed frequency of 1 Hz. A dynamic frequency sweep was then conducted by applying a constant strain of 1%, which was within the linear region of the emulsion, over an angular frequency range between 0.01 and 10 Hz. The dynamic mechanical spectra were obtained by recording the storage modulus (G′) and the loss modulus (G″) as functions of frequency. For steadyflow tests, apparent viscosity was obtained with an increasing a shear rate that ranged from 0.1 to 100 s−1 in 1 min.

Figure 2. Appearance of H−ESO−HEC-stabilized emulsions with different concentrations: (A) 1 day; (B) 80 days. The arrows in (B) mark the cream−serum boundary.

concentration of H−ESO−HEC aqueous suspensions has an influence on emulsion stability. According to Stoke’s law, the creaming stability of emulsion can be improved by reducing the droplet size, increasing the viscosity of the continuous phase, or minimizing the density differences between droplets and the continuous phase.13 Because of the associated behavior of the hydrophobic chains in the H−ESO−HEC structure, the emulsion became stable. When the concentration was increased from 0.05 to 0.4 wt %, the stability of the emulsion improved. At a concentration of 0.4% H−ESO−HEC, no creaming was detected for up to 80 days (Figure 2B). At high concentrations of H−ESO−HEC, an interfacial film, caused by the intermolecular association between the fatty acid alkane chains and the hydrophilic sodium carboxylate, formed in the emulsion and further enhanced the emulsion stability (Figure 1). The low concentration of H−ESO−HEC was used in stabilizing the oil-in-water emulsions because of the low critical micelle concentration (0.4 g/L) of H−ESO−HEC polymer in water. As shown in Figure 1, the effect of H−ESO−HEC in the oil-in-water emulsions can be divided into three parts. The fatty acid alkane chains of ESO, which acted as hydrophobic units, were distributed to the oil phase, and the hydrophilic sodium carboxylate of saponified ESO, which acted as a hydrophilic unit, was distributed to the aqueous phase. The HEC backbone, as an integument, was absorbed at the oil−water interface in an orderly arrangement (Figure 1). The combined effects of the three parts formed a tight interfacial film structure in the emulsion. Furthermore, the polymeric ESO that was grafted on the HEC backbone also enhanced the tightness of the oil−water interface. The arrangement of hydrophilic and hydrophobic groups became much tighter at the oil−water interface with increasing saponified polymeric ESO grafted on a HEC backbone, which enhanced the stability of the oil−water interface. This tight

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RESULTS AND DISCUSSION Emulsion Stability. Stability against Creaming and Aging Time. A preliminary experiment aimed to investigate the effects of H−ESO−HEC concentrations on emulsion stability with visual assessment. H−ESO−HEC-stabilized emulsion is evident in Figure 2. The oil fraction in all of the samples is 50 wt %, and the concentration of H−ESO−HEC in the aqueous phase was indicated previously. It is evident that the emulsion stability can be enhanced by increasing the concentration of H−ESO−HEC polymer in the aqueous phase as shown in Figure 2. For instance, at lower concentrations of H−ESO−HEC-stabilized oil-in-water emulsion (0.02 wt %), creaming was apparent within approximately 2 h, and the oil phase was completely floating on the surface (Figure 2A). In addition, the interfacial tension of ESO−water decreased from 18.20 to 9.35 mN/m when the concentration of H−ESO−HEC aqueous suspensions increased from 0.05 to 0.40%. The ESO-water interface can be emulsified significantly with lower interfacial tension. It is suggested that the 3499

DOI: 10.1021/acs.jafc.7b00662 J. Agric. Food Chem. 2017, 65, 3497−3504

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Journal of Agricultural and Food Chemistry interfacial film structure increased the stability of the emulsions. In addition, an entanglement network, caused by the HEC polymer, might occur in these emulsions.20 The creaming behavior and aging time of the H−ESO−HECstabilized emulsion at different concentrations are shown in Figure 3. The emulsions stabilized by low H−ESO−HEC

1 wt %.20 As an HEC derivative, H−ESO−HEC polymer has more advantages in terms of quality (0.4 wt %) compared with HMHEC and HM−EHEC. Furthermore, the emulsions stabilized by amorphous cellulose (AC) and microfibrillated cellulose (MFC) could be stored over 80 days, when the concentrations of polymers were all above 0.5 wt % in the water phase.15 However, the emulsions prepared by this kind of cellulose displayed a low oil content. The concentration of CNCs in the water phase that was used to prepare the oil-in-water emulsions was considerably less than 0.5 wt %. Also, the preparation of the CNCs and the modified CNCs is more sophisticated and noncommercial so far. Table 1 presents a comparison of the other parameters of cellulose polymers for oilin-water emulsions. On the whole, H−ESO−HEC can be considered as a promising product for emulsion preparation. Comparison of Emulsion Stability Prepared with Carboxymethyl Cellulose (CMC), HEC, and H−ESO−HEC. Figure 4

Figure 3. Influence of H−ESO−HEC concentration on the creaming index of 50 wt % H−ESO−HEC-stabilized emulsions during storage at room temperature for 80 days.

concentrations (0.02 wt %) started to cream in 2 h, and the oil phase was completely out of the mixtures after storage for 1 day. Figure 3 indicates that the creaming rate and index decreased when the H−ESO−HEC concentrations increased. Also, the emulsions stabilized with H−ESO−HEC concentrations of 0.05, 0.10, and 0.20 wt % showed delayed creaming over 1, 2, and 3 days, respectively. No creaming was observed for up to 80 days when the H−ESO−HEC concentration increased to 0.4 wt %. Previous studies reported that the oil-in-water emulsions were usually prepared through cellulose ether, natural cellulose, and cellulose nanocrystals in recent years. As shown in Table 1, the oil-in-water emulsions prepared by HMHEC could be stored for 30 days, in which the HMHEC concentration-in-water phase was

Figure 4. Appearance of emulsion prepared with CMC, HEC, and H− ESO−HEC after storage for 80 days at room temperature, respectively. The concentration of emulsifier is 0.4 wt %, the oil-volume fraction is 50 wt %, and the effective ultrasonic time is 10 s.

Table 1. Comparison of Various Cellulose Polymers for Emulsion Preparation cellulose category

origin

oil content

H−ESO−HEC HMHEC HM-EHEC AC regenerated cellulose MFC

HEC HEC HEC MCCd MCCd mangosteen rind

50 wt % soybean oil 50% paraffin oil 15 wt % olive oil 20% dodecane 30 wt % soybean oil 10 wt % soybean oil

CNCs

corncob cellulose Whatman filters wood pulp nata de coco CNCsg

10 wt % D-limonene

CNCs amphiphilic CNCs BCNse PDMAEMA-g-CNCsg poly(NIPAM)-gCNCsh

Ramie fibers

10 wt % hexadecane 10 wt % soybean oil 30% hexadecane 20 wt % heptane or toluene 50% heptane

cellulose contenta 0.4 wt % 1.0 wt % 1.0 wt % 0.83 wt % 1.0 wt % 0.5 wt %

preparation method 10 s sonication, 0.26 kWb 6000 rpm, 10 minc 25,000 rpm, 7 min, and 100 s sonication 10,000 rpm, 3 minc 15,000 rpm, 1.5 minc 11,000 rpm, 1 min; and 15,000 rpm, 4 minc 36 s sonication, 0.4 kWb

0.1 wt % 0.1 wt % >0.5 wt % 0.5 wt %

20 s sonication, b 10,000 rpm, 1 hc 12 s sonication, b 6000 rpm, 2 minc

>0.5%

6000 rpm, 60 sc

stability time (days)

reference

>80 >30 ≤10 90 >7 >80

this work 22 23 13 14 15 17

f 120

18 27 28 29 30

a Minimum concentration of cellulose to ensure emulsion stability. bUltrasonic method. cShearing method. dMicrocrystalline cellulose. eBacterial cellulose nanocrystals. fOver months. gPoly[2-(dimethylamino)ethyl methacrylate]-grafted cellulose nanocrystals. hCellulose nanocrystals grafted with thermoresponsive polymer brushes.

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Figure 5. Droplet diameter distribution of emulsions stabilized by H−ESO−HEC at different concentrations measured after storage for 1 day at room temperature: (A) 0.05 wt %; (B) 0.1 wt %; (C) 0.2 wt %; (D) 0.4 wt %. (Insets) Photomicrographs of the corresponding emulsions (scale bars = 20 μm).

Table 2. Influence of H−ESO−HEC Concentrations on d32 and d43 of H−ESO−HEC-Stabilized Emulsions Measured after Storage for 1 and 80 Days at Room Temperature, Respectively d32 (μm)

d43 (μm)

H−ESO−HEC concentration (wt %)

day 1

day 80

day 1

day 80

0.05 0.10 0.20 0.40

2.578 ± 0.003 2.236 ± 0.008 2.083 ± 0.007 1.779 ± 0.005

4.339 ± 0.055 4.085 ± 0.124 2.775 ± 0.019 2.159 ± 0.012

5.708 ± 0.040 4.696 ± 0.011 4.210 ± 0.011 2.9680 ± 0.017

16.814 ± 0.218 16.177 ± 0.639 9.1950 ± 0.044 3.2650 ± 0.040

photomicrograph images and droplet-size measurements of these emulsions after storage for 1 day at room temperature are shown in Figure 5. Bimodal distributions corresponding to two kinds of size ranges were observed for the emulsions prepared with 0.05−0.2 wt % H−ESO−HEC. The variation of peaks represented the change of droplet diameter of the emulsions. When the H−ESO−HEC concentration increased, the adsorption of the larger size range became weak in Figure 5, whereas the droplet diameter (>4 mm) of the emulsions decreased. Moreover, when the concentration increased to 0.4 wt %, the adsorption of the larger size range nearly ceased to exist. In general, the mean droplet diameter of the emulsions decreased when the H−ESO−HEC concentrations increased. The surface weight (d32) and volume weight (d43) meandroplet diameters of the oil-in-water emulsions (1 and 80 days) stabilized by H−ESO−HEC with different concentrations are given in Table 2; when the H−ESO−HEC concentration in the aqueous phase increased to 0.4 wt %, both d32 and d43 decreased significantly. Moreover, the d43 values were higher than the d32 values, particularly after storage for 80 days, because d32 and d43 values are sensitive to different aspects of droplet-size distribution. For instance, d32 is more sensitive to the presence of small particles, whereas d43 is more sensitive to the presence of

shows the appearance of emulsions prepared with CMC, HEC, and H−ESO−HEC polymers, respectively. As the previous research indicated, the H−ESO−HEC emulsifier was prepared by using HEC as raw material.22 In addition, the introduction of hydrophilic carboxyl groups made the H−ESO−HEC polymer more like cellulose ether, which was called CMC. Therefore, the stability of the emulsion prepared by H−ESO−HEC was compared with the CMC- and HEC-stabilized emulsions. It can be seen that, at the same emulsifier concentration (0.4 wt %), the emulsion stabilized with H−ESO−HEC showed good stability compared with the others. Besides, the CMC-stabilized emulsion was completely damaged after 12 h, and the HECstabilized emulsion started to cream after several days. One reason is that the emulsifying abilities of HEC and CMC are lower than that of H−ESO−HEC at low concentrations, although CMC and HEC are effective stabilizers.31,32 Additionally, the high surface activity of H−ESO−HEC polymer in water is another influencing factor for the emulsion stability.22 Droplet Size of H−ESO−HEC-Stabilized Oil-in-Water Emulsion. Effects of H−ESO−HEC Concentration. Figure 2 shows the instability in the emulsion prepared with 0.02 wt % H− ESO−HEC after 2 h. Therefore, only emulsions prepared with 0.05−0.4 wt % H−ESO−HEC were characterized. The 3501

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Journal of Agricultural and Food Chemistry large particles in the size distribution.1,33 The droplet size decreases when H−ESO−HEC polymer concentrations increase because more H−ESO−HEC polymer is available to stabilize the smaller droplets and a higher overall interfacial area requires stabilization.34 Effects of Ultrasonic Time. In general, the preparation condition of emulsions was a key factor for the droplet size of the emulsions. The droplet-size distributions of the H−ESO−HECstabilized emulsions, with different ultrasonic times after storage for 1 day at room temperature, are shown in Figure 6. The

Table 4. Results from Adsorption Experiments with Different H−ESO−HEC Concentration-Stabilized Emulsions H−ESO−HEC concentration (wt %)

d32 (μm)

Γ (mg/m2)

0.05 0.10 0.20 0.40

4.339 ± 0.055 4.085 ± 0.124 2.775 ± 0.019 2.159 ± 0.012

0.765 1.887 5.575 11.515

kinetic factors are likely to play a considerable role in the adsorption.35 The results calculated through eq 2 indicate that adsorption was controlled by the total surface area (d43) created during emulsification and the amount of H−ESO−HEC available for adsorption at the oil−water surface. Rheological Properties. The rheological properties of the H−ESO−HEC-stabilized oil-in-water emulsions as a function of H−ESO−HEC concentration were investigated by measuring their steady and dynamic shear behaviors. Steady Flow Behavior. For the steady shear measurement, the apparent viscosity of H−ESO−HEC-stabilized emulsions as a function of shear rate with different H-ES-HEC concentrations is shown in Figure 7A. Some fluctuations at the low shear rate occurred because of instrument error and minor viscosity. First, the apparent viscosity of the H−ESO−HEC -stabilized oil-inwater emulsions decreased as the shear rate increased, and the emulsions exhibited a pseudoplastic property at the low shear rate.36 Subsequently, the H−ESO−HEC-stabilized emulsions behaved like a Newtonian property, and the apparent viscosity remained constant when the shear rate reached a certain level. Moreover, the apparent viscosity increased when H−ESO−HEC concentrations increased. The increase of viscosity can also be caused by H−ESO−HEC molecules in the emulsions adsorbed at the oil−water interface.37 In other words, when the concentrations increased to higher values, the viscosity undoubtedly increased, and the prepared emulsions remained stable for longer periods. As shown in Figure 7B, the shear stress of H−ESO−HECstabilized oil-in-water emulsions increased when the shear rate increased. However, the slight fluctuations (below 20 s−1) defined the magnitudes of static yield stress, which is related to the strength of the interfacial film structure that must be broken down at higher shear rates to cause flow.15 Therefore, the emulsions displayed a non-Newtonian property when the shear rates were below 20 s−1. When the shear rate increased, the extent to which shear stress augmented gradually reduced when the H− ESO−HEC concentrations decreased (the decreasing slope). Additionally, the flow behavior of H−ESO−HEC-stabilized oilin-water emulsions changed from exhibiting pseudoplastic properties to Bingham properties. These results indicate that a stronger network formed in the emulsions containing higher H− ESO−HEC concentrations. Oscillatory Shear Properties. Oscillatory shear measurements were performed to determine the viscoelastic properties of the H−ESO−HEC-stabilized oil-in-water emulsions. The evolution

Figure 6. Droplet diameter distribution of emulsions stabilized by 0.4 wt % H−ESO−HEC with different ultrasonic times measured after storage for 1 day at room temperature.

droplet-size distribution of H−ESO−HEC became narrow and homogeneous when the ultrasound time increased gradually. The d32 and d43 mean-droplet diameters of the oil-in-water emulsions stabilized by 0.4 wt % H−ESO−HEC with different ultrasonic times are presented in Table 3. As predicted, the d32 and d43 of H−ESO−HEC-stabilized emulsions increased when ultrasonic times increased. Moreover, the emulsion droplets (d43) subjected to ultrasound treatment for 60 and 300 s showed little difference. In addition, the ultrasonic time had little effect on the d32 of H−ESO−HEC-stabilized emulsions. Because of the high energy consumption, the H−ESO−HEC-stabilized emulsions were prepared by ultrasound treatment for 10 s to investigate droplet-size distributions and rheology property. On the whole, the ultrasound times on the emulsion preparation were much shorter, whereas, in other papers, times reported were nearly longer than 20 s with the same ultrasonic power.25,26 Adsorption of the H−ESO−HEC Polymer at the Oil− Water Interface. To investigate the stabilizing mechanism of the emulsions, the adsorption amount of H−ESO−HEC polymer at the oil−water interface was measured ( Table 4). It can be seen that the adsorption amount of H−ESO−HEC at the oil−water interface increased when the H−ESO−HEC concentration increased. Adsorption of the H−ESO−HEC polymer to the surface of the emulsion droplets during the emulsification process occurs under nonequilibrium conditions, and thus

Table 3. Influence of Ultrasonic Time on d32 and d43 Mean-Droplet Diameters of H−ESO−HEC-Stabilized Emulsions Measured after Storage for 1 Day at Room Temperature ultrasonic time H−ESO−HEC stabilized emulsion (0.4 wt %)

10 s

20 s

30 s

60 s

300 s

d32 d43

1.779 ± 0.005 2.968 ± 0.017

1.706 ± 0.018 2.435 ± 0.004

1.576 ± 0.015 1.918 ± 0.005

1.384 ± 0.005 1.966 ± 0.007

1.710 ± 0.012 1.995 ± 0.012

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DOI: 10.1021/acs.jafc.7b00662 J. Agric. Food Chem. 2017, 65, 3497−3504

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Figure 7. Influence of H−ESO−HEC concentrations on (A) apparent viscosity, η, as a function of shear rate of H−ESO−HEC-stabilized emulsions, and (B) flow curve, shear stress as a function of shear rate. All measurements were performed after storage for 7 days at room temperature.

Figure 8. Influence of H−ESO−HEC concentration on (A) mechanical spectra, that is, storage modulus, G′ (solid symbols); (B) loss modulus, G″ (open symbols), as a function of frequency; and (C) loss tangent (tan η = G″/G′) as a function of frequency. All measurements were performed after storage for 7 days at room temperature.

of G′ and G″ with a frequency measured at 1% strain is shown in panels A and B, respectively, of Figure 8. The figure shows G′ > G″ with no crossover throughout the tested frequency range. This behavior could be classified as a typical gel, which behaves elastically.38 In addition, G′ and G″ of all the H−ESO−HEC emulsions showed a slight rise with increased testing frequency. The magnitudes of G′ and G″, which increased when H−ESO− HEC concentrations increased, indicate increased strength of the emulsion microstructure.15 Furthermore, the magnitudes of the dynamic mechanical-loss tangent (tan η = G″/G′) of H−ESO− HEC-stabilized oil-in-water emulsions as a function of frequency are shown in Figure 8C. The tan η represents the relationship between the viscous and the elastic components of the surface dilatational modulus.39 This rheological parameter provides information about the relative viscoelasticity of H−ESO−HECstabilized oil-in-water emulsions. It can be seen that tan η of all the H−ESO−HEC emulsions decreased when frequency increased, which indicates a predominantly elastic behavior.40,41 In conclusion, preparations of oil-in-water emulsions stabilized by H−ESO−HEC polymer with different concentrations showed that, at 0.4 wt % concentration, stability remained for >80 days. The H−ESO−HEC-stabilized oil-in-water emulsions showed increased stability when H−ESO−HEC concentrations increased. The mean droplet size of the emulsions ranged from 2 to 5 μm (d43) and decreased with increasing H−ESO−HEC concentration and extended ultrasonic time. The adsorption amounts of H−ESO−HEC related to the mean droplet size at the oil−water interface increased when H−ESO−HEC concentrations increased, which in turn increased the stability of emulsions. The rheological property revealed that the apparent viscosity of the H−ESO−HEC emulsions increased when the H−ESO−HEC concentrations increased. Steady flow curves provided evidence of interfacial film formation in the emulsions. The evolution of G′, G″, and tan η indicated that all emulsions

display a predominantly elastic behavior. In conclusion, these results have important implications for the design and production of polymers applied to food emulsions.

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AUTHOR INFORMATION

Corresponding Authors

*(H.L.) E-mail: [email protected]. Phone: 086-25-85482452. Fax: 086-25-85482499. *(S.S.) E-mail: [email protected]. ORCID

He Liu: 0000-0001-9177-9459 Funding

This work was supported by the Fundamental Research Funds of CAF (CAFYBB2017QB007), the Foundation of Jiangsu Province Biomass Energy and Material Laboratory (JSBEM-S201504), the National Natural Science Foundation of China (31200446), and the Central Special Foundation for Basic Research in Public Interest of the Chinese Academy of Forestry (CAFINT2014C07). Notes

The authors declare no competing financial interest.

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