Blocking and Blending: Different Assembly Models of Cyclodextrin and

Jul 31, 2015 - In route I, the protein, due to its higher affinity for the interface, adsorbs strongly at .... interface (route I), and the other is S...
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Blocking and Blending: Different Assembly Models of Cyclodextrin and Sodium Caseinate at the Oil/Water Interface Hua-Neng Xu,* Huan-Huan Liu, and Lianfu Zhang* State Key Laboratory of Food Science and Technology, Key Laboratory of Food Colloids and Biotechnology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu 214122, People’s Republic of China ABSTRACT: The stability of cyclodextrin (CD)-based emulsions is attributed to the formation of a solid film of oil−CD complexes at the oil/water interface. However, competitive interactions between CDs and other components at the interface still need to be understood. Here we develop two different routes that allow the incorporation of a model protein (sodium caseinate, SC) into emulsions based on β-CD. One route is the components adsorbed simultaneously from a mixed solution to the oil/water interface (route I), and the other is SC was added to a previously established CD-stabilized interface (route II). The adsorption mechanism of β-CD modified by SC at the oil/water interface is investigated by rheological and optical methods. Strong sensitivity of the rheological behavior to the routes is indicated by both steady-state and small-deformation oscillatory experiments. Possible β-CD/SC interaction models at the interface are proposed. In route I, the protein, due to its higher affinity for the interface, adsorbs strongly at the interface with blocking of the adsorption of β-CD and formation of oil−CD complexes. In route II, the protein penetrates and blends into the preadsorbed layer of oil−CD complexes already formed at the interface. The revelation of interfacial assembly is expected to help better understand CD-based emulsions in natural systems and improve their designs in engineering applications.

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interfacial films and consequently the stability of the emulsions. CDs differ fundamentally from rigid hard particles, regarding their interfacial assembly. The ICs can be formed spontaneously at the oil/water interface by a self-assembly process, without the need to overcome high adsorption barriers as found for hard particles. The ICs have been fully characterized in terms of shape, size, and physicochemical properties, which are dependent on the type of CD used.9 The properties make CDs ideal candidates for preparing emulsions with tunable stability. Sodium caseinate (SC) has been widely used as an emulsifier to facilitate the formation of emulsions, improve the stability of emulsions, and provide specific physicochemical properties to emulsions. SC contains various casein components (αs1-, αs2-, β-, and κ-caseins), with the majority of the proteins being random coil proteins.19 The distribution of hydrophobic and hydrophilic residues on each casein allows for rapid adsorption of the molecules at the oil/water interface and good stabilization of emulsion droplets through a combination of steric and electrostatic interactions. The existence of competitive adsorption between different components at the oil/ water interface in SC-stabilized emulsions has been documented: protein−protein, protein−surfactant, and protein− polysaccharide.20−25 It was found that there is a close relationship between the emulsion stability and the exposure methods of proteins to the interface, and the interfacial

INTRODUCTION Emulsions are of great practical importance due to their widespread use in food, pharmaceutical, and cosmetic products. Stable emulsions are normally prepared by adsorbing surfaceactive species such as surfactants, polymers, and particles at the liquid−liquid interface, which protect the emulsions against various mechanisms of destabilization such as creaming, flocculation, and coalescence.1,2 Hence, the development of emulsions with improved or novel physicochemical properties relies on understanding the interfacial behavior of adsorbed species and on elucidating the relationship between interfacial characteristics and bulk physicochemical properties of emulsions. Recently, cyclodextrins (CDs) have been used as emulsion stabilizers in replacement of surfactants.3−7 In CD-based emulsions, oil−CD inclusion complexes (ICs) are formed spontaneously by threading CDs from the aqueous phase on oil molecules from the emulsion drop surface. Such ICs can grow further into microcrystals, which remain attached on the surface of emulsion droplets and generate densely packed layers which resemble Pickering emulsions (i.e., solid stabilized emulsions).8−12 Pickering emulsions refer to emulsions that are physically stabilized by solid colloidal particles. Various types of colloidal particles with different sizes, shapes, and surface activities, including silica,13 cellulose nanocrystals,14 modified starch granules,15 and soy protein and zein particles,16−18 have been studied as emulsifiers in Pickering emulsions. The practical efficacy that the particles adsorb and arrange at the interface directly impacts the mechanical strength of the © XXXX American Chemical Society

Received: June 9, 2015 Revised: July 30, 2015

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emulsion with an oil volume fraction of 0.3. The emulsion was stirred at a rate of 100 rpm for at least 1 h for SC to diffuse to the β-CDstabilized interface. Optical Microscopy. The morphology of the emulsions was studied by using an Olympus BX 51 optical microscope (Japan), and the images were captured with a DP70 Olympus digital camera (Japan) using Image Pro Plus Software (United States).The emulsions were diluted 10 times with distilled water, and 10 μL was placed on a glass slide with a cover glass for microscopic observation. Measurement of the Emulsion Drop Size. The drop size distribution of the emulsions was determined using a computercontrolled bluewave laser particle analyzer (Microtrac, Montgomeryville, PA). An aliquot (1 mL) of each emulsion was diluted with 1000 mL of distilled water. The refractive index and adsorption of the dispersed phase were set as 1.456 and 0.001, respectively, and the refractive index of the continuous phase was 1.333. The average diameter (d4,3) is calculated as

composition and structure of mixed layers are predominantly determined by two mechanistic phenomenacompetitive adsorption from mixed solution and cooperative adsorption into multilayers. The formation of an interfacial film usually involves a stepwise process: diffusion of species to the interface, adsorption at the interface, and rearrangement at the interface. The technique of shear rheology can give useful and sensitive information about competitive adsorption and cooperative interactions in mixed protein films. For instance, the surface viscosity of the mixed protein system of β-casein and βlactoglobulin is highly dependent on the introduction sequence of the two proteins to the oil/water interface.26 The casein has a rather disordered structure, and hence undergoes rapid conformational changes and adsorption at the interface. When the mixtures are introduced together at the interface during emulsification, the casein invariably will dominate at the interface. Conversely, if β-lactoglobulin has previously formed a close-packed elastic layer at the interface, it is then extremely difficult to competitively displace the existing layer with βcasein added to the aqueous phase. The coexistence of CDs and proteins is also common in many food emulsions; thus, the formation of oil−CD ICs and the interfacial properties of their microcrystals may be particularly sensitive to the addition of proteins. However, to the best of our knowledge, no attempt has yet been made to examine the competitive interactions between CDs and proteins at the interface. In this study, two types of experiments of the addition of SC on the stability of CD-stabilized emulsions, as well as on the mechanical strength of the interfacial film, have been compared. One type is β-CD and SC adsorbed simultaneously from a mixed solution to the oil/water interface (route I), and the other is SC was added to a previously established CD-stabilized interface (route II). As the adsorption ability of the species greatly influences the interfacial viscoelasticity and thereby is responsible for emulsion stability, we make a shear rheology measurement that is sensitive to interactions and structural change within the interfacial films. We also present observable differences in the rheological properties and microstructure for the two types of emulsions. The assembly models of CD and SC at the oil/water interface can bring complementary information for the comprehension of Pickering-type emulsions in natural systems and improve their designs in engineering applications.

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d4,3 =

∑ nidi 4/∑ nidi 3

(1)

where ni is the number of droplets with diameter di. The results reported were obtained from the average of three readings of a testing sample. Confocal Laser Scanning Microscopy (CLSM). The microstructure observation of the samples was carried out with a confocal laser scanning microscope (Zeiss LSM 710, Carl Zeiss Inc., Braunschweig, Germany). A 0.01 g·L−1 solution of rhodamine B (Sigma Chemicals) was used to stain for the sodium caseinate.28,29 The excitation wavelength was 555 nm. The emulsion sample was placed in a welled slide with a coverslip. Confocal images were recorded within 10 min of posthomogenization sample preparation. Measurement of Dynamic Interfacial Tension. The dynamic interfacial tension (DIT) was measured through the pendant drop method on an OCA15EC tensiometer (Dataphysics Ltd., Germany) with a CMOS camera for drop-image processing. A drop of aqueous dispersion was formed in soybean oil, and then the dynamic interfacial tension was determined at 25 °C through rapid acquisition of the drop image, edge detection, and fitting of the Laplace−Young equation. Each measurement was repeated in triplicate, and the result was expressed as the average ± standard error. Measurement of Interfacial Shear Rheology. The interfacial shear moduli of β-CD/SC mixtures at soybean oil/water interfaces as a function of the SC concentration were obtained using the double-wallring method. Small-amplitude oscillatory experiments were studied using a stress-controlled rotational rheometer (AR-G2, TA Instruments) outfitted with a Du Noüy ring (radius R = 20 mm) suspended from the upper geometry mount. The ring was placed concentrically within the gap of a PTFE cup that was machined to provide a double Couette effect of interfacial shear on either side of the ring. The experimental apparatus has already been described elsewhere.30 Oscillatory frequency sweep tests covering the range from 0.1 to 10 rad s−1 were performed in the linear viscoelastic regime (a strain amplitude γ of 0.005%), as determined by dynamic strain sweep measurements. Each rheological measurement was repeated in triplicate, and the result was expressed as the average ± standard error. Rheological Measurement of Emulsions. Steady-shear and small-amplitude oscillatory experiments for the emulsions were performed on a stress-controlled rotational rheometer (AR-G2, TA Instruments) in parallel-plate geometry (plate diameter 40 mm). The gap between the two plates was set to 1.0 mm, and the experiments were carried out at 25 °C. Steady-shear rate flow curves were acquired covering the range from 0.1 to 100 s−1. Oscillatory frequency sweep tests covering the range from 0.1 to 100 rad s−1 were performed in the linear viscoelastic regime (0.4% strain), as determined by dynamic strain sweep measurements. Each rheological measurement was repeated in triplicate, and the result is expressed as the average ± standard error.

EXPERIMENTAL SECTION

Materials. β-Cyclodextrin (purity >98%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Soybean oil (purity >99%) was purchased from a local supermarket and purified with Florisil (60−100 mesh, Sigma-Aldrich) to remove surface-active impurities as described elsewhere.27 Sodium caseinate (5.86 wt % moisture, 0.014 wt % calcium) was obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). All the other chemical reagents used were of analytical grade and were used without any further purification. Preparation of Emulsions. Soybean oil-in-water emulsions were prepared with a high-speed homogenizer (FLUKO Equipment, Shanghai, China) operated at 19 000 rpm for 5 min. Emulsions containing 0.6 wt % β-CD and different concentrations of SC (0−1 wt %) were obtained in two different ways. An emulsion from route I was obtained by emulsification of oil with aqueous β-CD/SC solutions, and the volume fraction of oil was 0.3. The β-CD/SC solutions were prepared through mixing respective fresh stock solutions and stirred for at least 1 h before emulsification. An emulsion from route II was obtained by slow addition of a solution of SC to a β-CD-stabilized B

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Figure 1. Photographs of vessels containing CD-based emulsions with different SC concentrations for (a) route I and (b) route II (β-CD concentration of 0.6 wt %, SC concentration of 0−1 wt %).

Figure 2. Drop size (d4,3) of CD-based emulsions as a function of the SC concentration for route I and route II (β-CD concentration of 0.6 wt %, SC concentration of 0−1 wt %).

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RESULTS AND DISCUSSION The oil-in-water emulsions containing 0.6 wt % β-CD and different concentrations of SC (0−1 wt %) were prepared via simultaneous and sequential methods. Figure 1 shows the emulsions change in appearance from gel-like to liquid-like with an increase of the SC concentration. However, the emulsions of route II show liquid-like behavior when the SC concentration is higher than 0.5%, while the emulsions of route I show it only at 0.05%. The difference may stem from the different assembly models of CD and SC at the oil/water interface, which is discussed later. In the absence of SC, the adsorption of CD and the growth of IC microcrystals at interfaces can lead to the formation of a viscoelastic network, in which the emulsions become essentially immobile. The network might be

Figure 3. Optical microscopy images of CD-based emulsions with different SC concentrations for (a) route I and (b) route II (β-CD concentration of 0.6 wt %, SC concentration of 0−1 wt %).

disassembled by the SC, and the extent of the disassembly is a function of the protein concentration. The average drop size and morphology of the emulsion samples are shown in Figures 2 and 3, respectively. In route I, C

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Figure 4. CLSM images for emulsions of (a) β-CD, (b) a β-CD/SC mixture prepared by route I, and (c) a β-CD/SC mixture prepared by route II (β-CD concentration of 0.6 wt %, SC concentration of 0.05 wt %). The protein is stained with rhodamine B (excitation at 555 nm).

Figure 6. Equilibrium interfacial tension (EIT) of SC with different concentrations in the presence and absence of β-CD (β-CD concentration of 0.6 wt %, SC concentration of 0.05−1 wt %).

possible if the amount of emulsifier available is too low to fully cover the newly created interface. In the emulsion system containing mixtures of β-CD and SC, there is competitive adsorption at the interface between them. Because of its superior interfacial activity, SC can be adsorbed preferentially at the interface, and prevent or displace β-CD from being adsorbed from the interface during emulsification. This type of emulsion flocculation arising from competitive adsorption between stabilizing components of different surface activities is referred to as bridging flocculation.31,32 It was also reported that, for high volume fraction oil-in-water emulsions made with mixtures of SC and gelatin, the degree of flocculation in the emulsions was very sensitive to the emulsifier composition. The behavior was interpreted in terms of bridging flocculation caused by partial displacement of gelatin from the droplet surface by the more surface-active SC.31 In route II, however, the value of d43 is found to be essentially independent of the SC content, and the oil droplets are nicely dispersed without flocculation even at low SC concentration (Figure 3b). This indicates that by allowing the formation of IC microcrystals at the oil/water interface prior to the addition of SC, the emulsions become more resistant to coalescence and flocculation. The adsorption of SC may also have the effect of surface negative charges, which in turn increases the surface potential of emulsion droplets and thus prevents the droplets from getting closer to each other. It can therefore be inferred that different adsorbed film structures are present at the interface in the two routes. Figure 4 shows CLSM micrographs of emulsion samples containing only β-CD (Figure 4a) and β-CD/SC mixtures

Figure 5. Dynamic interfacial tension (DIT) curves for SC with different concentrations (a) in the presence of β-CD and (b) in the absence of β-CD (β-CD concentration of 0.6 wt %, SC concentration of 0.05−1 wt %).

as the SC concentration increases, the value of d4,3 rises to a maximum at a low concentration of 0.05 wt % and then decreases rapidly (Figure 2). At low SC concentration (e.g., 0.05 wt %), as there are not enough SC molecules present to fully cover the emulsion droplets, the emulsions may be destabilized by coalescence, and hence, the value of d4,3 increases. With an increase of the SC concentration, the emulsion droplets become effectively stabilized and the value of d4,3 decreases. The associated state of emulsion droplets can be found from the optical microscopy images (Figure 3a), which confirm the sensitivity of the degree of flocculation to the SC concentration. During the emulsion preparation, flocculation is D

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Figure 7. σ−t1/2 and σ−t−1/2 for SC and β-CD/SC solutions with different SC concentrations (β-CD concentration of 0.6 wt %, SC concentration of 0.05−1 wt %).

aggregates in the aqueous phase. Since the CD-stabilized emulsion was prepared before addition of SC, it is not expected that the β-CD would be displaced from the interface by addition of SC. Interfacial tension and interfacial rheological properties are fundamental quantities that relate to the assembly properties of adsorbed species at interfaces and play a crucial role in the process of emulsion formation and stabilization. To understand well the interfacial behavior of the β-CD/SC mixtures at the oil/water interface, the dynamic interfacial tension (DIT) was determined by the time-dependent drop-shape analysis at various SC concentrations. The results of DIT in the presence and absence of β-CD are shown in Figure 5, and the corresponding equilibrium interfacial tension (EIT) is shown in Figure 6. It is observed that the interfacial tension initially decreases quickly but then stabilizes within 1000 s following drop formation. The DIT decreases only very slowly over the rest of the measurement period (2500 s), reaching a final equilibrium value. The slow decrease in DIT indicates that some additional adsorption and reorganization of SC may happen at the oil/water interface. It is well-known that SC is an emulsifier that can decrease interfacial tension effectively. The decrease in DIT with an increase of the SC concentration suggests that the adsorbed amount of SC at the interface grows with the SC concentration. It also shows that the presence of βCD make no difference in the EIT, compared with that in the absence of β-CD (Figure 6), which confirms the low interfacial activity of β-CD. In addition to that, the EIT data can be used to ascertain whether β-CD and SC interact with each other in the aqueous phase. It was reported that β-CD can form an inclusion complex with β-casein, in which β-CD serves as the

Table 1. Effective Diffusion Coefficients for SC in the Absence and Presence of β-CD with Different SC Concentrations (β-CD Concentration of 0.6 wt %, SC Concentration of 0−1 wt %) effective diffusion coefficients (m2/s) t→0

concn of SC (wt %) 0.05 0.10 0.50 1 β-CD/SC 0.05 0.10 0.50 1

t→∞

(4.12 (4.74 (3.89 (4.99

± ± ± ±

SC 0.09) × 0.07) × 0.08) × 0.06) ×

(4.01 (4.75 (3.83 (4.94

± ± ± ±

0.07) 0.05) 0.08) 0.03)

10−11 10−12 10−14 10−13

(1.36 (7.25 (3.39 (5.88

± ± ± ±

0.08) 0.04) 0.05) 0.02)

× × × ×

10−13 10−14 10−15 10−16

× × × ×

10−11 10−12 10−14 10−13

(1.38 (7.21 (3.37 (5.80

± ± ± ±

0.01) 0.02) 0.04) 0.01)

× × × ×

10−13 10−14 10−15 10−16

prepared by route I and route II (Figure 4b,c). The concentrations of β-CD and SC are 0.6 and 0.05 wt %, respectively. The protein is colored red with rhodamine B, and the emulsion droplets appear as circular regions. In Figure 4b, some clear bright red rings are visible around the emulsion droplets and the aqueous phase region appears very dark. It seems reasonable to infer that these rings correspond to SC layers around the droplets, suggesting that, in route I, the SC is closely associated with the oil/water interface. In Figure 4c, the medium between the emulsion droplets does not appear to be as homogeneous in red intensity as that with β-CD alone (Figure 4a): that is, a noticeable pattern is formed in the dark regions, suggesting that, in route II, the SC may exist as some E

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Figure 10. Frequency dependence of elastic moduli (G′) and viscous moduli (G″) for emulsions of pure β-CD, route I and route II (β-CD concentration of 0.6 wt %, SC concentration of 0.05 wt %).

formation of inclusion complexes appears to involve some specific attribute, such as suitable size, shape, and polarity. An attempt has been made to fit the interfacial data at various SC concentrations using some conventional adsorption models, such as the Langmuir equation. However, the models provide a poor fit to the data. The time-dependent structural changes of adsorbed SC may decrease the suitability of the adsorption models compared to adsorption of common surfactants.34 Taking into account these particularities, here we have applied a reported diffusion-controlled adsorption model to the DIT results.34,35 The adsorption process is characterized by effective diffusion coefficients at short adsorption time (t → 0) and long adsorption time (t → ∞), which can be obtained as follows:35

Figure 8. Frequency dependence of interfacial elastic moduli (G′) for β-CD/SC mixtures with different SC concentrations: (a) route I and (b) route II (β-CD concentration of 0.6 wt %, SC concentration of 0− 1 wt %).

2 π ⎡ 1 ⎛ dσ ⎞ ⎤ ⎢ ⎥ ⎜ ⎟ Deff = 4 ⎢⎣ RTc ⎝ dt 1/2 ⎠t → 0 ⎥⎦

(2)

4

Deff = π

( ddlnσ c )

2

(RTc( )) dσ

dt

−1/2

t →∞

(3)

where σ is the interfacial tension, R is the universal gas constant, T is the temperature, and c is the concentration in the bulk phase. The σ−t1/2 and σ−t−1/2 dependencies for various SC concentrations are shown in Figure 7. It can be seen that there are linear dependencies for both cases (t → 0 and t → ∞), and therefore, the procedure for determining the effective diffusion coefficients as described above should be applicable. The effective diffusion coefficients obtained at different SC concentrations are shown in Table 1. They have almost the same values in the absence and presence of β-CD. The results show that the presence of β-CD has no effect on the performance of the SC at the oil/water interface. The elastic moduli (G′) measured during the frequency sweep experiments in the linear domain are plotted in Figure 8. Without SC, CD can quickly move to the interface and form a frequency-independent solid film with high elasticity, which can be explained in terms of the presence of strongly bound IC microcrystals. With the addition of SC, a remarkable loss in elasticity of the corresponding interfacial film is found. In route I, the elasticity decreases suddenly at a concentration as low as 0.05 wt % and becomes quite frequency dependent. The remarkable decrease in elastic moduli might be attributed to SC

Figure 9. Shear viscosity versus shear rate for emulsions of pure β-CD, route I and route II (β-CD concentration of 0.6 wt %, SC concentration of 0.05 wt %).

host and β-casein occupies the cavity in the CD as a guest molecule.33 Since SC possesses many hydrophobic groups that are probably exposed, it seems that the formation of CD-SC complexes might also occur. However, if they are complexed, differences in interfacial tension should be observed between the presence and absence of β-CD. On the basis of EIT results, it is conceivable that SC seems inefficient to form inclusion complexes with β-CD, which may be related to the existence of some interference effects such as steric hindrance. The F

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Figure 11. Schematic representation of the assembly model for the β-CD/SC system at the oil/water interface.

blocking the formation of a solid film of oil−CD ICs at the interface, which is due to the higher interfacial activity of SC molecules. In route II, however, the elastic moduli decrease rather slowly with the SC concentration and remain frequency independent. It is obvious that there is a gradual modification of the structure and composition of the interfacial layer. The different interfacial responses in the two routes imply not only distinct interfacial dynamics but also different stabilizing mechanisms for the emulsions. To determine and compare the viscoelastic properties of the emulsions for the two routes, rheological measurements of the emulsions at an SC concentration of 0.05 wt % have been conducted. Figure 9 shows the flow curves of the emulsions. It can be seen that the emulsions exhibit pseudoplasticity, i.e., a pronounced shear-thinning behavior with high viscosities at low shear rates. This can be interpreted as indicating the presence of weak attractive forces between the emulsion droplets, which give rise to the formation of a weak elastic gel-like network. The addition method of SC appears to have a significant effect on the viscosity of the emulsions. A reduction in shear viscosity at a high rate is observed upon the addition of SC in route I, while a large increase in shear viscosity is observed in route II. The different influences of SC on the emulsion viscosity reflect the different assembly models of SC and CD at the interface. One important observation that can be made is that the emulsions of route I appear to become even more shear thinning at higher shear rates. The large decrease in shear viscosity is presumably a reflection of restructuring of the transient network of flocculated emulsion droplets under flow. As there is a more extended aggregation of emulsion droplets and a certain

amount of continuous phase entrapped within the aggregated structure in route I, and thus under conditions of high shear rates, the aggregated structure breaks up and some trapped continuous phase is released. Consequently, increasing the shear rate leads to a greater decrease in the effective dispersedphase volume fraction and hence to a greater reduction in the apparent viscosity.36 Figure 10 shows the effects of SC on the viscoelastic properties of the emulsions. G′ is much larger than G″ over the entire frequency range, indicating that the behavior of the emulsions is predominantly elastic and there is a threedimensional network in the emulsions. Similar to the results of shear viscosity, the presence of SC induces a dramatic reduction in emulsion viscoelastic strength in route I, which might be attributable to the SC blocking the formation of a solid film of oil−CD ICs at the oil/water interface. Being less “structured”, these emulsions have lower elastic and viscous moduli. In contrast, route II shows that the addition of SC induces a remarkable increase in G′ and G″ compared with the values for the emulsion with pure β-CD. The increase in viscoelastic properties is rather unexpected and cannot be attributed to the displacement of oil−CD IC microcrystals by SC, as the displacement would normally be expected to produce a reduction in viscoelastic properties. It is possible that the formation of oil−CD IC microcrystals increases the ability of βCD to resist competitive displacement by SC. Consequently, the SC only blends with oil−CD IC microcrystals via coadsorption or complexation at the interface, resulting in a more viscoelastic emulsion. G

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On the basis of the experimental results, a schematic illustration of the changes in assembly models at the oil/ water interface for the two routes is depicted in Figure 11. In route I, with the addition of SC, the formation of oil−CD complexes is blocked. Hence, the emulsions are of a proteinstabilized type, compared with the Pickering type of pure β-CD. In route II, the emulsions have gel-like properties because the oil−CD IC microcrystals act as sticky particles and form threedimensional networks. With the addition of SC, entanglement and blending between SC and oil−CD IC microcrystals might happen, introducing an additional reinforcement on the threedimensional network of the emulsions. Similar competitive adsorption and cooperative interactions in a binary mixture of caseins and globular proteins at the oil/water interface have also been detected by surface shear rheology measurements.26 The surface shear properties of the mixed proteins are highly dependent on the sequence of introduction of the two proteins to the oil/water interface. It was found that there is a definite tendency for the caseins to adsorb in preference to the globular proteins, whereas the adsorbed globular proteins show a highly viscoelastic character that distinguishes them from the adsorbed caseins. The highly viscoelastic character of adsorbed layers of globular proteins is attributable to their high packing density and the strong protein−protein interactions, as compared with the loose packing and the weak interactions of casein layers. As the oil−CD ICs are formed spontaneously at the oil/water interface by a self-assembly process, the IC microcrystals thereby may lead to a more heterogeneous surface morphology than for the pure globular protein layers. Hence, in route II, the heterogeneity of IC microcrystals may allow the further adsorption of SC onto the interface, and subsequent formation of cohesive films. Overall, the results suggest that the rheological properties in CD-based emulsions can be controlled by adjusting the addition method of SC and its amount, which provide clues on how CD and an interfacial competitor can be mixed in emulsions to take advantage of their different interfacial properties. Therefore, this method can be viewed as a novel way of manipulating the microstructure and texture of CDbased emulsion systems.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 20906039) and the Key Laboratory of Food Colloids and Biotechnology (Grant No. JDSJ2013-04).

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CONCLUSION The addition of SC can efficiently tailor the interfacial structure and composition of CD-based emulsions, either via blocking the formation of oil−CD ICs in a simultaneous way or via blending with oil−CD ICs in a sequential way, which in turn leads to a transition of the emulsion type. This is interpreted in terms of a higher interfacial activity of SC than that of β-CD, which is witnessed by the interfacial tension data of SC in the absence and presence of β-CD. The presence of β-CD has no effect on the performance of the SC at the oil/water interface. The significant changes in the viscoelastic properties of the emulsions reveal that the introduction method of SC to the oil/ water interface is one of the decisive factors in designing the structure and properties of an interfacial film, which is very important to the general behavior of real CD-based emulsions.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b02111 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b02111 Langmuir XXXX, XXX, XXX−XXX