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Formation and characterization of oil-in-water emulsions stabilized by polyphenol-polysaccharide complexes: Tannic acid and β-glucan
Food Research International 123 (2019) 266–275
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Formation and characterization of oil-in-water emulsions stabilized by polyphenol-polysaccharide complexes: Tannic acid and β-glucan
T
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Ruyi Lia, Shengfeng Penga, Ruojie Zhangb, Taotao Daia, Guiming Fua, , Yin Wana, Chengmei Liua, ⁎ David Julian McClementsb, a b
State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang, Jiangxi 330047, PR China Biopolymers and Colloids Laboratory, Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Tannic acid β-Glucan Interfacial tension Emulsion stability Microstructure
Oil-in-water emulsions were prepared that were stabilized by a polyphenol (tannic acid, TA) and/or a polysaccharide (β-glucan, BG). The influence of TA concentration and solution pH on the physical stability and microstructure of the emulsions was investigated. Emulsions formed using only BG (1%) contained large droplets that were unstable to flocculation and coalescence. The stability of the emulsions (pH 5) could be enhanced by optimizing the ratio of TA-to-BG used to form them, i.e., TA/BG = 0.4 or 0.5. This effect was attributed to a combination of increased steric hindrance and reduced hydrogen bonding of TA to BG. Transmission electron microscopy (TEM) indicated that increasing the level of TA present increased the compactness and thickness of the TA-BG interfacial layers formed around the oil droplets. Furthermore, the stability of the emulsions to droplet flocculation and coalescence depended on the TA/BG ratio and solution pH. The impact of environmental conditions on emulsion stability was also investigated. Emulsions stabilized by TA-BG complexes (TA/ BG = 0.5) remained stable from pH 5 to 9 at ambient temperature and at temperatures ≤60 °C at pH 5, but were highly unstable to salt addition at pH 5. Our results may increase the breadth of applications of β-glucan as a functional ingredient in foods.
1. Introduction During the past decade or so, there has been growing interest in using plant-based emulsifiers, rather than synthetic or animal-based ones, in food products because of their potential benefits in terms of the environment, sustainability, human health, and reduction in animal cruelty (Bai, Huan, Li, & McClements, 2017; Lam & Nickerson, 2013; McClements & Gumus, 2016; Wan, Guo, & Yang, 2015). However, many plant-derived ingredients that are available in abundant quantities, such as the proteins from corn and wheat or the polysaccharides from starch and cellulose, are not naturally good emulsifiers, which may be due to their poor water-solubility characteristics, their low surface activity, or their poor emulsion stabilizing properties (McClements & Gumus, 2016; Wan et al., 2015). For this reason, many researchers have focused on enhancing the functional attributes of plant proteins and polysaccharides by utilizing covalent and/or noncovalent interaction with other types of plant-based materials, such as polyphenols (Foegeding, Plundrich, Schneider, Campbell, & Lila, 2017; Jakobek, 2015; Liu, Ma, Gao, & McClements, 2017). Moreover, some polyphenols have been reported to have a beneficial impact on human ⁎
health and food quality due to their anticarcinogenic, antioxidant, antibacterial, anti-inflammatory, and antiviral activities (Jakobek, 2015). For this reason, it may be possible to develop biopolymer-polyphenol ingredients that serve multiple functions in foods: emulsification, antioxidant, antimicrobial, and nutraceutical. Non-covalent interactions between polyphenols and plant biopolymers have been widely studied, e.g., chlorogenic acid with sunflower protein (Karefyllakis, Altunkaya, Berton-Carabin, van der Goot, & Nikiforidis, 2017), tannic acid with zein (Wang et al., 2016), resveratrol or rutin with soy protein (Chen, Wang, Yang, Qi, & Hou, 2016; Wan, Wang, Wang, Yuan, & Yang, 2014), phytic acid with whey protein (Pei et al., 2019), thymol and polysaccharides (De Fenoyl et al., 2018), tannic acid with methylcellulose (Patel, ten-Hoorn, Hazekamp, Blijdenstein, & Velikov, 2013), and tea polyphenols with oat β-glucan (Wu et al., 2011). These studies have demonstrated that the functional properties of non-covalent complexes formed from polyphenols and plant biopolymers can be improved compared to the individual components. This is because phenolic compounds have multiple reactive groups that can form cross-links with biopolymers, thereby enhancing their functional attributes, such as gel formation (Mamet, Yao, Li, & Li,
Corresponding authors. E-mail addresses: [email protected] (G. Fu), [email protected] (D.J. McClements).
https://doi.org/10.1016/j.foodres.2019.05.005 Received 26 March 2019; Received in revised form 29 April 2019; Accepted 2 May 2019 Available online 03 May 2019 0963-9969/ © 2019 Published by Elsevier Ltd.
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to prepare all solutions and emulsions.
2017). Also, colloidal particles assembled using non-covalent interactions between polyphenols and biopolymers can be used as functional ingredients to stabilize emulsions and foams (Dickinson, 2017). Based on this earlier research, we investigated the possibility of extending the functional attributes of β-glucan (BG) by forming non-covalent complexes with tannic acid (TA). BG is a water-soluble polysaccharide that is present at relatively high levels in a variety of foods from plant or microbial origin, including oat, barley, wheat, mushroom, and yeast (Zhu, Du, & Xu, 2016). BG has been reported to exhibit a broad range of health benefits, including lowering blood cholesterol, regulating blood glucose, inhibiting inflammatory, and preventing cancer (Zhu et al., 2016). Besides its health and nutritional benefits, BG also has various physicochemical attributes that make it useful as a functional ingredient in foods and beverages, such as thickening, stabilizing, and gelation (Ahmad, Anjum, Zahoor, Nawaz, & Dilshad, 2012). However, BG is predominantly a hydrophilic molecule that has little surface-activity and is therefore not a good emulsifier, which limits its applications in many products. This problem may be overcome by forming noncovalent complexes with other more hydrophobic or amphiphilic substances. As an example, when used alone, BG (1%) was found to exhibit poor performance as an emulsifier but when it was used in combination with whey protein (1%) it was able to reduce the droplet size and increase the emulsion stability (Burkus & Temelli, 2000). Tannic acid (TA), a commercial form of tannin suitable for use in foods, has been shown to have a diverse range of biological activities including antioxidant, antibacterial, and antiviral properties (Serrano, Puupponen-Pimiä, Dauer, Aura, & Saura-Calixto, 2009). In addition, it has been shown that it is capable of forming strong interactions with proteins, polysaccharides, alkaloids, and metal ions (Serrano et al., 2009). The complexation of TA with biopolymers has been shown to enhance their emulsifying properties and antioxidant activity (Huang, Li, Qiu, Teng, & Wang, 2017; Patel et al., 2013; Wang et al., 2016). TA-biopolymer complexes may therefore be suitable for application as multifunctional ingredients in foods and beverages. In a previous study, we successfully prepared TA-BG complexes in aqueous solutions and characterized their physicochemical and structural properties (Li et al., 2019). The size and stability of the complexes formed was shown to depend on the TA/BG ratio and the pH of the surrounding solution. The complexes formed were held together by non-covalent linkages, which meant that no chemical or biological cross-agent was required, which is beneficial in terms of ease of production, cost, safety, and food legislation (Huang et al., 2017). In the current study, we aimed to determine whether TA-BG complexes could be used as emulsifiers to form and stabilize oil-in-water emulsions. Emulsions were characterized by particle size distribution, microstructure, and storage stability measurements. Based on the fact that the TA-BG complexes are only held together by relatively weak non-covalent interactions, we also examined the impact of environment stresses on emulsion properties. This study should therefore provide useful insights into the potential application of TA-BG complexes as natural plant-based emulsifiers in foods and beverages.
2.2. Interfacial tension measurements The interfacial tension between the aqueous phase (TA, BG, or TABG solutions) and the oil phase (MCT) was determined using droplet shape analysis (DSA 100, Krüss GmbH, Hamburg, Germany). For water droplet formation, a straight-needle with a diameter of 1.088 mm was used to create a pendant drop. The water drop was extruded into a quartz cell containing MCT oil. All samples were measured for around 15 min, at 20 s intervals, which was long enough to ensure that the interfacial tension reached a steady value. Digital images were acquired using the instrument's camera. The instrument calculated the interfacial tension by analyzing the shape of the water drops and interpreting the data using the Young–Laplace equation. 2.3. Emulsion preparation Oil-in-water emulsions were fabricated by homogenizing an oil phase (5 wt%) with an aqueous phase (95 wt%). The oil phase consisted of MCT while the aqueous phase consisted of TA-BG mixtures (1% BG, with a TA/BG mass ratio of 0.1, 0.2, 0.3, 0.4, 0.5 and 0.8) in buffer solution (5 mM, pH 3, 5 or 7). Initially, an emulsion containing relatively large oil droplets was created by homogenizing the oil and aqueous phases using a high-shear mixing device (Bamix, Biospec Products, Bartlesville, OK) for 2 min at room temperature. Subsequently, the droplets in this emulsion were reduced in size by passing it through a high-pressure microfluidizer (M110Y, Microfluidics, Newton, MA) 3-times at a pressure of 12,000 psi. Sodium azide (0.02 wt%) was then added to the emulsions as a microbial preservative. 2.4. Emulsion storage stability Oil-in-water emulsions were poured into glass test tubes that were then incubated at room temperature (≈ 25 °C). The stability of the emulsions was then recorded at 0-day (soon after preparation), 1-day (stored for 24 h), and 7-day (stored for 7 days). The creaming stability was determined by visual observation. 2.5. Determination of particle size The particle size distribution and mean particle diameter (d4,3) were determined by static light scattering (Mastersizer 3000, Malvern Instruments Ltd., Malvern, Worcestershire, UK). Before being analyzed the emulsions were diluted with buffer solution of the same pH to avoid multiple scattering, which would interfere with data interpretation. The refractive index (RI) of the aqueous phase used in the calculations was 1.33 and that of the oil phase was 1.445. 2.6. Determination of ζ-potential The surface potential (ζ-potential) of the oil droplets in the emulsions was measured by microelectrophoresis (Zetasizer Nano ZS, Malvern Instruments, Worcestershire, UK). Again, the emulsions were diluted with buffer solutions of the same pH before being analyzed to avoid multiple scattering effects.
2. Materials and methods 2.1. Materials Medium chain triglycerides (MCT, Miglyol 812 N) were bought from the Warner Graham Co. (Cockeysville, MD). β-glucan powder was bought from the Yikang Co. (Hebei, China). The manufacturer reported that this ingredient contained around 6.7% moisture, 70.3% β-glucan, 3.6% protein, and < 5% ash. Tannic acid (from Chinese gall nuts) and sodium azide were purchased from Sigma-Aldrich Co. (St. Louis, MO). Citric acid, trisodium citrate dihydrate, sodium phosphate monobasic and sodium phosphate dibasic were purchased from Fisher Scientific Co. (Fair Lawn, NJ). Distilled and deionized water (Milli-Q®) was used
2.7. Confocal laser scanning microscopy (CLSM) CLSM imaging of the emulsions was carried out at room temperature using a confocal laser scanning microscope (Nikon D-Eclipse C1 80i, Nikon, Melville, NY, U.S.) with a 40 × objective lens. The lipid phase was stained with Nile Red dissolved in ethanol (1 mg/mL), which was mixed into the emulsions prior to observation. A small volume of sample was placed on a microscope slide for visualization. The 267
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fluorescent dye was excited by an Argon 488 nm laser and the emitted light was detected at 621 nm. 2.8. Transmission electron microscopy (TEM) The morphology of the emulsions was analyzed by taking images using a transmission electron microscope (JEM-2000FX, JEOL, Ltd., Tokyo, Japan). Emulsion samples were diluted (1:10, v/v) with buffer solution (5 mM, pH 3, 5 or 7). One drop of emulsion was placed on a 200-mesh carbon coated copper grid. After 3 min, the grid was stained with uranyl acetate solution (1%) for 4 min and air-dried at room temperature after removing the excess liquid with filter paper. The photographs were taken at various magnifications and 200 kV voltage. 2.9. Emulsion physical stability Freshly prepared emulsions were subjected to environmental stresses that emulsion-based food products might encounter in commercial applications, such as pH variations, salt addition, or heating, and then stored for 12 h at ambient temperature prior to analysis: pH: Emulsions were adjusted to various pH values (2–9) using NaOH and/or HCl solutions with continuously stirring. After 30 min, the pH was measured again and adjusted to the required value (if needed), and then the samples were transferred into glass test tubes. Thermal processing: Emulsions were placed into glass test tubes, which were then incubated in water baths set at temperatures ranging from 30 to 90 °C for 30 min, and then cooled to room temperature. Salt addition: Emulsions were placed in 50 mL beakers, adjusted to different salt levels (100–500 mM NaCl) by adding NaCl, and then transferred into glass test tubes. 2.10. Statistical analysis All measurements were repeated at least three times. The mean and standard deviation values were calculated from these measurements using statistical analysis software (SPSS 19.0 SPSS Inc., Chicago, IL). Analysis of variance (ANOVA) was used to determine significance at p < .05 by Tukey's HSD test. 3. Results and discussion 3.1. Interfacial properties
Fig. 1. A. Influence of TA and BG concentrations on the interfacial tension measured at an MCT-water interface at ambient temperature (pH 5). B. Timedependence of interfacial tension of BG (1%), TA (0.5%) and TA-BG (0.5% TA1% BG) samples at MCT-water interfaces (pH 5).
The interfacial properties of food emulsifiers play a critical role in determining their success at forming and stabilizing food emulsions (Dickinson, 2003). We therefore measured the interfacial tension of BG, TA, and TA-BG solutions to gain a better understanding of their interfacial properties at MCT oil-water interfaces (Fig. 1 and Table. 1). The interfacial tension of the clean oil-water was determined to be 26.18 ± 0.13 mN/m, which is close to the value reported by other researchers (Gomes, Costa, & Cunha, 2018). We then studied the impact of adding increasing levels of the two pure substances, either TA or BG, to the aqueous phase on the interfacial tension. The interfacial tension decreased as the concentration of both TA and BG increased (Fig. 1), which indicates that both molecules were surface active and absorbed to the MCT-water interface. The interfacial tension was higher for BG than TA at low concentrations (< 0.05%), but the opposite effect was observed at higher concentrations. This may have been because of differences in the surface hydrophobicity, molecular weight, and packing efficiency of the two types of molecules. The fact that both pure BG and pure TA were able to appreciably reduce the interfacial tension suggests that did have some surface-activity. However, the interfacial values reached at high emulsifier levels (around 14–16 mN/m) are considerably higher than those reported for biosurfactants or proteins
Table 1 Influence of TA-BG complexes with different mass ratio at different pH values on the interfacial tension measured at MCT-water interface. TA:BG (wt/wt)
Interfacial tension pH 3
1% BG 0.1 0.2 0.3 0.4 0.5 0.8 0.8% TA
14.25 14.51 14.24 14.20 13.95 13.84 13.61 15.93
pH 5 ± ± ± ± ± ± ± ±
0.23 0.11 0.10 0.09 0.09 0.10 0.11 0.13
BCb Ba BCb BCb CDb DEb Eb Ab
14.52 14.51 14.19 14.14 14.11 13.80 13.64 16.07
pH 7 ± ± ± ± ± ± ± ±
0.13 0.09 0.08 0.09 0.08 0.09 0.08 0.12
Bb Ba Cb Cb Cb Db Db Ab
15.07 14.84 14.63 14.67 14.50 14.39 14.00 17.55
± ± ± ± ± ± ± ±
0.16 Ba 0.11 BCa 0.10 CDa 0.13 CDa 0.11 Da 0.11 Da 0.10 Ea 0.12 Aa
Values are given as means ± SD from triplicate determinations; A–E means in the same column with different letters differ significantly (P < .05); a–b means in the same row with different letters differ significantly (P < .05).
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pH 3 > pH 5 > pH 7. The light scattering measurements also indicated that there were distinct differences in the particle size distributions (PSDs) of the emulsions depending on pH and TA-to-BG ratio (Fig. 2). For some commercial applications in the food industry, it is important for emulsions to have a narrow monomodal distribution, as this increases their stability to droplet aggregation and gravitational separation (e.g., soft drinks and nutritional beverages). Under some conditions narrow monomodal distributions were obtained, but under others broad bimodal distributions were observed (Fig. 2B to D). In the absence of TA, all of the emulsions were bimodal, which is undesirable. Monomodal distributions, however, could be produced at all three pH values provided that sufficient TA was included in the formulation. These results suggest that the TA-BG complexes adsorbed to the oil droplet surfaces and formed an interfacial coating that protected them from flocculation and coalescence (Dickinson, 2018).
(around 5–10 mN/m) (Bai, Huan, Gu, & McClements, 2016), which suggests that BG and TA are not strongly surface active. Previous studies have also shown that many food-grade polysaccharides are not particularly surface active because they are primarily hydrophilic (Bai et al., 2017; Dickinson, 2003; McClements & Gumus, 2016). Other studies have reported that some polyphenols, such as gallic acid, catechin, and quercetin, are also mildly surface-active (Di Mattia, Sacchetti, Mastrocola, Sarker, & Pittia, 2010). Conversely, numerous studies have reported that polyphenol-biopolymer complexes are highly effective at decreasing the interfacial tension (Chen et al., 2016; Karefyllakis et al., 2017; Pei et al., 2019). As shown in Fig. 1B, the TA-BG complexes adsorbed to oil-water interfaces more rapidly and deceased the interfacial tension more effectively than BG or TA alone at pH 5. For this reason, the interfacial characteristics of TA-BG complexes at different pH values were studied (Table 1). In general, all of the samples had fairly similar interfacial tensions, with the values ranging from around 14 to 18 mN/m. However, there were some differences depending on pH and sample composition. The interfacial tensions of all the samples were higher at pH 7 than pH 3 or 5, which suggests that the surface-activity of the components was pHdependent, as has been reported elsewhere (Camino, Sánchez, Patino, & Pilosof, 2011; Gharehkhani, Ghavidel, & Fatehi, 2019). This effect may have been due to changes in the conformation and/or charge of the TA and BG molecules, which altered their tendency to adsorb to the interfaces or modified their interfacial packing. Overall, the TA-BG complexes had lower interfacial tensions than the individual components, and the interfacial tension decreased with increasing TA/BG ratio. Other studies have also demonstrated that polyphenol-biopolymer complexes can reduce the interfacial tension more effectively than the individual parts (Chen et al., 2016; Karefyllakis et al., 2017; Pei et al., 2019). A possible reason for this phenomenon is that the smaller polyphenol molecules can pack into the spaces between the larger β-glucan molecules, thereby more effectively improving the interfacial activity of β-glucan. Measurements of the interfacial tension of substances are useful for establishing whether they are surface active or not, but they do not provide a good prediction of the ability of an emulsifier to form and stabilize emulsions. They are not sufficient to predict the ability of emulsifiers to form small droplets during homogenization because this is a highly dynamic process that takes place under complex fluid flow conditions, which is unlike the quiescent conditions used for interfacial tension measurements. Consequently, an emulsifier that is able to effectively decrease the interfacial tension under static conditions may not be able to adsorb to the surfaces of the oil droplets formed during homogenization (Bai et al., 2017; Walstra, 1993). Moreover, interfacial tension measurements do not provide insights into the stability of emulsifier-coated oil droplets to aggregation once the oil droplets have been formed. This is because these measurements do not give information about interfacial thickness or robustness, or about the colloidal interactions acting between the droplets. Consequently, it is always important to measure the performance of emulsifiers under conditions that simulate their application in commercial products.
3.3. Influence of TA-BG complexes on emulsion microstructure The structural organization, thickness, and rheology of the interfacial layers coating oil droplets is known to play an important role in determining emulsion stability (Li, Liu, Liu, Kong, & Diao, 2019; Zhang, Xiong, Chen, & Zhou, 2014). For this reason, we used electron microscopy to provide some insight into the nature of the interfacial layers in the emulsions. The TEM images of the emulsions stabilized by TA-BG complexes had the anticipated appearance of oil-in-water emulsions, i.e., oil droplets surrounded by water (Fig. 3). The oil droplets appeared to be roughly spherical in all the emulsions, but there were some differences in particle dimensions and structural organization depending on pH and TA:BG ratio. The TEM images suggested that the individual oil droplets were smaller at pH 7 than at pH 3 and pH 5 (Fig. 3), which is consistent with the light scattering measurements (Fig. 2). There were, however, some discrepancies between the microscopy and light scattering results in terms of the effects of TA/BG ratio. For instance, at pH 7, the TEM images indicated that smaller oil droplets were present at TA/BG = 0.5 than 0.1, but the light scattering results suggest the opposite. Similarly, at pH 3, the microscopy images suggest smaller oil droplets were present at TA/BG =0.1 than 0.5, but the light scattering results showed the opposite. These apparent discrepancies may be due to the different methods used to prepare the samples for the two analytical methods. For light scattering, the samples were diluted and stirred prior to analysis, whereas for TEM analysis they are dried onto a solid surface. In addition, the light scattering method may be measuring the size of other particles in the system, such as non-adsorbed complexes or flocs, as well as the individual oil droplets. The electron microscopy images also provided some useful insights into the nature of the interfacial structure, indicating the presence of a thick coating around the oil droplets, particularly at pH 3 and 5 (Fig. 3). The coatings formed at a TA/BG ratio of 0.5 appeared to be thicker and more densely packed than those formed at a TA/BG ratio of 0.1, which may have important consequences for emulsion stability. This could because increasing the TA concentration strengthened the interaction between TA and BG. In a previous study, we showed that TA-BG complexes (ratio = 0.5:1) could form colloid particles (Li et al., 2019), which absorbed to oil-water interfaces and formed densely packed coatings. For instance, the steric repulsion between emulsion droplets is known to increase as the interfacial layers become thicker and denser (McClements, 2015). Currently, it is unclear if the TA-BG complexes themselves are altered by the microfluidization process, as well as how this may impact their tendency to adsorb to the oil droplet surfaces and form interfacial coatings. In particular, the complexes may be partially or fully dissociated during microfluidization and then reform at the droplet surfaces after homogenization. Nevertheless, our TEM results do show that the end result is the creation of a TA-BG shell around the oil droplet
3.2. Influence of TA-BG complexes on emulsion formation For this reason, the impact of the TA-BG complexes on the formation of oil-in-water emulsions using high-pressure homogenization was measured. The size of the particles in emulsions fabricated using TA-BG complexes were measured at pH 3, 5 and 7 (Fig. 2). The mean particle diameter (d4,3) of the emulsions clearly depended on the pH and TA-toBG ratio of the emulsions (Fig. 2A). Smaller particles were produced using TA-BG complexes than using the individual components, but the minimum in the droplet size depended on the pH of the surrounding solution. The smallest particles were produced at TA:BG ratios of around 0.1, 0.2, and 0.4 at pH 7, 5, and 3, respectively. At a fixed TA:BG ratio, the measured particle size decreased in the following order: 269
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Fig. 2. A. Influence of pH and TA-BG ratio on the mean particle diameter (d4,3) of 5 wt% MCT emulsions (measured within 3 h of preparation). Particle size distributions of emulsions containing different mass ratios of TA and BG: pH 3 (B), pH 5 (C), and pH 7 (D).
microscopy images were in good agreement with the particle size and creaming stability results. The emulsions stabilized by only BG contained many highly flocculated oil droplets at all three pH values, especially after 7 days storage (Fig. 5A, B and C). Again, this result suggests that BG was not a good emulsifier when used on its own. These phenomena may have occurred for a number of reasons: (i) the emulsifier did not adsorb to the interfaces strongly enough; (ii) there was insufficient BG (1 wt%) to cover all the new interfaces created during homogenization; (iii) the large BG molecules did not absorb fast enough to prevent droplet coalescence during homogenization; (iv) the BG molecules promoted either bridging or depletion flocculation (McClements, 2015). The distribution of the oil droplets in the emulsions containing the complexes depended strongly on the TA/BG ratio and the pH of the solution. At the highest TA/BG ratio (0.8), extensive droplet aggregation was observed in all of the emulsions after 1-day storage. This effect may have been due to the depletion or bridging flocculation phenomenon mentioned earlier. The large particles observed in these emulsions would account for the rapid rate of creaming observed (Fig. 4A and B).
surfaces (Fig. 3). 3.4. Influence of TA-BG complexes on storage stability The long-term stability of emulsions is an important factor impacting the expected shelf life of many commercial products (McClements, 2015). Photographs of the appearance of the emulsions were therefore taken after they were stored for 0, 1 and 7 days (Fig. 4). With the exception of the emulsion with TA/BG = 0.8 at pH 3, all the emulsions appeared relatively stable to gravitational separation on the day of preparation (Fig. 4A). This suggests that the initial oil droplets were small enough to not undergo rapid creaming. After 1-day storage, a visible cream layer was observed in a number of the emulsions: TA/ BG = 0.8 at pH 3, 5 and 7; TA/BG = 0.5 at pH 3 and 7. After 7-day storage, creaming was observed in all the emulsions, with the exception of the following systems: TA/BG = 0.4 and 0.5 at pH 5 and TA/ BG = 0.1 at pH 7. An insight into the overall microstructures of the emulsions was also obtained using confocal fluorescence microscopy (Fig. 5). The 270
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Fig. 3. TEM images of emulsions stabilized by TA-BG complexes with TA/BG ratios of 0.1 and 0.5 at pH 3 (A), pH 5 (B) and pH 7 (C).
More limited droplet flocculation was observed in all the emulsions at intermediate TA/BG ratios, but there were conditions where the extent of flocculation was less. The lowest level of flocculation was observed at TA/BG ratios of 0.1, 0.5, and 0.3 at pH 7, 5, and 3, respectively, which is in good agreement with the light scattering measurements discussed earlier. The pH-dependence of emulsion stability was mainly attributed to changes in the molecular interactions between the TA and BG (see Scheme 1). The number and strength of the interactions between the two different kinds of molecule influences the thickness, density, and charge of the biopolymer coatings around the oil droplets, which in turn impacts the strength of the colloidal interactions operating in the system. An increase in TA-BG interactions might be expected to increase the thickness and density of the interfacial layer, which should increase
the steric repulsion between the droplets. On the other hand, it may also increase the interactions between molecules adsorbed to different oil droplets, thereby promoting droplet aggregation. Consequently, there may be a delicate balance of molecular interactions that leads to optimum stability. 3.5. Influence of environmental stress on emulsion stability Food emulsions should remain physically stable during processing, storage, transportation, and utilization. The droplets in emulsions may be exposed to alterations in pH, ionic strength, or temperature during their lifetime, which impacts their stability (McClements, 2015). We therefore examined the impact of these environmental stresses on emulsion stability. For these experiments, we focused on emulsions
Fig. 4. Visual appearance of emulsions stabilized by TA-BG complexes with different ratios at pH 3 (A), pH 5 (B) and pH 7 (C). The photos above show the emulsions at 0 day (fresh), the middle photos show the emulsions at 1 day (24 h), and the bottom photos show the emulsions at 7 days. All emulsions were stored at ambient temperature. 271
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Fig. 5. Confocal micrographs of emulsions stabilized by TA-BG complexes with different ratios at pH 3 (A), pH 5 (B) and pH 7 (C). The images above showed the emulsions at 0 day (fresh) and the images below showed the emulsions at 7 days. The scale bar corresponds to 50 μm.
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Scheme 1. Schematic representation of the main phenomena that MCT emulsion stabilized by BG or TA-BG complexes with TA/BG ratios of 0.5
earlier, neither TA nor BG is good at stabilizing emulsions on their own, and so when the complexes break down the emulsions become unstable to droplet aggregation. Salt stability: The salt-stability of the emulsions was determined by adding different levels of sodium chloride (0–500 mM) to them (Fig. 6C). The mean particle diameter increased progressively as the level of salt added was raised. In addition, visible observation indicated that a cream layer formed on top of the emulsions containing 200 mM NaCl or more. These measurements show that the emulsions were highly susceptible to droplet flocculation in the presence of salt. The addition of salt may weaken any hydrophobic or electrostatic interactions between the TA and BG molecules (Li et al., 2019), which could alter the structure of the interfacial coatings thereby altering the colloidal interactions between the droplets. Nevertheless, further studies are required to determine the precise origin of this effect.
formulated using TA-BG complexes (ratio = 0.5) at pH 5 because the emulsions formulated using BG alone were unstable. pH stability: The emulsions were unstable to droplet flocculation from pH 2 to 4, but stable at higher pH values, as demonstrated by changes in the measured particle size (Fig. 6A). The ζ-potential on the droplets was slightly negative across the entire pH range (0 to −4 mV). The fact that the absolute value of the surface potential was always relatively low suggests that electrostatic repulsion did not play a major role in determining emulsion stability. Instead, it is likely that steric repulsion was more important, which can be attributed to the presence of a relatively thick interfacial coatings around each of the oil droplets (Dickinson, 2018). Having said this, the decrease in magnitude of the ζ-potential when the pH was reduced below 5 corresponded to the increase in droplet flocculation, which suggests that electrostatic effects may have played some role. This phenomenon may have been because carboxylic acid groups on the TA or BG molecules became protonated at low pH values. As a consequence, the interactions between the molecules in the interfacial layer may have changed, which altered its thickness or density and therefore changed the steric interactions. In other words, changes in the molecular electrostatic interactions caused changes in the colloidal steric interactions. Thermal stability: Emulsions were heated at temperatures ranging from 30 to 90 °C for 30 min, cooled down to ambient temperature, stored for 12 h, and then their particle size was measured (Fig. 6B). The emulsions formulated using TA-BG complexes remained stable to droplet flocculation when held at temperatures of 60 °C or below, as demonstrated by the fact that there was no change in the particle size and appearance of the emulsions. However, the emulsions were unstable to aggregation and creaming when exposed to temperatures of 70 °C or higher. A recent study has shown that the turbidity of TA-BG complexes dispersed in aqueous solutions decreases with increasing temperature (Li et al., 2019), which can be attributed to disruption of the hydrogen bonding holding the TA and BG molecules together at elevated temperatures (Le Bourvellec, Guyot, & Renard, 2004). This phenomenon may therefore be the cause of the emulsion instability observed at higher temperatures. As mentioned
4. Conclusions The aim of this study was to establish whether complexes formed between a plant-based biopolymer (β-glucan) and polyphenol (tannic acid) could be used to successfully form and stabilize oil-in-water emulsions. Our results showed that TA-BG complexes could be used to produce emulsions with smaller oil droplets than using BG alone. However, the TA/BG ratio had to be optimized to create stable emulsions – if it was too high or too low, the emulsions were highly unstable to droplet flocculation. The optimum TA/BG ratio for forming stable emulsions depended on the pH of the surrounding solution, being lower at pH 7 than at pH 3 or pH 5. Once formed, the stability of the emulsions depended on the nature of the environmental stresses they experienced. The TA-BG coated droplets were unstable to aggregation under acidic conditions (< pH 5), high salts (≥ 100 mM), and when held at elevated temperatures (≥ 70 °C). These effects were attributed to the influence of these parameters on the interactions between the polyphenol and polysaccharide molecules, which caused a change in the thickness, density, and charge of the interfacial coatings. This study suggests that emulsions can be formulated using plant-based polyphenol-polysaccharide complexes but they are only stable under a limited range of 273
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Fig. 6. Influence of pH (A), temperature (B) and NaCl (C) on the mean particle diameter (d4,3) of MCT-in water emulsions stabilized by TA-BG complexes. corn fiber gum. Food Hydrocolloids, 66, 144–153. Bai, L., Huan, S. Q., Gu, J. Y., & McClements, D. J. (2016). Fabrication of oil-in-water nanoemulsions by dual-channel microfluidization using natural emulsifiers: Saponins, phospholipids, proteins, and polysaccharides. Food Hydrocolloids, 61, 703–711. Burkus, Z., & Temelli, F. (2000). Stabilization of emulsions and foams using barley betaglucan. Food Research International, 33, 27–33. Camino, N. A., Sánchez, C. C., Patino, J. M. R., & Pilosof, A. M. R. (2011). Hydroxypropylmethylcellulose at the oil–water interface. Part I. Bulk behaviour and dynamic adsorption as affected by pH. Food Hydrocolloids, 25, 1–11. Chen, X. W., Wang, J. M., Yang, X. Q., Qi, J. R., & Hou, J. J. (2016). Subcritical water induced complexation of soy protein and rutin: Improved interfacial properties and emulsion stability. Journal of Food Science, 81, C2149–C2157. De Fenoyl, L., Hirel, D., Perez, E., Lecomte, S., Morvan, E., & Delample, M. (2018). Interfacial activity and emulsifying behaviour of inclusion complexes between helical polysaccharides and flavouring molecules resulting from non-covalent interactions. Food Research International, 105, 801–811. Di Mattia, C. D., Sacchetti, G., Mastrocola, D., Sarker, D. K., & Pittia, P. (2010). Surface properties of phenolic compounds and their influence on the dispersion degree and oxidative stability of olive oil O/W emulsions. Food Hydrocolloids, 24, 652–658. Dickinson, E. (2003). Hydrocolloids at interfaces and the influence on the properties of
conditions. In future studies, it would be fruitful to examine different combinations of polyphenols and biopolymers to determine if more robust emulsions could be formed. Acknowledgements This study was funded by the fund of National Natural Science Foundation of P. R. China (No. 31460434) and Postgraduate Technology Innovation Project in Jiangxi Province (No. YC2016-B014). References Ahmad, A., Anjum, F. M., Zahoor, T., Nawaz, H., & Dilshad, S. M. R. (2012). Beta glucan: A valuable functional ingredient in foods. Critical Reviews in Food Science Nutrition, 52, 201–212. Bai, L., Huan, S., Li, Z., & McClements, D. J. (2017). Comparison of emulsifying properties of food-grade polysaccharides in oil-in-water emulsions: Gum arabic, beet pectin, and
274
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R. Li, et al.
Comprehensive Reviews in Food Science and Food Safety, 16, 76–95. Mamet, T., Yao, F., Li, K.-k., & Li, C.-m. (2017). Persimmon tannins enhance the gel properties of high and low methoxyl pectin. Food Research International, 86, 594–602. McClements, D. J. (2015). Food emulsions: Principles, practices, and techniques. CRC press. McClements, D. J., & Gumus, C. E. (2016). Natural emulsifiers - Biosurfactants, phospholipids, biopolymers, and colloidal particles: Molecular and physicochemical basis of functional performance. Advances in Colloid and Interface Science, 234, 3–26. Patel, A. R., ten-Hoorn, J. S., Hazekamp, J., Blijdenstein, T. B., & Velikov, K. P. (2013). Colloidal complexation of a macromolecule with a small molecular weight natural polyphenol: Implications in modulating polymer functionalities. Soft Matter, 9, 1428–1436. Pei, Y., Wan, J., You, M., McClements, D. J., Li, Y., & Li, B. (2019). Impact of whey protein complexation with phytic acid on its emulsification and stabilization properties. Food Hydrocolloids, 87, 90–96. Serrano, J., Puupponen-Pimiä, R., Dauer, A., Aura, A. M., & Saura-Calixto, F. (2009). Tannins: Current knowledge of food sources, intake, bioavailability and biological effects. Molecular Nutrition & Food Research, 53. Walstra, P. J. C. E. S. (1993). Principles of emulsion formation. Chemical Engineering Science, 48, 333–349. Wan, Z.-L., Guo, J., & Yang, X.-Q. (2015). Plant protein-based delivery systems for bioactive ingredients in foods. Food & Function, 6, 2876–2889. Wan, Z.-L., Wang, J.-M., Wang, L.-Y., Yuan, Y., & Yang, X.-Q. (2014). Complexation of resveratrol with soy protein and its improvement on oxidative stability of corn oil/ water emulsions. Food Chemistry, 161, 324–331. Wang, Y.-H., Wan, Z.-L., Yang, X.-Q., Wang, J.-M., Guo, J., & Lin, Y. (2016). Colloidal complexation of zein hydrolysate with tannic acid: Constructing peptides-based nanoemulsions for alga oil delivery. Food Hydrocolloids, 54, 40–48. Wu, Z., Ming, J., Gao, R., Wang, Y., Liang, Q., Yu, H., & Zhao, G. (2011). Characterization and antioxidant activity of the complex of tea polyphenols and oat β-glucan. Journal of Agricultural and Food Chemistry, 59, 10737–10746. Zhang, X., Xiong, Y. L., Chen, J., & Zhou, L. (2014). Synergy of licorice extract and pea protein hydrolysate for oxidative stability of soybean oil-in-water emulsions. Journal of Agricultural & Food Chemistry, 62, 8204–8213. Zhu, F., Du, B., & Xu, B. (2016). A critical review on production and industrial applications of beta-glucans. Food Hydrocolloids, 52, 275–288.
dispersed systems. Food Hydrocolloids, 17, 25–39. Dickinson, E. (2017). Biopolymer-based particles as stabilizing agents for emulsions and foams. Food Hydrocolloids, 68, 219–231. Dickinson, E. (2018). Hydrocolloids acting as emulsifying agents–How do they do it? Food Hydrocolloids, 78, 2–14. Foegeding, E. A., Plundrich, N., Schneider, M., Campbell, C., & Lila, M. A. (2017). Proteinpolyphenol particles for delivering structural and health functionality. Food Hydrocolloids, 72, 163–173. Gharehkhani, S., Ghavidel, N., & Fatehi, P. (JAN 21 2019). Kraft lignin-tannic acid as a green stabilizer for oil/water emulsion. ACS Sustainable Chemistry & Engineering, 7(2), 2370–2379. Gomes, A., Costa, A. L. R., & Cunha, R. L. (2018). Impact of oil type and WPI/Tween 80 ratio at the oil-water interface: Adsorption, interfacial rheology and emulsion features. Colloids and Surfaces B-Biointerfaces, 164, 272–280. Huang, Y., Li, A., Qiu, C., Teng, Y., & Wang, Y. (2017). Self-assembled colloidal complexes of polyphenol–gelatin and their stabilizing effects on emulsions. Food & Function, 8, 3145–3154. Jakobek, L. (2015). Interactions of polyphenols with carbohydrates, lipids and proteins. Food Chemistry, 175, 556–567. Karefyllakis, D., Altunkaya, S., Berton-Carabin, C. C., van der Goot, A. J., & Nikiforidis, C. V. (2017). Physical bonding between sunflower proteins and phenols: Impact on interfacial properties. Food Hydrocolloids, 73, 326–334. Lam, R. S. H., & Nickerson, M. T. (2013). Food proteins: A review on their emulsifying properties using a structure-function approach. Food Chemistry, 141, 975–984. Le Bourvellec, C., Guyot, S., & Renard, C. (2004). Non-covalent interaction between procyanidins and apple cell wall material: Part I. Effect of some environmental parameters. Biochimica et Biophysica Acta (BBA)-General Subjects, 1672, 192–202. Li, R., Zeng, Z., Fu, G., Wan, Y., Liu, C., & McClements, D. J. (June 2019). Formation and characterization of tannic acid/beta-glucan complexes: Influence of pH, ionic strength, and temperature. Food Research International, 120, 748–755. Li, Y., Liu, H., Liu, Q., Kong, B., & Diao, X. (2019). Effects of zein hydrolysates coupled with sage (salvia officinalis) extract on the emulsifying and oxidative stability of myofibrillar protein prepared oil-in-water emulsions. Food Hydrocolloids, 87, 149–157. Liu, F. G., Ma, C. C., Gao, Y. X., & McClements, D. J. (2017). Food-grade covalent complexes and their application as nutraceutical delivery systems: A review.
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Food Research International journal homepage: www.elsevier.com/locate/foodres
Formation and characterization of oil-in-water emulsions stabilized by polyphenol-polysaccharide complexes: Tannic acid and β-glucan
T
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Ruyi Lia, Shengfeng Penga, Ruojie Zhangb, Taotao Daia, Guiming Fua, , Yin Wana, Chengmei Liua, ⁎ David Julian McClementsb, a b
State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang, Jiangxi 330047, PR China Biopolymers and Colloids Laboratory, Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Tannic acid β-Glucan Interfacial tension Emulsion stability Microstructure
Oil-in-water emulsions were prepared that were stabilized by a polyphenol (tannic acid, TA) and/or a polysaccharide (β-glucan, BG). The influence of TA concentration and solution pH on the physical stability and microstructure of the emulsions was investigated. Emulsions formed using only BG (1%) contained large droplets that were unstable to flocculation and coalescence. The stability of the emulsions (pH 5) could be enhanced by optimizing the ratio of TA-to-BG used to form them, i.e., TA/BG = 0.4 or 0.5. This effect was attributed to a combination of increased steric hindrance and reduced hydrogen bonding of TA to BG. Transmission electron microscopy (TEM) indicated that increasing the level of TA present increased the compactness and thickness of the TA-BG interfacial layers formed around the oil droplets. Furthermore, the stability of the emulsions to droplet flocculation and coalescence depended on the TA/BG ratio and solution pH. The impact of environmental conditions on emulsion stability was also investigated. Emulsions stabilized by TA-BG complexes (TA/ BG = 0.5) remained stable from pH 5 to 9 at ambient temperature and at temperatures ≤60 °C at pH 5, but were highly unstable to salt addition at pH 5. Our results may increase the breadth of applications of β-glucan as a functional ingredient in foods.
1. Introduction During the past decade or so, there has been growing interest in using plant-based emulsifiers, rather than synthetic or animal-based ones, in food products because of their potential benefits in terms of the environment, sustainability, human health, and reduction in animal cruelty (Bai, Huan, Li, & McClements, 2017; Lam & Nickerson, 2013; McClements & Gumus, 2016; Wan, Guo, & Yang, 2015). However, many plant-derived ingredients that are available in abundant quantities, such as the proteins from corn and wheat or the polysaccharides from starch and cellulose, are not naturally good emulsifiers, which may be due to their poor water-solubility characteristics, their low surface activity, or their poor emulsion stabilizing properties (McClements & Gumus, 2016; Wan et al., 2015). For this reason, many researchers have focused on enhancing the functional attributes of plant proteins and polysaccharides by utilizing covalent and/or noncovalent interaction with other types of plant-based materials, such as polyphenols (Foegeding, Plundrich, Schneider, Campbell, & Lila, 2017; Jakobek, 2015; Liu, Ma, Gao, & McClements, 2017). Moreover, some polyphenols have been reported to have a beneficial impact on human ⁎
health and food quality due to their anticarcinogenic, antioxidant, antibacterial, anti-inflammatory, and antiviral activities (Jakobek, 2015). For this reason, it may be possible to develop biopolymer-polyphenol ingredients that serve multiple functions in foods: emulsification, antioxidant, antimicrobial, and nutraceutical. Non-covalent interactions between polyphenols and plant biopolymers have been widely studied, e.g., chlorogenic acid with sunflower protein (Karefyllakis, Altunkaya, Berton-Carabin, van der Goot, & Nikiforidis, 2017), tannic acid with zein (Wang et al., 2016), resveratrol or rutin with soy protein (Chen, Wang, Yang, Qi, & Hou, 2016; Wan, Wang, Wang, Yuan, & Yang, 2014), phytic acid with whey protein (Pei et al., 2019), thymol and polysaccharides (De Fenoyl et al., 2018), tannic acid with methylcellulose (Patel, ten-Hoorn, Hazekamp, Blijdenstein, & Velikov, 2013), and tea polyphenols with oat β-glucan (Wu et al., 2011). These studies have demonstrated that the functional properties of non-covalent complexes formed from polyphenols and plant biopolymers can be improved compared to the individual components. This is because phenolic compounds have multiple reactive groups that can form cross-links with biopolymers, thereby enhancing their functional attributes, such as gel formation (Mamet, Yao, Li, & Li,
Corresponding authors. E-mail addresses: [email protected] (G. Fu), [email protected] (D.J. McClements).
https://doi.org/10.1016/j.foodres.2019.05.005 Received 26 March 2019; Received in revised form 29 April 2019; Accepted 2 May 2019 Available online 03 May 2019 0963-9969/ © 2019 Published by Elsevier Ltd.
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to prepare all solutions and emulsions.
2017). Also, colloidal particles assembled using non-covalent interactions between polyphenols and biopolymers can be used as functional ingredients to stabilize emulsions and foams (Dickinson, 2017). Based on this earlier research, we investigated the possibility of extending the functional attributes of β-glucan (BG) by forming non-covalent complexes with tannic acid (TA). BG is a water-soluble polysaccharide that is present at relatively high levels in a variety of foods from plant or microbial origin, including oat, barley, wheat, mushroom, and yeast (Zhu, Du, & Xu, 2016). BG has been reported to exhibit a broad range of health benefits, including lowering blood cholesterol, regulating blood glucose, inhibiting inflammatory, and preventing cancer (Zhu et al., 2016). Besides its health and nutritional benefits, BG also has various physicochemical attributes that make it useful as a functional ingredient in foods and beverages, such as thickening, stabilizing, and gelation (Ahmad, Anjum, Zahoor, Nawaz, & Dilshad, 2012). However, BG is predominantly a hydrophilic molecule that has little surface-activity and is therefore not a good emulsifier, which limits its applications in many products. This problem may be overcome by forming noncovalent complexes with other more hydrophobic or amphiphilic substances. As an example, when used alone, BG (1%) was found to exhibit poor performance as an emulsifier but when it was used in combination with whey protein (1%) it was able to reduce the droplet size and increase the emulsion stability (Burkus & Temelli, 2000). Tannic acid (TA), a commercial form of tannin suitable for use in foods, has been shown to have a diverse range of biological activities including antioxidant, antibacterial, and antiviral properties (Serrano, Puupponen-Pimiä, Dauer, Aura, & Saura-Calixto, 2009). In addition, it has been shown that it is capable of forming strong interactions with proteins, polysaccharides, alkaloids, and metal ions (Serrano et al., 2009). The complexation of TA with biopolymers has been shown to enhance their emulsifying properties and antioxidant activity (Huang, Li, Qiu, Teng, & Wang, 2017; Patel et al., 2013; Wang et al., 2016). TA-biopolymer complexes may therefore be suitable for application as multifunctional ingredients in foods and beverages. In a previous study, we successfully prepared TA-BG complexes in aqueous solutions and characterized their physicochemical and structural properties (Li et al., 2019). The size and stability of the complexes formed was shown to depend on the TA/BG ratio and the pH of the surrounding solution. The complexes formed were held together by non-covalent linkages, which meant that no chemical or biological cross-agent was required, which is beneficial in terms of ease of production, cost, safety, and food legislation (Huang et al., 2017). In the current study, we aimed to determine whether TA-BG complexes could be used as emulsifiers to form and stabilize oil-in-water emulsions. Emulsions were characterized by particle size distribution, microstructure, and storage stability measurements. Based on the fact that the TA-BG complexes are only held together by relatively weak non-covalent interactions, we also examined the impact of environment stresses on emulsion properties. This study should therefore provide useful insights into the potential application of TA-BG complexes as natural plant-based emulsifiers in foods and beverages.
2.2. Interfacial tension measurements The interfacial tension between the aqueous phase (TA, BG, or TABG solutions) and the oil phase (MCT) was determined using droplet shape analysis (DSA 100, Krüss GmbH, Hamburg, Germany). For water droplet formation, a straight-needle with a diameter of 1.088 mm was used to create a pendant drop. The water drop was extruded into a quartz cell containing MCT oil. All samples were measured for around 15 min, at 20 s intervals, which was long enough to ensure that the interfacial tension reached a steady value. Digital images were acquired using the instrument's camera. The instrument calculated the interfacial tension by analyzing the shape of the water drops and interpreting the data using the Young–Laplace equation. 2.3. Emulsion preparation Oil-in-water emulsions were fabricated by homogenizing an oil phase (5 wt%) with an aqueous phase (95 wt%). The oil phase consisted of MCT while the aqueous phase consisted of TA-BG mixtures (1% BG, with a TA/BG mass ratio of 0.1, 0.2, 0.3, 0.4, 0.5 and 0.8) in buffer solution (5 mM, pH 3, 5 or 7). Initially, an emulsion containing relatively large oil droplets was created by homogenizing the oil and aqueous phases using a high-shear mixing device (Bamix, Biospec Products, Bartlesville, OK) for 2 min at room temperature. Subsequently, the droplets in this emulsion were reduced in size by passing it through a high-pressure microfluidizer (M110Y, Microfluidics, Newton, MA) 3-times at a pressure of 12,000 psi. Sodium azide (0.02 wt%) was then added to the emulsions as a microbial preservative. 2.4. Emulsion storage stability Oil-in-water emulsions were poured into glass test tubes that were then incubated at room temperature (≈ 25 °C). The stability of the emulsions was then recorded at 0-day (soon after preparation), 1-day (stored for 24 h), and 7-day (stored for 7 days). The creaming stability was determined by visual observation. 2.5. Determination of particle size The particle size distribution and mean particle diameter (d4,3) were determined by static light scattering (Mastersizer 3000, Malvern Instruments Ltd., Malvern, Worcestershire, UK). Before being analyzed the emulsions were diluted with buffer solution of the same pH to avoid multiple scattering, which would interfere with data interpretation. The refractive index (RI) of the aqueous phase used in the calculations was 1.33 and that of the oil phase was 1.445. 2.6. Determination of ζ-potential The surface potential (ζ-potential) of the oil droplets in the emulsions was measured by microelectrophoresis (Zetasizer Nano ZS, Malvern Instruments, Worcestershire, UK). Again, the emulsions were diluted with buffer solutions of the same pH before being analyzed to avoid multiple scattering effects.
2. Materials and methods 2.1. Materials Medium chain triglycerides (MCT, Miglyol 812 N) were bought from the Warner Graham Co. (Cockeysville, MD). β-glucan powder was bought from the Yikang Co. (Hebei, China). The manufacturer reported that this ingredient contained around 6.7% moisture, 70.3% β-glucan, 3.6% protein, and < 5% ash. Tannic acid (from Chinese gall nuts) and sodium azide were purchased from Sigma-Aldrich Co. (St. Louis, MO). Citric acid, trisodium citrate dihydrate, sodium phosphate monobasic and sodium phosphate dibasic were purchased from Fisher Scientific Co. (Fair Lawn, NJ). Distilled and deionized water (Milli-Q®) was used
2.7. Confocal laser scanning microscopy (CLSM) CLSM imaging of the emulsions was carried out at room temperature using a confocal laser scanning microscope (Nikon D-Eclipse C1 80i, Nikon, Melville, NY, U.S.) with a 40 × objective lens. The lipid phase was stained with Nile Red dissolved in ethanol (1 mg/mL), which was mixed into the emulsions prior to observation. A small volume of sample was placed on a microscope slide for visualization. The 267
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fluorescent dye was excited by an Argon 488 nm laser and the emitted light was detected at 621 nm. 2.8. Transmission electron microscopy (TEM) The morphology of the emulsions was analyzed by taking images using a transmission electron microscope (JEM-2000FX, JEOL, Ltd., Tokyo, Japan). Emulsion samples were diluted (1:10, v/v) with buffer solution (5 mM, pH 3, 5 or 7). One drop of emulsion was placed on a 200-mesh carbon coated copper grid. After 3 min, the grid was stained with uranyl acetate solution (1%) for 4 min and air-dried at room temperature after removing the excess liquid with filter paper. The photographs were taken at various magnifications and 200 kV voltage. 2.9. Emulsion physical stability Freshly prepared emulsions were subjected to environmental stresses that emulsion-based food products might encounter in commercial applications, such as pH variations, salt addition, or heating, and then stored for 12 h at ambient temperature prior to analysis: pH: Emulsions were adjusted to various pH values (2–9) using NaOH and/or HCl solutions with continuously stirring. After 30 min, the pH was measured again and adjusted to the required value (if needed), and then the samples were transferred into glass test tubes. Thermal processing: Emulsions were placed into glass test tubes, which were then incubated in water baths set at temperatures ranging from 30 to 90 °C for 30 min, and then cooled to room temperature. Salt addition: Emulsions were placed in 50 mL beakers, adjusted to different salt levels (100–500 mM NaCl) by adding NaCl, and then transferred into glass test tubes. 2.10. Statistical analysis All measurements were repeated at least three times. The mean and standard deviation values were calculated from these measurements using statistical analysis software (SPSS 19.0 SPSS Inc., Chicago, IL). Analysis of variance (ANOVA) was used to determine significance at p < .05 by Tukey's HSD test. 3. Results and discussion 3.1. Interfacial properties
Fig. 1. A. Influence of TA and BG concentrations on the interfacial tension measured at an MCT-water interface at ambient temperature (pH 5). B. Timedependence of interfacial tension of BG (1%), TA (0.5%) and TA-BG (0.5% TA1% BG) samples at MCT-water interfaces (pH 5).
The interfacial properties of food emulsifiers play a critical role in determining their success at forming and stabilizing food emulsions (Dickinson, 2003). We therefore measured the interfacial tension of BG, TA, and TA-BG solutions to gain a better understanding of their interfacial properties at MCT oil-water interfaces (Fig. 1 and Table. 1). The interfacial tension of the clean oil-water was determined to be 26.18 ± 0.13 mN/m, which is close to the value reported by other researchers (Gomes, Costa, & Cunha, 2018). We then studied the impact of adding increasing levels of the two pure substances, either TA or BG, to the aqueous phase on the interfacial tension. The interfacial tension decreased as the concentration of both TA and BG increased (Fig. 1), which indicates that both molecules were surface active and absorbed to the MCT-water interface. The interfacial tension was higher for BG than TA at low concentrations (< 0.05%), but the opposite effect was observed at higher concentrations. This may have been because of differences in the surface hydrophobicity, molecular weight, and packing efficiency of the two types of molecules. The fact that both pure BG and pure TA were able to appreciably reduce the interfacial tension suggests that did have some surface-activity. However, the interfacial values reached at high emulsifier levels (around 14–16 mN/m) are considerably higher than those reported for biosurfactants or proteins
Table 1 Influence of TA-BG complexes with different mass ratio at different pH values on the interfacial tension measured at MCT-water interface. TA:BG (wt/wt)
Interfacial tension pH 3
1% BG 0.1 0.2 0.3 0.4 0.5 0.8 0.8% TA
14.25 14.51 14.24 14.20 13.95 13.84 13.61 15.93
pH 5 ± ± ± ± ± ± ± ±
0.23 0.11 0.10 0.09 0.09 0.10 0.11 0.13
BCb Ba BCb BCb CDb DEb Eb Ab
14.52 14.51 14.19 14.14 14.11 13.80 13.64 16.07
pH 7 ± ± ± ± ± ± ± ±
0.13 0.09 0.08 0.09 0.08 0.09 0.08 0.12
Bb Ba Cb Cb Cb Db Db Ab
15.07 14.84 14.63 14.67 14.50 14.39 14.00 17.55
± ± ± ± ± ± ± ±
0.16 Ba 0.11 BCa 0.10 CDa 0.13 CDa 0.11 Da 0.11 Da 0.10 Ea 0.12 Aa
Values are given as means ± SD from triplicate determinations; A–E means in the same column with different letters differ significantly (P < .05); a–b means in the same row with different letters differ significantly (P < .05).
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pH 3 > pH 5 > pH 7. The light scattering measurements also indicated that there were distinct differences in the particle size distributions (PSDs) of the emulsions depending on pH and TA-to-BG ratio (Fig. 2). For some commercial applications in the food industry, it is important for emulsions to have a narrow monomodal distribution, as this increases their stability to droplet aggregation and gravitational separation (e.g., soft drinks and nutritional beverages). Under some conditions narrow monomodal distributions were obtained, but under others broad bimodal distributions were observed (Fig. 2B to D). In the absence of TA, all of the emulsions were bimodal, which is undesirable. Monomodal distributions, however, could be produced at all three pH values provided that sufficient TA was included in the formulation. These results suggest that the TA-BG complexes adsorbed to the oil droplet surfaces and formed an interfacial coating that protected them from flocculation and coalescence (Dickinson, 2018).
(around 5–10 mN/m) (Bai, Huan, Gu, & McClements, 2016), which suggests that BG and TA are not strongly surface active. Previous studies have also shown that many food-grade polysaccharides are not particularly surface active because they are primarily hydrophilic (Bai et al., 2017; Dickinson, 2003; McClements & Gumus, 2016). Other studies have reported that some polyphenols, such as gallic acid, catechin, and quercetin, are also mildly surface-active (Di Mattia, Sacchetti, Mastrocola, Sarker, & Pittia, 2010). Conversely, numerous studies have reported that polyphenol-biopolymer complexes are highly effective at decreasing the interfacial tension (Chen et al., 2016; Karefyllakis et al., 2017; Pei et al., 2019). As shown in Fig. 1B, the TA-BG complexes adsorbed to oil-water interfaces more rapidly and deceased the interfacial tension more effectively than BG or TA alone at pH 5. For this reason, the interfacial characteristics of TA-BG complexes at different pH values were studied (Table 1). In general, all of the samples had fairly similar interfacial tensions, with the values ranging from around 14 to 18 mN/m. However, there were some differences depending on pH and sample composition. The interfacial tensions of all the samples were higher at pH 7 than pH 3 or 5, which suggests that the surface-activity of the components was pHdependent, as has been reported elsewhere (Camino, Sánchez, Patino, & Pilosof, 2011; Gharehkhani, Ghavidel, & Fatehi, 2019). This effect may have been due to changes in the conformation and/or charge of the TA and BG molecules, which altered their tendency to adsorb to the interfaces or modified their interfacial packing. Overall, the TA-BG complexes had lower interfacial tensions than the individual components, and the interfacial tension decreased with increasing TA/BG ratio. Other studies have also demonstrated that polyphenol-biopolymer complexes can reduce the interfacial tension more effectively than the individual parts (Chen et al., 2016; Karefyllakis et al., 2017; Pei et al., 2019). A possible reason for this phenomenon is that the smaller polyphenol molecules can pack into the spaces between the larger β-glucan molecules, thereby more effectively improving the interfacial activity of β-glucan. Measurements of the interfacial tension of substances are useful for establishing whether they are surface active or not, but they do not provide a good prediction of the ability of an emulsifier to form and stabilize emulsions. They are not sufficient to predict the ability of emulsifiers to form small droplets during homogenization because this is a highly dynamic process that takes place under complex fluid flow conditions, which is unlike the quiescent conditions used for interfacial tension measurements. Consequently, an emulsifier that is able to effectively decrease the interfacial tension under static conditions may not be able to adsorb to the surfaces of the oil droplets formed during homogenization (Bai et al., 2017; Walstra, 1993). Moreover, interfacial tension measurements do not provide insights into the stability of emulsifier-coated oil droplets to aggregation once the oil droplets have been formed. This is because these measurements do not give information about interfacial thickness or robustness, or about the colloidal interactions acting between the droplets. Consequently, it is always important to measure the performance of emulsifiers under conditions that simulate their application in commercial products.
3.3. Influence of TA-BG complexes on emulsion microstructure The structural organization, thickness, and rheology of the interfacial layers coating oil droplets is known to play an important role in determining emulsion stability (Li, Liu, Liu, Kong, & Diao, 2019; Zhang, Xiong, Chen, & Zhou, 2014). For this reason, we used electron microscopy to provide some insight into the nature of the interfacial layers in the emulsions. The TEM images of the emulsions stabilized by TA-BG complexes had the anticipated appearance of oil-in-water emulsions, i.e., oil droplets surrounded by water (Fig. 3). The oil droplets appeared to be roughly spherical in all the emulsions, but there were some differences in particle dimensions and structural organization depending on pH and TA:BG ratio. The TEM images suggested that the individual oil droplets were smaller at pH 7 than at pH 3 and pH 5 (Fig. 3), which is consistent with the light scattering measurements (Fig. 2). There were, however, some discrepancies between the microscopy and light scattering results in terms of the effects of TA/BG ratio. For instance, at pH 7, the TEM images indicated that smaller oil droplets were present at TA/BG = 0.5 than 0.1, but the light scattering results suggest the opposite. Similarly, at pH 3, the microscopy images suggest smaller oil droplets were present at TA/BG =0.1 than 0.5, but the light scattering results showed the opposite. These apparent discrepancies may be due to the different methods used to prepare the samples for the two analytical methods. For light scattering, the samples were diluted and stirred prior to analysis, whereas for TEM analysis they are dried onto a solid surface. In addition, the light scattering method may be measuring the size of other particles in the system, such as non-adsorbed complexes or flocs, as well as the individual oil droplets. The electron microscopy images also provided some useful insights into the nature of the interfacial structure, indicating the presence of a thick coating around the oil droplets, particularly at pH 3 and 5 (Fig. 3). The coatings formed at a TA/BG ratio of 0.5 appeared to be thicker and more densely packed than those formed at a TA/BG ratio of 0.1, which may have important consequences for emulsion stability. This could because increasing the TA concentration strengthened the interaction between TA and BG. In a previous study, we showed that TA-BG complexes (ratio = 0.5:1) could form colloid particles (Li et al., 2019), which absorbed to oil-water interfaces and formed densely packed coatings. For instance, the steric repulsion between emulsion droplets is known to increase as the interfacial layers become thicker and denser (McClements, 2015). Currently, it is unclear if the TA-BG complexes themselves are altered by the microfluidization process, as well as how this may impact their tendency to adsorb to the oil droplet surfaces and form interfacial coatings. In particular, the complexes may be partially or fully dissociated during microfluidization and then reform at the droplet surfaces after homogenization. Nevertheless, our TEM results do show that the end result is the creation of a TA-BG shell around the oil droplet
3.2. Influence of TA-BG complexes on emulsion formation For this reason, the impact of the TA-BG complexes on the formation of oil-in-water emulsions using high-pressure homogenization was measured. The size of the particles in emulsions fabricated using TA-BG complexes were measured at pH 3, 5 and 7 (Fig. 2). The mean particle diameter (d4,3) of the emulsions clearly depended on the pH and TA-toBG ratio of the emulsions (Fig. 2A). Smaller particles were produced using TA-BG complexes than using the individual components, but the minimum in the droplet size depended on the pH of the surrounding solution. The smallest particles were produced at TA:BG ratios of around 0.1, 0.2, and 0.4 at pH 7, 5, and 3, respectively. At a fixed TA:BG ratio, the measured particle size decreased in the following order: 269
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Fig. 2. A. Influence of pH and TA-BG ratio on the mean particle diameter (d4,3) of 5 wt% MCT emulsions (measured within 3 h of preparation). Particle size distributions of emulsions containing different mass ratios of TA and BG: pH 3 (B), pH 5 (C), and pH 7 (D).
microscopy images were in good agreement with the particle size and creaming stability results. The emulsions stabilized by only BG contained many highly flocculated oil droplets at all three pH values, especially after 7 days storage (Fig. 5A, B and C). Again, this result suggests that BG was not a good emulsifier when used on its own. These phenomena may have occurred for a number of reasons: (i) the emulsifier did not adsorb to the interfaces strongly enough; (ii) there was insufficient BG (1 wt%) to cover all the new interfaces created during homogenization; (iii) the large BG molecules did not absorb fast enough to prevent droplet coalescence during homogenization; (iv) the BG molecules promoted either bridging or depletion flocculation (McClements, 2015). The distribution of the oil droplets in the emulsions containing the complexes depended strongly on the TA/BG ratio and the pH of the solution. At the highest TA/BG ratio (0.8), extensive droplet aggregation was observed in all of the emulsions after 1-day storage. This effect may have been due to the depletion or bridging flocculation phenomenon mentioned earlier. The large particles observed in these emulsions would account for the rapid rate of creaming observed (Fig. 4A and B).
surfaces (Fig. 3). 3.4. Influence of TA-BG complexes on storage stability The long-term stability of emulsions is an important factor impacting the expected shelf life of many commercial products (McClements, 2015). Photographs of the appearance of the emulsions were therefore taken after they were stored for 0, 1 and 7 days (Fig. 4). With the exception of the emulsion with TA/BG = 0.8 at pH 3, all the emulsions appeared relatively stable to gravitational separation on the day of preparation (Fig. 4A). This suggests that the initial oil droplets were small enough to not undergo rapid creaming. After 1-day storage, a visible cream layer was observed in a number of the emulsions: TA/ BG = 0.8 at pH 3, 5 and 7; TA/BG = 0.5 at pH 3 and 7. After 7-day storage, creaming was observed in all the emulsions, with the exception of the following systems: TA/BG = 0.4 and 0.5 at pH 5 and TA/ BG = 0.1 at pH 7. An insight into the overall microstructures of the emulsions was also obtained using confocal fluorescence microscopy (Fig. 5). The 270
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Fig. 3. TEM images of emulsions stabilized by TA-BG complexes with TA/BG ratios of 0.1 and 0.5 at pH 3 (A), pH 5 (B) and pH 7 (C).
More limited droplet flocculation was observed in all the emulsions at intermediate TA/BG ratios, but there were conditions where the extent of flocculation was less. The lowest level of flocculation was observed at TA/BG ratios of 0.1, 0.5, and 0.3 at pH 7, 5, and 3, respectively, which is in good agreement with the light scattering measurements discussed earlier. The pH-dependence of emulsion stability was mainly attributed to changes in the molecular interactions between the TA and BG (see Scheme 1). The number and strength of the interactions between the two different kinds of molecule influences the thickness, density, and charge of the biopolymer coatings around the oil droplets, which in turn impacts the strength of the colloidal interactions operating in the system. An increase in TA-BG interactions might be expected to increase the thickness and density of the interfacial layer, which should increase
the steric repulsion between the droplets. On the other hand, it may also increase the interactions between molecules adsorbed to different oil droplets, thereby promoting droplet aggregation. Consequently, there may be a delicate balance of molecular interactions that leads to optimum stability. 3.5. Influence of environmental stress on emulsion stability Food emulsions should remain physically stable during processing, storage, transportation, and utilization. The droplets in emulsions may be exposed to alterations in pH, ionic strength, or temperature during their lifetime, which impacts their stability (McClements, 2015). We therefore examined the impact of these environmental stresses on emulsion stability. For these experiments, we focused on emulsions
Fig. 4. Visual appearance of emulsions stabilized by TA-BG complexes with different ratios at pH 3 (A), pH 5 (B) and pH 7 (C). The photos above show the emulsions at 0 day (fresh), the middle photos show the emulsions at 1 day (24 h), and the bottom photos show the emulsions at 7 days. All emulsions were stored at ambient temperature. 271
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Fig. 5. Confocal micrographs of emulsions stabilized by TA-BG complexes with different ratios at pH 3 (A), pH 5 (B) and pH 7 (C). The images above showed the emulsions at 0 day (fresh) and the images below showed the emulsions at 7 days. The scale bar corresponds to 50 μm.
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Scheme 1. Schematic representation of the main phenomena that MCT emulsion stabilized by BG or TA-BG complexes with TA/BG ratios of 0.5
earlier, neither TA nor BG is good at stabilizing emulsions on their own, and so when the complexes break down the emulsions become unstable to droplet aggregation. Salt stability: The salt-stability of the emulsions was determined by adding different levels of sodium chloride (0–500 mM) to them (Fig. 6C). The mean particle diameter increased progressively as the level of salt added was raised. In addition, visible observation indicated that a cream layer formed on top of the emulsions containing 200 mM NaCl or more. These measurements show that the emulsions were highly susceptible to droplet flocculation in the presence of salt. The addition of salt may weaken any hydrophobic or electrostatic interactions between the TA and BG molecules (Li et al., 2019), which could alter the structure of the interfacial coatings thereby altering the colloidal interactions between the droplets. Nevertheless, further studies are required to determine the precise origin of this effect.
formulated using TA-BG complexes (ratio = 0.5) at pH 5 because the emulsions formulated using BG alone were unstable. pH stability: The emulsions were unstable to droplet flocculation from pH 2 to 4, but stable at higher pH values, as demonstrated by changes in the measured particle size (Fig. 6A). The ζ-potential on the droplets was slightly negative across the entire pH range (0 to −4 mV). The fact that the absolute value of the surface potential was always relatively low suggests that electrostatic repulsion did not play a major role in determining emulsion stability. Instead, it is likely that steric repulsion was more important, which can be attributed to the presence of a relatively thick interfacial coatings around each of the oil droplets (Dickinson, 2018). Having said this, the decrease in magnitude of the ζ-potential when the pH was reduced below 5 corresponded to the increase in droplet flocculation, which suggests that electrostatic effects may have played some role. This phenomenon may have been because carboxylic acid groups on the TA or BG molecules became protonated at low pH values. As a consequence, the interactions between the molecules in the interfacial layer may have changed, which altered its thickness or density and therefore changed the steric interactions. In other words, changes in the molecular electrostatic interactions caused changes in the colloidal steric interactions. Thermal stability: Emulsions were heated at temperatures ranging from 30 to 90 °C for 30 min, cooled down to ambient temperature, stored for 12 h, and then their particle size was measured (Fig. 6B). The emulsions formulated using TA-BG complexes remained stable to droplet flocculation when held at temperatures of 60 °C or below, as demonstrated by the fact that there was no change in the particle size and appearance of the emulsions. However, the emulsions were unstable to aggregation and creaming when exposed to temperatures of 70 °C or higher. A recent study has shown that the turbidity of TA-BG complexes dispersed in aqueous solutions decreases with increasing temperature (Li et al., 2019), which can be attributed to disruption of the hydrogen bonding holding the TA and BG molecules together at elevated temperatures (Le Bourvellec, Guyot, & Renard, 2004). This phenomenon may therefore be the cause of the emulsion instability observed at higher temperatures. As mentioned
4. Conclusions The aim of this study was to establish whether complexes formed between a plant-based biopolymer (β-glucan) and polyphenol (tannic acid) could be used to successfully form and stabilize oil-in-water emulsions. Our results showed that TA-BG complexes could be used to produce emulsions with smaller oil droplets than using BG alone. However, the TA/BG ratio had to be optimized to create stable emulsions – if it was too high or too low, the emulsions were highly unstable to droplet flocculation. The optimum TA/BG ratio for forming stable emulsions depended on the pH of the surrounding solution, being lower at pH 7 than at pH 3 or pH 5. Once formed, the stability of the emulsions depended on the nature of the environmental stresses they experienced. The TA-BG coated droplets were unstable to aggregation under acidic conditions (< pH 5), high salts (≥ 100 mM), and when held at elevated temperatures (≥ 70 °C). These effects were attributed to the influence of these parameters on the interactions between the polyphenol and polysaccharide molecules, which caused a change in the thickness, density, and charge of the interfacial coatings. This study suggests that emulsions can be formulated using plant-based polyphenol-polysaccharide complexes but they are only stable under a limited range of 273
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Fig. 6. Influence of pH (A), temperature (B) and NaCl (C) on the mean particle diameter (d4,3) of MCT-in water emulsions stabilized by TA-BG complexes. corn fiber gum. Food Hydrocolloids, 66, 144–153. Bai, L., Huan, S. Q., Gu, J. Y., & McClements, D. J. (2016). Fabrication of oil-in-water nanoemulsions by dual-channel microfluidization using natural emulsifiers: Saponins, phospholipids, proteins, and polysaccharides. Food Hydrocolloids, 61, 703–711. Burkus, Z., & Temelli, F. (2000). Stabilization of emulsions and foams using barley betaglucan. Food Research International, 33, 27–33. Camino, N. A., Sánchez, C. C., Patino, J. M. R., & Pilosof, A. M. R. (2011). Hydroxypropylmethylcellulose at the oil–water interface. Part I. Bulk behaviour and dynamic adsorption as affected by pH. Food Hydrocolloids, 25, 1–11. Chen, X. W., Wang, J. M., Yang, X. Q., Qi, J. R., & Hou, J. J. (2016). Subcritical water induced complexation of soy protein and rutin: Improved interfacial properties and emulsion stability. Journal of Food Science, 81, C2149–C2157. De Fenoyl, L., Hirel, D., Perez, E., Lecomte, S., Morvan, E., & Delample, M. (2018). Interfacial activity and emulsifying behaviour of inclusion complexes between helical polysaccharides and flavouring molecules resulting from non-covalent interactions. Food Research International, 105, 801–811. Di Mattia, C. D., Sacchetti, G., Mastrocola, D., Sarker, D. K., & Pittia, P. (2010). Surface properties of phenolic compounds and their influence on the dispersion degree and oxidative stability of olive oil O/W emulsions. Food Hydrocolloids, 24, 652–658. Dickinson, E. (2003). Hydrocolloids at interfaces and the influence on the properties of
conditions. In future studies, it would be fruitful to examine different combinations of polyphenols and biopolymers to determine if more robust emulsions could be formed. Acknowledgements This study was funded by the fund of National Natural Science Foundation of P. R. China (No. 31460434) and Postgraduate Technology Innovation Project in Jiangxi Province (No. YC2016-B014). References Ahmad, A., Anjum, F. M., Zahoor, T., Nawaz, H., & Dilshad, S. M. R. (2012). Beta glucan: A valuable functional ingredient in foods. Critical Reviews in Food Science Nutrition, 52, 201–212. Bai, L., Huan, S., Li, Z., & McClements, D. J. (2017). Comparison of emulsifying properties of food-grade polysaccharides in oil-in-water emulsions: Gum arabic, beet pectin, and
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Comprehensive Reviews in Food Science and Food Safety, 16, 76–95. Mamet, T., Yao, F., Li, K.-k., & Li, C.-m. (2017). Persimmon tannins enhance the gel properties of high and low methoxyl pectin. Food Research International, 86, 594–602. McClements, D. J. (2015). Food emulsions: Principles, practices, and techniques. CRC press. McClements, D. J., & Gumus, C. E. (2016). Natural emulsifiers - Biosurfactants, phospholipids, biopolymers, and colloidal particles: Molecular and physicochemical basis of functional performance. Advances in Colloid and Interface Science, 234, 3–26. Patel, A. R., ten-Hoorn, J. S., Hazekamp, J., Blijdenstein, T. B., & Velikov, K. P. (2013). Colloidal complexation of a macromolecule with a small molecular weight natural polyphenol: Implications in modulating polymer functionalities. Soft Matter, 9, 1428–1436. Pei, Y., Wan, J., You, M., McClements, D. J., Li, Y., & Li, B. (2019). Impact of whey protein complexation with phytic acid on its emulsification and stabilization properties. Food Hydrocolloids, 87, 90–96. Serrano, J., Puupponen-Pimiä, R., Dauer, A., Aura, A. M., & Saura-Calixto, F. (2009). Tannins: Current knowledge of food sources, intake, bioavailability and biological effects. Molecular Nutrition & Food Research, 53. Walstra, P. J. C. E. S. (1993). Principles of emulsion formation. Chemical Engineering Science, 48, 333–349. Wan, Z.-L., Guo, J., & Yang, X.-Q. (2015). Plant protein-based delivery systems for bioactive ingredients in foods. Food & Function, 6, 2876–2889. Wan, Z.-L., Wang, J.-M., Wang, L.-Y., Yuan, Y., & Yang, X.-Q. (2014). Complexation of resveratrol with soy protein and its improvement on oxidative stability of corn oil/ water emulsions. Food Chemistry, 161, 324–331. Wang, Y.-H., Wan, Z.-L., Yang, X.-Q., Wang, J.-M., Guo, J., & Lin, Y. (2016). Colloidal complexation of zein hydrolysate with tannic acid: Constructing peptides-based nanoemulsions for alga oil delivery. Food Hydrocolloids, 54, 40–48. Wu, Z., Ming, J., Gao, R., Wang, Y., Liang, Q., Yu, H., & Zhao, G. (2011). Characterization and antioxidant activity of the complex of tea polyphenols and oat β-glucan. Journal of Agricultural and Food Chemistry, 59, 10737–10746. Zhang, X., Xiong, Y. L., Chen, J., & Zhou, L. (2014). Synergy of licorice extract and pea protein hydrolysate for oxidative stability of soybean oil-in-water emulsions. Journal of Agricultural & Food Chemistry, 62, 8204–8213. Zhu, F., Du, B., & Xu, B. (2016). A critical review on production and industrial applications of beta-glucans. Food Hydrocolloids, 52, 275–288.
dispersed systems. Food Hydrocolloids, 17, 25–39. Dickinson, E. (2017). Biopolymer-based particles as stabilizing agents for emulsions and foams. Food Hydrocolloids, 68, 219–231. Dickinson, E. (2018). Hydrocolloids acting as emulsifying agents–How do they do it? Food Hydrocolloids, 78, 2–14. Foegeding, E. A., Plundrich, N., Schneider, M., Campbell, C., & Lila, M. A. (2017). Proteinpolyphenol particles for delivering structural and health functionality. Food Hydrocolloids, 72, 163–173. Gharehkhani, S., Ghavidel, N., & Fatehi, P. (JAN 21 2019). Kraft lignin-tannic acid as a green stabilizer for oil/water emulsion. ACS Sustainable Chemistry & Engineering, 7(2), 2370–2379. Gomes, A., Costa, A. L. R., & Cunha, R. L. (2018). Impact of oil type and WPI/Tween 80 ratio at the oil-water interface: Adsorption, interfacial rheology and emulsion features. Colloids and Surfaces B-Biointerfaces, 164, 272–280. Huang, Y., Li, A., Qiu, C., Teng, Y., & Wang, Y. (2017). Self-assembled colloidal complexes of polyphenol–gelatin and their stabilizing effects on emulsions. Food & Function, 8, 3145–3154. Jakobek, L. (2015). Interactions of polyphenols with carbohydrates, lipids and proteins. Food Chemistry, 175, 556–567. Karefyllakis, D., Altunkaya, S., Berton-Carabin, C. C., van der Goot, A. J., & Nikiforidis, C. V. (2017). Physical bonding between sunflower proteins and phenols: Impact on interfacial properties. Food Hydrocolloids, 73, 326–334. Lam, R. S. H., & Nickerson, M. T. (2013). Food proteins: A review on their emulsifying properties using a structure-function approach. Food Chemistry, 141, 975–984. Le Bourvellec, C., Guyot, S., & Renard, C. (2004). Non-covalent interaction between procyanidins and apple cell wall material: Part I. Effect of some environmental parameters. Biochimica et Biophysica Acta (BBA)-General Subjects, 1672, 192–202. Li, R., Zeng, Z., Fu, G., Wan, Y., Liu, C., & McClements, D. J. (June 2019). Formation and characterization of tannic acid/beta-glucan complexes: Influence of pH, ionic strength, and temperature. Food Research International, 120, 748–755. Li, Y., Liu, H., Liu, Q., Kong, B., & Diao, X. (2019). Effects of zein hydrolysates coupled with sage (salvia officinalis) extract on the emulsifying and oxidative stability of myofibrillar protein prepared oil-in-water emulsions. Food Hydrocolloids, 87, 149–157. Liu, F. G., Ma, C. C., Gao, Y. X., & McClements, D. J. (2017). Food-grade covalent complexes and their application as nutraceutical delivery systems: A review.
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