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Pickering emulsions stabilized by native starch granules
Colloids and Surfaces A: Physicochem. Eng. Aspects 431 (2013) 142–149
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Pickering emulsions stabilized by native starch granules Chen Li, Yunxing Li, Peidong Sun, Cheng Yang ∗ The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
h i g h l i g h t s
g r a p h i c a l
• Some native starch granules had
At the low starch concentrations, the droplet surface coverage also was low and particles formed monolayer close-packed “clumps”. When the starch concentration progressed, the droplet surface coverage increased and became monolayer dense packing.
• • • •
emulsifying ability. Emulsifying ability of starch granules seemed inversely proportional to their sizes. The emulsion stabilized by rice granule was very stable to against coalescence. The close-packed “clumps” and dense packing of starch granules at interface were found. The droplet surface coverage increased with the starch concentration.
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Article history: Received 23 January 2013 Received in revised form 16 April 2013 Accepted 17 April 2013 Available online 24 April 2013 Keywords: Pickering emulsion Starch granules Emulsifier Food-grade
a b s t r a c t
a b s t r a c t The surface properties of native starch, namely rice, waxy maize, wheat and potato starch were characterized by contact angle measurement, zeta-potential, and then the emulsifying ability of these native starch granules was investigated by using liquid paraffin as oil phase and starch granules as sole emulsifier. It was found that emulsions could be prepared by using rice, waxy maize and wheat starch granules as emulsifier, but not potato starch, which had no emulsifying ability even when the concentration of starch granules as high as 15 wt% relative to water phase. Rice starch was the best emulsifier among these native starches. The emulsion stabilized by rice granules proved to be stable to coalescence for several months when the starch concentration was above 3 wt% relative to water phase. With the increasing of the rice starch concentration, the sizes of droplets decreased and the stability of emulsions enhanced. The salt and pH values had no significant influence on the packing structure of native starch granules at interface. © 2013 Elsevier B.V. All rights reserved.
1. Introduction An emulsion is a system of dispersed droplets of one immiscible liquid in another stabilized by emulsifier. A particle-stabilized emulsion, usually referred to as Pickering emulsions, was first reported in 1900s by Pickering. Because of their long-term stability and current activity in nanoparticle technology for producing new material, the interest in Pickering emulsion has been renewed
∗ Corresponding author. Tel.: +86 510 85917090; fax: +86 510 85917763. E-mail address: [email protected] (C. Yang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.04.025
during past decade. The unique properties of this kind of emulsion are due to the almost irreversible adsorption energy and the strong particle–particle interactions [1–3]. Number of different particle emulsifiers have been reported, such as silica [4,5], clays [6,7], microgels [8,9] and latex [10,11]. Recently, environmentally benign particle-emulsifiers have received reasonable attention due to their novel application, especially in food, cosmetics and pharmaceutics. It has been found that cellulose microparticles [12,13], cellulose nanoparticles [14], chitin nanocrystals [15], hydrophobic modified starch granules [16–19], and starch nanoparticles [20,21] could stabilize emulsions. The number of reported studies on environmentally benign
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particle-emulsifiers is much less than those on synthesized organic particles and inorganic particles. On the other hand, compared to the synthesized organic particle and inorganic particle emulsifiers, the emulsifying of environmentally benign particle-emulsifiers is less studied partly due to the difficulty in controlling the size, shape and surface properties of these particles. The size and shape of native starch granules vary among different botanical sources. Especially, the size of native starch granules ranges from several to tens of micrometers and can be seen in the light microscope, which makes them a good model for investigating the emulsion mechanism of the emulsifiers from environmentally benign sources. Starch is the second most abundant natural polymer after cellulose and is widely used in food and pharmaceutics. This also makes starch particles a good candidate for food-grade particle emulsifier. This paper reports the emulsifying ability of different native starch granules.
2.1. Materials Four commercial starches were used in this work. The waxy maize starch (Ash ≤ 0.05%) and wheat starch (Ash ≤ 0.5%) were purchased from Fluka, the potato starch was purchased from Acros Organics, and the rice starch was purchased from Sigma. The paraffin liquid (CR), hydrochloric acid (AR), sodium hydroxide(AR), sodium chloride(AR) and sodium azide were purchased from Guoyao Co. Ltd., Shanghai, China. 2.2. Preparation of emulsions The volume ratio of water to paraffin liquid was kept at a constant 1:1. A certain mass of starch granules was initially dispersed in 7 ml water containing 0.02% sodium azide, followed by adding 7 ml of paraffin liquid, and the mixture was homogenized at 10,000 rpm for 4 min using an XHF-D homogenizer (Ningbo Xinzhi, China) fitted with a dispersing tool the outer diameter equal to 1.4 cm. The starch granule concentrations are expressed as weight percentage relative to the water phase. A series of emulsions were prepared by adjusting them to different pH before homogenization using HCl (1 N) or NaOH (1 N) solution. The series of emulsions with different ionic strength were prepared by adding the NaCl. The emulsion type was identified by the drop test. After homogenization, a drop of the emulsion was added to a small volume of the oil and aqueous phase separately. An emulsion which dispersed in the aqueous phase but not in the oil phase was assessed as water continuous (O/W); conversely, an emulsion dispersing singularly in the oil phase was assessed as oil continuous. 2.3. Optical microscopy The optical micrographs of the emulsions were captured by a VHX-1000 digital microscope (Keyence Int. Trading Co. Ltd., Japan). The emulsion samples were placed directly on a microscope slide. The droplet size and distribution were obtained by measuring over 200 droplets from the digital microscopy image. The surface mean diameter (d32d ) and the volume mean diameter (d43d ) of droplets were calculated from Eqs. (1) and (2).
3 di d32d = 2
(1)
4 di 3
(2)
d43d =
di
Table 1 The size and size distribution of starch granules. Species
d32s (m)
d43s (m)
PDI
Rice starch Waxy maize starch Wheat starch Potato starch
4.5 10.8 22.0 47.0
5.2 11.3 23.8 52.1
1.16 1.15 2.32 1.56
where di is the diameter of a droplet. The d43d is sensitive to the presence of large droplets. The polydispersity index of droplet sizes (PDI) could be characterized by PDI =
d
43d
di/N
(3)
where N is the total number of droplets. 2.4. Scanning electron microscopy (SEM)
2. Materials and methods
di
143
An environmental scanning electron microscope (ESEM) (S4800, Hitachi, Japan) was used at a high voltage (1.0 kV) to capture the morphology and size of native starch granules and microcapsules. The surface mean diameter (d32s ) and the volume mean diameter (d43s ) and size distribution of native starch granules were obtained by Eqs. (1)–(3). 2.5. Measurement of contact angle The measurement of contact angles of water on starch in air were performed at room temperature by using an OCA40 optical contact angle measuring device (DataPhysics Instrument GmbH. Germany) equipped with a CDD camera and the WINDROP software. The volume of the drop was equal to 2 l. The smooth surface of starch samples were prepared according to Thielemans et al. [22] by compacting the powder under a pressure of 10 metric tons applied by a KBr press. Fig. 1 shows the pressed tablets of different starch granules have a comparably smooth surface. All the measurements were performed in triplicates. 2.6. Measurement of zeta potential Starch (0.01 wt%) were dispersed in water with homogenization. The zeta potentials were measured using a commercial Zetasizer (ZetaPALS, Brookhaven, America) at 25 ◦ C. All the measurements were performed in triplicates. 3. Results and discussion 3.1. Characterization of starch granules Fig. 2 shows the size and morphology of native starch granules. The average size of different native starch granules was determined from SEM of starch granules. Table 1 shows the size and size distribution of the native starch granules. They were different from each other in size and shape. Rice starch was polygonal in shape and its d43s , ∼5.2 m, was smallest among the tested starches. Waxy maize starch was spherical and polygonal in shape and had a d43s of ∼11.3 m. Wheat and potato starch had both oval and spherical shapes. The average size of wheat starch granules was larger than that of waxy maize starch and the d43s of potato starch, ∼52.1 m, was largest. Our results are in agreement with the previously reported work [23]. Fig. 3 shows the contact angle of water on starch. The contact angle of water on rice, waxy maize, wheat and potato starch was 48◦ , 45◦ , 29◦ and 63◦ , respectively. All these contact angles were smaller than 90◦ . Obviously, the native starch was hydrophilic but
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Fig. 1. SEM of pressed tablets of starch granules for contact angle measurements (rice starch (a), waxy maize starch (b), wheat starch (c) and potato starch (d)).
the hydrophilicity of different starch granules was varied. Potato starch had the largest contact angle, whereas wheat starch had the lowest contact angle. This indicates that potato starch is more hydrophobic than other starches, and wheat starch is the most
hydrophilic. Starch granule-associated protein and lipid are by far the most abundant of the minor components of starch [23]. The presence of surface protein and lipid may have significant effects on the properties of the starch. By dry heating, the hydrophobicity
Fig. 2. SEM of rice starch (a), waxy maize starch (b), wheat starch (c) and potato starch (d).
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Table 2 The size and size distribution of droplets stabilized by various starch (15 wt%) with different storage time. Species
Rice starch wx maize starch Wheat starch
d43d (m) (PDI) 1 day
2 weeks
4 weeks
8 weeks
188 (1.26) 1590 (1.82) 3131(1.63)
190 (1.22) 2215 (1.73) 3670 (1.72)
194 (1.30) 2876 (2.00) 5030 (2.31)Some oil droplets broken
200 (1.28) Some oil droplets broken Most oil droplets broken
stabilized by wheat or waxy maize starch granules were only stable to coalescence for several weeks to 2 months. To adsorb at interface, particles need to be partly wetted by both phases. The long-term stability of particle-stabilized emulsion was believed due to the almost irreversible adsorption energy and the strong particle–particle interaction. The energy required to move the particle from interface can be estimated from Eq. (4) if the gravity effects are neglected: Fig. 3. Contact angles of (a) rice starch, (b) waxy maize starch, (c) wheat starch and (d) potato starch.
of starch would be improved due to the starch granule surface proteins changing character from hydrophilic to hydrophobic [18,24]. The zeta-potential of starch granules was −20.6 mV, −19.0 mV, −20.3 mV, and −4.0 mV for rice, waxy maize, wheat and potato starch, respectively. Potato starch granules had much lower charge density than other starches did. It was reported that ∼90% in cereal starch granules surface and ∼95% in potato starch granules surface were carbohydrate, granule surface protein was to represent around 5% of the cereal starch surface and 0.05% of the potato starch surface, both starch types might have up to 5% lipid at granule surfaces [23]. Therefore, the lower charge density of potato starch granules might be attributed to the lower surface protein content of potato starch. 3.2. Characterization of emulsions Fig. 4 shows the emulsions stabilized by native starch granules at 15 wt% relative to water phase. Except potato starch, which could not stabilize oil droplets, the emulsions could be prepared by using native starch particles as emulsifier. Table 2 shows the droplet size changes during storage time. All these emulsions were unstable and creaming happened quickly after homogenization. This is the typical character of particle-stabilized emulsion due to the large droplets. However, the creaming and the size and size distribution of the droplets had no significant change during long-term storage, indicating these emulsions were stable to coalescence for long periods. The emulsion stabilized by rice starch granules was stable to coalescence for several months, whereas the emulsions
E = r 2 ˛ˇ (1 ± cos)
2
(4)
where E is detachment energy, r is the radius of particles, ˛ˇ is the oil–water interface tension and is the contact angle of particle at interface. The desorption energy of particle is positively dependent on the square of particle size, so that the large particles can almost irreversibly anchor at interface. The emulsions have the largest stability when the contact angle is about 90◦ , whereas emulsion can not form when the contact angle is close to 0◦ or 180◦ . Although the contact angle of water on starch measured in this experiment was not equal to the contact angle of particles at oil–water interface, the contact angle of water on starch was positively related to the contact angle of starch granules at oil–water interface. Therefore, it is reasonable to use the contact angle of water on starch for discussing the detachment energy of starch granules at interface. According to Eq. (4), potato starch granules had the largest desorption energy due to their largest particle size and largest contact angle. Other researchers indeed found that hydrophobic modification of starch granules would improve their emulsifying power [17,18]. However, emulsion did not form when potato starch was used as emulsifier, even though its contact angle and size favor the stablest emulsion compared with other starches. Native rice and waxy corn starch granules had rougher surfaces, whereas wheat and potato starch had smooth surface (as shown in Fig. 2). It was found that the surface roughness had a negative impact on emulsifying power possibly because the reduced surface contact considerably lessens the interfacial potential well holding smooth particles at the interface [25]. For non-spherical particles, the aspect ratio of the particles has strong influence on emulsion stability. It has been found that long particles with an aspect ratio over 4 are more effective emulsifier than less elongated ones with similar wettability [26]. The oval shape of potato and
Fig. 4. Vessels containing emulsion stabilized by rice starch (a), waxy maize starch (b), wheat starch (c) and potato starch (d) at 2 months (left) and 3 months (right) after homogenization (15 wt% starch granules).
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wheat starch granules was also expected to favor their emulsifying power compared with rice and waxy corn starch with polygonal shape. However, the emulsifying capability of native and waxy corn starch granules was stronger than that of wheat and potato starch granules. This indicates that surface morphology is not the primary factor contributing to the emulsifying ability of native starch granules. The rice, waxy corn, wheat and potato starch granules have varying emulsifying ability, each weaker than the one before it, with the exception of rice, whereas the granule sizes are progressively larger in the same given order. It seems in this experiment that the smaller starch granules exhibit a stronger emulsifying ability. It has been found that the size of particles which can be used to successfully stabilize emulsion ranges from nanometer to micrometer, the overall stability being inversely proportional to particle size, with smaller particles giving a higher packing efficiency, and producing a more homogenous layer [3,11,27,28]. By using the maximum surface coverage concept, the required starch mass per volume of oil
Fig. 5. Vessels containing emulsion stabilized by rice starch granules at different concentration. (From left to right: 3 wt%, 9 wt%, 15 wt%, 30 wt%, 45 wt%, 5 months after homogenization.)
Fig. 6. The optical micrography of emulsion stabilized by starch granules at different concentrations: (a) 3 wt% rice starch, (b) magnified from (a), (c) 30 wt% rice starch, (d) magnified from (c), (e) 15 wt% waxy maize starch, scale bar = 50 m.
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to generate a starch granule stabilized emulsion of a given drop size could be roughly estimated from Eq. (5) [17,18]. Cso = 4sg ϕ
d32s d32d
(5)
where Cso is the starch to oil ratio (mg/ml), sg the starch density, ϕ the packing density, d32s the surface mean diameter of starch granules and d32d the surface mean diameter of the oil droplets to be stabilized. According to equation (5), the Cso could be scaled to d32s as Cso ∼ d32s. The larger the granules the higher the required mass to cover an equal interfacial area. The d32s of rice starch granules is about 0.4, 0.2, and 0.09 of that of waxy corn, wheat and potato starch granules, respectively. Therefore, to cover an equal interfacial area, the required mass of waxy corn, wheat and potato starch granules are respectively about 2.4, 5 and 10 times that of rice starch. This is the dominating reason why the rice starch has highest emulsifying power whereas potato starch has lowest. Fig. 5 shows the emulsions stabilized by rice starch at different concentrations. When starch concentration was above 3 wt%, the emulsion could be prepared. The low critical starch concentrations which can stabilize an emulsion increased with the starch granule size and were much higher than that of nanoparticles [14,15,20,21]. This also indicates that smaller particles have more efficient emulsifying abilities. The emulsions stabilized by rice starch were very stable to coalescence. However, the creaming was observed after homogenization. Meanwhile, the creaming decreased with the increased starch concentration. This is because the surface coverage of oil droplets increased with the starch concentration, leading to increased creaming stability. On the other hand, the creaming might be inhibited due to aggregation of particles between droplets forming a network of particle-bridged droplets [15,16,18,29]. Fig. 6 shows the optical micrography of emulsion stabilized by native starch granules. It could be clearly seen that starch granules indeed adsorbed on the surface of oil droplets. Table 3 shows the droplet
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Table 3 The droplet size and distribution of emulsions stabilized by rice starch granules with different storage time. Starch content
d43d (m) (PDI) 1 day
3 wt% 9 wt% 15 wt% 30 wt% 45 wt%
802 (1.36) 342 (1.38) 188 (1.26) 67 (1.21) 43 (1.18)
2 weeks 823 (1.57) 353 (1.27) 190 (1.22) 72 (1.17) 48 (1.19)
8 weeks 850 (1.72) 368 (1.31) 200 (1.28) 78 (1.19) 50 (1.26)
1 year 1982 (1.81) 432 (1.42) 280 (1.34) 122 (1.28) 71 (1.28)
sizes of emulsions with different rice starch granule content. The d43d of droplets stabilized by rice starch decreased with the increasing of starch content. A higher concentration of particles can stabilize a larger total interfacial area which in turn means that a smaller average droplet size can be achieved. Fig. 6 indeed shows that droplet surface coverage increased with the starch concentration. Two packing types of particle at interface were observed. At the low starch concentration, the droplet surface coverage also was low and particles formed monolayer close-packed “clumps” as shown in Fig. 6(b) [25]. When the starch concentration progressed, the droplet surface coverage increased and became monolayer dense packing as shown in Fig. 6(d). In the emulsion stabilized by waxy maize starch, the dense packing of granules was also found for small droplets as shown in Fig. 6(e). This dense packing would be a strong steric barrier to inhibit coalescence when the droplets were brought contact. Very recently, Timgren et al. [18] reported, by using 100 mg per ml oil of native rice and waxy rice varieties, only millimeter sized droplets was formed and no dense packing of granules was observed. In this experiment, we used much higher concentration of native rice starch (∼450 mg per ml oil of starch) so that smaller droplets and dense packing was observed. This was evidence that the poor coverage of the droplets did not originate from the lower surface affinity of starch granules instead from the
Fig. 7. The optical micrography of emulsion stabilized by 15 wt% rice starch at pH (a) 2.4, (b) 10.6 and (c) 12.2 (Polarized light), scale bar = 50 m.
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Fig. 8. The optical micrography and photography (inset) of emulsions stabilized by 15 wt% rice starch granules after heating at 80 ◦ C for 2 h, scale bar = 50 m.
low concentration of starch granules used. Multilayer packing of particles at interface was not observed, which was possible because starch granules were too large to form a multilayer of absorbed particles at oil–water interface. For charged particles at the oil–water interface, the hexagonal could be formed due to the self-assembly of particles [3,30]. The hexagonal packing structure was not found in our experiments. Long-range electrostatic repulsion prevented the starch granules from aggregation; however, van der Waals and capillary attractive forces induced starch granules aggregation. Both van der Waals and capillary attractive forces increase with the particle size. Especially, the gravity-induced lateral capillary force is not negligible when radius of particles is above 5 m [3,31,32]. In this experiment, only the average radius of rice starch granules (∼2.5 m) was smaller than 5 m, other starch granules were larger than 5 m. It is reasonable to suppose that the van der Waals and capillary were the dominant forces responsible for the interfacial granule aggregation. It was found the ionic strength and pH values would influence the packing structure of charged particles at interface due to the electrostatic repulsion of particles being screened [15,30]. In the case of emulsions stabilized by rice starch granules, when increasing the salt concentration (NaCl) up to 500 mM there was no significant change in the emulsion stability, size of droplets and packing structure of granules at interface. Similar result has been reported by other researches [18]. It was inferred that there was no significant effect of salt concentration on the emulsion. Similarly, the pH value had also no influence on the emulsion when pH values were from 3 to 10. As has been discussed above, the electrostatic repulsion of starch granules at interface was weak so that the pH value and salt had no significant influence on the emulsion stabilized by starch granules. However, the average size of droplets decreased sharply to 16.4 m when pH value was above 12 and optical micrography revealed few starch particles adsorbed at the droplets’ interface, as shown in Fig. 7. This is due to the native starch granules breaking in the high pH-value aqueous solution, the released starch polymer chain adsorbed at the interface. In this case, the emulsion was stabilized by starch polymer chain and a little remained starch granules instead of sole native starch granules. When the emulsion stabilized by rice starch was heated at 60 ◦ C for 2 h, it displayed no change in its state. However, after heating the emulsion at 80 ◦ C for 2 h, the starch gelatinized to become gel as shown in Fig. 8. It was interesting to find that the emulsion was still stable after gelatinization of starch and the oil droplets were trapped by starch gel. The gel-like emulsion was transferred by centrifugation into ether to remove the oil phase and then dried in air [33]. It was found the hollow microcapsules formed as shown in
Fig. 9. The SEM of the microcapsules prepared by heating rice starch granules stabilized emulsion.
Fig. 9. This is a potential for constructing a novel delivery system for active molecules and drugs. 4. Conclusions The morphology and surface chemistry of native starch granules had no significant impact on the emulsifying power of native starch granules. The emulsifying capability of native starch granules seemed to be inversely related to the size of starch granules. To stabilize a given emulsion interface, large size starch granules needed much more amount than smaller starch granules did, consequently they were not efficient particle emulsifier. Rice starch granule proved to be a good particle emulsifier and could stabilize emulsion to against coalescence for several months. With the increasing of rice starch concentration, the packing density of granules at interface increased and droplet size diminished. The negative charge at the surface of starch granules was too weak to prevent the aggregation of starch granules at the interface and pH and salt had no significant effect on emulsion. After heating the emulsion at gelatinization temperature of starch, the emulsion did not break, but turned into gel-like emulsion. Acknowledgment The financial support of Qing Lan Project is gratefully acknowledged. References [1] F. Leal-Calderon, V. Schmitt, Solid-stabilized emulsion, Curr. Opin. Colloid Interface Sci. 13 (2008) 217–227. [2] S. Tcholakova, N.D. Denkov, A. Lips, Comparison of solid particles, globular proteins and surfactants as emulsifiers, Phys. Chem. Chem. Phys. 160 (2008) 1608–1627. [3] T.N. Hunter, R.J. Pugh, G.V. Franks, G.J. Jameson, The role of particles in stabilising foams and emulsions, Adv. Colloid Interface Sci. 137 (2008) 57–81. [4] B.P. Binks, C.P. Whitby, Silica particle-stabilized emulsions of silicone oil and water: aspects of emulsification, Langmuir 20 (2004) 1130–1137. [5] B.R. Midmore, Preparation of a novel silica-stabilized oil/water emulsion, Colloids Surf. A Physicochem. Eng. Aspects 132 (1998) 257–265. [6] N.P. Ashby, B.P. Binks, Pickering emulsions stabilised by laponite clay particles, Phys. Chem. Chem. Phys. 2 (2000) 5640–5646. [7] Y. Nonomura, N. Kobayashi, Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles, J. Colloid Interface Sci. 330 (2009) 463–466. [8] Z. Li, T. Ming, J. Wang, T. Ngai, High internal phase emulsion stabilized solely by microgel particles, Angew. Chem. Int. Ed. 48 (2009) 8490–8493. [9] B. Brugger, B.A. Rosen, W. Richtering, Microgels as stimuli-responsive stabilizers for emulsions, Langmuir 24 (2008) 12202–12208. [10] N.P. Ashby, B.P. Binks, V.N. Paunov, Bridging interaction between a water drop stabilised by solid particles and a planar oil/water interface, Chem. Commun. 4 (2004) 436–437.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Pickering emulsions stabilized by native starch granules Chen Li, Yunxing Li, Peidong Sun, Cheng Yang ∗ The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
h i g h l i g h t s
g r a p h i c a l
• Some native starch granules had
At the low starch concentrations, the droplet surface coverage also was low and particles formed monolayer close-packed “clumps”. When the starch concentration progressed, the droplet surface coverage increased and became monolayer dense packing.
• • • •
emulsifying ability. Emulsifying ability of starch granules seemed inversely proportional to their sizes. The emulsion stabilized by rice granule was very stable to against coalescence. The close-packed “clumps” and dense packing of starch granules at interface were found. The droplet surface coverage increased with the starch concentration.
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 16 April 2013 Accepted 17 April 2013 Available online 24 April 2013 Keywords: Pickering emulsion Starch granules Emulsifier Food-grade
a b s t r a c t
a b s t r a c t The surface properties of native starch, namely rice, waxy maize, wheat and potato starch were characterized by contact angle measurement, zeta-potential, and then the emulsifying ability of these native starch granules was investigated by using liquid paraffin as oil phase and starch granules as sole emulsifier. It was found that emulsions could be prepared by using rice, waxy maize and wheat starch granules as emulsifier, but not potato starch, which had no emulsifying ability even when the concentration of starch granules as high as 15 wt% relative to water phase. Rice starch was the best emulsifier among these native starches. The emulsion stabilized by rice granules proved to be stable to coalescence for several months when the starch concentration was above 3 wt% relative to water phase. With the increasing of the rice starch concentration, the sizes of droplets decreased and the stability of emulsions enhanced. The salt and pH values had no significant influence on the packing structure of native starch granules at interface. © 2013 Elsevier B.V. All rights reserved.
1. Introduction An emulsion is a system of dispersed droplets of one immiscible liquid in another stabilized by emulsifier. A particle-stabilized emulsion, usually referred to as Pickering emulsions, was first reported in 1900s by Pickering. Because of their long-term stability and current activity in nanoparticle technology for producing new material, the interest in Pickering emulsion has been renewed
∗ Corresponding author. Tel.: +86 510 85917090; fax: +86 510 85917763. E-mail address: [email protected] (C. Yang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.04.025
during past decade. The unique properties of this kind of emulsion are due to the almost irreversible adsorption energy and the strong particle–particle interactions [1–3]. Number of different particle emulsifiers have been reported, such as silica [4,5], clays [6,7], microgels [8,9] and latex [10,11]. Recently, environmentally benign particle-emulsifiers have received reasonable attention due to their novel application, especially in food, cosmetics and pharmaceutics. It has been found that cellulose microparticles [12,13], cellulose nanoparticles [14], chitin nanocrystals [15], hydrophobic modified starch granules [16–19], and starch nanoparticles [20,21] could stabilize emulsions. The number of reported studies on environmentally benign
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particle-emulsifiers is much less than those on synthesized organic particles and inorganic particles. On the other hand, compared to the synthesized organic particle and inorganic particle emulsifiers, the emulsifying of environmentally benign particle-emulsifiers is less studied partly due to the difficulty in controlling the size, shape and surface properties of these particles. The size and shape of native starch granules vary among different botanical sources. Especially, the size of native starch granules ranges from several to tens of micrometers and can be seen in the light microscope, which makes them a good model for investigating the emulsion mechanism of the emulsifiers from environmentally benign sources. Starch is the second most abundant natural polymer after cellulose and is widely used in food and pharmaceutics. This also makes starch particles a good candidate for food-grade particle emulsifier. This paper reports the emulsifying ability of different native starch granules.
2.1. Materials Four commercial starches were used in this work. The waxy maize starch (Ash ≤ 0.05%) and wheat starch (Ash ≤ 0.5%) were purchased from Fluka, the potato starch was purchased from Acros Organics, and the rice starch was purchased from Sigma. The paraffin liquid (CR), hydrochloric acid (AR), sodium hydroxide(AR), sodium chloride(AR) and sodium azide were purchased from Guoyao Co. Ltd., Shanghai, China. 2.2. Preparation of emulsions The volume ratio of water to paraffin liquid was kept at a constant 1:1. A certain mass of starch granules was initially dispersed in 7 ml water containing 0.02% sodium azide, followed by adding 7 ml of paraffin liquid, and the mixture was homogenized at 10,000 rpm for 4 min using an XHF-D homogenizer (Ningbo Xinzhi, China) fitted with a dispersing tool the outer diameter equal to 1.4 cm. The starch granule concentrations are expressed as weight percentage relative to the water phase. A series of emulsions were prepared by adjusting them to different pH before homogenization using HCl (1 N) or NaOH (1 N) solution. The series of emulsions with different ionic strength were prepared by adding the NaCl. The emulsion type was identified by the drop test. After homogenization, a drop of the emulsion was added to a small volume of the oil and aqueous phase separately. An emulsion which dispersed in the aqueous phase but not in the oil phase was assessed as water continuous (O/W); conversely, an emulsion dispersing singularly in the oil phase was assessed as oil continuous. 2.3. Optical microscopy The optical micrographs of the emulsions were captured by a VHX-1000 digital microscope (Keyence Int. Trading Co. Ltd., Japan). The emulsion samples were placed directly on a microscope slide. The droplet size and distribution were obtained by measuring over 200 droplets from the digital microscopy image. The surface mean diameter (d32d ) and the volume mean diameter (d43d ) of droplets were calculated from Eqs. (1) and (2).
3 di d32d = 2
(1)
4 di 3
(2)
d43d =
di
Table 1 The size and size distribution of starch granules. Species
d32s (m)
d43s (m)
PDI
Rice starch Waxy maize starch Wheat starch Potato starch
4.5 10.8 22.0 47.0
5.2 11.3 23.8 52.1
1.16 1.15 2.32 1.56
where di is the diameter of a droplet. The d43d is sensitive to the presence of large droplets. The polydispersity index of droplet sizes (PDI) could be characterized by PDI =
d
43d
di/N
(3)
where N is the total number of droplets. 2.4. Scanning electron microscopy (SEM)
2. Materials and methods
di
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An environmental scanning electron microscope (ESEM) (S4800, Hitachi, Japan) was used at a high voltage (1.0 kV) to capture the morphology and size of native starch granules and microcapsules. The surface mean diameter (d32s ) and the volume mean diameter (d43s ) and size distribution of native starch granules were obtained by Eqs. (1)–(3). 2.5. Measurement of contact angle The measurement of contact angles of water on starch in air were performed at room temperature by using an OCA40 optical contact angle measuring device (DataPhysics Instrument GmbH. Germany) equipped with a CDD camera and the WINDROP software. The volume of the drop was equal to 2 l. The smooth surface of starch samples were prepared according to Thielemans et al. [22] by compacting the powder under a pressure of 10 metric tons applied by a KBr press. Fig. 1 shows the pressed tablets of different starch granules have a comparably smooth surface. All the measurements were performed in triplicates. 2.6. Measurement of zeta potential Starch (0.01 wt%) were dispersed in water with homogenization. The zeta potentials were measured using a commercial Zetasizer (ZetaPALS, Brookhaven, America) at 25 ◦ C. All the measurements were performed in triplicates. 3. Results and discussion 3.1. Characterization of starch granules Fig. 2 shows the size and morphology of native starch granules. The average size of different native starch granules was determined from SEM of starch granules. Table 1 shows the size and size distribution of the native starch granules. They were different from each other in size and shape. Rice starch was polygonal in shape and its d43s , ∼5.2 m, was smallest among the tested starches. Waxy maize starch was spherical and polygonal in shape and had a d43s of ∼11.3 m. Wheat and potato starch had both oval and spherical shapes. The average size of wheat starch granules was larger than that of waxy maize starch and the d43s of potato starch, ∼52.1 m, was largest. Our results are in agreement with the previously reported work [23]. Fig. 3 shows the contact angle of water on starch. The contact angle of water on rice, waxy maize, wheat and potato starch was 48◦ , 45◦ , 29◦ and 63◦ , respectively. All these contact angles were smaller than 90◦ . Obviously, the native starch was hydrophilic but
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Fig. 1. SEM of pressed tablets of starch granules for contact angle measurements (rice starch (a), waxy maize starch (b), wheat starch (c) and potato starch (d)).
the hydrophilicity of different starch granules was varied. Potato starch had the largest contact angle, whereas wheat starch had the lowest contact angle. This indicates that potato starch is more hydrophobic than other starches, and wheat starch is the most
hydrophilic. Starch granule-associated protein and lipid are by far the most abundant of the minor components of starch [23]. The presence of surface protein and lipid may have significant effects on the properties of the starch. By dry heating, the hydrophobicity
Fig. 2. SEM of rice starch (a), waxy maize starch (b), wheat starch (c) and potato starch (d).
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Table 2 The size and size distribution of droplets stabilized by various starch (15 wt%) with different storage time. Species
Rice starch wx maize starch Wheat starch
d43d (m) (PDI) 1 day
2 weeks
4 weeks
8 weeks
188 (1.26) 1590 (1.82) 3131(1.63)
190 (1.22) 2215 (1.73) 3670 (1.72)
194 (1.30) 2876 (2.00) 5030 (2.31)Some oil droplets broken
200 (1.28) Some oil droplets broken Most oil droplets broken
stabilized by wheat or waxy maize starch granules were only stable to coalescence for several weeks to 2 months. To adsorb at interface, particles need to be partly wetted by both phases. The long-term stability of particle-stabilized emulsion was believed due to the almost irreversible adsorption energy and the strong particle–particle interaction. The energy required to move the particle from interface can be estimated from Eq. (4) if the gravity effects are neglected: Fig. 3. Contact angles of (a) rice starch, (b) waxy maize starch, (c) wheat starch and (d) potato starch.
of starch would be improved due to the starch granule surface proteins changing character from hydrophilic to hydrophobic [18,24]. The zeta-potential of starch granules was −20.6 mV, −19.0 mV, −20.3 mV, and −4.0 mV for rice, waxy maize, wheat and potato starch, respectively. Potato starch granules had much lower charge density than other starches did. It was reported that ∼90% in cereal starch granules surface and ∼95% in potato starch granules surface were carbohydrate, granule surface protein was to represent around 5% of the cereal starch surface and 0.05% of the potato starch surface, both starch types might have up to 5% lipid at granule surfaces [23]. Therefore, the lower charge density of potato starch granules might be attributed to the lower surface protein content of potato starch. 3.2. Characterization of emulsions Fig. 4 shows the emulsions stabilized by native starch granules at 15 wt% relative to water phase. Except potato starch, which could not stabilize oil droplets, the emulsions could be prepared by using native starch particles as emulsifier. Table 2 shows the droplet size changes during storage time. All these emulsions were unstable and creaming happened quickly after homogenization. This is the typical character of particle-stabilized emulsion due to the large droplets. However, the creaming and the size and size distribution of the droplets had no significant change during long-term storage, indicating these emulsions were stable to coalescence for long periods. The emulsion stabilized by rice starch granules was stable to coalescence for several months, whereas the emulsions
E = r 2 ˛ˇ (1 ± cos)
2
(4)
where E is detachment energy, r is the radius of particles, ˛ˇ is the oil–water interface tension and is the contact angle of particle at interface. The desorption energy of particle is positively dependent on the square of particle size, so that the large particles can almost irreversibly anchor at interface. The emulsions have the largest stability when the contact angle is about 90◦ , whereas emulsion can not form when the contact angle is close to 0◦ or 180◦ . Although the contact angle of water on starch measured in this experiment was not equal to the contact angle of particles at oil–water interface, the contact angle of water on starch was positively related to the contact angle of starch granules at oil–water interface. Therefore, it is reasonable to use the contact angle of water on starch for discussing the detachment energy of starch granules at interface. According to Eq. (4), potato starch granules had the largest desorption energy due to their largest particle size and largest contact angle. Other researchers indeed found that hydrophobic modification of starch granules would improve their emulsifying power [17,18]. However, emulsion did not form when potato starch was used as emulsifier, even though its contact angle and size favor the stablest emulsion compared with other starches. Native rice and waxy corn starch granules had rougher surfaces, whereas wheat and potato starch had smooth surface (as shown in Fig. 2). It was found that the surface roughness had a negative impact on emulsifying power possibly because the reduced surface contact considerably lessens the interfacial potential well holding smooth particles at the interface [25]. For non-spherical particles, the aspect ratio of the particles has strong influence on emulsion stability. It has been found that long particles with an aspect ratio over 4 are more effective emulsifier than less elongated ones with similar wettability [26]. The oval shape of potato and
Fig. 4. Vessels containing emulsion stabilized by rice starch (a), waxy maize starch (b), wheat starch (c) and potato starch (d) at 2 months (left) and 3 months (right) after homogenization (15 wt% starch granules).
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wheat starch granules was also expected to favor their emulsifying power compared with rice and waxy corn starch with polygonal shape. However, the emulsifying capability of native and waxy corn starch granules was stronger than that of wheat and potato starch granules. This indicates that surface morphology is not the primary factor contributing to the emulsifying ability of native starch granules. The rice, waxy corn, wheat and potato starch granules have varying emulsifying ability, each weaker than the one before it, with the exception of rice, whereas the granule sizes are progressively larger in the same given order. It seems in this experiment that the smaller starch granules exhibit a stronger emulsifying ability. It has been found that the size of particles which can be used to successfully stabilize emulsion ranges from nanometer to micrometer, the overall stability being inversely proportional to particle size, with smaller particles giving a higher packing efficiency, and producing a more homogenous layer [3,11,27,28]. By using the maximum surface coverage concept, the required starch mass per volume of oil
Fig. 5. Vessels containing emulsion stabilized by rice starch granules at different concentration. (From left to right: 3 wt%, 9 wt%, 15 wt%, 30 wt%, 45 wt%, 5 months after homogenization.)
Fig. 6. The optical micrography of emulsion stabilized by starch granules at different concentrations: (a) 3 wt% rice starch, (b) magnified from (a), (c) 30 wt% rice starch, (d) magnified from (c), (e) 15 wt% waxy maize starch, scale bar = 50 m.
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to generate a starch granule stabilized emulsion of a given drop size could be roughly estimated from Eq. (5) [17,18]. Cso = 4sg ϕ
d32s d32d
(5)
where Cso is the starch to oil ratio (mg/ml), sg the starch density, ϕ the packing density, d32s the surface mean diameter of starch granules and d32d the surface mean diameter of the oil droplets to be stabilized. According to equation (5), the Cso could be scaled to d32s as Cso ∼ d32s. The larger the granules the higher the required mass to cover an equal interfacial area. The d32s of rice starch granules is about 0.4, 0.2, and 0.09 of that of waxy corn, wheat and potato starch granules, respectively. Therefore, to cover an equal interfacial area, the required mass of waxy corn, wheat and potato starch granules are respectively about 2.4, 5 and 10 times that of rice starch. This is the dominating reason why the rice starch has highest emulsifying power whereas potato starch has lowest. Fig. 5 shows the emulsions stabilized by rice starch at different concentrations. When starch concentration was above 3 wt%, the emulsion could be prepared. The low critical starch concentrations which can stabilize an emulsion increased with the starch granule size and were much higher than that of nanoparticles [14,15,20,21]. This also indicates that smaller particles have more efficient emulsifying abilities. The emulsions stabilized by rice starch were very stable to coalescence. However, the creaming was observed after homogenization. Meanwhile, the creaming decreased with the increased starch concentration. This is because the surface coverage of oil droplets increased with the starch concentration, leading to increased creaming stability. On the other hand, the creaming might be inhibited due to aggregation of particles between droplets forming a network of particle-bridged droplets [15,16,18,29]. Fig. 6 shows the optical micrography of emulsion stabilized by native starch granules. It could be clearly seen that starch granules indeed adsorbed on the surface of oil droplets. Table 3 shows the droplet
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Table 3 The droplet size and distribution of emulsions stabilized by rice starch granules with different storage time. Starch content
d43d (m) (PDI) 1 day
3 wt% 9 wt% 15 wt% 30 wt% 45 wt%
802 (1.36) 342 (1.38) 188 (1.26) 67 (1.21) 43 (1.18)
2 weeks 823 (1.57) 353 (1.27) 190 (1.22) 72 (1.17) 48 (1.19)
8 weeks 850 (1.72) 368 (1.31) 200 (1.28) 78 (1.19) 50 (1.26)
1 year 1982 (1.81) 432 (1.42) 280 (1.34) 122 (1.28) 71 (1.28)
sizes of emulsions with different rice starch granule content. The d43d of droplets stabilized by rice starch decreased with the increasing of starch content. A higher concentration of particles can stabilize a larger total interfacial area which in turn means that a smaller average droplet size can be achieved. Fig. 6 indeed shows that droplet surface coverage increased with the starch concentration. Two packing types of particle at interface were observed. At the low starch concentration, the droplet surface coverage also was low and particles formed monolayer close-packed “clumps” as shown in Fig. 6(b) [25]. When the starch concentration progressed, the droplet surface coverage increased and became monolayer dense packing as shown in Fig. 6(d). In the emulsion stabilized by waxy maize starch, the dense packing of granules was also found for small droplets as shown in Fig. 6(e). This dense packing would be a strong steric barrier to inhibit coalescence when the droplets were brought contact. Very recently, Timgren et al. [18] reported, by using 100 mg per ml oil of native rice and waxy rice varieties, only millimeter sized droplets was formed and no dense packing of granules was observed. In this experiment, we used much higher concentration of native rice starch (∼450 mg per ml oil of starch) so that smaller droplets and dense packing was observed. This was evidence that the poor coverage of the droplets did not originate from the lower surface affinity of starch granules instead from the
Fig. 7. The optical micrography of emulsion stabilized by 15 wt% rice starch at pH (a) 2.4, (b) 10.6 and (c) 12.2 (Polarized light), scale bar = 50 m.
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Fig. 8. The optical micrography and photography (inset) of emulsions stabilized by 15 wt% rice starch granules after heating at 80 ◦ C for 2 h, scale bar = 50 m.
low concentration of starch granules used. Multilayer packing of particles at interface was not observed, which was possible because starch granules were too large to form a multilayer of absorbed particles at oil–water interface. For charged particles at the oil–water interface, the hexagonal could be formed due to the self-assembly of particles [3,30]. The hexagonal packing structure was not found in our experiments. Long-range electrostatic repulsion prevented the starch granules from aggregation; however, van der Waals and capillary attractive forces induced starch granules aggregation. Both van der Waals and capillary attractive forces increase with the particle size. Especially, the gravity-induced lateral capillary force is not negligible when radius of particles is above 5 m [3,31,32]. In this experiment, only the average radius of rice starch granules (∼2.5 m) was smaller than 5 m, other starch granules were larger than 5 m. It is reasonable to suppose that the van der Waals and capillary were the dominant forces responsible for the interfacial granule aggregation. It was found the ionic strength and pH values would influence the packing structure of charged particles at interface due to the electrostatic repulsion of particles being screened [15,30]. In the case of emulsions stabilized by rice starch granules, when increasing the salt concentration (NaCl) up to 500 mM there was no significant change in the emulsion stability, size of droplets and packing structure of granules at interface. Similar result has been reported by other researches [18]. It was inferred that there was no significant effect of salt concentration on the emulsion. Similarly, the pH value had also no influence on the emulsion when pH values were from 3 to 10. As has been discussed above, the electrostatic repulsion of starch granules at interface was weak so that the pH value and salt had no significant influence on the emulsion stabilized by starch granules. However, the average size of droplets decreased sharply to 16.4 m when pH value was above 12 and optical micrography revealed few starch particles adsorbed at the droplets’ interface, as shown in Fig. 7. This is due to the native starch granules breaking in the high pH-value aqueous solution, the released starch polymer chain adsorbed at the interface. In this case, the emulsion was stabilized by starch polymer chain and a little remained starch granules instead of sole native starch granules. When the emulsion stabilized by rice starch was heated at 60 ◦ C for 2 h, it displayed no change in its state. However, after heating the emulsion at 80 ◦ C for 2 h, the starch gelatinized to become gel as shown in Fig. 8. It was interesting to find that the emulsion was still stable after gelatinization of starch and the oil droplets were trapped by starch gel. The gel-like emulsion was transferred by centrifugation into ether to remove the oil phase and then dried in air [33]. It was found the hollow microcapsules formed as shown in
Fig. 9. The SEM of the microcapsules prepared by heating rice starch granules stabilized emulsion.
Fig. 9. This is a potential for constructing a novel delivery system for active molecules and drugs. 4. Conclusions The morphology and surface chemistry of native starch granules had no significant impact on the emulsifying power of native starch granules. The emulsifying capability of native starch granules seemed to be inversely related to the size of starch granules. To stabilize a given emulsion interface, large size starch granules needed much more amount than smaller starch granules did, consequently they were not efficient particle emulsifier. Rice starch granule proved to be a good particle emulsifier and could stabilize emulsion to against coalescence for several months. With the increasing of rice starch concentration, the packing density of granules at interface increased and droplet size diminished. The negative charge at the surface of starch granules was too weak to prevent the aggregation of starch granules at the interface and pH and salt had no significant effect on emulsion. After heating the emulsion at gelatinization temperature of starch, the emulsion did not break, but turned into gel-like emulsion. Acknowledgment The financial support of Qing Lan Project is gratefully acknowledged. References [1] F. Leal-Calderon, V. Schmitt, Solid-stabilized emulsion, Curr. Opin. Colloid Interface Sci. 13 (2008) 217–227. [2] S. Tcholakova, N.D. Denkov, A. Lips, Comparison of solid particles, globular proteins and surfactants as emulsifiers, Phys. Chem. Chem. Phys. 160 (2008) 1608–1627. [3] T.N. Hunter, R.J. Pugh, G.V. Franks, G.J. Jameson, The role of particles in stabilising foams and emulsions, Adv. Colloid Interface Sci. 137 (2008) 57–81. [4] B.P. Binks, C.P. Whitby, Silica particle-stabilized emulsions of silicone oil and water: aspects of emulsification, Langmuir 20 (2004) 1130–1137. [5] B.R. Midmore, Preparation of a novel silica-stabilized oil/water emulsion, Colloids Surf. A Physicochem. Eng. Aspects 132 (1998) 257–265. [6] N.P. Ashby, B.P. Binks, Pickering emulsions stabilised by laponite clay particles, Phys. Chem. Chem. Phys. 2 (2000) 5640–5646. [7] Y. Nonomura, N. Kobayashi, Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles, J. Colloid Interface Sci. 330 (2009) 463–466. [8] Z. Li, T. Ming, J. Wang, T. Ngai, High internal phase emulsion stabilized solely by microgel particles, Angew. Chem. Int. Ed. 48 (2009) 8490–8493. [9] B. Brugger, B.A. Rosen, W. Richtering, Microgels as stimuli-responsive stabilizers for emulsions, Langmuir 24 (2008) 12202–12208. [10] N.P. Ashby, B.P. Binks, V.N. Paunov, Bridging interaction between a water drop stabilised by solid particles and a planar oil/water interface, Chem. Commun. 4 (2004) 436–437.
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