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Coalescence in Highly Concentrated Coarse Emulsions George A. van Aken* and Franklin D. Zoet Wageningen Centre for Food Sciences (WCFS), c.o. NIZO food research, P.O. Box 20, 6710 BA Ede, The Netherlands Received January 14, 2000. In Final Form: June 21, 2000 This study was focused on coalescence in highly concentrated emulsions stabilized by a whey protein isolate or by sodium dodecyl sulfate. For both stabilizers, coalescence of the emulsion at rest occurred below a critical concentration of the stabilizer. This concentration was related to full coverage of the droplet interface. Coalescence proceeded until the internal area of the emulsion was sufficiently reduced to obtain full coverage by the stabilizer. Above the critical stabilizer concentration, this mechanism of coalescence was almost completely inhibited. Coalescence could also be induced by subjecting the emulsion to external forces that made the emulsion flow. Coalescence induced by flow occurred much more readily for the protein-stabilized emulsion than for the surfactant-stabilized emulsion and also occurred above the critical stabilizer concentration. This demonstrated that coalescence induced by flow and coalescence at rest proceeded through different mechanisms.
Introduction Coarse emulsions (hereafter referred to as “emulsions”) are thermodynamically unstable mixtures of two liquid phases, of which one (the dispersed phase) is present as droplets dispersed in the other (the continuous phase). The surface tension between the two phases usually remains relatively high (typically >1 mN‚m-1). When droplets merge, the area is reduced, which is energetically favorable; therefore coalescence readily occurs when the liquid interiors of the droplets come in contact. Coalescence is impeded when the droplet surfaces are coated by a protective layer of surface-active material, in this way producing a metastable emulsion that may persist for a long time. Good control of the stability against coalescence is of much practical importance. For this reason the mechanisms of coalescence have been studied for a long time. The motivation of the present work was to improve our insight into coalescence of food-type emulsions. These emulsions are often stabilized by macromolecular substances, such as proteins and polysaccharides. It is well documented in the literature that these are able to stabilize emulsions almost indefinitely against coalescence, which has often been attributed to the mechanical properties of the adsorbed layer.1 On the other hand, it has been pointed out by several authors that mechanical agitation of proteinstabilized emulsions strongly accelerates coalescence.2-4 Despite a large number of experimental and theoretical studies, our understanding of this subject has remained limited.5,6 This is mainly due to differences in experimental conditions, which may involve different mechanisms of coalescence and may also accentuate different steps in the sequence of processes that finally lead to coalescence. * To whom correspondence should be addressed. E-mail: aken@ nizo.nl. (1) Kitchener, J. A.; Musselwhite, P. R. In Emulsion Science; Sherman, P., Ed.; Academic Press: London, 1968; pp 117-120. (2) Britten, M.; Giroux, H. J. J. Food Sci. 1991, 56, 792. (3) Klemaszewski, J. L.; Das, K. P.; Kinsella, J. E. J. Food Sci. 1992, 57, 366. (4) Kumar, S.; Narshimhan, G.; Ramkrishna, D. Ind. Eng. Chem. Res. 1996, 35, 3155. (5) Walstra, P. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York 1996; Vol. 4, pp 1-61. (6) Malhotra, A. K.; Wasan, D. T. In Thin Liquid Films; Ivanov, I. B., Ed.; Marcel Dekker: New York 1988; pp 829-890.
These steps are droplet encounter by diffusion or convection, outflow of the continuous phase from the spacing between the droplets, thin film formation, and thin film rupture.7 This work reports an experimental study of coalescence in emulsions stabilized by whey protein isolate (WPI) as a model system for emulsions in food applications. In view of the expected relation between the emulsion stabilizing properties and the mechanical properties of the adsorbed layer, we studied coalescence of a highly concentrated emulsion, in which the rate-determining step for coalescence will be the rupture of the thin films between the emulsion droplets. For comparison, some of the experiments were also carried out with sodium dodecyl sulfate (SDS) as the stabilizer, representing an ionic surfactant with a high hydrophilic-lipophilic balance (HLB) value.8 Emulsions are said to be highly concentrated whenever the volume fraction of the droplets exceeds the limit of close packing. As a consequence, thin films between emulsion droplets are continuously present, and rupture of the thin film would dominate the coalescence rate. Highly concentrated emulsions stabilized by surfactants were studied in detail by Princen et al.9 These authors distinguished compressed and uncompressed highly concentrated emulsions. A highly concentrated emulsion is said to be uncompressed if it is in equilibrium with a bulk phase with the same composition as the continuous phase. In this case, attractive interactions between the droplets result in a concentrated phase of aggregated droplets in equilibrium with its continuous phase. Compressed emulsions on the other hand are not in equilibrium with a bulk continuous phase and swell when they are brought in contact with their continuous phase. The excess pressure that needs to be exerted on the emulsion in order to keep it in equilibrium with its continuous phase is the osmotic pressure of the emulsion. This osmotic pressure equals the “osmotic stress” by which the droplets are pushed together.10 (7) Binks, B. P. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry: Cambridge, 1998; p 1. (8) Kabalnov, A. S. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry: Cambridge, 1998; p 205. (9) Princen, H. M.; Aronson, M. P.; Moser, J. C. J. Colloid Interface Sci. 1980, 75, 246. (10) Bibette, J. Langmuir 1992, 8, 3178.
10.1021/la0000419 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/12/2000
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It will be shown that in highly concentrated emulsions stabilized by WPI, coalescence may proceed through different mechanisms. Which of the mechanisms is prevailing depends on the protein concentration and on presence of external forces acting on the emulsion. Experimental Section Materials. Whey protein isolate (WPI, brand name “Bipro”) was obtained from Domo Food Ingredients (Beilen, The Netherlands) and consisted of β-lactoglobulin (71%), R-lactalbumin (12%), immunoglobulin (5%), bovine serum albumin (5%), salts (2%), lactose (1%), and water (4%). SDS (sodium dodecyl sulfate, Pharma copoeia) was purchased from Merck (Hohenbrunn, Germany). Reddy Sunflower oil (Vandenmoortele, Roosendaal, The Netherlands) was purchased from a local retailer. Methods. Initial Emulsions. Emulsions containing 50% (w/ w) oil, referred to as “initial emulsions” in the remainder of the paper, were prepared by mixing equal volumes of sunflower oil and an aqueous solution of 0.10 M NaNO3 containing either WPI or SDS. Pre-homogenization was carried out with a rotor-stator type mixer (Ultra Turrax, Kinematica, Kriens, Switzerland), and subsequent high-pressure homogenization was carried out with a laboratory homogenizer (8.30H, Rannie, APV, Wilmington, U.S.A.) operated at a pressure of 50 bar (10 passes). A small amount of thiomersal (sodium ethylmercurithiosalicylate, Fluka Chemie AG, Buchs, Switzerland) was added to prevent microbial growth. The pH was adjusted to 6.7 by adding very small amounts of either 0.1 M HCl or 0.1 M NaOH. In some experiments protein-stabilized initial emulsions were mixed with other solutions in order to alter their protein content. We noted that, especially at low protein content, stirring often led to oil separation in these emulsions. To avoid this as much as possible, very gentle mixing was applied. Highly Concentrated Emulsions. These were prepared by ultracentrifugation of the initial emulsions in Polyalomer centrifuge tubes (large, 25 × 89 mm, or small, 14 × 89 mm; Beckman instruments, Palo Alto, CA) placed in a SW28 (large tubes) or an SW41-TI (small tubes) rotor in a ultracentrifuge (L8-70M, Beckman instruments, Palo Alto, CA) thermostated to 20 °C and operated at either 28 000 or 17 500 rpm. Due to the centrifugal force, the emulsion droplets (with lower density than those in the continuous phase) move toward the axis and form a stacked layer. The centrifugal force acting on droplet i induces a pressure difference across droplet i of
∆Pi ) ∆Fω2ri
Vi Oi
where ∆F is the density difference between the droplets and the medium (approximately 80 kg/m3), ω is the rotational speed, ri is the distance of the droplet to the axis, Vi is the droplet volume, and Oi is the cross-sectional area of the projection of the droplets onto the upper surface of the highly concentrated emulsion. Summing of these pressure differences leads to an equilibrium osmotic stress exerted at emulsion droplets at the top of the stacked layer (this is closest to the centrifugal axis) equal to
P ) ∆Fω2
∑
stack of droplets i
ri
Vi Oi
≈ -∆Fω2
∫
r)r0-h
r)r0
r dr )
(
∆Fω2h r0 -
1 2
)
h
For both tube sizes r0 was approximately 115 mm and h was approximately 45 mm; this leads to calculated equilibrium osmotic pressures of 29 and 11 bar for centrifugational speeds of 28 000 and 17 500 rpm, respectively. After ultracentrifugation, the bottom part of the centrifuge tube was cut off with a hot knife, and the highly concentrated emulsion, together with the remaining cylindrical portion of the centrifuge tube, was stored in a small airtight flask. For further handling, the highly concentrated emulsion could be easily released from the tube. Highly concentrated emulsions were seen to rapidly lose water on exposure to air, and this was followed by oil separation.
Microscopic examination of these emulsions revealed that the start of oil separation coincides with the crystallization of salt that was dissolved in the aqueous phase. Therefore care was taken to avoid exposure of highly concentrated emulsions to air as much as possible. Droplet Size Distribution. Droplet size distributions of SDSand WPI-stabilized emulsions were measured by first gently dissolving a sample in a 0.02 M (ca. 0.6 wt %) SDS solution, to deflocculate (WPI) and stabilize (SDS and WPI) the emulsion. Probably SDS molecules (partially) displace adsorbed protein molecules and form a stabilizing adsorbed layer of SDS. It was checked that this procedure did not lead to changes in the size distribution for droplets with radii below 100 µm. A small amount of the SDS-deflocculated and stabilized emulsion was then injected into a circulating stream of water that passed through the measuring cell of an optical particle sizer (Mastersizer-X, Malvern Instruments Ltd., Malvern, Worcestershire, England). The measured size distribution was limited to approximately 100 µm due to fast creaming of larger droplets. The measured size distribution was sometimes characterized by averages D3,2 and D4,3.7 To investigate variations in droplet-size distribution along the height of the centrifuge tube, the highly concentrated emulsion produced in a small-sized ultracentrifuge tube was carefully cut into equal slices along its height, using a wire frame cutter. The slices were subsequently redispersed in a 0.02 M SDS solution, and the size distributions were determined (see below). Extent of Coalescence. Two methods were used to estimate the extent of coalescence in an emulsion sample. The first method was a direct measurement of the separation of a distinct oil layer on top of the highly concentrated emulsion. These measurements were carried out with the large-sized centrifuge tubes. The fraction of oil separated after ultracentrifugation was calculated from the measured volume of the oil layer. The method was checked against an accurate measurement of the fraction of oil remaining in emulsified state. The second method was a qualitative observation of the change of the droplet size distribution for SDS-deflocculated emulsions. Coalescence was indicated by a broadening of the size distribution toward larger droplet radii or by the occurrence of a second peak in the size distribution at larger droplet radii. Often very large droplets or a distinct oil layer were seen floating on top of the SDS-deflocculated emulsion. This “free oil” was due to coalescence but was not represented in the particle size distribution due to the large dimensions of the oil volumes. The volume of the separated oil layer was measured, and from this the fraction of oil separated was calculated. Application of Flow To Induce Coalescence. The application of controlled deformation to the emulsion was carried out by transferring a slice of the highly concentrated emulsion, produced by ultracentrifugation with the large tube size, into a syringe (Plastipack 50 mL syringe with luer tip, Becton Dickinson, Drogheda, Ireland). A volume of 8.5 mL of the emulsion was transferred to the syringe and subsequently compressed at a constant speed (0.3 mm‚s-1) using a texture analyzer (TA-XT2, Stable Micro Systems, Haslemere, Surrey, England). In this way the emulsion was squeezed through the tip of the syringe, leading to a flow containing shear and especially elongational flow components. The flow-treated emulsion was collected in a 0.02 M SDS solution for stabilization and redispersion. Confocal Scanning Laser Microscopy (CSLM) Imaging. This was carried out using a Leica TCS SP CSLM (Leica, Heidelberg, Germany). The CSLM was equipped with an air-cooled mixedgas Ar/Kr laser. Light at 488 nm was used to excite the Rhodamine B, which resulted in the emission of 550 nm light by fluorescence. Rhodamine B stained the protein that was present at relatively high concentration at the surface of protein-stabilized emulsion droplets. For CSLM observation 1% of an aqueous solution of 0.05% Rhodamine B (Aldrich Chem. Co., Milwaukee, WI) was mixed through the initial emulsion. After ultracentrifugation, a thin (∼1 mm) slice was gently cut from the highly concentrated emulsion, transferred onto a cover slip, and covered by a second cover slip. The specimen was compressed manually to approximately half its original height, in this way exerting a biaxial squeeze deformation upon the sample.
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Figure 1. Droplet size distributions as a function of cp in the initial emulsion immediately after preparation. The maximum of the peak for cp ) 2 wt % is displaced due to slight variation of the conditions during homogenization. The curves have been shifted along the vertical axis to avoid overlap.
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Figure 2. Average water content in the highly concentrated emulsion layer as a function of centrifugation time for a proteinstabilized emulsion, containing 50 wt % oil and cp ) 1 wt %, centrifuged at 17 500 and 28 000 rpm.
Results Figure 1 shows droplet size distributions measured directly after preparation for a series of initial emulsions stabilized by WPI with varying overall protein content cp. For cp > 1 wt % (thus 1 g of protein per 100 g of emulsion), the droplet size distribution showed one peak at 1.4 µm. The position of this peak was subject to (small) fluctuations due to variations of the homogenization conditions (e.g., homogenization pressure and temperature). At cp < 0.6 wt % a second peak was visible around 10 µm, which may have been due to recoalescence of smaller droplets after homogenization, caused by an insufficient amount of protein available to cover the interface (similar to the mechanism proposed by Taisne et al.11 for SDS-stabilized emulsions). Sometimes three peaks were observed; however this may have been an artifact of the particle-sizing equipment. At cp < 0.6 wt %, a further lowering of cp was seen to result in an increasing tendency to droplet aggregation. This behavior was also reduced if more protein was added to such an emulsion after homogenization (to be published). Figure 2 shows the average water content of proteinstabilized emulsions after centrifugation. Depending on the experimental conditions (speed and duration for ultracentrifugation), the water content was reduced to values between 0.5 and 20 wt %. At the lower water contents, the dimensions of the thin films and Plateau borders were so small that the highly concentrated emulsions were optically translucent or even almost transparent. When brought in contact with its continuous phase, such an emulsion slowly swelled and became more turbid due to absorption of this phase, confirming that such emulsions were of the “compressed” type. Proteinstabilized highly concentrated emulsions did not readily redisperse in the continuous phase, which indicated that the droplets remained in an aggregated state. However, the emulsions were readily redispersible in a 0.02 M SDS solution, after which the droplets were completely deflocculated. The highly concentrated emulsions were semisolid and yielded if a sufficiently large external force was applied. (11) Taisne, L.; Walstra, P.; Cabane, B. J. Colloid Interface Sci. 1996, 184, 378.
Figure 3. Oil separation as a function of cp, for a highly concentrated emulsion centrifuged for 30 min at 28000 rpm.
During flow of a highly concentrated emulsion, oil was seen to separate, especially for protein-stabilized emulsions. Therefore, a distinction must be made between coalescence of a highly concentrated emulsion in rest and coalescence of a highly concentrated emulsion induced by flow. Coalescence at Rest. Figure 3 shows the fraction of oil separated as a distinct oil layer after centrifugation for 30 min at 28 000 rpm for a series of initial emulsions with varying protein content cp. A linear decrease of the oil separated with increasing protein concentration to a zero value at approximately 0.07 wt % protein was observed. The sharp transition in the slope of the curve suggests a transition between two regimes occurring at a critical value, cp*, of the protein concentration in the initial emulsion. Repeats of these experiments have been carried out, and from these it appears that cp* varies with homogenization conditions, which were also reflected in variations of D3,2. An average value cp* ) 0.1 wt % was found. Determination of the protein concentration in the continuous phase, obtained by centrifugation of proteinstabilized emulsions, revealed that the fraction of protein remaining in solution was much smaller than the fraction adsorbed for emulsions with cp ≈ cp*. Consequently, cp* ) 0.1 wt % implies a surface excess of about 0.8 mg‚m-2 for an emulsion with D3,2 ) 1.3 µm at a weight fraction
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Figure 4. Oil separation as a function of time for proteinstabilized emulsions, cp ) 0.05 wt %. Centrifugation at 28 000 rpm. Oil separation determined (b) directly after preparation and (2) after 1 day. Fitted curve: oil separation (%) ) 62 - 86 exp(- time of centrifugation (min)/28).
0.50 of oil of density 920 kg‚m-3. This value corresponds well to the value of 1 mg‚m-2 typically found for adsorbed layers of various proteins, at the point where a plot of the surface pressure versus the surface excess shows a sharp increase. At this value of the surface excess, unfolded protein molecules would just be able cover the interface.12,13 The effect of the centrifugation time on oil separation is shown in Figure 4 for cp < cp*. The oil separation reached a limiting value at a rate that decreased exponentially with centrifugation time. When centrifugation was stopped before this point, oil separation of the highly concentrated emulsion continued, and after 1 day approximately the same amount of oil was separated independent of the centrifugation time. This shows that ultracentrifugation speeds up the oil separation but does not influence the amount of oil that eventually separates. For an emulsion with cp ) 0.05 wt %, coalescence was seen to stop after 60% of the oil had separated (Figure 4). This suggests that coalescence stops when sufficient protein is present to cover the remaining emulsified oil. Also the addition of protein to an initial emulsion with cp ) 0.06 wt % effectively reduces the oil separation as can be seen from Figure 5. The reduction of the oil separation closely follows the curve of Figure 3. The oil separation remaining at cp > 0.1 wt % in Figure 4 is probably due to coalescence induced by flow during mixing of the emulsion with the solution containing extra protein. To investigate the occurrence of coalescence at protein contents above cp*, the initial emulsions were subjected to a centrifugation time of 18 h at 28 000 rpm. No oil separation was observed for any emulsion with cp > cp*. Measured droplet size distributions at various heights in the centrifuge tube after 16 h of centrifuging at 28 000 rpm are shown for an emulsion with cp ) 1 wt % in Figure 6. No sign of coalescence is observed from these measurements. Even after 48 h of centrifugation at 28 000 rpm, no sign of oil separation or any change of the droplet size distribution was observed for an emulsion with cp ) 1 wt %. Special attention was given to protein-stabilized emulsions with cp in the vicinity of cp*. Figure 7A shows the measured size distributions at various heights within a highly concentrated emulsion produced from an initial (12) Bull, H. B. Adv. Protein Chem. 1947, 3, 95. (13) De Feijter, J. A.; Benjamins, J. J. Colloid Interface Sci. 1982, 90, 289.
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Figure 5. Effect of the addition of protein to a protein-stabilized initial emulsion on the oil separation measured after 30 min of centrifugation at 28 000 rpm. Protein content of the initial emulsion: cp ) 0.06 wt %. The dashed line corresponds to the drawn line in Figure 3. Symbols: b, emulsion without added extra protein; 2, emulsions with added protein.
Figure 6. Variation of the droplet size distribution with the height in the centrifuge tube, for a protein-stabilized emulsion, cp ) 1 wt %, compressed by 16 h centrifugation at 28 000 rpm. Arrow indicates the direction toward the axis of centrifugation. The curves have been shifted along the vertical axis to avoid overlap.
emulsion with cp ) 0.1 wt %, which was right at the critical concentration. Only a very slight amount of oil was seen to separate on ultracentrifugation. The size distribution appeared to vary with the height, and the peak corresponding to the largest droplet sizes increased in size and shifted to larger radii. Part of this variation with the height will have been due to fractionation of the droplets in the centrifugal field; however, also the average distribution was clearly shifted toward larger droplets compared to the emulsion before ultracentrifugation. Parts B and C of Figure 7 show the same emulsion as in Figure 7A, but now additional amounts of protein had been added to the initial emulsion directly after homogenization. All three emulsions were left to equilibrate for 2 h prior to ultracentrifugation. They were treated simultaneously, to avoid differences in aging between the samples as much as possible. It can be seen that the droplets of the large size range were almost absent after the addition of 0.1 wt % protein (Figure 7B) and were completely absent after the addition of 1.9 wt % protein (Figure 7C). Correspondingly, no oil separation was found in these samples. Note that although the emulsions were produced from the same
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Figure 8. Oil separation as a function of csa for an SDSstabilized emulsion centrifuged for 30 min at 28 000 rpm.
Figure 7. Variation of the droplet size distribution with the height in the centrifuge tube for a protein-stabilized emulsion, after compression for 30 min at 28 000 rpm: Symbols: /, emulsion before compression; arrow indicates the upward direction in the compressed emulsions; A, cp ) 0.1 wt %; B, cp initially 0.1 wt %, increased to 0.2 wt %; C, cp initially 0.1 wt %, increased to 2.0 wt %. The curves have been shifted along the vertical axis to avoid overlap. Table 1. Droplet Size Distributions of SDS-Stabilized Emulsions
a
csaa (wt %)
D3,2a (µm)
D4,3a (µm)
0.24 0.26 0.28 0.3 0.8 2 4
1.09 0.78 0.76 0.72 0.79 0.62 0.59
1.94 1.19 1.13 1.09 1.07 0.79 0.84
Defined in text.
initial emulsion, the size distributions were different even before ultracentrifugation. The peak observed at large droplet radii for the emulsion to which no protein was added indicates that some coalescence had already taken place in this emulsion during the 2 h of equilibration before ultracentrifugation. For comparison, some experiments were repeated with SDS instead of WPI as the emulsion stabilizer. Table 1 shows the variation of the droplet size distribution with the SDS concentration in the initial emulsion, csa. For all concentrations a single peaked distribution was found. D3,2 was seen to shift to lower values for higher csa. Figure 8 shows the oil separation as a function of the csa as found by ultracentrifugation. Extrapolation of a linear fit through the decreasing series of data points yields a critical SDS concentration, csa*, of approximately 0.27 wt %. This critical value for the SDS concentration also applied to the stability of the initial emulsion at rest: at csa < csa* oil separation became visible within 1 h, while at csa > csa* no oil separation was detected. If all SDS would be
adsorbed to the surface of the droplet, the experimental value csa* ) 0.27 wt % in combination with D3,2 ) 0.77 µm would correspond to a surface excess of 4.4 µmol‚m-2. This value is close to that corresponding to saturation adsorption of SDS, for which a surface excess of 3.44 µmol‚m-2 has been measured for the cyclohexane/water interface.14 The difference may be explained by the presence of some SDS in the continuous phase and inaccuracy in the measured D3,2 value. Coalescence Induced by Flow. Coalescence could be induced in highly concentrated emulsions stabilized by WPI or SDS by deforming the highly concentrated emulsions by an external force. It was observed microscopically that if a sample of the highly concentrated emulsion was squeezed between microscope cover slips, a process occurred in slip regions, which led to coalescence of many droplets into one much larger droplet (Figure 9). Samples of highly concentrated emulsions were deformed strongly by extrusion at constant speed through a small orifice, as described in the Experimental Section. The droplet size distribution after this treatment showed two peaks: the first peak closely resembled the size distribution of the initial emulsion, while the second peak corresponded to much larger droplets, each of them formed by coalescence of many droplets of the initial emulsion (Figure 10). Some of the oil had separated as free oil, which is out of the measured range in Figure 10. Note that the first peak has shifted toward smaller droplet radii, which suggests that larger droplets are more prone to flowinduced coalescence. Flow-induced coalescence was found to increase gradually with increasing oil content (Figure 11). Oil separation almost vanished above a water content of approximately 20 wt %; however flow-induced coalescence still occurred at higher water content, indicated by the presence of a second peak in the droplet size distribution (not shown). Apparently, at volume fractions of the emulsion droplets lower than approximately 80 wt %, the coalescence process involves a relatively small number of droplets, and therefore droplets formed by coalescence remain too small to lead to separation of free oil. Further investigation on this point is in progress. Flow-induced coalescence was found to decrease gradually as a function of cp (Figure 12). Note that, in clear contrast to Figure 3, the oil separation does not vanish at a certain protein content. SDS-stabilized highly concentrated emulsions were also seen to be sensitive to the extrusion treatment, although to a much lesser extent. The standard extrusion treatment (14) Rehfield, S. J. J. Phys. Chem. 1967, 71, 738.
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Figure 9. CSLM image of a protein-stabilized highly concentrated emulsion (cp ) 1 wt % and compressed by 30 min of centrifugation at 28 000 rpm) after flow treatment (see text): A, region where the emulsion remained visually unaffected by the flow treatment; B, region inside a macroscopic crack in the highly concentrated emulsion, filled with oil that separated as a result of coalescence; C, region containing protein “hulls” remaining after coalescence and collapse of droplets.
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Figure 11. Oil separation found after flow treatment of highly concentrated emulsions with cp ) 1 wt % and varying water content. The water content was varied by varying the time and intensity of centrifugation: b, 17500 rpm; 2, 28000 rpm.
Figure 12. Effect of cp on the oil separation found after flow treatment of highly concentrated protein-stabilized emulsions.
Figure 10. Effect of flow treatment on the measured droplet size distribution for a protein-stabilized emulsion, cp ) 1 wt %, that had been compressed by 30 min of centrifugation at 28 000 rpm.
resulted in some oil separation (
