Langmuir 2002, 18, 5007-5014
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Technique for Studying the Effects of Rapid Surface Expansion on Bubble Stability Brent S. Murray,*,‡ Iain Campbell,†,§ Eric Dickinson,‡ Krystel Maisonneuve,‡ Phillip V. Nelson,‡ and Ingrid So¨derberg‡ Food Colloids Group, Procter Department of Food Science, University of Leeds, Leeds, LS2 9JT, United Kingdom, and Unilever Research, Colworth House, United Kingdom Received January 18, 2002. In Final Form: March 15, 2002 A new apparatus is described that allows measurement of the effects of rapid surface expansion, due to a drop in pressure, on the stability of air bubbles beneath a planar air-water (A-W) interface. The fraction, Fc, of bubbles coalescing on expansion has been studied as a function of bulk protein concentration, expansion rate, and surface age of the interface before expansion. Two commercial protein samples were used: a whey protein isolate (WPI) and a sodium caseinate (SC). For 0.04 wt % WPI, when bubbles were slowly compressed and then rapidly expanded back to their original area, Fc was found to increase with decreasing bulk protein concentration (Cb) and increasing bubble size, but all values of Fc were relatively low, that is, less than 0.1, under conditions where bubbles were reasonably stable on injection. Much higher values of Fc were observed when bubbles were rapidly expanded from their original size, that is, with no prior compression stage. Here Fc was also observed to increase with decreasing bulk protein concentration and increasing expansion rate. Within experimental error, Fc was not very dependent on the age of the planar A-W interface, except for 0.2% WPI, when there was a slight increase in Fc with increasing surface age. In contrast, once stable bubbles had been formed at the A-W interface with SC as the stabilizer, they remained very stable to coalescence, that is, with Fc ≈ 0, for all bulk concentrations, expansion rates, and surface ages studied. The results can be explained mainly in terms of the adsorption kinetics and surface rheology of the two proteins at the A-W interface, although a number of observations suggest that other phenomena, such as the variable tendency for the bubbles to move and to aggregate in the interface, also have an influence on bubble stability.
Introduction Many important commercial products are foams,1 with the dispersion of bubbles commonly contributing to the desired bulk and texture. Important examples are food product foams2 such as bread, ice cream, whipped products, and mousses, where the dispersion of gas bubbles contributes to the overall structural stability and particularly to the texture perceived by the consumer. In addition, foams form a very important processing medium in many other industries, such as minerals processing and biotechnology, or conversely, foaming may create difficult problems during processing.3 In foods, the predominant foam-stabilizing molecules are proteins,4,5 and the complexity of the structure and interactions of these molecules means that there is still some way to go in obtaining a full explanation of how they can stabilize foams under the wide variety of processing and storage conditions that exist. Unlike with other types of dispersions, the structural units of foams can in effect “disappear” through the processes of disproportionation and coalescence of bubbles. * Corresponding author. E-mail:
[email protected]. † Coauthors in alphabetical order. ‡ University of Leeds. § Unilever Research. (1) Foams: Physics, Chemistry and Structure; Wilson, A. J., Ed.; Springer-Verlag: London, 1989. (2) Bubbles in Food; Campbell, G. M., Webb, C., Pandiella, S., Niranjan, K., Eds.; Eagan Press: St. Paul, MN, 1999. (3) Defoaming: theory and industrial applications; Garrett, P. R., Ed.; Marcel Dekker: New York, 1993. (4) Turan, S.; Kirkland, M.; Bee, R. In Food Emulsions and Foams Interfaces, Interactions and Stability; Dickinson, E., Rodriguez Patino, J. M., Eds.; Royal Society of Chemistry: Cambridge, 1999. (5) Walstra, P. In Foams: Physics, Chemistry and Structure; Wilson, A., Ed.; Springer-Verlag: London, 1989.
Of these two processes, coalescence is by far the most catastrophic under conditions where good foam volume and uniformity of bubble size are desired. Coalescence can lead to very rapid collapse and loss of the foam structure. One of the factors controlling coalescence (and also disproportionation) is the surface rheology of the film of adsorbed molecules on the surface of the bubbles.6-8 As the basis of the Gibbs-Marangoni mechanism of foam (and emulsion) stability is intimately connected with the dynamic surface dilatational rheology of the interface, many workers have sought to demonstrate the relationship between stability and surface rheology via theoretical and experimental means.6,8,9 Measurement of dilatational rheology involves subjecting an interface to a change in area (surface strain) and measuring the corresponding change in interfacial tension (surface stress). Most methods can make measurements at only low rates (or frequencies) of area change, a possible exception being the maximum bubble pressure method.10 Certainly, there have been very few direct measurements of the effects of rapid area change on the coalescence behavior of bubbles. One area of foam processing where there can be a significantly high rate of bubble expansion is in the exit of foam from a mixing or aeration chamber, where a pressure drop occurs. In the manufacture of many foamed products the gas is introduced under pressure, and so the foam exits the aerator at an even higher (6) Dickinson, E.; Ettelaie, R.; Murray, B. S.; Du, Z. J. Colloid Interface Sci., in press. (7) Murray, B. S. In Proteins at Liquid Interfaces; Miller, R., Mo¨bius, D., Eds.; Elsevier Science: Amsterdam, 1998. (8) Kloek, W.; van Vliet, T.; Meinders, M. J. Colloid Interface Sci. 2001, 237, 158. (9) Dickinson, E. Int. Dairy J. 1999, 9, 305. (10) Fainerman, V. B.; Makievski, A. V.; Miller, R. Colloids Surf., A 1994, 87, 61.
10.1021/la025555h CCC: $22.00 © 2002 American Chemical Society Published on Web 05/10/2002
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Figure 1. Schematic diagram of bubble apparatus: (a) microscope; (b) upper window; (c) mica float; (d) observation chamber; (e) bubbles injected into aqueous phase; (f) lower window; (g) light source; (h) pressurization chamber; (i) O-ring; (j) bubble syringe; (k) piston drive.
pressure. Exit from the aerator will then be accompanied by simultaneous expansion of the bubbles over the time that the pressure of the foam falls back to atmospheric pressure. In practice, this time will be controlled by the flow rate out of the aerator and the dimensions of the exit pipe: pressure drops of up to 5 atm in less than a second are typical. Such rapid expansions could lead to significant bubble coalescence and consequently detrimental effects on product quality.4 Thus, to understand better foam formation under industrial processing conditions, it is apparent that there is a need to study the coalescence stability of bubbles under rapid surface expansion conditions. In this paper, we describe an experimental arrangement that goes some way to enable such measurements to be made, and we report some results on protein systems of widespread commercial significance. Recently, we also reported6 on how the same apparatus can be used to investigate the disproportionation of bubbles at a planar air-water (AW) interface. To a limited extent, interfacial rheology and changes in pressure and bubble area can also affect the growth and shrinkage of bubbles. However, these effects occur over much longer time scales than those considered here. Materials and Methods Materials. Commercial whey protein isolate (WPI; PSDI 2400) with a β-lactoglobulin content of >95% was obtained from MD Food Ingredients (Vidabaek, Denmark). Commercial spraydried sodium caseinate (>82% dry protein, 100 µm); “medium” (40 µm e diameter e 100 µm and “small” (diameter < 40 µm). This was necessary because of the spread in the bubble size distributions and the fact that it was difficult to accurately size some bubbles via ImageTool when they were close-packed or very small. The corresponding number of bubbles studied in each size class, for three different film ages (5, 10, and 30 min)
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Table 1. Total Numbers of Bubbles Studied (Ntotal) for 0.04 wt % WPI at Different Planar Film Agesa age/min
Ntotal
Nsmall
Nmedium
Nlarge
5 10 30
923 2126 1640
325 1273 1098
490 788 492
108 65 50
a Also shown is the number of small (N small), medium (Nmedium), and large (Nlarge) bubbles studied, as defined in the text.
Figure 6. Fraction of bubbles coalescing on expansion (Fc) for 0.04 wt % WPI as a function of interface age: Fc recorded immediately after expansion (b); Fc recorded 1 min after the end of expansion (O).
Figure 7. Fraction of bubbles coalescing on expansion (Fc) as a function of bulk concentration of WPI (Cb) for an interface age of 10 min: Fc recorded immediately after expansion (b); Fc recorded 1 min after the end of expansion (O).
before the bubbles were injected, is given in Table 1. The data come from at least eight separate injections of bubbles. Figure 5 clearly shows that large bubbles are less stable than small bubbles for all three film ages. This is expected if the buoyant force is the major factor driving coalescence, because the buoyant force is proportional to the cube of the bubble radius. The 5 min old film seems to give a slightly higher value of Fc for the small bubbles, but for the medium and large bubbles the differences in Fc are not so clear. One must also bear in mind the different numbers of bubbles analyzed in each case (see Table 1). The fact that it was much easier to produce small stable bubbles means that there are far more data for small bubbles, and so the statistical uncertainty in Fc is less for the smaller bubbles. In Figure 5, the values of Fc given are the final values, that is, 60 s after the end of the expansion. Figure 6 shows the values of Fc immediately after expansion and 60 s after the expansion for all bubble sizes as a function of the planar film age, for 0.04 wt % WPI. A significant amount of coalescence can occur in the 60 s immediately after the expansion. In fact, in all experiments with WPI, this “delayed” coalescence, when it occurred, did so in the first 20 s. Coalescence more than 60 s after the end of expansion was not observed. The effect of bulk protein concentration on bubble stability was also studied. Figure 7 shows Fc immediately after expansion and 60 s after the expansion for all bubble sizes as a function of protein concentration for the 10 min old planar films. Results for concentrations of 0.2, 0.04, 0.03, and 0.02 wt % are shown, where the corresponding numbers of bubbles studied were 1072, 2126, 317, and 50,
respectively. For reasons given earlier, it was not feasible to gather enough data at concentrations lower than these, due to the poor stability of bubbles on injection. As the bulk concentration is increased, there is a clear increase in stability, but even at 0.2 wt % WPI there remains a significant amount of instability. During the C-E experiments, one expects the adsorbed film to be compressed during the compression stage, so that the protein surface load will increase. Significant removal of protein from the interface over the time scale of the experiment is not expected, as protein desorption is generally quite slow.16,17 It is possible that the compression may lead to enhanced cross-linking between protein molecules already adsorbed and hence to a stronger interfacial film. Conversely, all protein films have a collapse surface pressure beyond which further surface compression may render them insoluble,17 under which conditions they will not form a particularly strong or coherent film at all, because inhomogeneities in the film thickness and density develop that make it more liable to fracture and/or buckling. But, at the end of the rapid expansion, it is not expected that the surface load will be any lower than before the cycle, when the bubbles were perfectly stable. It is therefore highly significant that there is any coalescence at all under such conditions, particularly at concentrations as high as 0.2 wt % (see Figure 7), as this must mean that inhomogeneities develop in the protein layer during the expansion, which cannot be healed by rearrangement within the film or by new protein adsorbing to the surface. In this way, interfacial regions must develop which are unstable to coalescence. The delayed coalescence is also probably a manifestation of the relatively long time scale of relaxation processes within the film compared to the time scale of the expansion. This explanation has been substantiated18 by Brewster angle microscopy of films of the same WPI sample subjected to similar compression-expansion cycles, which show long length scale inhomogeneities which may take minutes or even hours to disappear. Although the compressionexpansion cycle used here is somewhat arbitrary, bubbles in food foams are expected to be subjected to a range of compression and expansion cycles during their processing lifetime. Hence, these effects are not without practical significance. An added complication in the data analysis is the variable tendency of bubbles to aggregate at the A-W interface (see above). It might be expected that the coalescence of one bubble, which is quite a violent event, sending visible compression waves throughout the neighboring bubbles, might affect the probability of coalescence of a closely neighboring bubble. A rudimentary analysis of the data for such effects, performed for the large bubble class only, has revealed that for both short (5 min) and long (30 min) surface aging times, 68 ( 2% of those bubbles that coalesced were visibly seen to be touching at least one other bubble (irrespective of whether the other bubble coalesces). In other words, there seems to be a significantly higher probability of coalescence of a bubble if it is touching another bubble. However, this tentative conclusion awaits confirmation by detailed image analysis of the data, as the precise number, exact juxtaposition, and relative size of neighboring bubbles have so far not been taken into account. The effect of interfacial aging is seen to be not a particularly significant factor. But since this is the age of (16) Murray, B. S. Prog. Colloid Polym. Sci. 1997, 103, 441. (17) MacRitchie, F. Adv. Colloid Interface Sci. 1986, 254, 341. (18) Murray, B. S.; Cattin, B.; Ozlem, Z.; Schu¨ler, E. Manuscript in preparation.
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Table 2. Average Fraction of Bubbles Coalescing on Injection (Fc) for SCa SC concentration (wt %)
Ntotal
Fc
σ
0.05 0.004
945 1107
0.018 0.005
0.031 0.0086
a The total number of bubbles studied (N total) and the standard deviation (σ) between experiments is shown.
the film at the planar interface and not at the bubble surface, this is perhaps not so surprising, as the planar film does not itself undergo a compression-expansion cycle. We have not performed experiments where both the bubble (after injection) and the planar interface are aged for longer and therefore approximately similar times. This is because there can be significant shrinkage of the bubbles over time scales of an hour or so, due to disproportionation.6 Compression-Expansion (C-E) Experiments with SC. In contrast to the C-E experiments on WPI, negligible coalescence occurred with SC at the same concentrations, even at 0.004 wt %. In fact, statistically significant coalescence could only be produced by increasing the strain rate in the expansion phase to 2.17 s-1 (compared to 1.26 s-1 employed for WPI). The average results for eight separate injections on 0.004 wt % SC and nine separate injections on 0.05 wt % SC are shown in Table 2. The mean bubble sizes were 320 and 180 µm, respectively. The mean Fc was still low, though the relatively large spread in data, represented by the standard deviation, should be noted. However, taking the error into account it is still clear that the SC-stabilized bubbles were far more stable to the C-E cycle than were the WPI-stabilized bubbles. This is no doubt related to the difference in coalescence stability on injection noted earlier, where bubbles appeared to be either stable or not stable for SC above a certain concentration, whereas a much higher concentration of WPI was required to obtain a reasonable number of stable bubbles on injection. The same reasoning therefore applies in explaining this result. The SC adsorption is probably more rapid than that of WPI, and the adsorbed SC film does not possess such high surface viscoelasticity, or coherence, as does that of WPI. Therefore, film structure and coverage with SC appear to be more reversible on C-E cycling, and consequently no surface concentration gradients arise. Expansion (E) Experiments with WPI. In the C-E experiments with both WPI and SC, larger Fc values would probably be obtained by using more rapid or larger pressure changes, but as this would take the measurements outside the limits of the usual pressure changes associated with aeration, this aspect has not been pursued up to now. However, straight expansion of an adsorbed film, without any prior compression, might be expected to lead to greater instability, since it would be determined more by adsorption kinetics to the newly created bubble surface rather than by rearrangements within an existing protein film. Such a process is also of direct relevance to quiescent bubbles in a system that suddenly undergoes a pressure release. For these reasons, for comparison with the C-E experiments, experiments were also performed where the bubbles were just expanded after injection at 1 atm, by reducing pressure below 1 atm. Two examples of the theoretical variation in bubble area as function of time in these expansion experiments are shown in Figure 8, calculated as previously described, for the lowest and highest expansion rates used. In all of the E experiments, the final A/A0 ratio was 1.60, but different expansion rates were investigated. In Figure 8, the plots
Figure 8. Relative expansion (ln(A/A0)) versus time (t) in expansion experiments: the highest (full line) and lowest (dashed line) rates used are shown, corresponding to average strain rates of 0.12 and 2.17 s-1, respectively.
Figure 9. Fraction of bubbles coalescing on expansion (Fc) versus mean expansion rate (d ln A/dt) in expansion experiments for 0.03 wt % WPI, with an interface age of 5 min.
of relative area change versus time are almost linear. In the following, the linear regression fits to such ln(A/A0) versus time plots are used in referring to these (mean) dilatational strain rates. For example, the corresponding values of the strain rates for the lowest and highest expansion rates shown in Figure 8 are 0.12 and 2.17 s-1, respectively. In all of the E experiments described, the age of the planar A-W interface was 5 min at bubble injection. The initial WPI concentration studied was 0.03 wt %, and Figure 9 shows the values of Fc as a function of expansion rate. The line drawn through the points is to guide the eye. Values of Fc were evaluated from the images obtained 3-5 s after the start of expansion. In contrast to the C-E experiments, no delayed coalescence was observed, so these values of Fc represent the plateau value in Fc(t) after expansion. An additional feature of these experiments was the appearance of new bubbles on expansion, due either to nucleation as the pressure was released or to growth of bubbles which, prior to expansion, were simply too small to be observed. This was found to complicate the image analysis somewhat, with particular care being required in the counting of the smallest bubbles, for example. It is obvious from Figure 9 that the E experiment generates considerably higher Fc values than the corresponding C-E experiment at equivalent concentrations of WPI (cf. Figure 7). Within experimental error, the results also show a decreasing tendency for Fc to increase substantially further at higher expansion rates. The reason for this is not clear, but it suggests that different mechanisms of instability may operate at high and low expansion rates. As the strain rate increases, higher interfacial tension gradients will develop within the interface, which may inhibit the development of gross film inhomogeneities via the Gibbs-Marangoni stabilization mechanism. The highest expansion rate in Figure 9 was chosen to study the effect of bulk WPI concentration on Fc. The results are shown in Figure 10. Within experimental error, there is a clear trend for the extent of coalescence to increase as the bulk concentration decreases. However,
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Figure 10. Fraction of bubbles coalescing in expansion experiments (Fc) as a function of bulk protein concentration (Cb): WPI at an expansion rate of 2.17 s-1 (2); SC at an expansion rate of 1.79 s-1 (O); SC at an expansion rate of 0.50 s-1 (b). In all cases, the interface age was 5 min.
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viscoelasticity of the WPI films at higher concentrations, which leads to increased film inhomogeneity, particularly on more rapid expansion, and therefore greater instability. Expansion Experiments with SC. Figure 10 also shows the corresponding Fc results for SC (film age 5 min) as a function of bulk protein concentration. The values of Fc are strikingly lower for SC than for WPI at all the concentrations studied. Again, this fits in with the picture of the SC protein molecules being able to adsorb and possibly rearrange within the interface much more easily and quickly than the WPI molecules. No significant variation in Fc with expansion rate was measurable, though this may have been partly due to the low values of Fc overall, which implies that a vastly larger number of bubbles would have to be studied before any other trends could become statistically significant. Conclusions
Figure 11. Fraction of bubbles coalescing in expansion experiments (Fc) as a function of interface age for 0.2 wt % WPI at expansion rates of 0.50 s-1 (O) and 1.79 s-1 (2).
at 0.03 wt % WPI, the bubbles were found to be considerable smaller, more monodisperse, and also less likely to aggregate at the A-W interface. This seems to have an effect on reproducibility, the standard deviation in Fc being lower for the larger number of smaller bubbles studied. The C-E experiments did not reveal much dependence of Fc on the age of the planar A-W interface, for the reasons given earlier, despite the well-known long time dependence of the surface viscoelasticity of globular protein films.7,12 For comparison purposes, the effect of film age on Fc in the E experiments was studied for 0.2 wt % WPI. Figure 11 shows the results at a low and high expansion rate. The lines drawn through the points are to guide the eye. Within experimental error, Fc was found to increase slightly at the higher expansion rate, whereas there was little significant change in Fc at the lower expansion rate. Similarly, there was no significant change in Fc at 0.04 wt % at either expansion rate (data not shown). The slight tendency for Fc to increase with increasing interface age is again possibly related to increased cross-linking and
An apparatus has been developed that enables measurement of the coalescence behavior of bubbles at a planar A-W interface when the bubbles are subjected to compression and/or expansion, due to an externally applied pressure change. Overall, in the range of concentrations where initially stable bubbles can be produced at the planar interface, SC-stabilized bubbles are far less susceptible to coalescence on expansion (or compressionexpansion) than WPI-stabilized bubbles. Stability appears to increase with decreasing bubble size, increasing WPI concentration, and decreasing expansion rate, but stability does not vary greatly with SC concentration, over the range of conditions studied. The differences in coalescence behavior produced by the two proteins are probably a reflection of differences in the structure and viscoelasticity of their adsorbed films and thus of consequent differences in film inhomogeneity on expansion. However, a number of other effects remain to be investigated, such as the influences of individual bubble mobility and bubble aggregation within the interface. Also required for a full interpretation are corresponding measurements of the (dynamic) surface tension and surface viscoelasticity of these films under the same conditions of the surface compression and expansion. These matters will form the subject of a separate publication.18 Acknowledgment. The research was supported by BBSRC Research Grant 24/D10926 and by Unilever Research, Colworth House, U.K. LA025555H
