4622
Langmuir 2005, 21, 4622-4630
Coalescence of Protein-Stabilized Bubbles Undergoing Expansion at a Simultaneously Expanding Planar Air-Water Interface Brent S. Murray,* Eric Dickinson, Cathy Ka Lau, Phillip V. Nelson, and Estelle Schmidt Food Colloids Group, The Procter Department of Food Science, The University of Leeds, Leeds LS2 9JT, United Kingdom Received October 30, 2004. In Final Form: February 16, 2005 A novel design of apparatus is described that allows observation of the coalescence stability of bubbles at a planar interface when the planar interface and the bubble surface both expand. Bubbles are introduced beneath the planar air-water interface contained within a square barrier made of perfluorocarbon rubber. The bubbles are then expanded by reducing the air pressure above the interface, while at the same time the rubber barrier is mechanically expanded, maintaining its square shape, to give the same rate and extent of expansion of the planar interface. The area can typically be increased by a factor of three over time scales as short as 0.2 s. This arrangement has been designed to mimic the behavior of aerated products when they exit from a pressurized aeration unit or product dispenser. Compared to results obtained via a previous technique, where it was only possible to expand the bubbles but not the planar interface, the bubbles are less stable. The apparatus has been used to compare the stabilizing effects of ovalbumin, β-lactoglobulin, whey protein isolate, and sodium caseinate, in a model aqueous food system thickened with 40% invert sugar. Stability improved with increasing concentration of all the proteins and with a decrease in expansion rate, but considerable instability remained even at protein concentrations as high as 4 to 6 wt % and also at very low expansion rates, though the systems were stable in the absence of expansion. However, the stability was greatly improved by the replacement of the above proteins by the hydrocolloids gelatine or polypropylene glycol alginate. Detailed analysis revealed that the coalescence of individual bubbles in clusters of bubbles were not strongly correlated in distance or time, but larger bubbles and bubbles toward the outside of a cluster were found to be, on average, less stable than smaller bubbles and bubbles located more toward the interior of a cluster. The different degrees of stability are discussed in terms of local deformation, fracture behavior, and time-dependent composition of the adsorbed layers.
Introduction There continues to be much interest, both from the practical and from the theoretical points of view, in the properties of foams. Air is seen as a renewable and cheap ingredient that can be easily incorporated into products to provide bulk volume as well as desired texture, stability, and appearance. In foods, for example, it may be seen as a zero calorie replacement for fat, while at the same time it is a traditional structural component of many foodstuffs such as mousses, ice cream, whipped toppings, meringues, and leavened products such as bread, cakes, and so forth. As well as forming products with desirable edibility, there are of course numerous other applications where the unique properties of foams are exploited to advantage, such as firefighting,1 enhanced oil recovery,2 and mineral processing.3,4 Theoretical interest ranges from their use as models of condensed matter in flow5,6 to studies of volcanic eruptions.7 In many industrial applications, it is the transient existence of foams that can be either an advantage or a disadvantage. Apart from instances where * To whom correspondence should be addressed. Tel.: +44 (0)113 343 2962. Fax: +44 (0)113 343 2982. E-mail: b.s.murray@ food.leeds.ac.uk. (1) Figueredo, R. C. R.; Sabadini, E. Colloids Surf., A 2003, 215, 77. (2) Rossen, W. R. Colloids Surf., A 2003, 225, 1. (3) Tao, D. Sep. Sci. Technol. 2004, 39, 741. (4) Mathe, Z. T.; Harris, M. C.; O’Connor, C. T.; Franzidis, J. P. Miner. Eng. 1998, 11, 397. (5) Weaire, D. Adv. Eng. Mater. 2002, 4, 723. (6) Weaire, D.; Hutzler, S.; Cox, S.; Kern, N.; Alonso, M. D.; Drenckhan, W. J. Phys.: Condens. Matter 2003, 5, S65-S73. (7) Parfitt, E. A. J. Volcanol. Geotherm. Res. 2004, 134, 77.
the continuous phase of a foam is solidified, when compared to emulsions and solid dispersions, the disperse phase of a foam is generally easy to dispel, literally disappearing into thin air, if and when this is required. On the other hand, premature collapse of a foam structure can lead to catastrophic loss of foam properties at the point of use. A key reason for this transient nature is that the bubbles in a foam are generally more deformable than emulsion droplets (or certainly solid particles), and this makes them more susceptible to coalescence. For instance, the volume of a bubble undergoing a pressure change during processing must increase or decrease in inverse proportion to the pressure, changing the surface coverage of stabilizing surfactant. In addition, the gas bubbles in many foamed products are large enough that they can be much more easily deformed than emulsion droplets by the hydrodynamic forces commonly operating during processing. Bubbles can also be quite unstable due to dissolution of the gas into the continuous phase, leading to coarsening (disproportionation) of the bubble size distribution.8 However, coalescence can often be much more rapid than coarsening, especially if the bubbles are subjected to mechanical disturbance. Coalescence could be considerably accelerated by a rapid drop in pressure, with catastrophic bursting of bubbles taking place over the time scale of the pressure drop. Instances of foam processing (8) 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; p 151.
10.1021/la047333k CCC: $30.25 © 2005 American Chemical Society Published on Web 04/09/2005
Coalescence Stability of Expanding Bubbles
Figure 1. Schematic illustration of the practical issue of bubble stability and pressure drop. (a) Foam is initially generated in a sealed, pressurized vessel, by injection of gas under pressure/ and or agitation. (b) Foam exits the vessel down a pipe, with a drop in pressure. Bubbles expand (possibly distorting and relaxing as well) and coalesce with each other and with the surface of the product. (c) Foam is additionally or further expelled from the system, resulting in further bubble expansion, distortion, relaxation, and coalescence with the surface of the foam.
where there can be a significantly high rate of bubble expansion due to a pressure drop are the exit of foam from a mixing or aeration chamber and the extrusion of an aerated product from a nozzle, in dispensing and filling, and so forth. In the manufacture of many foamed products the gas is introduced under pressure, and so the foam exits the aerator at an even higher pressure than that induced by the impeller motion alone. Such expansions could be expected to lead to significant bubble-bubble coalescence or to coalescence of bubbles with the surface of the product, assuming that the surface is exposed to the outside atmosphere (at lower pressure). The nature of the problem is illustrated in Figure 1. Under such conditions, the probability of coalescence is expected to be related to the mechanical resistance to deformation of the adsorbed film of surfactant on the surface of the bubbles, the kinetics of surfactant adsorption and rearrangement at the surface, and the rate and extent of deformation of the surface.9 However, up to now, there have been relatively few studies10-13 that have investigated this problem, either experimentally or theoretically. Ideally one would like to observe and measure such coalescence in a jet of aerated product exiting from a nozzle, including coalescence between bubbles and between the bubbles and the surface of the jet. On further consideration, however, the experimental obstacles to such in situ measurements are obvious: tracking the behavior of individual bubbles in rapid motion and expansion; observing moving bubbles both below and at the surface;
Langmuir, Vol. 21, No. 10, 2005 4623
accounting for instability and turbulence of the jet; knowing exactly the rate of expansion/pressure drop at the point of observation; and so on. As our first attempt to try and tackle this problem, we previously developed an apparatus10 that allowed one to observe and measure the behavior of air bubbles undergoing expansion (or compression and then expansion) due to changes in pressure of the order of a factor of 5 (which seems to be the typical range in many processes).10-12 It was shown10 that rapid expansion of protein-stabilized bubbles beneath a planar air-water (A-W) interface resulted in a significant fraction, Fc, of the bubbles coalescing with the interface, whereas without expansion the bubbles were completely stable. This behavior was even observed at quite high protein concentrations, where at first sight the kinetics of protein adsorption might not be expected to be limiting, because there should be enough time for protein readsorption to stabilize the film. Bubbles stabilized by sodium caseinate were considerably more stable to coalescence on expansion than bubbles stabilized by globular proteins such as β-lactoglobulin and ovalbumin.10-12,14 This was explained by the more rapid adsorption of caseins compared to globular proteins. However, further recent work14 on the addition of sucrose and polysaccharide thickeners to such protein-stabilized systems has highlighted the fact that the rate of protein diffusion in the bulk aqueous phase is probably not the sole factor determining stability where the bubbles undergo fairly fast expansion. Rather, the results point to the additional effects of the mechanical response of the interfacial film under such stresses. Thus, fracture and breakage of interfacial films at critical extents of deformation may also be important. Susceptibility to fracture and breakage may be related to the degree of aggregation and cross-linking within the film. As far as we know, no other work of this type has yet been reported to compare with these measurements or to suggest alternative techniques to make such observations. However, one weakness of our previous experimental arrangement was that, although the rate and extent of the bubble expansion could be varied (by varying the pressure), the bubbles were still expanded against a planar interface of constant surface area. This was not very realistic compared to the situation in foam processing in practice, where the rates of expansion of the two gasliquid interfaces might be expected to be the same (in the case of two bubbles) or similar (in the case of a bubble in contact with the expanding product surface). The main purpose of this paper is to describe a measurement technique by which we have surmounted this weakness in the previous methodology. At the same time we present here some preliminary results illustrating the potential of the new technique for obtaining a more thorough understanding of coalescence stability during product expansion due to a pressure drop. A range of food proteins of different structural types and surface active properties was studied to highlight the effects that these properties might have on bubble stability under these conditions. Materials and Methods
(9) Murray, B. S. Curr. Opin. Colloid Interface Sci. 2002, 7, 426. (10) Murray, B. S.; Campbell, I.; Dickinson, E.; Maisonneuve, K.; Nelson, P. V.; So¨derberg, I. Langmuir 2002, 18, 5007. (11) So¨derberg, I.; Dickinson, E.; Murray, B. S. Colloids Surf., B 2003, 30, 237. (12) Murray, B. S.; Dickinson, E.; Du, Z.; Ettelaie, R.; Maisonneuve, K.; So¨derberg, I. In Food Colloids, Biopolymers and Materials, Dickinson, E., van Vliet, T., Eds.; Royal Society of Chemistry: Cambridge, 2003; p 165. (13) Hu, B. J.; Nienow, A. W.; Pacek, A. W. Colloids Surf., A 2003, 31, 3.
Materials. Bovine β-lactoglobulin (crystallized three times, lyophilized, desiccated, lot no. 21K7079), containing variants A and B, and ovalbumin (crystallized three times, lyophilized, desiccated, lot no. 127H7037) were purchased from Sigma-Aldrich (Poole, U.K.). Gelatine powder (product code 44045) was purchased from BDH Chemicals, Ltd., Poole, U.K. The commercial (14) Murray, B. S.; Dickinson, E.; Gransard, C.; So¨derberg, I. Food Hydrocolloids, in press.
4624
Langmuir, Vol. 21, No. 10, 2005
Murray et al.
Figure 3. Detailed view of the elastic barrier arrangement, from above. Molded barrier material (a); one of the four pins (b) that move diagonally to give dilatational expansion of the interface (c).
Figure 2. Schematic side view of the sample cell module, together with the two key components of the power transmission assembly. Sample cell body (A); central rod mechanism (B); side and plan views of elastic barrier (C); window unit (D); platform (E) and turntable (F) of power transmission and viewing assembly; microscope objective lens (G). Upper air phase (H) and lower aqueous phase (I). whey protein isolate (WPI; BiPro) was from Davisco Foods International, Inc. (Maine, U.S.A.) containing 97.7% protein, 0.3% fat, 1.9% ash, and 4.8% moisture. Tween 20 was from Sigma Chemical Co. (Poole, Dorset). PSDI-2400, with a β-lactoglobulin content >95%, was obtained from MD Food Ingredients (Vidabaek, Denmark). Spray-dried sodium caseinate (>82 wt % dry protein,
