CRYSTAL GROWTH & DESIGN
Melting Point Depression of the Surface Layer in n-Alkane Emulsions and Its Implications for Fat Destabilization in Ice Cream Malcolm J. W.
Povey,*,†
Scott A.
Hindle,†
Asbjorn
Aarflot,‡
and Harald
2006 VOL. 6, NO. 1 297-301
Hoiland‡
Department of Food Science, UniVersity of Leeds, Leeds, LS2 9JT, U.K., and UniVersity of Bergen, N-5020 Bergen, Norway ReceiVed June 17, 2004
ABSTRACT: Evidence of phase changes occurring at the surface of droplets but not in the bulk is presented for n-alkane emulsions. It is postulated that these phase changes are the result of the formation of a monolayer thick solid solution of the oil phase and surfactant. The role of surfactant is shown to extend to controlling the rate of collision between crystallized and uncrystallized droplets and a secondary nucleation mechanism due to droplet collision. Ultrasound velocity methods for the study of emulsion crystallization are shown capable of giving quantitative data regarding nucleation kinetics such as Gibbs free energy, concentration of catalytic impurities, size of catalytic impurities, and nucleation rates. We show that destabilization of a stabilizing protein film at the surface of a droplet can occur through crystallization of an oil droplet and does not require competitive absorption processes at the droplet surface involving small molecule surfactants. This destabilization process may be an important factor in the “aging” of ice cream emulsions. Introduction n-Alkane crystallization has been studied for years and was thought to be a well-known system.1-5 We have previously studied these systems using ultrasound techniques that are very sensitive to the nucleation and initial freezing stages, giving quantitative data of the solids content in these regions.6-9 Use of these techniques has revealed details in the freezing and thawing of emulsion that had previously been unreported. Recent publications by Sirota and co-workers10-14 have illustrated that the melting and supercooling of n-alkanes show an odd/even effect in that bulk supercooling only occurs for even alkanes. The freezing temperatures do not display this odd/even effect. The reason for this is that the even alkanes have to go through a rotator phase (Figure 1) before eventually crystallizing into the triclinic phase (Figure 2). As shown in Figures 3 and 4, the rotator phase is transient, metastable, or stable, depending on the chain length. Emulsion crystallization is a very effective way of studying nucleation kinetics and determining catalytic impurities. The reason for this is that it is possible by dispersing the bulk phase as particles to control the concentration of catalytic impurities by increasing or decreasing the number of particles for a given dispersed phase volume.15 This is done by controlling particle size through the concentration of surfactant and the energy applied to comminute the particles. Emulsions can be produced that are stable for weeks and months, are stable to freeze-thaw, exhibit a large undercooling ∼15°, and have a high latent heat. There are various methods available to study crystallization in these emulsions including DSC, time-dependent X-ray scattering, microscopy, ultrasound velocity measurements, and ultrasound attenuation measurements (particle sizing). The advantages of ultrasound are that we can observe changes in properties of emulsion during cooling-melting, assess the stability of the emulsion by following the size distribution during freezing and thawing, the equipment is inexpensive, and it is suitable for * Corresponding author. Phone: +44 113 343 2963. Fax: +44 113 343 2982. E-mail:
[email protected]. † University of Leeds. ‡ University of Bergen.
Figure 1. Rotator.
Figure 2. Triclinic.
chemometrics as one can easily obtain multidimensional data such as velocity, attenuation, temperature, and time. Methods Sound velocity is measured by the pulse echo technique with a single piezoelectric transducer operating as both generator and receiver (Figure
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Figure 3. Schematic showing the relative free energy (relative to the rotator phase) vs temperature for (a) stable rotator phase (i.e., C24), (b) metastable rotator phase (C20), and (c) transient rotator phase (C16). The dashed lines represent the equilibrium free energy curves for the three phases. The heavy and light solid line represent the path taken on cooling and heating, respectively. The bracket represents the difference between TTfR and TRfT.
Figure 5. Diagram of the UVM1 ultrasound velocity meter.
Figure 4. Phase diagram of n-alkanes. Tf is the freezing point where the first crystal appears, T, R, and L denote the triclinic, rotator. and liquid phases, respectively. 5). The speed of sound (V) of a pure liquid is simply determined by
V)
s t
where s is distance and t is time. t is measured, and s is found by measuring t in a pure liquid for which the speed of sound is known. In the apparatus of Figure 5, s ) 8 cm and the desired precision is 0.1 m/s. So for water, the error in t (∆t) must be less than 3 ns. Therefore accurate reference data are needed. We use del Gross and Mader16 and Bilaniuk and Wong17,18 data for the calibration. However, the most significant errors come through measurement and calibration of temperature. In particular, it is necessary to compensate for the appropriate temperature scale, and we recalculated our reference data for the ITS-90 temperature scale instead of the 1968 (ITS-68) scale on which the original data were based. Regarding impurities in the water when used as a calibrant, the isotopic composition and dissolved gas are not a problem19 if accuracy below 0.15 m/s is acceptable, which is the case here. The typical sample was a 20 vol % (oil in aqueous phase) n-hexadecane (99% supplied by Sigma) in water with 2 mass % Tween 20 of the total aqueous phase used as an emulsifier. The emulsions were prepared by forcing the mixture through a Shields homogenizer five times. The surface weighted mean particle size (d32) of an emulsion prepared in this way is about 0.17 µm and very repeatable. This emulsion is stable for weeks and months if not left exposed to air for long periods of time. The speed of sound was measured with the UVM1 (Figure 5) in a continuous temperature scan ranging from a temperature of about 10 °C above the melting point (18 °C) and 5 °C below the point of nucleation.
Figure 6. 20 vol % n-hexadecane in waterscalibration curve. The vertical dashed line indicates the bulk melting point (18 °C). Crosses are cooling data, circles are heating data. The solid line is the extrapolated liquid line (Vl ) -0.034444T2 + 2.8786T + 1377.7), the dashed curve is the extrapolated solid line (Vs ) -0.13023T2 + 4.3262T + 1464.5). Once freeze-thaw data are gathered, one can determine the freezing point of the emulsion, the relative supercooling of the alkane, and more importantly, the volume fraction of solid droplets. The solid volume fraction, φ(T), is a function of the extrapolated velocity of solid emulsion and liquid emulsion, Vs(T) and vl(T), respectively, and the actual velocity, V(T):
φ(T) )
[
V(T)-2 - V1(T)-2
Vs(T)-2 - Vl(T)-2
]
Results and Discussion n-Hexadecane Crystallization Experiments. A representative result is displayed in Figure 6. On cooling a considerable supercooling is observed, much greater than expected in the bulk oil, and this is due to the very low probability of finding a crystal nucleation center in each droplet. Eventually crystallization does occur, and as can be shown, the temperature at which this occurs depends on the concentration of catalytic impurities and the time.20,21 On heating, the droplets remain frozen above the temperature at which they froze, and melting is complete at the bulk melting point; however, note that melting
Melting Point Depression of Surface Layer
Figure 7. 20 vol % n-hexadecane in waterssolids content curve derived from Figure 6. The solid line is the cooling curve, and the dashed line is the heating curve. The bulk melting point is indicated by the vertical arrow. Some melting continues above the melting point, presumably due to impurities. The dotted rectangular box represents a volume fraction (0.1) equivalent to one fatty acid chain deep in the surface layer of the droplets.
Figure 8. Diagram of surfactant molecules with their lauric acid tails dissolved in the oil.
begins 8° below the bulk melting point. The temperature can be raised and lowered in this way many times, and the same temperature-velocity trajectory is followed without any sign of a change in particle size or any other obvious changes. The solid content derived from Figure 6 is plotted in Figure 7. As already stated, the heating curve in Figure 6 can be cycled with complete repeatability up to 15 °C and back. The repeatability of the cycling leads us to conclude that a proportion of each droplet melts, rather than complete melting of a proportion of the drops. If the latter occurred, the solid content would gradually fall as cycling continued, and we have no evidence that this occurs. We can calculate the proportion of the volume of the oil that comprises oil with dissolved lauric acid chains given that the Sauter mean diameter for the oil droplets is 170 nm and lauric acid chain length is 3 nm:22
∆Vp 1/2πd2∆d 6 nm ∆d ≈ 0.1 ) 1 )3 )3 3 Vp d 170 nm / πd 6
It is not unreasonable to assume that the dissolution of the hydrophobic lauric acid chains of the Tween 20 in the oil causes melting point depression of the surface in which they are dissolved (Figure 7). On this assumption, between 10 and 14 °C, 10% of each droplet melts, corresponding to a depth of one lauric acid, the hydrophobic component of Tween 20. Between 14 and 15 °C a further 20% melts corresponding to a layer three lauric acid chains deep, and between 15 and 18 °C almost all the melting is completed. Presumably the small amount of material that has not melted at 18 °C is due to impurities in the oil.
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Figure 9. 20 vol % n-octadecane in emulsion behavior with changes in velocity below the melting point. Light squares, cooling; dark circles, heating. Two heating-cooling cycles are incorporated, but the data repeat so closely it is not possible to distinguish the two measured data sets. The arrows indicate the direction of change of temperature. The arrow labeled 1 indicates missing data where the attenuation was too high to take measurements. The vertical dotted line indicates the bulk melting point as determined by Kraack et al.,14 and the vertical dashed line is the emulsion freezing point as determined by Kraack et al.
While this is a reasonable interpretation of our data, it is possible that other explanations may apply. For example, in the cases of n-alkanes,13 water, and ice, it is well-known that the melting behavior at surfaces is quite different than that in bulk.23,24 These other phenomena may combine with the effects of dissolved surfactant to produce quite complicated effects. However, it is clear that the ultrasound technique potentially provides information about the behavior of a monomolecular layer in the surface of emulsion droplets. n-Octadecane Crystallization Experiments. Experiments were also conducted on n-octadecane emulsions prepared in the same way as the n-hexadecane emulsions. This emulsion showed overall similar characteristics to the n-hexadecane emulsions and some differences in the behavior of the surface layer. A supercooling of about 15 °C was observed before the liquid n-octadecane droplets crystallized, in agreement with Kraack et al.14 (see Figure 9). However, a second-phase transition was observed at about 7 °C (Figures 9 and 10). The change in speed of sound was not more than 3 m/s but still significant due to its repeatability. For confirmation a DSC scan was also completed. The nature of these phase transitions requires investigation by X-ray techniques. However, as in the case of the n-hexadecane experiments (Figures 6 and 7), the size of the change in the velocity of sound is inconsistent with participation of the bulk droplet in the phase change; rather it is consistent with a phase change occurring in a monomolecular layer at the surface of each droplet (see Figure 11). It has already been suggested by Sirota and Herhold,12 Kraack et al.,14 and Shimazu and Yamamotoa25 that the surface of the paraffins has special properties regarding phase transitions when compared to the bulk. Perhaps as suggested above, these special properties are further modified by interactions between the paraffin and the surfactant. Taking a clue from the surface melting phenomena observed in n-hexadecane, we hypothesize that this is a modification of the rotator phase which occurs due to chain length (mis?)match between the lauric acid (C12) chain that forms the hydrophobic portion of the Tween 20 and the C16/ C18 chains of n-hexadecane and n-octadecane. Note that the apparently significant changes in velocity (labels 1-6 in Figure 10) correspond to tiny changes in solid content when Figure 10 is translated into solid content terms (Figure 11) and that all these changes are consistent with participation by a monomolecular surface layer. In Figure 11, the step change in solid
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Figure 12. Crystallized solids content in a 20 vol % cocoa butter oilin-water emulsion determined from sound velocity measurements.25 Figure 10. Details of figure: 1, rotator phase nucleation; 2, transformation to triclinic; 3, an unknown melting phase transition probably in a monomolecular layer in the surface (see Figure 11); 4, another unknown phase transition; 5, a further unidentified phase transition; 6, probable melting of the most stable phase originally observed at point 2 and fitted by line B (Vs ) -0.1413T2 + 4.6T + 1489.9). Line A is the n-hexadecane solid fit displaced upward by 30.9 m/s (Vs ) -0.13T2 + 4.33T + 1495).
Figure 11. Solid content plot derived from Figure 10. ×, cooling plot; *, heating plot. The transition at 25 °C in the heating plot is an artifact of the absence of data referred to in Figure 9. The vertical dotted line is the bulk melting point as given by Kraack et al.14 The dashed vertical line is the emulsion freezing point as observed by Kraack et al.14 The rectangular dashed grid delineates the volume occupied by one monomolecular layer of lauric acid molecules.
content on the heating part of the curve above 25 °C is an artifact due to the interpolation procedure used and the effect of data missing as a result of high attenuation during melting of the bulk of the droplets. Cocoa Butter Crystallization. We used similar techniques as previously to study the crystallization of cocoa butter. In this case our interest was in the postulated presence of seed crystals in the cocoa butter that catalyzed cocoa butter crystallization. However, our data demonstrated a very interesting surfactant effect. While the initial nucleation was identical in emulsions containing sodium caseinate as surfactant and Tween 20 as surfactant, the later behavior diverged. We have shown elsewhere (Hindle et al.20,21) that both primary and secondary nucleation is occurring in these emulsions. Initially primary nucleation occurs in those droplets that contain catalytic impurities and then the primary nucleated droplets collide with supercooled droplets that do not contain catalytic impurities and in a small proportion of cases cause nucleation (Figure 12). The protein (sodium caseinate) is a much more effective steric stabilizer than Tween 20 so the secondary nucleation process is slower in this case. But eventually the casein layer is destabilized, and the secondary nucleation rate then rises until
it catches up with the less effectively stabilized Tween emulsion. From isothermal plots of solid content against time and undercooling and particle size, we obtain the following data: (i) there are few seed crystals whose size exceeds 0.28 µm at 80 °C; (ii) there were between 3 × 1016 and 3 × 1017 seed crystals per m3; (iii) average size is less than 0.1 µm; (iv) 0.1 mJ m-2 is obtained for the Gibbs free energy of the nucleating surface; (iv) and from X-ray evidence there is strong evidence that it is the R form of POS that comprises the nucleating layer in the seed crystal. Furthermore, there is no evidence that surfactant influences the primary nucleation of cocoa butter oil crystals. It is clear that the surfactant has a big effect on the kinetics of the secondary nucleation process, mediated by droplet collision. Ice Cream. How are the above data relevant to ice cream? The continuous phase is a concentrated aqueous solution of milk proteins, sugars, milk salts, and stabilizers. The fat is dispersed as oil-in-water emulsion, stabilized by protein (caseins) and added emulsifiers. Immediately following homogenization, the ice cream mix is cooled rapidly to 4 °C for aging. The kinetics of fat crystallization during this process is not well-understood, although supercooling occurs followed by crystallization. It is well-known that aging for 3 h or more is essential to the overall physical structure of ice cream. Before aging, the surface of the fat droplet is rough due to coverage by proteins. After aging, the surface of the fat droplet is smooth,26 and this is presumed to be caused by displacement of the protein from the surface by surfactant. An important observation in this paper is that the stabilizing protein film can be destabilized (Figure 12) by crystallization of the oil phase alone. Conventionally it has been thought that the protein is destabilized by migration of small molecule surfactants that displace the protein from the surface.27 Of course this does not mean that there is not a role for the displacement of protein by small molecule surfactants, but perhaps this is not essential for destabilization of the protein stabilizing layer. Perhaps the addition of small molecule emulsifiers accelerates the destabilization process that would occur in any case given long enough. In particular, we can envisage the following processes in ice cream that do not require the presence of small molecule surfactants or any other competitive absorption process: (i) During crystallization, initially the protein remains at the interface. (ii) Later, the stabilizing interface fails, allowing fat droplet coalescence. This of course begs the following questionshow is protein destabilized at the interface; is it “expelled” by the bulk liquid/
Melting Point Depression of Surface Layer
solid-phase transition, by a separate transition at the interface, by small molecule surfactants, or by all these processes in combination? Conclusion It is very clear that complex processes occur at the boundary between oil and water (a) due to surfactant (melting point depression and modification of molecular packing); (b) due to energetics of the boundary layer; and (c) due to phase changes at the boundary. Using emulsion crystallization techniques, we can determine the Gibbs free energy of nucleation and energy barrier due to surfactant coating. Significantly, we can also observe effects in the surface layer of droplets just a few molecules thick and begin to elucidate these complex processes that are of great industrial significance. Acknowledgment. Work supported by Syngenta (Huddersfield, Dr. Neil George) and Unilever (Colworth, Dr. Kevin Smith). References (1) Dickinson, E.; Goller, M. I.; McClements, D. J.; Peasgood, S.; Povey, M. J. W. J. Chem. Soc., Faraday Trans. 1990, 86, 1147-1148. (2) McClements, D. J.; Dickinson, E.; Povey, M. J. W. Chem. Phys. Lett. 1990, 172, 449-452. (3) McClements, D. J.; Dickinson, E.; Dungan, S. R.; Kinsella, J. E.; Ma, J. G.; Povey, M. J. W. J. Colloid Interface Sci. 1993, 160, 293297. (4) Dickinson, E.; Goller, M. I.; McClements, D. J.; Povey, M. J. W. In Food Polymers, Gels and Colloids; Dickinson, E., Ed.; Royal Society of Chemistry: London, 1991; pp 171-179. (5) Coupland, J. N.; Dickinson, E.; McClements, D. J.; Povey, M. J. W.; de Rancourt de Mimmerand, C. In Food Colloids and Polymers: Stability and Mechanical Properties; Dickinson, E., Walstra, P., Eds.; Royal Society of Chemistry: London, 1993; pp 2443-249. (6) Dickinson, E.; McClements, D. J.; Povey, M. J. W. J. Colloid Interface Sci. 1991, 1142, 103-110.
Crystal Growth & Design, Vol. 6, No. 1, 2006 301 (7) Dickinson, E.; Goller, M. I.; McClements, D. J.; Povey, M. J. W. In Food Polymers, Gels and Colloids; Dickinson, E., Ed.; Royal Society of Chemistry: Cambridge, 1991; pp 171-179. (8) Dickinson, E.; Kruizenga, F.-J.; Povey, M. J. W.; van der Molen, M. Colloids Surf. A 1993, 81, 273-279. (9) McClements, D. J.; Povey, M. J. W.; Dickinson, E. Ultrasonics 1993, 31, 433-437. (10) Sirota, E. B. Langmuir 1998, 14, 3133-3136. (11) Sirota, E. B.; King, H. E.; Singer, D. M.; Shao, H. H. J. Chem. Phys, 1993, 98, 5809-5824. (12) Sirota, E. B.; Herhold, A. B. Science 1999, 283, 529-532. (13) Sirota E. B.; Herhold, A. B. Polymer 2000, 41, 8781-8789. (14) Kraack, H.; Sirota, E. B.; Deutsch, M. J. Chem. Phys. 2000, 112, 6873-6875. (15) Povey, M. J. W. In Modern Characterization Methods of Surfactant Systems Vol. 83; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; pp 551-593. (16) del Grosso, V. A.; Mader, C. W. J. Acoust. Soc. Am. 1972, 52, 14421445. (17) Bilaniuk, N.; Wong, G. S. K. J. Acoust. Soc. Am. 1993, 93, 16091612. (18) Bilaniuk, N.; Wong, G. S. K. J. Acoust. Soc. Am. 1996, 99, 3257. (19) Marczak W. J. Acoust. Soc. Am. 1997, 102, 2776-2779. (20) Hindle, S. A.; Povey, M. J. W.; Smith, K. W. J. Colloid Interface Sci. 2000, 232, 370-380. (21) Hindle, S. A.; Povey, M. J. W.; Smith, K. W. J. Am. Oil Chem. Soc. 2002, 79, 993-1002. (22) Sato K.; Ueno S. In Crystallization Processes in Fats and Lipid Systems; Garti. N., Sato, K., Ed.; Dekker: New York, 2001; pp 177210. (23) Dosch, H.; Lied, A.; Bilgram, J. H. Surf. Sci. 1996, 366, 43-50. (24) Lang, P. J. Phys. Condens. Matter 2004, 16, 699-720. (25) Shimizu, T.; Yamamotoa, T. J. Chem. Phys. 2000, 113, 3351-3359. (26) Bucheim, W. In Proceedings of the PennsylVania State Ice Cream Centennial Conference; Kroger, M., Ed.; Penn State University: Happy Valley, PA, 1992; pp 281-197. (27) Adelman, R.; Hartel, R. W. In Crystallization Processes in Fats and Lipid Systems; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 2001; pp 329-355.
CG030051I
