Effect of Minor Components and Temperature Profiles on Polymorphism in Milk Fat Mazzanti,§,†
Guthrie,†
Sirota,‡
Gianfranco Sarah E. Eric B. Alejandro G. Marangoni,§,† and Stefan H. J. Idziak*,†
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1303-1309
Department of Food Science, University of Guelph, Guelph, Ontario, Canada, Department of Physics, and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario, Canada, and ExxonMobil Research and Engineering Company, Corporate Strategic Research, Route 22 East, Annandale, New Jersey 08801-0998 Received July 12, 2004;
Revised Manuscript Received September 9, 2004
ABSTRACT: Time-resolved synchrotron X-ray diffraction techniques were used to study the effect of cooling rates and temperature on the crystallization dynamics and polymorphism of anhydrous milk fat (AMF) and milk fat triacylglycerols (MFTs). The crystallization of AMF at fast cooling rates and low final temperatures proceeded through the metastable phase R and resulted in the formation of a mixture of phases β′ and β. Slow cooling rates and high final temperatures resulted in the formation of phase β′ only, with no formation of phase R nor phase β. Moreover, in the absence of polar lipids, MFTs had a decreased tendency to form the R and β phases. The formation of the β phase was largely dependent on the initial amount of R phase formed. At high cooling rates and low crystallization temperatures, polar lipids may have initiated the crystallization process together with a high melting fraction of milk fat in the R phase. In time, this metastable phase transformed to the β phase. When no R phase was initially formed (high temperatures and/or slow cooling rates), no β phase was formed either. Small-angle X-ray diffraction could be used to monitor compositional variation of crystals over time, and demonstrated that minor polar lipids present in the milk fat delayed the onset of crystallization and reduced the rate of crystal growth. Introduction The stability and sensorial attributes of milk fat, like many other natural plastic fats, are strongly dependent on the polymorphism of the fat crystals that constitute their solid matrix.1,2 The term polymorph is used to indicate a particular type of lateral packing of the aliphatic chains of the triacylglycerols (TAGs),3 but it does not imply that two polymorphic crystalline phases occurring in the material have exactly identical chemical composition. In fact, natural fats, being multicomponent systems, usually produce mixed crystals and phases with somewhat different composition.4-9 As with many other fats, milk fat crystals are composed of domains formed by the piling of flat lamellae whose structure is given by a combination of the lateral packing of TAGs and their longitudinal stacking. This results in three types of crystalline phases, termed R, β′, or β in order of melting point, which have hexagonal, orthorhombic, and triclinic unit cells, respectively. The structure formed depends on the conditions of crystallization.5,10-12 As mentioned before, anhydrous milk fat (AMF) can form compound or mixed crystals during crystallization.4,13,14 Breitschuh et al.13 showed that a greater degree of supercooling induces mixed crystal formation that has a narrower melting range and lower peak melting temperature.7,13 Van Aken et al.15,16 summarized the crystallization characteristics of milk fat between 10 and 20 °C by observing that phase R has a clear point, which normally corresponds to the equilibrium melting temperature, of 20 °C, and at that temperature the high melting fraction * Corresponding author. E-mail:
[email protected]. § University of Guelph. † University of Waterloo. ‡ ExxonMobil Research and Engineering Company.
forms phase β′ after some 35 min, while the medium melting fraction forms phase β′ after more than 60 min; below 20 °C the medium and low melting fractions cocrystallize together in a β′ phase after 20-40 min. Several detailed X-ray diffraction studies of the crystallization of bulk milk fat under static conditions have been conducted using in-house or synchrotron radiation sources.5-7,10,12 A very detailed account of the different processes that occur upon cooling to low (4 °C) or very low (-8 °C) temperatures and the subsequent melting at different rates has been provided by several papers from Lopez et al. on milk fat5,6,12,17 and cream.11,12,18,19 In these reports, the simultaneous use of synchrotron X-ray diffraction and in-situ DSC allowed the identification of several phases, resulting from the segregation of different TAG molecules during crystallization, pointing again to the presence of crystals of mixed composition. The functional properties of milk fat and milk fatcontaining food products such as butter, bakery products, and ice cream depend strongly on processing conditions, which in turn influence the type of fat crystal polymorph obtained. Milk fat is composed roughly of 97% TAGs and 3% minor components, namely, polar lipids, mostly di- and monoacylglycerols, cholesterol, phospholipids and traces of free fatty acids.20,21 The composition of the sample used in this study can be found in Wright et al.21 The effect of these minor components on the crystallization of bulk fats has been object of several recent studies.9,21-24 Wright et al.21 and Herrera et al.9 agree on the fact that polar lipids delay the onset time of crystallization at higher temperatures. Their observations regarding growth rates, however, are contradictory. Wright et al. reported that polar lipids slowed crystal growth, while Herrera et al. reported the
10.1021/cg0497602 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/21/2004
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opposite. Even though this discrepancy could be attributed to compositional differences, the question remains open. It is important to note that the milk fat triacylglycerols (MFT) sample used in this study is the same as that used in Wright et al.’s study,21 and so is the composition of the polar lipids. In this work, we present a detailed comparison of crystallization dynamics and polymorphic transformations seen in both AMF and MFTs using time-resolved synchrotron diffraction techniques. This allowed a direct study of the effect of minor components on the system. Experimental Procedures The MFT were prepared by stripping the native milk fat from its minor polar components using a Florisil column, as described in the literature.9,21 The composition of the AMF and the MFT used in this study are common to other milk fat systems,21 composed mostly of TAGs with carbon numbers between 24 and 52, with a smooth bimodal distribution. Both AMF and MFT had very similar TAG composition.20,21 The melted sample of fat was loaded in a 1-mm thin glass capillary and placed in a temperature controlled X-ray compatible holder. The capillary was heated to 50 °C for 30 min prior to each experiment. The capillary was then cooled to the final temperature at a controlled rate using Peltier thermoelectric coolers. Two cooling rates were explored, 3 and 0.5 °C/ min. The final temperatures studied at the end of the cooling ramp were 17.5, 20, 22.5, and 25 °C. The same fat sample loaded in a capillary was used for all temperatures and cooling rates. The experiments were conducted at the ExxonMobil beamline X10A at the National Synchrotron Light Source in Brookhaven National Laboratory, Upton, NY. A Bruker 1500 two-dimensional CCD detector was used to capture diffraction patterns with an exposure time of 50 s. The X-rays had a wavelength λ ) 1.075 Å for the wide angle experiments, with the detector located 157 mm from the capillary, and λ ) 1.602 Å for the small angle experiments, with the detector situated 734 mm from the capillary. With a beam size of 0.5 × 0.5 mm the instrumental resolution was respectively 0.019 and 0.0027 Å-1. Radial averages of the resulting isotropic diffraction patterns were obtained using a custom plugin for ImageJ25 developed by us, which also allows for proper normalization of the intensities. The resulting one-dimensional powder diffraction profiles from the small angle experiments were then fitted to a combination of Gaussian and Lorentzian peak functions using a modified Levenberg-Marquardt algorithm. The background scattering from the capillary, air, and the Kapton windows of the temperature stage and the flight path were subtracted from the resulting radial plots. The X-ray diffracted intensity is plotted as a function of the reciprocal lattice spacing q, where q ) 2π/d ) 4π sin(θ)/λ, d is the interplanar spacing and 2θ is the Bragg angle.
Results and Discussion In our wide angle experiments, summarized in Figure 1 by selected radial plots from diffraction patterns at 17.5, 20, and 22.5 °C, the three phases R, β′, and β were easily identified according to the position of their characteristic strong diffraction peaks, presented in Table 1. Phase R has a single wide angle reflection at q ) 1.514 Å-1 observed above the broad X-ray scattering from the liquid fat, as seen in Figure 1a,b for both AMF and MFT 5 min after reaching 17.5 °C. There are two strong reflections from the β′ phase reported in the literature,26 and a few weaker reflections. Both β′ strong reflections are indicated in Figure 1c, and are present in Figure 1d,e,f as well. These two strong reflections
Figure 1. Wide-angle X-ray diffraction patterns from AMF and MFT crystallized at different temperatures after cooling at 3 or 0.5 °C/min. The characteristic crystalline diffraction peaks are visible above the broad scattering due to the liquid fat. The diffraction peak from phase R, after 5 min at 17.5 °C is presented in (a) and (b). The diffraction peaks for phase β′ and β obtained at the different cooling rates and temperatures are visible in the subsequent frames. The profiles at 17.5 and 20 °C were taken 50 min after reaching that temperature, and show a mixture of phases β′ and β for AMF at both cooling rates, and for MFT at the fast cooling rate; the profiles at 22.5 °C were taken 30 min after reaching that temperature and did not show the presence of phase β. Table 1. Characteristic Strong Reflections for Milk Fat Polymorphs Extracted from the Literaturea,b q, Å-1 (d spacing, Å) polymorph
small angle reflections
wide angle reflections
R β′
0.137 (47.2) 0.151 (41.6)
β
0.166 (38.0)
1.514 (4.15) 1.654 (3.80) 1.472 (4.27) 1.366 (4.60)
a
Refs 12 and 26. b Note that there are two main strong β′ wide angle reflections.
always appeared together in our experiments indicating the presence of phase β′. Phase β is characterized by its own strong peak at q ) 1.366 Å-1, as indicated in Figure 1c. This reflection was present in Figure 1c,d,e at 17.5 and 20 °C, but absent at 22.5 °C and absent as well in Figure 1f. The scans presented in Figure 1c,d at 17.5 and 20 °C were taken 50 min after reaching the final temperature; the profiles at 22.5 °C were taken 30 min after reaching 22.5 °C. At 17.5 and 20 °C, the crystallization of AMF started with the nucleation of phase R, followed by the phase transition to a mixture of phases β and β′, at both cooling rates. In all the experiments at 22.5 °C pure phase β′ was formed directly from the melt. Phase R was detected when cooling MFT at 3 °C/min to 17.5 and 20 °C, but it was absent at 22.5 °C and in the experiments at slow cooling rate for MFT. The relative proportion of phase β to phase β′, as determined by the relative intensity of the wide angle diffraction peaks, was smaller when the crystallization temperature was higher or the cooling rate was slower. This seems contrary to general observations on fat
Polymorphism in Milk Fat
crystallization, where the more stable phases such as phase β (with higher melting point and lower free energy) are generally found by slow cooling to high temperatures and not by rapid cooling to low temperatures. However, phase β was observed only in those cases where phase R had been detected and more or less in amounts proportional to it. Phase β was also present in higher proportion in AMF than MFT for the same crystallization conditions. The presence of mixtures of phases β and β′ is not uncommon in milk fat, but is usually associated with crystallization at high temperatures, slow cooling rates, and/or long storage periods, as observed in samples stored at 4 °C by Lopez et al.12,17 The general trends and positions of the wide-angle X-ray diffraction peaks are similar for AMF and MFT. However, the peaks for MFT appear narrower and better defined, perhaps as a consequence of the formation of more perfect crystals, due to the absence of the polar lipids. The peaks of the slowly cooled samples appear also better defined than those from the fast cooled samples, again perhaps due to a more perfect organization of the TAG chains. Although the increased presence of phase β at low temperatures and fast cooling rates seems counterintuitive, there is a possible explanation. The formation of phase β is largely dependent on the previous amount of phase R formed. Possibly at high cooling rates and low crystallization temperatures the polar lipids act as nucleation sites to induce the initial crystallization of the high melting fraction TAGs in phase R, and most of it then transforms into phase β because its composition is more stable in such a crystalline arrangement. Meanwhile, another phase, mostly composed of molecules directly crystallized from the melt, is formed with a composition that is already stable in phase β′. At higher temperatures and/or slower cooling rates, the polar lipids can be segregated from the TAGs, which preferentially crystallize in the stable phase β′. In any case, the formation of phase β is dependent on the initial nucleation of phase R. The lower angle diffraction peaks appeared as an intense ring around the beamstop near the center, as seen in Figure 2. In the small angle region, both phases had an intense (001) peak and a weaker (003) peak (not shown). The (002) peak was not detected above the background. The position of the (001) reflection of β and β′ cannot be resolved. In the small angle experiments it was easy to discern the phase R reflection from the subsequently formed phase, as illustrated in Figure 2, at least until the R reflection had become a very small shoulder on the peak of the subsequently formed phase. However, it was not possible to determine if that other phase was just β′ or a mixture of β′ and β. The time evolution of the characteristics of the (001) diffraction peak from phase R is presented in Figure 3. The zero value for time in the graphs was assigned to the moment when the cooling ramp ended and the crystallization temperature was reached; hence, at the slow cooling rates the onset of the phase appears as a “negative” time, meaning that crystals appeared as the cooling was still taking place. As mentioned before, phase R did not appear at the higher crystallization temperatures of 22.5 or 25 °C, and did not appear in MFT at the slow cooling rate. This
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Figure 2. Selected two-dimensional small angle diffraction patterns of milk fat taken after 3 min (a), 21 min (b), and 48 min (c) of reaching the crystallization temperature of 17.5 °C, showing the transition from phase R to phase β′, likely mixed with a small amount of phase β. Radially averaged intensity profiles from the two-dimensional images are shown in panels (d), (e), and (f), respectively. From the wide angle data, we know that there was some amount of phase β present as well after the transition, but its peak could not be resolved from the strong diffraction peak of phase β′.
confirms the suggestion by Wright et al.21 that the crystallization mechanism was different below and above 20 °C, since no evidence of formation of phase R was found through our X-rays observations above 20 °C in AMF and MFT. Thus, the melting point of phase R in this system is somewhere above 20 °C and below 22.5 °C. The effect of temperature and cooling rate are not altogether independent, because at the slow cooling rate the crystallization effectively started at a higher temperature than at the fast cooling rate. However, phase R was formed in AMF even at the slow cooling rate when the temperature went to 20 or 17.5 °C, while it was not formed in MFT in those cases. Why is it that AMF is more prone to form phase R than MFT? The formation of phase R is nature’s temporary solution to the need of quickly packing diverse TAG molecules in a crystalline state, as a response to a large undercooling. This crystalline form is intrinsically less ordered than phase β′. With less undercooling conditions, the drive to form a crystalline state is reduced, and more time is available to pack and organize the molecules. The absence of the polar lipids relieves somewhat the constraint of having to accommodate a wide diversity of molecules. Therefore, the TAG molecules in MFT are more free to accommodate themselves in the more stable phase β′, and hence the phase transition between R and β′ happens readily and quickly, as seen in Figure 3b, while in AMF the presence of the polar lipids makes it more difficult to form the stable phase and thus the R phase tends to last longer, as seen in Figure 3a. The integrated intensity of the diffraction peak from phase R (Figure 3a,b,c) grew only until the moment when the formation of phase β′
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Figure 3. Integrated intensity (a-c), fwhm (d-f), and peak position (g-i) of the (001) diffraction peaks from phase R. Phase R did not appear at 22.5 or 25 °C. It did not appear either in MFT at the slow cooling rate. The integrated intensity of the diffraction peak from phase R (a, b, and c) grew only until the moment when the formation of phase β′ started, and then decreased, and is plotted until the time where the shoulder on the peak of the β′ reflection could no longer be resolved.
started, and then decreased, consistent with the above description, and is plotted until the time where the shoulder on the peak of the β′ reflection could no longer be resolved. The full width half-maximum (fwhm) of the diffraction peaks (Figure 3d,e,f) characterizes the coherence length27 of the crystals. The fwhm from phase R showed a clear decreasing trend only in AMF at 17.5 °C and at fast cooling rate, consistent with the increase in coherence length and the crystal domain size over time. In the experiments at the slow cooling rate there was a weak decreasing tendency. The values of fwhm correspond to an average coherence length of 630 Å. If all the domains had the same size, the coherence length would coincide with the minimum thickness possible for a crystallite. However, it is more likely that there is a size distribution, and that the observed value is the resulting average from all the contributing domains. The peak position of phase R (Figure 3g,h,i) showed a tendency to increase over time, indicating that the interplanar spacing was decreasing, and its values were lower at 20 °C than at 17.5 °C. The crystallization at higher temperatures favors the incorporation of a higher proportion of the high melting fraction components, which in the case of milk fat have average longer fatty acid chains,20 resulting in slightly thicker lamellae and hence smaller peak position values. The slow cooling rate also resulted in a reduction of the peak position, an effect equivalent to an increase in temperature. Another effect of the increased temperature was that the initial d spacings increase at 17.5 °C and decrease at 20.0 °C, probably because the types of molecules that
are being preferentially crystallized are slightly different at the different temperatures. The average values of the d spacing are consistent with the formation of 2 L lamellar structures of thickness between 47.15 and 47.64 Å. The ratio between coherence length and lamellar thickness indicates that the coherence length corresponds approximately to 13 lamellae. The characteristics of the (001) diffraction peaks for the phase β′ are summarized in Figure 4, with the caveat that it is probably mixed with phase β in the cases where phase R had been present. The integrated intensity indicates an acceleration of the kinetics in the absence of polar lipids, as seen in Figure 4a-d. The onset times for phase β′ were longer for AMF than for MFT. At 25 °C, AMF showed a plateau for both cooling rates at approximately 31 min, suggesting the consecutive crystallization of two different fractions. The solid fat content (SFC) values estimated from the literature9 were up to 13% for the experiments at 20 °C in both systems, and for the experiments at 25 °C up to 9% for MFT and 7% for AMF. The integrated intensity was roughly proportional to the SFC in the range where it was possible to compare our data with the published SFCs. The integrated intensity curves of phase β′, representing the SFC, were fitted to the Avrami growth model used by Wright et al.21, I ) Imax[1 - exp(-ktn)], where I is the integrated intensity at any given time t after the onset of the phase β′ (not after reaching the final temperature), Imax is the maximum integrated intensity, k represents a crystallization rate constant,
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Figure 4. Integrated intensity (a-d), fwhm (e-h), and peak position (i-l) of the (001) diffraction peaks from phase β′ (likely including some phase β at low temperatures and fast cooling rate) from both AMF and MFT and both cooling rates.
and n represents the nucleation and/or growth mode. Integer values of n include a value of 1 for sporadic nucleation or 0 for instantaneous nucleation, plus 1, 2, or 3 if the crystals grow in 1, 2, or 3 dimensions, respectively. Thus, the theoretical values of n are between 1 and 4. Most curves fit well only for the first 30 min or less after the onset of phase β′, suggesting that there had been a change in the crystallization regime after that time. The curves for AMF at 25 °C were fitted to two sequential events, before and after the plateau at 31 min, since they did exhibit two growth stages. The results are presented in Table 2. The exponents “n” were mostly between 1.5 and 2.5, often close to 2, with the exception of the two initial segments of AMF at 25 °C. At the slow cooling rate the exponent n and the constant k had larger values for MFT than for AMF. At the fast cooling rate, there was no obvious trend of the values, although visual inspection of the curves shows that MFT was crystallizing faster than AMF, at least in terms of the integrated intensity. The difference in the growth exponent “n” for AMF at 25 °C before and after 31 min supports the possibility of two different growth modes in this material, related to the presence of polar lipids. The different modes may correspond to the formation of different phases, with different composition or poly-
Table 2. Avrami Constant k and Exponent n for the Growth of Phase β′ + β AMF
MFT
cooling rate
temperature, °C
n
k × 10-3
n
K × 10-3
3 °C/min
17.5 20 22.5 25 (t < 31 min) 25 (t > 31 min) 17.5 20 22.5 25 (t < 31 min) 25 (t > 31 min)
1.9 2.5 1.7 3.0 2.0 2.2 2.0 1.9 3.0 1.7
5.4 0.3 2.7 3.3 0.8 0.8 2.1 2.1 24 1.7
2.4 1.6 2.1 2.0
3.1 4.1 4.7 2.6
2.5 2.1 2.2 2.1
0.9 4.4 2.2 2.2
0.5 °C/min
morphic form. The values found by Wright et al.,21 which are different from ours, were computed using the SFC values obtained through pulsed nuclear magnetic resonance, and therefore did not distinguish between the different phases. The values computed in this work correspond only to the growth of the phase β′, probably combined with small amounts of phase β at the lower temperatures and fast cooling rates. The fwhm of the scattered X-rays offers an indication of the approximate averaged size of the coherence length of the domains that constitute the crystallites, with larger domains producing narrower peaks. The value is an indicator of the average size of the domains
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perpendicular to the lamellae within the crystal mass being observed. In our case, the fwhm of well-resolved peaks is decreasing, indicating an average increase in the size of the coherence length, or size of these domains, as seen in Figure 4e-h. This is consistent with the observed growth of the total crystalline mass. The sizes of the domains were between 400 and 800 Å. The growth of the thickness of the domains did not seem too different at the fast cooling rate between AMF and MFT, but are different at the slow cooling rate. MFT reached a steady state size relatively quickly, while AMF did not, indicating that the lack of minor components in MFT facilitates the quick growth of large stable crystallites. The values of Avrami exponents around 2 then come from the growth perpendicular to the lamellae, plus the growth of the surface of the platelets, which may have happened by a screw mechanism, equivalent in terms of “n” to a linear growth. This would mean that most of the nucleation took place at the beginning of the crystallization. These “n” values, however, come from the fitting of single curves, and thus must be taken with caution, although the interesting fact is that they all seem to be in a relatively narrow range. Well-resolved peaks displayed a progressive increase in the value of the peak position in reciprocal space, i.e., the averaged lamellar thickness decreased as time went by, as seen in Figure 4i-l. This suggests that the material being crystallized initially contained a higher proportion of long molecules than the material crystallized at a later time. The averaged peak positions for the β′ phase correspond to d spacing values that ranged from 40.93 to 42.45 Å. This is consistent with typical 2L type structures6 of milk fat. As pointed out by Lopez et al.6,12 the crystallization process of a multicomponent system such as milk fat involves molecular segregation, or fractionation. As mentioned before, in the small-angle X-ray diffraction experiments it was not possible to differentiate between the reflections from the β′ and β phases, although they were probably present together in the experiments at 17.5 and 20 °C and fast cooling rate. The position of the diffraction peak was higher at lower crystallization temperatures, consistent with the crystallization of a larger proportion of smaller molecules. Yet, the position of the diffraction peak at 17.5 °C is considerably higher than would be expected from the effect of temperature. This would be consistent with the presence of phase β, which has a smaller d spacing,12 resulting in an increase in the measured position of the diffraction peak in reciprocal space. In most cases, the observed averaged peak position increased in time, indicating that the molecules incorporated to the crystalline mass at the beginning of the crystallization formed thicker lamellae (larger d spacing) than those incorporated at later stages in the crystallization. The peak position captured by the detector is the weighted average of all the d spacings present in the system, and the crystalline phase acquires most of its mass in the first 10 min after the onset, which is the period when the largest change in peak position happens. The large amount of material at the beginning formed the thickest lamellae, reflected in smaller q values for the peak position. The material added later must have much smaller d spacing to produce a visible
Mazzanti et al.
increase of the averaged peak position. This is consistent with the continuum situation when large TAG molecules crystallize initially, followed by smaller TAGs, resulting in a decrease in the average d spacing, due to the incorporation of the smaller molecular species from the medium and lower melting fractions.20 The d spacing of the lamellae seen in MFT reached an equilibrium value within 20 min, again consistent with the rapid formation of crystallites, as discussed earlier, providing additional evidence that the minor components in AMF hinder the growth of uniform large crystals. The peak position observed from MFT was generally higher than the position observed for AMF under the same conditions, indicative of a smaller lamellar thickness, either due to shorter crystallized molecules, or a tighter packing of the TAGs molecules in MFT. The number of lamellae per domain started at 12 and grew in some cases up to 20. It was always a higher value for MFT than for AMF. It has been suggested in the literature, or rather “taken for granted” that the presence of polar lipids in the structure of the TAG would produce more imperfect crystals, and this would be the explanation for the slower kinetics observed, as well as other properties such as higher melting points. If the number of lamellae in a coherent domain can be used as an estimator of “crystalline perfection”, then more lamellae in the coherent domains, the more perfect a crystal is. It turned out that phase β′ of MFT had always more lamellae per domain than AMF. To our knowledge, this would the first direct structural evidence that such is indeed the case in milk fat. Conclusion The crystallization of AMF at fast cooling rate and low final temperatures resulted in the formation of a mixture of phases β′ and β after the transition from the metastable phase R. Slow cooling rate and high final temperatures resulted in the formation only of phase β′, with no formation of phase R or phase β. MFTs showed less tendency to form phases R and β. This suggests that the polymorphic transformation into phase β at high degrees of supercooling depends on the initial formation of phase R, which in turn is favored by the presence of the minor components. The presence of the minor components at their natural levels slowed the overall kinetics of crystal nucleation, growth, and polymorphic transformation. Acknowledgment. We thank Steve Bennett for his technical assistance at the beamline, and Dr. Amanda Wright for the sample of MFT. Funding was provided by the Canadian agencies National Sciences and Engineering Research Council (NSERC), Dairy Farmers of Ontario (DFO), Ontario Ministry for Agriculture and Food (OMAF), and National Centres for Excellence (NCE). Research carried out in part at the NSLS, Brookhaven National Laboratory, NY, U.S.A., which is supported by the U.S. DOE, Division of Materials Sciences and Division of Chemical Sciences. References (1) Wright, A. J.; Scanlon, M. G.; Hartel, R. W.; Marangoni, A. G. J. Food Sci. 2001, 66, 1056-1071.
Polymorphism in Milk Fat (2) Narine, S. S.; Marangoni, A. G. Food Res. Intl. 1999, 32, 227-248. (3) Sato, K.; Ueno, S. In Crystallization Processes in Fats and Lipid Systems; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 2001. (4) Walstra, P.; Vanberesteyn, E. C. H. Neth. Milk Dairy J. 1975, 29, 238-241. (5) Lopez, C.; Lavigne, F.; Lesieur, P.; Bourgaux, C.; Ollivon, M. J. Dairy Sci. 2001, 84, 756-766. (6) Lopez, C.; Lavigne, F.; Lesieur, P.; Keller, G.; Ollivon, M. J. Dairy Sci. 2001, 84, 2402-2412. (7) Breitschuh, B.; Windhab, E. J. J. Am. Oil Chem. Soc. 1998, 75, 897-904. (8) Dimick, P. S.; Manning, D. S. J. Am. Oil Chem. Soc. 1987, 64, 1663-1669. (9) Herrera, M. L.; Gatti, M. D.; Hartel, R. W. Food Res. Int. 1999, 32, 289-298. (10) ten Grotenhius, E.; Aken, G. A. v.; Malssen, K. F. v.; Schenk, H. J. Am. Oil Chem. Soc. 1999, 76, 1031-1039. (11) Lopez, C.; Lesieur, P.; Bourgaux, C.; Keller, G.; Ollivon, M. J. Colloid Interface Sci. 2001, 240, 150-161. (12) Lopez, C.; Bourgaux, C.; Lesieur, P.; Ollivon, M. Lait 2002, 82, 317-335. (13) Walstra, P. In Food Structure and Behaviour; Lillford, P., Blanshard, J. M. V., Eds.; Academic Press: London, 1987; pp 67-85. (14) Marangoni, A. G.; Lencki, R. W. J. Agric. Food Chem. 1998, 46, 3879-3884. (15) van Aken, G. A.; ten Grotenhuis, E.; van Langevelde, A. J.; Schenk, H. J. Am. Oil Chem. Soc. 1999, 76, 1323-1331.
Crystal Growth & Design, Vol. 4, No. 6, 2004 1309 (16) van Aken, G. A.; Visser, K. A. J. Dairy Sci. 2000, 83, 19191932. (17) Lopez, C.; Bourgaux, C.; Lesieur, P.; Bernadou, S.; Keller, G.; Ollivon, M. J. Colloid Interface Sci. 2002, 254, 64-78. (18) Lopez, C.; Lesieur, P.; Keller, G.; Ollivon, M. J. Colloid Interface Sci. 2000, 229, 62-71. (19) Lopez, C.; Riaublanc, A.; Lesieur, P.; Bourgaux, C.; Keller, G.; Ollivon, M. J. Am. Oil Chem. Soc. 2001, 78, 1233-1244. (20) Herrera, M. L.; Hartel, R. W. J. Am. Oil Chem. Soc. 2000, 77, 1177-1187. (21) Wright, A. J.; Hartel, R. W.; Narine, S. S.; Marangoni, A. G. J. Am. Oil Chem. Soc. 2000, 77, 463-475. (22) Litwinenko, J. W.; Singh, A. P.; Marangoni, A. G. Cryst. Growth Des. 2004, 4, 161-168. (23) Tietz, R. A.; Hartel, R. W. J. Am. Oil Chem. Soc. 2000, 77, 763-771. (24) Siew, W. L.; Ng, W. L. J. Sci. Food Agric. 1996, 71, 496500. (25) Rasband, W. S., ImageJ Image Analysis Software, National Institutes of Health, Bethesda, Maryland, U.S.A., http:// rsb.info.nih.gov/ij/, 1997-2004. (26) D’Souza, V.; deMan, L.; deMan, J. J. Am. Oil Chem. Soc. 1990, 67, 835-843. (27) Cullity, B. D.; Stock, S. R. Elements of X-ray Diffraction, 3rd ed.; Prentice Hall: New Jersey, 2001.
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