Effects of Shear Rate Variation on the ... - ACS Publications

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Effects of Shear Rate Variation on the Nanostructure of Crystallizing Triglycerides Gianfranco Mazzanti,*,† Mengyu Li,† Alejandro G. Marangoni,‡ and Stefan H. J. Idziak§ †

Process Engineering and Applied Science, Dalhousie University, Halifax, Nova Scotia B3J 2X4, Canada Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada § Department of Physics & Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada ‡

ABSTRACT: Shear flow affects the kinetics of crystallization and modifies the composition of the solid and liquid phases obtained during crystallization. Research on the effects of shear flow on the crystallization is necessary to understand how triglycerides form crystalline structures. In this Article, palm oil, milk fat, and milk fat triglycerides were cooled at 3 or 0.5 °C/min from the melt at 50 °C down to 17.5 or 17 °C and left to crystallize under a shear rate of 90 s
1 in a Couette cell system. The shear rate was then increased to 1440 s
1. Synchrotron X-ray diffraction patterns were captured during the crystallization process. The integrated intensity, average thickness, and average lamellar spacing of the crystalline nanoplatelets were modified by the shear rate step. Increased thickness and crystalline orientation were evident after the shear rate step-up. Similar experiments with milk fat in a Rheo-NMR instrument provided additional information to interpret the orientation and thickness changes observed. These effects of shear step-up on crystallizing fats offer new options for the industrial processing of chocolate, dairy, margarine, and shortening production, allowing more freedom for tailoring their desired crystalline nanostructures.

’ INTRODUCTION Triglycerides (TAGs) play a major role in our diet and in our health. They also display unique polymorphic crystallization characteristics that make them very interesting materials.1 TAGs are crystallized industrially under a variety of conditions of temperature and shear to obtain textures appropriate for their use. They are consumed as raw materials for other industries, such as baking or fillings, or directly by institutions and individuals. The industrial processing practices for crystallization of natural lipid materials are still a combination of considerable empirical technical lore and sound scientific principles. This is particularly true when it comes to understanding the effect that shear flow has on the crystallization of these multicomponent lipid systems. Thus, in recent years, the study of food processing under shear from the perspective of materials science, and in particular TAGs crystallization, has gained considerable attention.2
6 These studies had the invaluable side result of highlighting a fact only vaguely suspected before them. This fact is that triglycerides spontaneously form nanosized platelet crystals under most crystallization conditions. The application of shear flow and the subsequent observation of the orientation of the crystalline domains,7 together with the measurement of the domain sizes using X-ray diffraction,5,8,9 led to the formulation of the hypothesis that the basic building blocks of the lipid networks were nanoplatelets. This hypothesis was proven in a very recent study.10,11 Thus, a change in paradigm is required as to what is called a “crystal” in triglyceride r 2011 American Chemical Society

crystallization. In most of the previous literature, a crystal was referred to as an entity observed under polarized light microscopy, with sizes ranging from one to hundreds of micrometres.12 These “crystals” are really very large aggregates of innumerable nanosized platelets. This has enormous implications for the development of manufacturing methods, because with this knowledge we can harness the natural tendency of these materials to form nanoparticles and the way they aggregate hierarchically to form networks. Although the study of the crystallization of natural bulk lipids under shear flow using synchrotron or in house X-ray diffraction is very recent,3,7,8,13
17 shear flow has been used industrially for a long time to improve heat transfer and to provide textural characteristics for margarines and other products. Shear can dramatically affect the kinetics of crystallization and modify substantially the composition of the solid and liquid phases as compared to those obtained under static crystallization conditions. We also know that shear will affect the dynamics of aggregation and segregation between crystalline nanoplatelets and their clusters. However, the effect of shear variations on the crystalline nanoplatelets during the crystallization has not been studied yet. This Article offers a first exploratory foray into the Received: June 21, 2011 Revised: July 26, 2011 Published: August 23, 2011 4544

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Figure 1. Diffraction pattern (Debye ring) from the β0 polymorph of AMF crystallized at 17 °C under a shear rate of 1440 s
1, captured approximately 2 h after reaching the crystallization temperature. The arrow q indicates the radial direction, and the curved arrow χ indicates the azimuthal direction.

vast landscape that combined shear and temperature profiles provide for the formation and hierarchical organization of the nanoplatelets. This was done by observing the effects of a step shear rate variation on the crystallization of anhydrous milk fat (AMF), milk fat triacylglycerols (MFT), and palm oil (PO). The experiments were conducted by changing the applied shear rate on the crystallizing material after the phase transition from the metastable phase α to a more stable phase β0 . Because the metastable phase α was present only for a short time, the shear step was applied after the phase transition. Several very interesting observations resulted from this stepwise change, chiefly a clear effect on the thickness and orientation of the nanoparticles.

’ METHODS The melted sample was placed in the gap (δ = 1 mm) between the cylinders of a Couette shear cell described previously.7,15 The shear rate, _ can be approximated by dividing the relative velocity u between the γ, shearing cylinders by the gap (γ_ ≈ u/δ). The values of the applied shear rates were 90 and 1440 s
1. The samples were cooled at 3 or 0.5 °C/min from the melt at 50 °C down to the crystallization temperatures (17.5 or 17 °C). Previous studies have shown that this temperature was enough to erase crystal memory,5,8,9 because the material did not go directly into the β0 form. After 50 min, the shear rate was raised from 90 to 1440 s
1. 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 exposure times of 20 or 50 s. A typical diffraction pattern is shown in Figure 1. The X-rays had a wavelength λ = 1.097 Å, and the detector was located at a distance L = 1132 mm from the cell axis for the small-angle X-ray diffraction experiments. A beam size of 0.5  0.5 mm gave an instrumental resolution of 0.0023 Å
1. The X-ray diffraction intensity from each 2D diffraction image was radially averaged and plotted as a function of the reciprocal lattice spacing q (Figure 2a), where q = 2π/d = (4π/λ) sin θ, d is the interplanar spacing, and 2θ is the Bragg angle. The radial averaging was done by circular integration of the X-ray intensity at a fixed radius and was repeated for all radial distances. These radial averages were obtained with a custom plugin for the open source ImageJ software.18 The plug-in normalized the intensities with respect to the incident beam and corrected the absorption

Figure 2. (a) Averaged intensity in normalized units as a function of the radial reciprocal space coordinate q in inverse angstrom, obtained from Figure 1. A schematic drawing of a nanoplatelet indicates the physical nanostructural dimensions obtained from the diffraction peak. (b) Azimuthal plot of the intensity at the maximum of the scattering peak against azimuthal position χ in degrees. The plot starts at the upper vertical point in the Debye ring in Figure 1, that is twelve o’clock, and is taken counter clockwise. distortion introduced by the Couette cell. The resulting one-dimensional powder diffraction profiles (e.g., Figure 2a) were fit to a Gaussian
Lorentzian peak profile using a modified Levenberg
Marquardt algorithm, to determine the peak position q0, the full-width-at-half-maximum Δq, and the area under the peak. The peak position q0 is related to the thickness d of the lamellae by d = 2π/q0, as shown in Figure 2a. The thickness of the lamellae is determined by the length and arrangement of two TAG molecules stacked longitudinally. The correlation length ξ can be estimated from the full width at half-maximum of the diffraction peak, Δq, using a simplification of the Scherrer approximation19 for small angles ξ = 2π/Δq (Figure 2a). The correlation length ξ from the (001) peak corresponds to the size of a single crystalline domain, which in this case gives an approximate thickness of the crystalline nanoplatelets, schematically shown in Figure 2a, because the long axis of the molecules is almost perpendicular to the flat surface of the crystal. On the other hand, the interplanar distance d depends on the size of the molecules aggregated to the lamellae. Thus, the two nanostructural parameters ξ and d provide information on the characteristic composition and size of the nanoplatelets.10,11 4545

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Figure 3. Diffraction pattern and slice radial plots from palm oil crystallized at 17 °C and 1440 s
1 showing the (001) peak of phase β0 captured 32 min after reaching 17 °C. The slices are 90° apart and span a 10° angle in the χ direction. To evaluate the crystalline orientation in the sample, the ImageJ plug-in provided azimuthal plots, by recording the intensity along the circumference (χ angle in Figure 1) of the 2D diffraction rings, as illustrated in Figure 2b, which was obtained from the diffraction pattern in Figure 1. The plots were fit to a sum of Gaussian functions with full width at half-maximum Δχ. The orientation ratio χr is defined as the ratio of the area above the dotted line over the total area. Small Δχ and large χr are the hallmarks of a larger proportion of oriented nanoplatelets. The proportion of “free” nanoplatelets versus “clustered” nanoplatelets coupled with the hydrodynamic conditions thus determines the azimuthal shape of the diffraction pattern. For an unoriented, polycrystalline material, the area under the X-ray diffraction peak seen in the radial plots in Figure 2a, referred to as the integrated X-ray intensity, is proportional to the total crystalline mass (also called solid fraction, or SF) of a given phase present in the volume illuminated by the X-rays.19 In our measurements taken at higher shear rates, orientation of the nanoplatelets is observed for wide-angle and for small-angle X-ray diffraction rings. It is not possible to know what proportion of nanoplatelets are in a cluster and what proportion is free. However, the presence of orientation requires that the nanoplatelets be oriented with the flow, either individually or in “coherent clusters” in which all of the platelets are parallel. The degree of orientation in azimuthal plots like Figure 2b was not very high (Δχ > 50°), even when the fraction of oriented crystals (χr) coexisting with the unoriented nanoplatelets was high (0.8). We therefore assumed in previous publications5,8,14,16,20 that this weak orientation did not impact strongly on the relationship between SF and total integrated intensity. In the present study, we compared SF with integrated intensity in a particular case and found that the orientation under high shear rates is, in fact, an important factor in their relationship, as explained in the following section. The crystallization was allowed to proceed until the phase transition from α to β0 had ended. Some residual amount of form α remained with the β0 , as shown by a very weak shoulder on the diffraction peak. Eventually, the shoulders became very small. Thus, for the purpose of this Article, we have fitted the diffraction peak as a single one. The average integrated intensity, the lamellar thickness d, and the thickness ξ were determined from the radial plots from the whole diffraction pattern. However, to investigate differences in nanostructural parameters at different orientations, we obtained as well radial profiles at both extremes of the orientation patterns, as indicated in the inset of Figure 3. Radial profiles were taken from vertical (axial, parallel to the axis of rotation) and horizontal (equatorial, perpendicular to the axis of

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Figure 4. Nanostructural information obtained from the diffraction patterns of the (001) peak of the β0 phase of AMF cooled at 0.5 °C/min to 17 °C under a shear rate of 90 s
1. After approximately 50 min, the shear rate was increased to 1440 s
1. After 120 min, the shear flow was stopped. (a) Integrated intensity in normalized units n.u. for the horizontal slice (0), the vertical slice (9), and the total Debye circle (g). (b) Average thickness of the thickness in nanometers for the horizontal slice (4) and the vertical slice (2). (c) Average lamellar spacing d in nanometers for the horizontal slice (O) and the vertical slice (b). rotation and the X-ray beam) slices. The slices spanned a 10° angle in the χ direction. The slices showed differences at the higher shear rates, apparent in the plots in Figure 3 for the horizontal (dotted line) and vertical (solid line) profiles. The higher intensity of the horizontal portion was expected, due to the preferred orientation of the nanoplatelets. The results became even more interesting due to the observed changes in the lamellar distance and the crystal thickness. We found that they also depended on the direction of the profile. As will be discussed in the following sections, this means that nanoplatelets that were preferentially oriented in different directions did not have the exact same nanostructural characteristics. Solid fraction (SF) measurements were taken with a Bruker Minispec20, modified for both shear flow and temperature control. The shear was applied to the sample using a Teflon shear shaft as described in previous publications.21
23 Milk fat samples were melted at 80 °C and placed in the mini-Couette shear cell to a level higher than the height of the radio frequency coil in the NMR. Once the liquid sample was in the cell, it was exposed to shear rates and temperature profiles described above (e.g., kept melted at 50 °C and crystallized at 17 °C).

’ RESULTS AND DISCUSSION A. Integrated Intensity Change under Shear Flow. The effect of an increase in shear rate on the nanostructural parameters of the crystalline β0 phase of milk fat is summarized in Figure 4. The sample was cooled at 3 °C/min from the melt at 50 °C down to 17 °C and left to crystallize under a shear rate of 90 s
1. After approximately 50 min, the shear rate was increased to 1440 s
1. After 120 min, the shear flow was stopped, and a few minutes later the sample was melted. Figure 4a shows the integrated intensity that is proportional to the crystalline mass. The horizontal slice (0), the vertical slice (9), and the total Debye circle (g) are plotted together. During the first 50 min, there is almost no difference in the integrated intensity between these signals. This is consistent with the absence of crystalline orientation observed during this part of the crystallization 4546

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Figure 5. SF (%) versus time (minutes) for milk fat crystallized in the Rheo-NMR mini-Couette cell at 17.5 °C under two shear regimes (averages of three replicates).

process under a shear rate of 90 s
1. However, upon increasing the shear rate to 1440 s
1, a sudden reduction of the average integrated intensity was observed. The new value of total integrated intensity just after the step-up of the shear rate was similar to the value at about 20 min into the crystallization, and the reduction was observed in both the vertical and the horizontal slices. The most probable causes of this sudden reduction are a partial melting due to viscous heat generation and a reorientation of the nanoplatelets away from the X-ray diffraction Bragg angle. It is unlikely, however, that upon increasing the shear rate the larger platelets will orient out of the scattering angle, as indicated by all our observations that they actually accumulate preferentially around the shear plane and increase the scattering. After the step-up, the shear rate was maintained at 1440 s
1, and we observed an increase in the average integrated intensity as time progressed. However, the increase was not the same in the vertical and horizontal slices. The intensity from the horizontal slice grew fast, whereas the one from the vertical slice decreased. This is simply consistent with the change in distribution of crystalline mass at different orientations as time went on. Upon stopping the shear, a sudden small reduction of the average intensity was again observed, before the melting of the sample started. This reduction cannot be attributed to an increase in viscous heating, because the mechanical energy input had ceased. It must therefore come from the redistribution of the nanoplatelets into other orientations that cannot be observed with our scattering geometry as the imposed orientation field was removed. Inertial effects on the nanoplatelets are insignificant at these very low Reynolds numbers.24 Thus, the redistribution of orientation is produced by Brownian motion25,26 and by attractive interparticle forces.7 The shear field, by promoting a particular orientation, increases the proportion of the crystalline population that is at an angle capable of producing a Bragg reflection. It therefore affects the amount of free nanoplatelets and coherent clusters with respect to the number of randomly packed “ball clusters”. In this particular material, once the shear field is removed, there is a tendency to recover a random distribution in three dimensions, and therefore a portion of the mass becomes “invisible” as these orientations can no longer be observed using our scattering geometry. As the orientation of the nanoplatelets is reduced, due to Brownian motion and random interactions, the difference between the horizontal and vertical slices is very much reduced. A certain amount of crystallization happens during the no-shear 5 min, due to the absence of viscous heating, which allows for a local temperature decrease. It is not possible to determine, based

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only on the X-ray diffraction integrated intensity for highly orientable materials, what is the proportion in which growth and orientation are happening after the shear rate increase. A complete study of the behavior requires an independent method for the determination of the SF under shear flow.21
23 Using a prototype for such measurements,23 experiments with AMF were started at a shear rate of 90 s
1 and then raised to 1100 s
1, because we could not reach a shear rate of 1440 s
1 in the prototype. The results summarized in Figure 5 show an obvious SF decrease when the shear rate changed from 90 to 1100 s
1 and a moderate increase when the shear stopped. The decrease in SF after the step is consistent with the reduction of integrated intensity, and so is the increase after stopping the shear. However, the increase in integrated intensity during the high shear period happened while the SF remained essentially unchanged. This happens because upon increasing the shear rate smaller younger crystals melted due to the increasing heat generation, whereas the SF recovered somewhat after the shear stopped. At high shear rates, the remaining thicker and older large crystalline platelets, as seen by the thickness ξ, became easier to orient because of their larger size7 (see following subsection). This is consistent with the increase in integrated intensity for the horizontal slice while the one for vertical slice decreased at the same time. Combining both trends, the average integrated intensity increased even though the SF remained constant under high shear rates. B. Average Thickness Change under Shear Flow. The thicknesses of the nanoplatelets for the horizontal slice (4) and the vertical slice (2) are presented in Figure 5b. It can be seen that as the nanoplatelets grow under a 90 s
1 shear rate, there is little difference in the size between both slices. However, as the shear rate is increased to 1440 s
1, there is a clear separation between larger nanoplatelets in the horizontal slice and smaller nanoplatelets in the vertical slice. Crystals with a larger size are more readily oriented than smaller ones, and thus the shear field imposed produces not only an overall orientation, but also an orientational segregation based on size. This shows that the suspension of nanoplatelets in the liquid fraction is polydisperse (the crystalline population has a distribution of sizes). Although the difference in size is not large, this makes the problem of relating crystallinity (SF) to X-ray integrated intensity even more difficult for lipid systems under high shear rate fields. It is particularly interesting that upon stopping the shear flow, the average size of the nanoplatelets in both slices becomes the same. It can be clearly seen too that the size of nanoplatelets after the shear increase is considerably larger than it would have been if the shear rate had been kept at 90 s
1. This may appear counterintuitive at first glance. It has been reported repeatedly in the literature that shear flow or mixing produces smaller crystals.12 However, the evidence presented here is clearly telling the opposite for this particular case. The first clarification that needs to be made is that the term “crystal” used in many previous publications refers to microscopic aggregates of large numbers of nanoplatelets. Upon shear increase, these clusters are disrupted and become smaller. The likely explanation for the process observed in this study is that upon increasing the shear rate, a sudden burst of viscous heat is generated that results in the melting of the smaller crystals of the polydisperse population. As time continues, a ripening process happens where smaller crystals melt and larger crystals grow. This is not an artifact linked to the orientation of the nanoplatelets. The thickness did not change substantially when shear was stopped, as did the integrated 4547

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Figure 6. Nanostructural information obtained from the diffraction patterns of the (001) peak of the β0 phase of MFT cooled at 3 °C/min to 17.5 °C under a shear rate of 90 s
1. After approximately 60 min, the shear rate was increased to 1440 s
1. After 120 min, the shear flow was stopped. (a) Integrated intensity in normalized units n.u. for the horizontal slice (0), the vertical slice (9), and the total Debye circle (g). (b) Average thickness of the thickness in nanometers for the horizontal slice (4) and the vertical slice (2). The average values for AMF are plotted as gray triangles for comparison. (c) Average lamellar spacing d in nanometers for the horizontal slice (O) and the vertical slice (b). The average values for AMF are plotted as gray circles for comparison.

Figure 7. Thickness in nanometers versus time at 17 °C for palm oil crystallized under shear flow. (a) Triangles are the horizontal (4) and vertical (2) slices at a shear rate of 90 s
1, and squares initially at 90 s
1 increasing after 60 min to 1440 s
1. (b) Triangles are the horizontal (4) and vertical (2) slices at a shear rate of 1440 s
1, and squares initially at 90 s
1 increasing after 60 min to 1440 s
1.

intensity. The final thickness of the nanoplatelets is around 65 nm, as compared to the 47 nm that they would have reached if the shear rate was kept at 90 s
1. On the other hand, if the whole crystallization process was carried out under a shear rate of 1440 s
1, then the thickness reached was 52 nm, slightly larger than the 47 nm obtained at 90 s
1 but still less than 65 nm. Another experiment at 17.5 °C stepping up the shear rate from 45 to 1440 s
1 for 10 min showed an increase from 46 to 56 nm. The drastic increase in the thickness was also observed in an experiment at 20 °C when increasing the shear rate from 90 to 1440 s
1. The thickness increased from 56 to 69 nm. At 20 °C,

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the overall sizes were larger, even though the total integrated intensity (SF) was lower. The slower kinetics at 20 °C favors the formation of fewer larger crystals. Milk fat triglycerides (MFT), which were obtained by removing all of the polar lipids from AMF,27 showed a response to the shear step change that was in general qualitative terms similar to the behavior observed in AMF, as seen in Figure 6. However, some interesting details are different from AMF. The initial orientation at 90 s
1 in MFT is present from the onset of the crystallization. Although there is a clear increase in the nanocrystalline thickness, there is very little difference in the vertical a horizontal direction. Similar behavior is observed for the lamellar distance. Experiments with palm oil showed results similar to those of AMF for the integrated intensity and lamellar distance. The thicknesses of the nanoplatelets from experiments with palm oil are presented in Figure 7. Figure 7a shows the thickness of the system crystallized continuously at 90 s
1, together with the system crystallized initially at 90 s
1 and later increased to 1440 s
1. Figure 7b shows the system crystallized continuously at 1440 s
1 together with the system crystallized initially at 90 s
1 and later increased to 1440 s
1. The arrow in both figures shows the time when the shear was stepped up. In the case of palm oil, there was not much difference in the nanoplatelet thickness at both shear rates (90 or 1440 s
1 all of the time), when each shear rate was applied continuously. In contrast, the change in shear rate produced a very notorious change in the nanoplatelet sizes. Thus, the increase in nanoplatelets thickness after a sudden shear step-up is an observed phenomenon of general nature, as far as crystallizing TAGs under shear is concerned. The detail that is different in the two systems is that palm oil tends to reach a single crystalline size in the vertical and horizontal scattering directions, whereas in milk fat they tend to segregate. The thickness distribution of the nanoplatelets is therefore different between the two materials. The absolute size of the milk fat nanoplatelets is smaller than the palm oil nanoplatelets. Although we do not have a measurement of the aspect ratios of both materials, we know from previous research that palm oil exhibits weaker orientation patterns than does milk fat.5,7,15,16,28,29 We also know that the aspect ratios seem to remain constant for a given material regardless of its size.11 The tendency to be oriented increases both with aspect ratio and absolute size,30 and the absolute size of the palm oil nanoplatelets in this work and previous ones is larger.5,8,16 It follows that the aspect ratio of the palm oil nanoplatelets must be considerably smaller than that of milk fat. This will have to be experimentally confirmed and quantified by procedures similar to those used by Acevedo.10,11 A comparison of the AMF experiments with an experiment that had a temperature spike is presented in Figure 8. Both experiments were done at the same cooling rate and to the same final temperature. However, in the experiment represented by the filled symbols, there was a temperature spike about 15 min after reaching the final temperature. The temperature spike rose to about 25 °C and took about 5 min to go back to 17.5 °C. The integrated intensity (9) suffered a dip as a consequence of this sudden temperature spike, but soon recovered and followed a growing trend similar to that of the experiment that had not been affected by the spike (0). The thickness of the nanoplatelets affected by the temperature spike (2) increased as a result of the spike. The thickness, however, did not go back to the values of the original experiment (4). Although it follows a similar growing trend, the values of the nanoplatelets thickness remained higher for the course of the experiment. The sudden shear rate 4548

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Figure 8. Three y axes plotting the thickness ξ of the crystalline domains in nanometers (left axis, triangles), the integrated intensity in normalized units (n.u.) (right axis, squares), and the temperature in °C (second right axis, diamonds) versus time for two experiments done with AMF cooled at 3 °C/min to 17.5 °C under a shear rate of 1440 s
1. Filled symbols are for the first experiment with a temperature spike, the second one for the experiment with normal temperature profile. Open symbols are for the experiment with normal temperature profile.

increase in the experiments discussed previously produced the same effect; thus it is clear that the shear step resulted in a sudden temperature change on top of the hydrodynamic and orientation changes induced in the system. C. Average Lamellar Spacing Change under Shear Flow. Figure 4c shows that there was a small difference in the average lamellar spacing d in nanometers between the horizontal slice (O) and the vertical slice (b) during the crystallization under a shear rate of 90 s
1. During crystallization of AMF, the decrease in average lamellar thickness over time is a consequence of the incorporation of shorter molecules into the crystalline structure.9 Because smaller molecules of homologous series tend to have a lower melting point, they are incorporated later as compared to the longer molecules in the multicomponent liquid. However, after the shear rate was increased to 1440 s
1, the first reaction of the lamellar thickness was to go back to a larger value, similar to that present at 20 min, consistent with the observed decrease in integrated intensity. This indicates that the younger and smaller nanoplatelets formed using the shorter molecules were melted as a consequence of the shear rate increase. The older and thicker nanoplatelets survived the melting, losing only their outer recent layers, yielding an overall thicker crystalline population. After the shear change, both the horizontal and the vertical slices show a similar trend of continued decrease in lamellar distance, but the difference between the two is consistently large. The horizontal slice corresponds to the slightly larger nanoplatelets and to the most oriented ones. These nanoplatelets had a smaller lamellar thickness as compared to the vertical slice. This also means that the peak position of both axes is slightly different, and therefore there is a small elliptic deformation in the Debye ring, because is not produced by a fully random collection of crystallites, but rather by a distribution of crystals affected by size, which is related to composition. This small deformation corresponds to an aspect ratio of 0.998. This very small value will not affect estimated average lamellar distances or nanoplatelet thickness. The reader must bear in mind that the two-dimensional detector provides unparalleled spatial resolution of the diffraction patterns.

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Figure 9. Oriented fraction χr (4, left axis) and full width halfmaximum Δχ of the chi-plot (9, right axis) versus time in minutes for AMF crystallized at 17 °C and 90 s
1 after the increase of the shear rate to 1440 s
1 at time 50 min followed by the suppression of shear at 120 min.

The detector used in these experiments had approximately 1200 measuring pixels per radial position at the locations of our peaks. Thus, we can obtain very precise measurements of peak positions, and therefore of lamellar spacings d. Static diffraction patterns did not show this small difference; therefore, it cannot be attributed to a misalignment of the detector. The segregation due to orientation is related to size, and the size is related to how old the nanoplatelets are. The older crystals are expected to have compositional differences with respect to the younger ones because these are multicomponent systems and some fractionation happens during crystallization. Therefore, it is not surprising that these differences can be observed. Upon stopping the shear rate, the randomization of the nanoplatelets happened rather quickly, and the final value of both vertical and horizontal slices was nearly the same. This final value, 4.134 nm, was different from the 4.144 nm value that the system would have reached if the shear step had not been introduced. It is a very small difference; however, it may result in a somewhat different stability and thermal behavior of the final product. This difference did not exist in the MFT. Therefore, the polar lipids present in AMF27 induced a more polydisperse population by facilitating sporadic nucleation and slowing the overall growth, as was noticed in previous experiments both statically and under shear flow.5,9 D. Effect of the Shear Flow on the Overall Orientation of AMF. To compare the effect of the shear step on the overall orientation of the AMF sample, we calculated the oriented fraction χr as well as the full width at half-maximum Δχ of the azimuthal plots. The results are presented in Figure 9. The χr at 90 s
1 was essentially zero because there was no obvious orientation, and the Δχ had no meaning, that is, tended to infinity. After the shear rate was stepped up to 1440 s
1, the orientation began to increase, indicated by an increase in χr and a decrease in Δχ. The values of both parameters for the material crystallized only under a shear rate of 1440 s
1 are indicated for comparison as horizontal dotted lines. The oriented fraction reached a value of 0.52, similar to the value reached by the material crystallized continuously at 1440 s
1. However, the azimuthal peaks reached a Δχ of 68°, while the system crystallized at 1440 s
1 had a Δχ of 78°. Thus, the azimuthal peaks at the end of the experiment were narrower for the shear step treatment, indicating that a higher 4549

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Crystal Growth & Design degree of orientation was obtained, consistent with platelets with a larger size. E. General Conclusion. The nanoplatelets constitute the building blocks of the hierarchical structures of plastic natural triglyceride networks. It is therefore very important that the concept of “nanoplatelets” be used to describe these materials, rather than a loose “crystal” term that refers to microscopic aggregates of nanoplatelets. Otherwise, explanations of the crystallization phenomena and their potential applications will not reflect the reality of the material’s structures. The judicious use of combined temperature and shear rate profiles can be used to tailor the size and aggregation characteristics of the nanoplatelets. For instance, the thermo-mechanical process of applying a sudden increase in shear rate can cause sudden temperature spikes, which will melt small crystals. This is akin to many tempering methods, except that the energy here is delivered mechanically throughout the sample in a very short time, rather than through the boundary of the crystallizing vessel via heat transfer, offering opportunities for many new processes. Much research is needed to develop methods to measure and control the real temperature of the material at high shear rates, and the larger temperature gradients that will be present in the material as it is being processed. The combined shear
temperature mechanism can also affect the composition of the nanoplatelets, as well as the total amount of crystallized material. The processes of crystallization of multicomponent systems are inherently fractionation processes as well. Therefore, by changing the path through which the final solid fraction of the material is reached, the mechanical, thermal, diffusional, and oil binding capacity of the final material will be affected. The design of proper shear and temperature sequences in a laminar shear crystallizer31,32 can therefore result in products with very different properties.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ REFERENCES (1) Sato, K. Crystallization behaviour of fats and lipids - a review. Chem. Eng. Sci. 2001, 56, 2255–2265. (2) Donald, A. Food for thought. Nat. Mater. 2004, 3, 579–581. (3) Mezzenga, R.; Schurtenberger, P.; Burbidge, A.; Michel, M. Understanding foods as soft materials. Nat. Mater. 2005, 4, 729–740. (4) de Graef, V.; van Puyvelde, P.; Goderis, B.; Dewettinck, K. Influence of shear flow on polymorphic behavior and microstructural development during palm oil crystallization. Eur. J. Lipid Sci. Technol. 2009, 111, 290–302. (5) Mazzanti, G.; Marangoni, A. G.; Idziak, S. H. J. Synchrotron study on crystallization kinetics of milk fat under shear flow. Food Res. Int. 2009, 42, 682–694. (6) Toro-Vazquez, J. F.; Rangel-Vargas, E.; Dibildox-Alvarado, E.; Charo-Alonso, M. A. Crystallization of cocoa butter with and without polar lipids evaluated by rheometry, calorimetry and polarized light microscopy. Eur. J. Lipid Sci. Technol. 2005, 107, 641–655. (7) Mazzanti, G.; Guthrie, S. E.; Sirota, E. B.; Marangoni, A. G.; Idziak, S. H. J. Orientation and phase transitions of fat crystals under shear. Cryst. Growth Des. 2003, 3, 721–725. (8) Mazzanti, G.; Marangoni, A. G.; Idziak, S. H. J. Modeling of a two-regime crystallization in a multicomponent lipid system under shear flow. Eur. Phys. J. E 2008, 27, 135–144.

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dx.doi.org/10.1021/cg200786k |Cryst. Growth Des. 2011, 11, 4544–4550