Published: April 28, 2011 r2011 American Chemical Society 2335 dx.doi.org/10.1021/cg200014w | Cryst. Growth Des. 2011, 11, 2335–2345 ARTICLE pubs.acs.org/crystal Laminar Shear Effects on Crystalline Alignments and Nanostructure of a Triacylglycerol Crystal Network Fatemeh Maleky, Alexandra K. Smith, and Alejandro Marangoni* Department of Food Science, University of Guelph, Guelph, Ontario, Canada ’ INTRODUCTION The study of microstructure in polycrystalline materials such as edible fats has become increasingly important since the macro- scopic properties of such materials depend on the structure of their crystal networks. 1À7 Edible fats are a class of plastic materials com- posed of a continuous network of crystalline fat suspended in an oil phase. 8,9 It is widely held that the balance between van der Waals attractions and Brownian motion causes the suspended crystals to aggregate and form a three-dimensional (3-D) net- work via diffusion-limited clusterÀcluster aggregation. 10,11 The properties of this 3-D network depend not only on the amount and distribution of the network mass but also on the properties of the individual particles. These include the size, shape, and arrangement of the crystals. A significant amount of research has shown that the habit of fat crystals is greatly affected by heat, mass, and momentum transfer conditions established during the crystallization process. 6,12À14 Therefore, by modifying proces- sing conditions (i.e., crystallization temperature, cooling rate, and agitation rate), the crystal habit and the subsequent properties of the crystal network can be tailored. It has been shown that the crystallization temperature affects the packing of the triacylgly- cerol (TAG) molecules and thus influences the microstructure of the crystallized fats. 6,15,16 Marangoni and McGauley (2003) reported that while cocoa butter crystallized at different tem- peratures showed similar polymorphism, the resultant material exhibited dissimilar microstructure. 6 Several research groups have demonstrated the effects of cooling rate on the nucleation, polymorphism, and aggregation of small crystalline particles in fat materials. 5,17À20 They observed that the slow cooling of milk fat results in a small number of large crystals, whereas rapid cool- ing produces numerous small crystallites with a fairly uniform size distribution. The effects of shear on the development of a fat crystal net- work have been widely studied. Several research groups have found that subjecting the melt to shear stress while it crystallizes accelerates the rate of solid-state phase transformations. 21À27 Furthermore, it has been found that the application of shear during crystallization hinders crystal aggregation such that the size of the formed crystallite clusters was reduced. 24,28,29 A few studies also reported that the application of shear during crystal- lization resulted in internal rearrangements of the crystalline lamella. This was confirmed by X-ray diffraction studies. 25,29À32 Although the combined influence of these processing condi- tions on the microstructural properties of the network is rela- tively well understood, nanoscale characteristics have yet to be investigated in full. 33,34 The purpose of the present study is to develop a suitable imag- ing technique for the visualization of the structure of crystallized fat at the meso- and nanoscale levels. Particular attention will be paid to the alignment and distribution of crystal mass within the network in addition to observing the habit of the crystals (size and morphology). These observations can furnish data for Received: January 6, 2011 Revised: April 5, 2011 ABSTRACT: The effects of laminar shear on crystalline orienta- tion and the nanostructure of triglyceride crystal networks were quantified by using different microscopic techniques. Cocoa butter (CB) was crystallized in the presence and absence of an external shear field. Two different dynamic samples were crystal- lized under a shear rate of approximately 340 s À1 by using a continuous Couette-type laminar shear crystallizer and by using a standard paddle mixer. To improve imaging resolution, liquid oil was removed from crystallized samples using isobutanol and aqueous solutions of different surfactants such as AOT, Teepol, and Fatsolve. Using cryogenic scanning electron microscopy (Cryo-SEM), oriented sheets of crystalline cocoa butter were observed in the sample obtained in the laminar shear crystallizer, while spherulitic structures were observed in the statically crystallized sample. The strong influence of the applied laminar shear on the nanoscale structure is demonstrated by characterization of CB platelet size using cryogenic transmission electron microscopy (Cryo-TEM). Shear crystallization caused a reduction in the platelets’ length from 2000 to 300 nm and width from 165 to 130 nm. The platelets’ thickness, obtained from Scherrer analysis of the 002 SAXS reflection, yielded a domain size of 54.8 nm for the specimen crystallized under laminar shear and 58.2 nm for the statically crystallized sample. This work demonstrates the large effects of shear during the crystallization process on the microstructure of polycrystalline materials.