Relationship between Crystallization Behavior and Structure in Cocoa Butter Alejandro G. Marangoni* and Sara E. McGauley Department of Food Science, University of Guelph, Guelph, Ontario N1G2W1, Canada Received September 9, 2002; Revised Manuscript Received November 5, 2002 ABSTRACT: Cocoa butter was crystallized statically from the melt to various temperatures in the range of -20 to 26 °C and annealed for up to 45 days. During this period, the polymorphism of the solid state was monitored using differential scanning calorimetry and powder X-ray diffraction. Moreover, the microstructure of the materials was imaged using polarized light microscopy. Below -15 °C, a mixture of the transient metastable γ and R phases was observed. Between -15 and 20 °C, the material nucleated initially into an R form and then gradually transformed into more stable phases. The lifetime of the R phase was at least 7 days at and below 0 °C and decreased gradually above 0 °C to 30 min at 15 and 20 °C. The R phase transformed into the ′ phase, which was stable for 28 days between 0 and 15 °C. Above 15 °C, the lifetime of the ′ phase decreased gradually to 10 h at 24 °C. The ′ form could be formed directly from the melt above 20 °C. Above 15 °C, the ′ phase could transform into the  phase. Interestingly, cocoa butter crystals did not nucleate directly from the melt into the  phase and did not form under any conditions below 20 °C. The  polymorph could only be formed via the ′ form. Microstructural studies indicated that cocoa butter initially nucleated as metastable γ or R phases below 15 °C remained granular in appearance, ir- respective of further phase transformations into other more stable forms. The microstructures of the ′ form could thus appear granular, clustered, and needlelike, depending on whether they were formed through the R form or di- rectly from the melt. The microstructure of the  form was complex and varied, from granular to needlelike to featherlike. Crystallization kinetics was quantified from solid fat content-time curves at the different crystallization temperatures using the Avrami model. Changes in the Avrami exponent and the induction time of crystallization were correlated with certain polymorphic transformations, particularly the ′ to  transition. Microstructure was quantified using a box-counting fractal dimension. Changes in microstructure as a function of time at 20, 22, and 26 °C correlated with changes in the fractal dimension. A particularly interesting finding in this work was the fact that the fractal dimension was directly related to the rate of nucleation as well as inversely related to the Avrami exponent. Introduction Triglycerides are known to crystallize in a number of different polymorphic forms depending on processing conditions (heat, mass, and momentum transfer) and chemical composition. 1,2 Cocoa butter, the main struc- turing material in chocolate and confections, displays complex crystallization behavior in that six different polymorphs have been identified. These solid phases have characteristic melting ranges (Table 1) and wide angle X-ray reflections (Table 2). In a few cases, distinct microstructures have been associated with particular polymorphic forms. Essential to the manufacturing of good quality chocolate is the ability to induce the formation and stabilization of certain polymorphic forms in cocoa butter. This in turn will lead to the formation of a fat crystal network with desirable mechanical and thermal properties. 3 Modern studies on cocoa butter polymorphism have included real-time powder X-ray diffraction (XRD). 4-10 Results from these studies suggest that the meta- stable γ (orthorhombic subcell) and R (hexagonal sub- cell) phases, as well as the more stable ′ (orthorhom- bic subcell) phase, can crystallize directly from the melt, whereas the stable  (triclinic) polymorph can only be obtained via phase transformation from the ′ form. Moreover, the different solid phases display wide melting ranges rather than unique melting points. Of particular importance is the static isothermal phase transition scheme constructed by van Malssen et al. 8 using real-time powder XRD. This state diagram cata- logues the polymorphism of the solid state in cocoa butter at different time-temperature combinations. The study of microstructure in fats has become in- creasingly important since many macroscopic properties of fats and fat-containing products depend on their fat Table 1. Melting Ranges of the Different Polymorphic Forms Found in Cocoa Butter (as Reported by van Malssen et al. 8 ) polymorphic form (I-IV) M.R. (°C) polymorphic form (V-VI) M.R. (°C) γ (sub-R) -5 to +5 2 29-34 R 17-22 1 2′ 20-27 1′ Table 2. Characteristic Powder XRD Wide Angle Reflections (Short Spacings) for the Various Polymorphic Forms of Cocoa Butter 27 a polymorphic form short spacings (Å) γ (sub-R) 3.87(m), 4.17(s) R 4.20(vs)  2′ 3.87(vw), 4.20(vs) 1′ 3.75(m), 3.88(w), 4.13(s), 4.32(s) 2 3.65(s), 3.73(s), 3.87(w), 3.98(s), 4.22(w), 4.58(vs), 5.13(w), 5.38(m) 1 3.67(s), 3.84(m), 4.01(w), 4.21(vw), 4.53(vs), 5.09(vw), 5.37(m) a The relative intesity is noted as very strong (vs), strong (s), medium (m), weak (w), or very weak (vw). CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 1 95 - 108 10.1021/cg025580l CCC: $25.00 © 2003 American Chemical Society Published on Web 11/23/2002