A Conceptual Model for Shear-Induced Phase Behavior in Crystallizing Cocoa Butter

• Published in: Crystal growth & design 2007-07, Vol.7 (7), p.1230-1241
• Main Authors: Mazzanti, Gianfranco, Guthrie, Sarah E, Marangoni, Alejandro G, Idziak, Stefan H. J
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Physics; Single-crystal and powder diffraction; Solid-solid transitions; Specific phase transitions; Structure of solids and liquids; crystallography; X-ray diffraction and scattering
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg050467r

We propose a conceptual model to explain the quantitative data from synchrotron X-ray diffraction experiments on the shear-induced phase behavior of cocoa butter, the main structural component of chocolate. We captured two-dimensional diffraction patterns from cocoa butter at crystallization temperatures of 17.5, 20.0, and 22.5 °C under shear rates from 45 to 1440 s-1 and under static conditions. From the simultaneous analysis of the integrated intensity, correlation length, lamellar thickness, and crystalline orientation, we postulate a conceptual model to provide an explanation for the distribution of phases II, IV, V, and X and the kinetics of the process. As previously proposed in the literature, we assume that the crystallites grow layer upon layer of slightly different composition. The shear rate and temperature applied define these compositions. Simultaneously, the shear and temperature define the crystalline interface area available for secondary nucleation by promoting segregation and affecting the size distribution of the crystallites. The combination of these factors (composition, area, and size distribution) favors dramatically the early onset of phase V under shear and determines the proportions of phases II, IV, V, and X after the transition. The experimental observations, the methodology used, and the proposed explanation are of fundamental and industrial interest, since the structural properties of crystalline networks are determined by their microstructure and polymorphic crystalline state. Different proportions of the phases will thus result in different characteristics of the final material.