Relationship between Crystallization Behavior and Structure in Cocoa Butter

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 α phases was observed. Between −15 and 20 °C, the material nucleated initially into an α form and then gradually transformed into more stable phases. The lifetime of the α 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 α phase transformed into the β‘ phase, which was stable for 28 days between 0 and 15 °C. Above 15 °C, the lifetime of the β‘ phase decreased gradually to 10 h at 24 °C. The β‘ form could be formed directly from the melt above 20 °C. Above 15 °C, the β‘ phase could transform into the β phase. Interestingly, cocoa butter crystals did not nucleate directly from the melt into the β phase and did not form under any conditions below 20 °C. The β polymorph could only be formed via the β‘ form. Microstructural studies indicated that cocoa butter initially nucleated as metastable γ or α phases below 15 °C remained granular in appearance, irrespective of further phase transformations into other more stable forms. The microstructures of the β‘ form could thus appear granular, clustered, and needlelike, depending on whether they were formed through the α form or directly from the melt. The microstructure of the β 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 β‘ to β 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.