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Raman spectroscopy of the polymorphic forms and liquid state of cocoa butter



Raman spectroscopy was used to characterize the polymorphs and liquid state of cocoa butter, with emphasis placed on the evolution of the ester carbonyl stretching region (1800–1700 cm−1), along with complementary analysis and comparison of the Raman-active C[BOND]C (1200–1000 cm−1), C[DOUBLE BOND]C (1660 cm−1), C[BOND]H (3000–2700 cm−1) and CH2 (1500–1250 cm−1) vibrational modes. Unique Raman signatures were obtained for all cocoa butter polymorphs, with their identity confirmed using DSC and XRD. The ester carbonyl region permitted polymorph discrimination due to differences in the number of modes, their relative frequencies and their full-widths at half-maximum. The C[BOND]C stretching modes, which provided insight into the trans/gauche content, were polymorph-independent. C[BOND]H stretching generally increased with polymorph stability, indicating the dominance of antisymmetric C[BOND]H methylene vibrations as the cocoa butter crystal lattice became more ordered. The change in the intensities of the C[BOND]H stretching bands used to probe the order–disorder transition of forms IV, V and VI hinted at pre-melt structural changes mostly in forms IV and V. Overall, Raman spectroscopy clearly demonstrated that the different functional groups studied could be characterized independently, allowing for the understanding of their role in cocoa butter polymorphism.

Practical applications: Fat bloom is the unwanted, uncontrolled re-crystallization or polymorphic transition of CB form V crystals into form VI normally caused by the migration of lower-melting fats (e.g. in centre-filled products) and/or temperature fluctuations during storage. In its mildest form, it appears as an overall dulling of the chocolate surface. In its extreme form, the appearance of the chocolate deteriorates significantly with the development of distinct white patches. Though forms V and VI can be clearly distinguished via XRD, we present evidence that Raman spectral characterization of the ester carbonyl stretching (1800–1700 cm−1), C[BOND]C (1200–1000 cm−1), C[DOUBLE BOND]C (1660 cm−1), C[BOND]H (3000–2700 cm−1) and CH2 (1500–1250 cm−1) vibrational modes yields distinct liquid–solid and polymorph-dependent differences in CB. From a practical standpoint, the unique signatures associated with forms V and VI offer novel possibilities in the study of fat bloom formation, such as the development of predictive tools.