Observation on structure in the surface region of cocoa butter, POP, SOS and POS by X-ray diffraction

Abstract

Cocoa butter, prepared from ground roasted cacao beans, has six polymorphisms characterized by different melting points in thermal analysis. It is known to consist of several oils and fats, e.g., POP (sn-1,3-dipalmitoyl-2-oleoylglycerol), SOS (sn-1,3-dialmitoyl2-stearoyl-glycerol) and POS (1,3-rac-palmitoylstearoyl-2oleoylglycerol), etc. Since there is some resemblance in molecular structure among them, POP, SOS and POS exhibit six or five polymorphisms like cocoa butter. They are also known to acquire stable structures throughout phase transformation via quasi-stable polymorphisms. In the present study, we observe structures in surface region to a depth of 10nm from the surface and those of thin films of polymorphisms of POP, SOS, POS and natural cocoa butter by surface-sensitive X-ray diffraction techniques. The aim of our research is to clarify the peculiar molecular interactions and structures emerging only in the surface and thin films, which would also serve as basic information on melting and oxidation of chocolate. X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) were exploited with high precision diffract meters on rotating anode X-ray generators (SLX2000+UltraX, TTR-450, Rigaku Co.). Thin films on Si (100) were prepared by spin-coating method with acetone as a solvent. For cocoa butter, uniform layers were easily formed by annealing, yet dewetted layers are obtained for some oils even after the annealing. A strong surface-induced preferred orientation is shown for all the samples, indicating anisotropy in intermolecular interaction. Furthermore, a distinct transformation from a double-layer structure to a single-layer structure is observed. We consider that those structures might correspond to smectic phases of liquid crystals.