Structure evolution and bloom formation in tempered cocoa butter during long-term storage

摘要

Structural evolution in tempered cocoa butter (CB) and CB mixed with a cocoa butter equivalent (CBE) was examined during 26 wk of storage (at 25 deg C) using atomic force microscopy,X-ray powder diffraction,colorimetry and pulsed nuclear magnetic resonance.The form V-to-VI polymorphic transition in CB started after 1 week of storage.However,fat bloom was not detected until week 3 when large crystals started to appear on the CB surface.Changes in surface topography coincided with an increase in the surface whiteness index.Addition of CBE delayed bloom development by 1-2 wk.The solid fat content (SFC) of both CB and CB + CBE increased gradually during the early wk of storage before reaching a summit and then decreasing slowly with time (at 25 deg C).Concurrently,the surface roughened and the whiteness index increased for both CB and CB + CBE.We postulate that,upon bloom formation,parallel phenomena took place: (i) There was exclusion of triglyceride molecules from the CB and CB + CBE fat crystal networks due to continued contraction,and (ii) less stable crystals melted due to the heat release from the (re)crystallization of liquid fat onto existing surface crystals and from the ongoing form V->VI polymorphic transition.These events resulted in the gradual decrease in SFC seen at longer storage times.In conclusion,this study demonstrated that kinetic and thermodynamic phenomena take place in CB long after it has been tempered.