α-Melt Structure of 1,3-Dipalmitoyl-2-oleoyl-sn-glycerol (POP) under a Thermal Thawing Process Studied by Infrared Spectroscopy.

The temperature thawing, as called tempering, of triacylglycerols (TAGs) is an important processing method in food productions, such as chocolates, cream, confections, and spreads. Especially, melt-mediation by temperature thawing is famous in chocolate production for controlling the polymorphic crystalline forms and accelerating crystallization. In the present study, we investigated the α-melt structure of 1,3-dipalmitoyl-2-oleoyl-sn-glycerol (POP), one of the major continuants of cacao butter, under a phase transition from its melt to γ-crystal with in-situ attenuated total reflection-infrared (ATR-IR) spectroscopy. The differential IR spectrum between α-melt via temperature thawing (α-melt mediation) and melt via simple cooling revealed that crystal-like local ordered structures remained in part in the α-melt, and that they acted as nuclei for a rapid phase transition to the γ-crystal. The changes to the γ-crystal occur in the local ordered structures at first from the glycerol moiety to the acyl chains in the crystallization, providing an important suggestion concerning the mechanism for the acceleration of crystallization to the γ-form via α-melt mediation.