Thermal and Structural Behavior of Anhydrous Milk Fat. 2. Crystalline Forms Obtained by Slow Cooling

¹ Equipe Physico-Chimie des Systèmes Polyphasés, UMR 8612 du CNRS, 5 rue J. B. Clément, 92296 Châtenay-Malabry, France and Arilait Recherches, 42 rue de Châteaudun, 75314 Paris cedex 09, France
² Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Bât. 209D, Université Paris-Sud, 91898 Orsay, France
³ Equipe Physico-Chimie des Systèmes Polyphasés, UMR 8612 du CNRS, 5 rue J. B. Clément, 92296 Châtenay-Malabry, France

The crystallization behavior of milk fat has been examined on slow cooling at 0.1°C/min from 50 to –15°C, to determine the variations of triacylglycerol organizations as a function of temperature. The experiments have been conducted with an instrument allowing coupled X-ray diffraction (XRD) at both small and wide angles and high-sensitivity differential scanning calorimetry (DSC) recordings from the same sample by taking advantage of the high-energy flux of a synchrotron. On slow cooling, milk fat triacylglycerols sequentially crystallize in four different lamellar structures with double-chain length of 41.5, 48.3, and 39.2 Å and a triple-chain length of 62.2 Å stackings. Simultaneous wide-angle XRD has shown that initial nucleation occurs in a packing of ß' type at about 24°C. For temperature < 13°C, triacylglycerols crystallize in an hexagonal subcell of type, leading to the coexistence of the ß' + polymorphic forms, which is recorded until –15°C. Thermal analysis allowed to correlate the formation of the different crystalline species monitored by XRDT (XRD as a function of temperature) to the exothermal events recorded simultaneously by differential scanning calorimetry. The evolution of the species formed during crystallization was also monitored on heating at 2°C/min. The absence of polymorphic evolution on heating, as well as the high final melting point observed, about 40 to 41°C, confirmed that cooling at 0.1°C/min leads to quasi equilibrium.

Key Words: triacylglycerols • polymorphism • differential scanning calorimetry • X-ray diffraction

Submitted on March 20, 2001
Accepted on June 6, 2001

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