Thermal and structural behavior of anhydrous milk fat. 2. Crystalline forms obtained by slow cooling.

The crystallization behavior of milk fat has been examined on slow cooling at 0.1 degrees C/min from 50 to -15 degrees 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 A and a triple-chain length of 62.2 A stackings. Simultaneous wide-angle XRD has shown that initial nucleation occurs in a packing of beta' type at about 24 degrees C. For temperature < 13 degrees C, triacylglycerols crystallize in an hexagonal subcell of alpha type, leading to the coexistence of the beta' + alpha polymorphic forms, which is recorded until -15 degrees 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 degrees C/min. The absence of polymorphic evolution on heating, as well as the high final melting point observed, about 40 to 41 degrees C, confirmed that cooling at 0.1 degrees C/min leads to quasi equilibrium.