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

Abstract

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 β′ type at about 24°C. For temperature < 13°C, triacylglycerols crystallize in an hexagonal subcell of α type, leading to the coexistence of the β′ + α 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.