Anhydrous Goat's Milk Fat: Thermal and Structural Behaviors Studied by Coupled Differential Scanning Calorimetry and X-ray Diffraction. 2. Influence of Cooling Rate

摘要

Crystallization and melting properties of triacylglycerols (TGs) in anhydrous goat's milk fat (AGMF) are investigated by X-ray diffraction as a function of temperature (XRDT) coupled with high-sensitivity differential scanning calorimetry (DSC), using synchrotron radiation and Microcalix. The polymorphic behavior of AGMF was monitored by varying the cooling rates between 5 and 1 °C/min from 45 to -20 °C with their subsequent melting at 1 °C/min. Quenching of AGMF at -20 °C was also examined to determine the metastable polymorphic form of AGMF. At intermediate cooling rates, TGs in AGMF crystallize, from about 18 °C in two different lamellar structures with triple chain length 3L alpha stacking of 72 A and a double chain length 2L alpha stacking of 48 A, which are correlated to two overlapped exothermic peaks recorded by DSC. A reversible transition sub alpha reversible alpha was observed. Subsequent heating at 1 °C/min shows numerous structural rearrangements before final melting. At fast cooling of AGMF (5 °C/min), similar unstable crystalline varieties are formed while three endotherms are recorded. Several new unstable lamellar structures are observed after quenching. All of these data are compared to those previously reported at slow cooling (0.1 °C/min) showing a relative stability of the structures formed. In spite of general similitude, the thermal and structural behavior of the goat's milk is more complex than that of the cow's milk.