Dromedary milk fat: thermal and structural properties 2. Influence of cooling rate

Abstract

The crystallization behavior of anhydrous dromedary milk fat (ADMF) was examined using Microcalix (Centre National de la Recherche Scientifique, Châtenay-Malabry, France), an instrument coupling time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) at both small and wide angles and high-sensitivity differential scanning calorimetry (DSC) at an intermediate (1 °C·min-1) and at a fast (5 °C·min-1) cooling rate from 60 to -20 °C, together with their subsequent melting at 1 °C·min-1. Upon cooling at 1 °C·min-1, the crystallization of ADMF started at about 24 °C with the formation of a lamellar structure with a 46.9 A period corre- sponding to a double-chain length molecular stacking and hexagonal chain packing (2L1 α). From 20 °C, a major crystalline species also with double-chain lengths of 42.5 A and orthorhombic chain packing (2L2 β') was observed. The successive crystallizations of varieties 2L1 and 2L2 were cor- related with the apparition of two large exothermal events recorded simultaneously by thermal ana- lysis (DSC). However, comparison of XRDT and DSC recordings revealed the presence of a third exothermal peak probably related to a transition α→β '. At fast cooling (5 °C·min-1) of ADMF, three crystalline varieties were formed: the 2L1 α (47.6 A), 2L2 β' (42.15 A) and a new crystalline structure with 4 chain lengths: 4L β' of period 84.5 A, while three endotherms were recorded by DSC. This last crystalline form which had never been characterized until now, likely corresponds to the accumulation of defects in the stable 2L β' form (42.15 A) resulting from fast cooling and then to a local lack of symmetry in the 4L cell along the c axis of the crystals. This lack of symmetry was released during a transition 4L → 2L observed upon heating. A polymorphic transition α→β ' was always observed upon ADMF heating following cooling at either 5 or 1 ° C·min-1. Compared with the slow cooling at 0.1 °C·min-1 which was studied in the previous paper of this series, ADMF presented different structural and thermal properties. Indeed, the cooling at 1 °C·min -1 and 5 °C·min -1 generated an unstable crystalline form (α) that was not found at slow cooling at 0.1 °C·min-1. How- ever, the major stable form 2L β' was always observed in the three types of cooling. polymorphism / X-ray diffraction / differential scanning calorimetry / camel's milk