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 Å period corresponding to a double-chain length molecular stacking and hexagonal chain packing (2L 1 α). From 20 °C, a major crystalline species also with double-chain lengths of 42.5 Å and orthorhombic chain packing (2L 2 β') was observed. The successive crystallizations of varieties 2L 1 and 2L 2 were correlated with the apparition of two large exothermal events recorded simultaneously by thermal analysis (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 2L 1 α (47.6 Å), 2L 2 β' (42.15 Å) and a new crystalline structure with 4 chain lengths: 4L β' of period 84.5 Å, 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 Å) 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 . However, 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 * Corresponding author ( ): michel.ollivon@cep.u-psud.frArticle published by EDP Sciences and available at http://www.edpsciences.org/lait or http://dx

La matière grasse du lait de dromadaire : composition, microstructure et polymorphisme. Une revue

“…Furthermore, it is of interest to note that a weak XRD bump corresponding to the formation of a new variety with a thickness of 84.5 Å is recorded during cooling of ADMF at 5°C/min. This structure has not been observed on cooling previously neither at the slow cooling of 0.1°C/min [17] nor at that of 1°C/min [24]. Moreover, such crystalline structure 4L have never been met in the anhydrous bovine milk fat when it is cooled at different rates [30,32,36].…”

“…As observed for other fats [29][30][31][32][33][34][35][36][37], X-ray diffraction (XRD) and differential scanning calorimetry (DSC) are the best techniques to characterize, respectively, the structural and thermal properties of ADMF [17,24]. Extensive descriptions of TG polymorphism based on data obtained using these techniques can be found in the literature [38,39].…”

“…The occurrence of the different crystalline forms strongly depends on the thermal history of the TG. Recently, Karray et al [17,24] by using coupling of DSC and XRDT, characterized the specific thermal and structural properties of ADMF. These authors examined the crystalline structures formed by TG of ADMF at slow (0.1°C/min), intermediate (1°C/min) and fast (5°C/min) cooling rates and their evolution as a function of temperature during subsequent heating at 1°C/min.…”

“…Moreover, from the absence of any new line at small and wide angles during heating there is no evidence of any b' → b transition. Karray et al [24] showed that cooling of ADMF in fast conditions (5°C/min), or at intermediate cooling rate (1°C/min), first leads to the crystallization of a form with the existence of longitudinal stacking of the TG molecules corresponding to double chain length structures (47.6 Å and 46.9 Å, respectively, likely the same structure with slightly different TG compositions) ( figure 7A). This lateral organization of FA chains in hexagonal cell has not been observed on slow cooling at 0.1°C /min that directly generates the stable b' form 2L (42.15 Å) ( figure 7A).…”

“…The lipid content of lipid dromedary milk were the subject of many studies [6,15,[18][19][20][21][22][23]. Recently, Karray et al [17,24] studied physical properties of dromedary milk fat. The aim of this paper is to examine the present state of knowledge on the composition, the microstructure and essentially the physical properties of dromedary milk fat.…”