Thermal and Structural Behavior of Milk Fat. 1. Unstable Species of Anhydrous Milk Fat

Abstract

The thermal and structural behavior of anhydrous milk fat (AMF) was studied by a technique that allowed simultaneous time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) and high sensitivity differential scanning calorimetry (DSC) to be carried out in the same apparatus from the same sample. In this paper (the first of a series), the less stable crystalline structures made by triacylglycerols (TG) of bulk AMF after its liquid quenching down to -8 degrees C are addressed The coexistence of two lamellar structures characterized by sharp long spacing reflections corresponding to well-defined 3L (70 A) and 2L (47 A) longitudinal stackings but broad short spacing lines related to poorly ordered hexagonal (alpha) lateral packing is shown for the first time, The bilayered structure was very unstable, since it disappeared during a 20-min isothermal recording. Simultaneous DSC and X-ray monitoring of AMF heating in the range -8, +50 degrees C at a rate of 2 degrees C/min allows the same sample to be followed on the evolution of these unstable forms to more stable varieties. The 3L stacking transforms into a new 2L crystalline structure characterized by broad LS reflections corresponding to a ill-defined 2L (37 A) longitudinal stacking but a more compact orthorhombic (beta') lateral packing. A delimitation of the domains of existence of the crystalline structures resulted from the comparison of detailed analysis of the evolutions of positions, intensities, and widths of X-ray peaks as a function of temperature to microcalorimetry recording.

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

“…Experiments were conducted using Microcalix (Centre National de la Recherche Scientifique, Châtenay-Malabry, France), an apparatus allowing simultaneous timeresolved synchrotron X-ray diffraction as a function of temperature (XRDT) and high sensitivity differential scanning calorimetry (DSC) to be carried out on the same sample, as reported previously [4,9,15,19,23]. The recording of XRDT patterns and the calibration of the detectors and the calorimeter were conducted as previously reported [4,9,10,19,23].…”

“…The recording of XRDT patterns and the calibration of the detectors and the calorimeter were conducted as previously reported [4,9,10,19,23]. The analysis of the XRDT patterns was performed as previously described [9,14].…”

“…ADMF is an essential nutriment in desert areas with a complex fatty acid composition [2,5,8], which has already been discussed in the first article of this series [9]. As observed for other fats and in the previous study [9], the coupling of X-ray diffraction (XRD) as a function of temperature (XRDT) and differential scanning calorimetry (DSC) is the best technique to characterize the structural and thermal behavior of ADMF [14][15][16][17][18][19][20][21]25]. XRD permits the study of the various lamellar structures formed by triacylglycerols (TG).…”

“…XRD permits the study of the various lamellar structures formed by triacylglycerols (TG). The longitudinal stacking of the TG molecules and the lateral packing of their acylglycerol chains are recorded at small and wide angles, respectively, as detailed in [14]. DSC permits characterization of the thermal properties of complex fat which depend on the thermal treatments applied to fat [6,27,28].…”

Dromedary milk fat: thermal and structural properties 1. Crystalline forms obtained by slow cooling

“…Understanding of the functionality of ADMF in food products necessitates the knowledge of its physical properties. As observed for other fats, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) are the best techniques to characterize, respectively, the structural and thermal properties of ADMF [18][19][20][21][22][23][24][25]32]. Extensive descriptions of TG polymorphism based on data obtained using these techniques can be found in the literature [9,29].…”