| Abstrakt: |
The thermal and structural behavior of anhydrous dromedary milk fat (ADMF) 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). As the camel probably represents the mammalian species best adapted to the hottest temperatures on earth as well as to harsh climatic conditions, the fatty acid composition of ADMF and its thermal properties are of special interest. In this paper, the first of a series, the crystalline organizations made by the triacylglycerols (TG) of ADMF during a slow cooling rate at |dT/dt| = 0.1 ?C?min?1are addressed. The crystalline form obtained from about 29.5 ?C corresponds to a lamellar structure with a double-chain length longitudinal organization of the TG molecules (2L = 42.3 ?) associated with a ?? lateral packing of the chains. The crystallization range which spreads over about 75?80??C, is similar to that observed for bovine milk fat, although the final melting temperature is higher by about 5?8 ?C. DSC scans revealed the existence of up to 6 exotherms on slow cooling. A similar complex behavior was also observed on heating. PEAKFIT analysis of the XRDT individual patterns and their evolutions observed at small angles also showed the existence of additional molecular packings. However, XRDT analysis was unable to resolve individual structures. A possible epitaxy phenomenon during crystallization is discussed. Propri?t?s thermiques et structurales de la mati?re grasse de lait de dromadaire. 1. Formes cristallines obtenues lors d?un refroidissement lent. Les propri?t?s thermiques et structurales de la mati?re grasse anhydre extraite du lait de dromadaire (ADMF) ont ?t? ?tudi?es par une nouvelle technique qui permet des mesures simultan?es, sur un m?me ?chantillon, des signaux de microcalorim?trie diff?rentielle ? balayage (DSC) et de diffraction des rayons X en fonction du temps et de la temp?rature. L??tude des propri?t?s thermiques de la mati?re grasse du lait de chamelle, en relation avec sa composition, rev?t un int?r?t particulier si l?on consid?re les conditions extr?mes de temp?rature auxquelles ce mammif?re est adapt?. Cet article, qui est le premier d?une s?rie, est consacr? ? l??tude des organisations cristallines form?es par les triacylglyc?rols (TG) de l?ADMF lors d?un refroidissement ? 0,1 ?C?min?1. Les formes cristallines obtenues ? partir de 29,5??C correspondent ? une structure lamellaire ? deux longueurs de cha?nes (2L = 42,3 ?) et ? un empilement lat?ral des cha?nes d?acides gras de type ??. La plage de cristallisation couvre un domaine d?environ 75?80 ?C, similaire ? celui observ? pour la mati?re grasse laiti?re bovine, bien que la temp?rature finale de fusion soit plus ?lev?e pour le dromadaire d?environ 5 ? 8 ?C. Les enregistrements de DSC observ?s ? la cristallisation comme ? la fusion r?v?lent l?existence d?environ 6??v?nements thermiques qui contrastent avec l?apparente simplicit? du comportement structural. Une analyse fine des pics de diffraction des rayons X et de leurs ?volutions a aussi montr? l?existence d?autres empilements longitudinaux mais sans pouvoir diff?rencier ces structures de mani?re univoque. Une croissance cristalline de type ?pitaxie est propos?e pour expliquer ce comportement structural. |
