Crystallisation and melting properties of dromedary milk fat globules studied by X-ray diffraction and differential scanning calorimetry. Comparison with anhydrous dromedary milk fat

Crystallisation and melting properties of triacylglycerols (TG) in dromedary milk fat (DMF) globules are studied by X-ray diffraction (XRD) as a function of temperature coupled with high-sensitivity differential scanning calorimetry (DSC), using synchrotron radiation. The crystallisation properties studied on cooling at |dT/dt| = 1°C.min-1 from 60°C to -8°C show the successive formation of two double-chain length (2L) lamellar structures: α 2L1 (46.7 Å) from 22°C and β′ 2L2 (41.7 Å) from 9°C, which coexist until the end of the cooling process. These two types of crystals are related to the two exothermal peaks recorded simultaneously by DSC. The melting properties are studied on subsequent heating of DMF globules at dT/dt = 1°C.min -1. The detailed analysis of the intensity and position of the XRD peaks recorded as a function of time shows that (i) the α 2L1 structure melts first, (ii) some TG from 2L1 melt and are incorporated again in the 2L2 structure, and (iii) the 2L2 structure melts with a final melting temperature over 44°C. The influence of the dispersion of TG molecules in individual fat globules, with the presence of an interface with water, was determined by comparing their crystallisation behaviour to that of the same TG extracted from cream (anhydrous DMF) under the same experimental conditions. Results show that the same lamellar structures, α 2L1 and then β′ 2L2, are formed on cooling in the dispersed and bulk states. However, crystallisation in the unstable α form is favoured in fat globules. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA.