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1 Equipe Physico-Chimie des Systèmes Polyphasés, UMR 8612 du CNRS, 5 rue J.B. Clément 92296, Châtenay-Malabry, France
2 Laboratoire pour l’Utilisation du Rayonnement Electromagnétique, Bât. 209D Université Paris-sud, 91898 Orsay, France
3 Physico-Chimie des Colloïdes, Université Henri Poincaré, UMR7565 BP 239, 54506 Vandoeuvre-lès-Nancy, France
Corresponding author: C. Lopez; e-mail: Christelle.Lopez{at}rennes.inra.fr.
The crystallization behavior of anhydrous milk fat has been examined with a new 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. Crystallizations were monitored at cooling rates of 3 and 1°C/ min from 60 to –10°C to determine the triacylglycerol organizations formed. Simultaneous thermal analysis permitted the correlation of the formation/melting of the different crystalline species monitored by XRDT to the thermal events recorded by differential scanning calorimetry. At intermediate cooling rates, milk fat triacylglycerols sequentially crystallize in 3 different lamellar structures with double-chain length of 46 and 38.5 Å and a triple-chain length of 72 Å stackings of 
type, which are correlated to 2 exothermic peaks at 17.2 and 13.7°C, respectively. A time-dependent slow sub- 
reversible transition is observed at –10°C. Subsequent heating at 2°C/min has shown numerous structural rearrangements of the varieties into a single ß' form before final melting. This polymorphic evolution on heating, as well as the final melting point observed (~39°C), confirmed that cooling at 3°C/min leads to the formation of crystalline varieties that are not at equilibrium. An overall comparison of the thermal and structural properties of the crystalline species formed as a function of the cooling rate and stabilization time is presented. The influence on crystal size of the cooling rates applied in situ using temperature-controlled polarized microscopy is also determined for comparison.
Key Words: triacylglycerol • polymorphism • differential scanning calorimetry • x-ray diffraction
Abbreviation key: AMF = anhydrous milk fat, DSC = differential scanning calorimetry, 2L = bi-layered stacking, 3L = tri-layered stacking, SAXD = small-angle XRD, T = temperature, TG = triacylglycerol, WAXD = wide-angle XRD, XRD = x-ray diffraction, XRDT = XRD as a function of T
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