Systematic investigation of lard polymorphism using combined DSC and time-resolved synchrotron X-ray diffraction

The polymorphic behavior of lard was systematically investigated by differential scanning calorimetry (DSC) while simultaneously monitoring the formation of the different crystal forms with X-ray diffraction (XRDT). To interpret the complex polymorphic evolution of the sample analyzed by regular DSC, both XRD patterns and DSC curves were recorded at the same time from the same sample (20 mg) using a laboratory-made calorimeter (http://www.umr-cnrs8612.u-psud.fr/Francais/pdf/MICROCALIX.pdf) installed at a synchrotron radiation bench capable of both small- and wide-angle X-ray diffraction and time-resolved experiments. Lard exhibits a large melting range (-30 degrees C <= T <= 50 degrees C) with three main crystallization peaks. In this range, lard polymorphism was investigated by varying the cooling and heating rates between 10 and 0.15 K/min, and by isothermal recrystallization at -10 and 15 C. XRD lines observed by SAXS at about 35, 43.8 and 48.2 angstrom are identified to subcells alpha, beta'(1), beta'(2) and beta, and attributed to the main DSC peaks. The crystalline forms appearing during the cooling depend on the cooling rate and on the final temperature. Higher cooling rates lead to unstable a forms that transform subsequently upon heating and/or storage into beta' forms. The persistence of a monotropic transformation at -10 degrees C that was monitored during isothermal storage is explained by the presence of a liquid phase at this temperature.