Thermal and structural behaviour of crude palm oil: Crystallisation at very slow cooling rate

• Published in: European journal of lipid science and technology 2007-04, Vol.109 (4), p.410-421
• Main Authors: Chong, Chiew Let, Kamarudin, Zukarinah, Lesieur, Pierre, Marangoni, Alejandro, Bourgaux, Claudie, Ollivon, Michel
• Format: Article
• Language: English
• Subjects: differential scanning calorimetry; Palm oil; polymorphism; triacylglycerols; X-ray diffraction
• ISSN: 1438-7697; 1438-9312
• DOI: 10.1002/ejlt.200600249

The crystallisation behaviour of crude palm oil (CPO) has been examined with a new instrument coupling time-resolved synchrotron X-ray diffraction with high-sensitivity differential scanning calorimetry at cooling rates of 0.1 and 0.4 °C/min from melt temperature to -20 °C, in order to determine the triacylglycerol (TAG) organisations formed as a function of temperature and thermal treatment. At slow cooling rate, CPO TAG sequentially crystallise in apparently two different lamellar structures with double-chain length of 41.9 Å and a triple-chain length of 62.8Å stackings of β' type, which are correlated to two main exothermic peaks at 26 and 8 °C, respectively. While a time-dependent slow β'[rightward arrow]β irreversible transition is observed even at low temperature, it only affects a few percent of the TAG population. Subsequent heating at 1 °C/min showed no structural rearrangements of the crystalline forms observed before final melting, indicating that the main moiety of the system is close to equilibrium. Crystallisation at 0.4 °C/min has shown the formation of many additional structures, some of which are evanescent. An unexpected finding was the vicinity of the two lines corresponding to the two 2L (bilayered) structures and the isosbestic point located at 4.42 Å. Only careful analysis allowed the separation of both lines, the coexistence of which was related to the epitaxy/eutectic phenomenon, as evidenced by the simultaneous appearance of the 3L (trilayered) stackings at the same time of the second burst of the 2L stacking and the observation of the isosbestic point. The coexistence of β and β' forms even at high temperature demonstrates that the whole system is not in a two-phase domain but rather in a three-phase one (β + β' + liquid) in which all TAG molecules could not fit into a single β' structure. This is likely related with the granular phase separations observed in margarine.