Thermal and structural behavior of anhydrous milk fat. 3. Influence of cooling rate.
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 degrees C/ min from 60 to -10 degrees 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 A and a triple-chain length of 72 A stackings of alpha type, which are correlated to 2 exothermic peaks at 17.2 and 13.7 degrees C, respectively. A time-dependent slow sub-alpha <--> alpha reversible transition is observed at -10 degrees C. Subsequent heating at 2 degrees C/min has shown numerous structural rearrangements of the alpha varieties into a single beta' form before final melting. This polymorphic evolution on heating, as well as the final melting point observed (approximately 39 degrees C), confirmed that cooling at 3 degrees 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.
Lopez
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Equipe Physico-Chimie des Systèmes Polyphasés, UMR 8612 du CNRS, 5 rue J.B. Clément 92296, Châtenay-Malabry, France. Christelle.Lopez@rennes.inra.fr
Lesieur
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P
Bourgaux
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Ollivon
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M
D000818:Animals; D002152:Calorimetry, Differential Scanning; D003080:Cold Temperature; D003460:Crystallization; D006358:Hot Temperature; D007700:Kinetics; D008055:Lipids / Q000737:chemistry*; D008859:Microscopy, Polarization; D008892:Milk / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
D008055:Lipids