Evidence of critical cooling rates in the nonisothermal crystallization of triacylglycerols: a case for the existence and selection of growth modes of a lipid crystal network.
The isoconversional method, a model-free analysis of the kinetics of liquid-solid transformations, was used to determine the effective activation energy of the nonisothermal crystallization of melts of pure and complex systems of triacylglycerols (TAGs). The method was applied to data from differential scanning calorimetry (DSC) measurements of the heat of crystallization of purified 1,3-dilauroyl-2-stearoyl-sn-glycerol (LSL) and commercially available cocoa butter melts. The method conclusively demonstrated the existence of specific growth modes and critical rates of cooling at specific degrees of conversion. The existence of critical rates suggests that the crystallization mechanism is composed of growth modes that can be effectively treated as mutually exclusive, each being predominant for one range of cooling rates and extent of conversion. Importantly, the data suggests that knowledge of the critical cooling rates at specific rates of conversion can be exploited to select preferred growth modes for lipid networks, with concomitant benefits of structural organization and resultant physical functionality. Differences in transport phenomena induced by different cooling rates suggest the existence of thresholds for particular growth mechanisms and help to explain the overall complexity of lipid crystallization. The results of this model-free analysis may be attributed to the relative importance of nucleation and growth at different stages of crystallization. A mechanistic explanation based on the competing effects of the thermodynamic driving force and limiting heat and transport phenomena is provided to explain the observed behavior. This work, furthermore, offers satisfactory explanations for the noted effect of cooling-rate-induced changes in the physical functionality of lipid networks.
Trent University Biomaterials Research Program, Department of Physics and Astronomy, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada.
Laziz
L
Bouzidi
Suresh S
SS
Narine
D002152:Calorimetry, Differential Scanning; D003460:Crystallization; D014280:Triglycerides / Q000737:chemistry*
C552880:1,3-dilauroyl-2-stearoylglycerol; D014280:Triglycerides