Relationships between molecular structure and kinetic and thermodynamic controls in lipid systems. Part I: propensity for oil loss of saturated triacylglycerols.

Pure saturated triacylglycerols (TAGs) in canola oil were used as model systems to analyse oil loss in structured oil both from thermodynamic and kinetic perspectives. Two important parameters which effectively and predictively measure the relative propensity of a solid network to lose/hold oil were defined: (1) the rate of oil loss, K, which is a quantified representation of the kinetics of oil loss and (2) the initial amount of oil susceptible to be lost, i.e., the propensity for oil loss (POL), which is a representation of the thermodynamics of oil binding. It was found that the POL and K values do not always trend in the same fashion, suggesting that the mechanism of oil binding is complex, depending on the structurant's crystalline form locked within the oil network. The two parameters were, however, correlated to the melting and thermal behavior of the structurants, to the polymorphic structures that are obtained during the cooling process and to the habit (shape, size and morphology) of the crystalline phase in the oil. Both POL and K had a strong correlation to the oil loss.

Lubricating and waxy esters, I. Synthesis, crystallization, and melt behavior of linear monoesters.

Four pure jojoba wax-like esters (JLEs), having carbon chain length of 36, 40 (two isomers) and 44, were prepared by Steglish esterification of fatty acids (or acid chlorides) with fatty alcohols at room temperature. Calorimetric and diffraction data was used to elucidate the phase behavior of the esters. The primary thermal parameters (crystallization and melting temperatures) obtained from the DSC of the symmetrical molecules correspond well with the carbon numbers of the JLEs. However, the data also suggests that carbon number is not the only factor since the symmetry of the molecule also plays a significant role in the phase behavior. Overall, the JLEs show very little polymorphic activity at the experimental conditions used, suggesting that they are likely to transform the same way during melting as well as crystallization, a characteristic which may be useful in designing new waxes and lubricants. The XRD data clearly show that the solid phase in all samples consists of a mixture of a ß-phase and a ß'-phase; fully distinguishable by their characteristic diffraction peaks. Subtle differences between the subcell patterns and phase development of the samples were observed. Different layering of the samples was also observed, understandably because of the chain length differences between the compounds. The long spacings were perfectly linearly proportional to the number of carbon atoms. The length of the ester layers with n carbon atoms can be calculated by a formula similar to that used for the layers in linear alkane molecules.

Relationships between molecular structure and kinetic and thermodynamic controls in lipid systems. Part II: Phase behavior and transformation paths of SSS, PSS and PPS saturated triacylglycerols--effect of chain length mismatch.

The kinetic phase behavior and phase transformation paths of purified tristearoylglycerol (SSS), 3-palmitoyl-1,2-distearoyl-sn-glycerol (PSS) and 1,2-dipalmitoyl-3-stearoyl-sn-glycerol (PPS) were investigated in terms of polymorphism, crystallization and melting. The details of the phase transformation paths were obtained using the heating cycles of two sets of experiments: (a) cooling rate was varied and heating rate fixed and (b) cooling rate was fixed and heating rate varied. Kinetic effects were manifest in all measured properties, underscoring the complexity of the phase transformation paths for each TAG, and the intricate thermodynamics-molecular relationships. For the first time, XRD data obtained for SSS, PSS and PPS TAGs, cooled at rates higher than 0.5°C/min, suggested the formation of a transient structure similar to the so-called α(2)-phase which has been observed in mixed saturated-unsaturated TAGs quenched from the melt. The more stable phases (ß' in PSS and PPS, and ß in SSS) were only observed for cooling rates lower than 1.0°C/min. The kinetic and thermodynamic differences observed in the crystallization, structure and melting of SSS, PSS and PPS are proposed to be mainly due to the disturbances introduced at the "terrace" level via methyl-end group interactions, i.e., the missing of two or four CH(2) groups compared to SSS. The symmetrical SSS with a relatively flat "terrace" crystallizes preferably in the most stable ß-form. Two missing CH(2) groups at the sn-1 position (PSS) introduces enough structural disturbances to promote the relative prevalence and persistence of the ß'-phase, and four missing CH(2) groups at the sn-1 and sn-2 positions (PPS) is relatively too large of a disturbance and therefore favors the α-form.

Relationships between molecular structure and kinetic and thermodynamic controls in lipid systems. Part III. Crystallization and phase behavior of 1-palmitoyl-2,3-stearoyl-sn-glycerol (PSS) and tristearoylglycerol (SSS) binary system.

The phase behavior of 1-palmitoyl-2,3-distearoyl-sn-glycerol (PSS)/tristearoylglycerol (SSS) binary system was investigated in terms of polymorphism, crystallization and melting behavior, microstructure and solid fat content (SFC) using widely different constant cooling rates. Kinetic phase diagrams were experimentally determined from the DSC heating thermograms and analyzed using a thermodynamic model to account for non-ideality of mixing. The kinetic phase diagram presented a typical eutectic behavior with a eutectic point at the 0.5(PSS) mixture with a probable precipitation line from 0.5(PSS) to 1.0(PSS), regardless of the rate at which the sample was cooled. The eutectic temperature decreased only slightly with increasing cooling rate. PSS has a strong effect on the physical properties of the PSS-SSS mixtures. In fact, the overall phase behavior of the PSS-SSS binary system was determined, for a very large part, by the asymmetrical TAG. Moreover, PSS is a key driver of the high stability observed in crystal growth, polymorphism and phase development. Levels as low as 10% PSS, when cooled slowly, and 30% when cooled rapidly, were found to be sufficient to suppress the effect of thermal processing.

Quantification of oil binding capacity of structuring fats: A novel method and its application.

A robust, well-defined and reproducible method to accurately measure the oil binding capacity (OBC) of structuring fats was developed. The method was validated using two oil/fat model systems, i.e., fully hydrogenated canola oil (FHCO) in canola oil (CO) (FHCO/CO) and fully hydrogenated soybean oil (FHSO) in CO (FHSO/CO). The mixtures were crystallized from the melt down to three different temperatures (15, 25 and 35 degrees C) at constant rates of cooling and the OBC was measured after different periods of storage time. The critical concentration of hard fat at which the solid fat network is stable and effectively binds oil has been also measured for mixtures crystallized at temperatures close to room temperature, i.e., 25 degrees C. Crystal structure, melting behavior, microstructure, and solid fat content of these binary systems have been investigated in relation to the OBC of the solid fat network using X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide-line pulsed nuclear magnetic resonance (pNMR) techniques. The two model systems exhibited similar trends in OBC over time, a behavior attributed to their similar TAG composition and polymorphism. However, relatively smaller OBC values were achieved by the CO/FHSO compared to CO/FHCO samples, largely due to differences in their solid network structure. Four successive decreasing linear segments, identifying successive mechanisms of oil migration/binding, were observed in the experimental OBC versus fat weight fraction curves. The critical concentration of hard fat, at which the solid fat network is effective in binding oil, was also determined and found to be approximately 6wt% for both systems.

The binary phase behavior of 1,3-dilauroyl-2-stearoyl-sn-glycerol and 1,2-dilauroyl-3-stearoyl-sn-glycerol.

The binary phase behavior of purified 1,3-dilauroyl-2-stearoyl-sn-glycerol (LSL) and 1,2-dilauroyl-3-stearoyl-sn-glycerol (LLS) was investigated at a slow (0.1 degrees C/min) and a relatively fast (3.0 degrees C/min) cooling rate in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Much of the behavior of the system is explained by its polymorphism and the influence of thermal processing. The alpha-form and the beta'-form of a double chain length structure were detected in the mixtures cooled at 3.0 degrees C/min, whereas only the beta'-form was detected in those cooled at 0.1 degrees C/min. X-ray diffraction data as well as thermodynamic data propose that the most stable phases are promoted by the symmetrical LSL. The measured trends in structural characteristics, thermal properties, SFC, relative hardness and microstructure delimit three groups of mixtures which imply a competition between the stabilizing effect of LSL and disordering introduced by kinetic effects: (a) LLS-rich mixtures with LSL molar fractions (X(LSL)) less than 0.3, (b) mixtures with X(LSL) clustered around 0.5 and (c) LSL-rich mixtures with X(LSL)>or=0.7. The balance between ordering and kinetic effects determines the polymorphism of the mixtures, which in turn determines the behavior of the LSL/LLS system. The kinetic phase diagram of the LSL/LLS binary system constructed using heating differential scanning calorimetry thermograms displayed a singularity at the 0.5(LSL) molar fraction which delimits two distinct behaviors: eutectic behavior in one region and monotectic behavior in the other. The molecular interactions, as depicted by a non-ideality parameter of mixing obtained from a thermodynamic model based on the Hildebrand equation, suggests an almost ideal mixing behavior and a moderate tendency to the formation of unlike-pairs in the liquid state.

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.

The binary phase behavior of 1,3-dicaproyl-2-stearoyl-sn-glycerol and 1,2-dicaproyl-3-stearoyl-sn-glycerol.

The phase behavior of a binary system constituted of purified 1,3-dicaproyl-2-stearoyl-sn-glycerol (CSC) and 1,2-dicaproyl-3-stearoyl-sn-glycerol (CCS) was investigated at a very slow (0.1 degrees C/min) and a relatively fast (3.0 degrees C/min) cooling rate using differential scanning calorimetry (DSC), low resolution NMR, X-ray diffraction (XRD), and polarized light microscopy (PLM). Related forms of the beta' polymorph were detected for all mixtures as well as a beta form for CSC-rich mixtures. A double chain length (DCL) stacking of the non-mixed CCS-CCS and CSC-CSC phases and a triple chain length (TCL) stacking of mixed CCS-CSC structure were detected for the different beta' forms. The kinetic phase diagram demonstrated an apparent eutectic at the 0.5(CSC) composition when cooled at 0.1 degrees C/min and at the 0.25(CSC) composition when cooled at 3.0 degrees C/min. The application of a thermodynamic model based on the Hildebrand equation suggests that compounds CSC and CCS are not fully miscible. In addition, the miscibility changes according to the structure of the growing solid phase which is dependent on CSC molar ratio as well as on the kinetics. It was also shown that the miscibility is concentration dependent and that the solid phase, which is growing at conditions well away from equilibrium, is determined kinetically. The molecular interactions were found to be strong and to favor the formation of CSC-CCS pairs in the liquid state. CSC and CCS were also shown to be immiscible in the solid state. Depressions in solid fat content (SFC) were observed for both rates. Relatively complex networks made of needle-like, spherulitic and granular crystals were observed in the CSC/CCS system. A pure CSC phase was found to be instrumental in promoting a higher SFC, and more stable polymorphic forms. The microstructure was shown to be strongly dependent on the cooling rate and was linked to the different polymorphic forms observed by DSC and XRD. Correlations between SFC and the eutectic behavior have been observed for the 3.0 degrees C/min cooling rate, but not directly in the case of the 0.1 degrees C/min cooling rate, where slower kinetics which favors the metastable to stable phase conversion processes prevented the same shifts in behavior.

Crystallization and phase behavior of fatty acid esters of 1,3 propanediol III: 1,3 propanediol dicaprylate/1,3 propanediol distearate (CC/SS) and 1,3 propanediol dicaprylate/1,3 propanediol dipalmitate (CC/PP) binary systems.

The phase behavior of the 1,3 propanediol dicaprylate/1,3 propanediol distearate (CC/SS) and the 1,3 propanediol dicaprylate/1,3 propanediol dipalmitate (CC/PP) binary systems were investigated using different techniques. The two systems presented essentially the same overall features. XRD measurements detected CC-CC, PP-PP and SS-SS bilayers which crystallized in beta forms but no mixed bilayers for all mixtures. The phase diagrams of both systems were comparable and displayed a monotectic behavior. As strongly evidenced by XRD data, both phase diagrams suggested that CC, PP and SS formed largely separate phases but were probably not completely immiscible. Avrami analysis of SFC vs. time indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei. However, noticeable differences in the manifestation of the molecular interactions have been detected at all levels of structure and confirmed by the interchange coupling determined by the enthalpy of melt, the final SFC and the hardness data. This was obviously related to the difference in chain size between SS and PP molecules. The effect on texture was highlighted by drastic microstructural differences between the two systems. Furthermore, the differences in nucleation and crystal growth, the more pronounced tendency for phase separation in the CC/SS system compared to the CC/PP system, and the relatively better crystallization of the CC/PP mixtures, particularly visibly for x(CC)< or =0.3 compared to the CC/SS mixtures were associated with the chain length difference.

Crystallization and phase behavior of 1,3-propanediol esters II. 1,3-propanediol distearate/1,3-propanediol dipalmitate (SS/PP) and 1,3-propanediol distearate/1,3-propanediol dimyristate (SS/MM) binary systems.

Polymorphic influences on the phase behavior of two types of binary mixtures of saturated monoacid 1,3-propanediol esters (PADEs), dipalmitate/distearate (PP/SS) and dimyristate/distearate (MM/SS) were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by solid fat content (SFC), hardness and microscopy measurements. Three stacking modes have been found in the PP/SS binary system. Mixed SS-PP bilayers were detected in all mixtures, SS-SS bilayers in x(PP)=0.0-0.4 mixtures and PP-PP bilayers in x(PP)=0.6-0.1 mixtures. Two different but close beta polymorphs and one beta' polymorph were detected for this system. beta' was only detected in x(PP)=0.5-0.9 mixtures for the mixed bilayers. For the MM/SS binary system, only MM-MM and SS-SS bilayers were detected and both solid phases crystallized in two different beta forms. XRD data evidenced clearly that the MM and SS components were completely immiscible in the solid state. The phase diagrams constructed using DSC data, exhibited a typical eutectic-type phase boundary. The presence of eutectics, the shape of the solidus lines as well as the analysis of the individual enthalpies of melting indicated typical phase separation for both systems. A thermodynamic study based on the Hildebrand equation and using the Bragg-Williams approximation for non-ideality of mixing confirmed the phase separation in the solid phase and suggested that the PP and SS were miscible in the liquid phase and that SS formed an ideal mixing with MM. Avrami analysis of SFC vs. time curves indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei, and suggested that the nucleation rate was higher for the mixture at the eutectic composition. The relative hardness was correlated with the enthalpies, the final SFC and the microscopy measurements.