Characterization, polymorphism and structuring properties of trisaturated triacylglycerols containing palmitic and stearic acids

Pure tripalmitoylglycerol (PPP), 1,3-dipalmitoyl-2-stearoyl-sn-glycerol (PSP), 1-palmitoyl-2,3-distearoyl-sn-glycerol (PSS) were successfully synthesized according to a newly developed processing route. The kinetic phase behavior and phase transformation paths of these pure TAGs, binary and ternary mixtures thereof, were investigated at 10.0°C min-1 (fast cooling) and 1.0°C min-1 (slow cooling) cooling rates, and re-heating at 5.0°C min-1 and 1.0°C min-1 by differential scanning calorimetry and X-Ray diffraction. Polarized light microscopy was used to study the microstructure of different model application blends. At the higher cooling rate, all samples crystallized in α polymorph, while at the lower cooling rate, a different behavior was observed. For pure PPP, the most stable form obtained was the β-form, whereas for PSS and PSP, the most stable form achieved was the β′-form.
In PPP-PSP binary mixture, samples containing XPSP ≤ 0.3 crystallized in the α-polymorph and samples containing XPSP ≥ 0.4 crystallized in the β′-polymorph. During heating all samples transformed to a more stable β′-form and melted finally in the β-form (XPSP ≤ 0.3) or in the β′-form (XPSP ≥ 0.4). Kinetic phase diagrams were constructed by using the melting and transformation peak temperatures from the DSC heating thermograms and XRD patterns and displayed an apparently typical eutectic behavior with a eutectic point at XPSP = 0.3, irrespective of the rate at which the samples were cooled and re-heated. The eutectic temperature was independent of the cooling and heating rates used.
In PPP-PSS and PSS-PSP binary systems, all samples crystallized in the α-polymorph and transformed into the more stable β′-form via melt mediated crystallization or direct re-crystallization from the α-phase. In PSS-PSP, samples containing XPSP ≥ 0.5 tended to crystallize in the βʹ2-form and transformed during heating to the more stable βʹ1-form. In PPP-PSS, samples containing high PPP concentration ultimately transformed to the most stable β-form. The kinetic phase diagram of PPP-PSS mixtures displayed an apparently typical eutectic behavior with a eutectic point at XPSS = 0.3, when the samples were melted at the higher heating rate, which shifted to higher concentration, XPSS = 0.5, when the lower heating rate was applied. The eutectic temperature (~58.4°C) was independent of the cooling and heating rates used. The kinetic phase diagram of PSS-PSP mixtures displayed two types of behavior, a probable monotectic behavior in the most stable phase and a eutectic behavior in the meta-stable phase when the samples were melted at higher heating rate.
To study the most stable form, samples were incubated at different temperature and incubation time. At lower incubation temperature (15°C), most stable forms were not obtained, whereas when stored at temperature close to the melting point (50°C) of the least stable polymorph, samples transformed to the most stable form, although for samples containing higher amount of PPP a mixture of polymorphs was observed.
Different ternary blends were derived based on the composition of hydrogenated palm oil. These ternary blends showed similar phase behavior as was observed for the binary mixtures. For these blends, βʹ-form is desired for food application, but when the PPP content was high, the mixtures transformed to the β-form during incubation period. These results showed that samples having high PPP content are not suitable for use as hard stock fat for structuring purposes.
Finally, three selected ternary systems were studied in model application blends, and showed different behavior based on the liquid fat used. It seemed that the presence of a high content of OOO facilitated the transformation to the more stable β-form. The microstructure of all the mixtures containing palm oleine (lower OOO content) as the liquid phase consisted of small spherulitic crystals, while the blend containing rapeseed oil (higher OOO content) as the liquid phase showed well-defined dendritic aggregations of crystallites. All samples formed a continuous network under these processing conditions.
This study showed that mixed-acid TAGs (PSP and PSS) have a strong effect on the physical properties of the systems studied and dominated a large part of the phase behavior of the mixtures. At lower concentrations of these TAGs, the mixtures transformed to the most stable β-form, but at increasing PSP or PSS concentration, the mixtures stabilized in the β′-phase, which is preferred in many food application because this results in a smooth texture.