Polymorphism and Kinetic Behavior of Binary Mixtures of Trisaturated Triacylglycerols Containing Palmitic and Stearic Acid Under Non‐Isothermal Conditions

Abstract

The kinetic phase behavior and phase transformation paths of pure tripalmitoylglycerol (PPP), 1‐palmitoyl‐2,3‐distearoyl‐sn‐glycerol (PSS), and 1,3‐dipalmitoyl‐2‐stearoyl‐sn‐glycerol (PSP) and binary mixtures thereof, are investigated at 10.0 °C min−1 (fast cooling) and 1.0 °C min−1 (slow cooling) cooling rates, and further re‐heating at 5.0 and 1.0 °C min−1. Mixtures are studied in terms of polymorphism, crystallization, and melting behavior. Kinetic phase diagrams are determined from DSC heating thermograms and XRD data. Independent of the cooling rate, most samples crystallized in the α‐polymorph and transformed into the more stable β′‐form via melt‐mediated crystallization or direct recrystallization from the α‐phase. Samples containing a high amount of PSP crystallized in the β′‐form at slow cooling rate and samples containing high PPP concentration ultimately transform to the most stable β‐form. The kinetic phase diagram of PPP‐PSS mixtures present an apparently typical eutectic behavior with a eutectic point at XPSS = 0.3, when the samples are melted at the higher heating rate, which shifts to higher concentration, XPSS = 0.5, when the lower heating rate is applied. The eutectic temperature (58.4 ± 0.3 °C) is independent of the cooling and heating rates used. The kinetic phase diagram of PSS‐PSP mixtures display two types of behaviors, a probable monotectic behavior in the most stable phase and a eutectic behavior in the meta‐stable phase when the samples are melted at higher heating rate. This study shows that PSS has a strong effect on the physical properties of PPP‐PSS blends and affects a large part of the phase behavior of this binary system. At lower PSS concentrations, the mixtures transform to the most stable β form, but at increasing PSS concentration, the samples stabilize in the β′‐phase. In the PSS‐PSP binary systems, all the mixtures stabilize in the β′‐phase, which is preferred in many food applications since it is associated with a smooth texture. Practical Applications: This research extends a previous study on the phase behavior of the most common trisaturated triacylglycerols and the results are useful for the food industry as they can help in understanding the functionality of hard fat based structuring agents in end applications, and in developing new triacylglycerol based crystallization and structuring agent for specific food applications. The kinetic phase behavior of trisaturated triacylglycerol containing stearic and palmitic acid were studied under three sets of conditions. The binary systems showed different behaviors at the conditions studied. An apparent eutectic phase behavior was obtained for mixtures containing mono‐acid and symmetric triacylglycerols. However, the samples containing asymmetric triacylglycerols showed a probable monotectic phase behavior in the most stable β′1‐phase.

An Insight into the Solid-State Miscibility of Triacylglycerol Crystals

“…Conversely, when the heating rate was decreased to 1 °C·min −1 , Χ E shifted to the 50PPP/50PSS composition and β polymorphs were still visible at a Χ PSS of 40%, which is in great agreement with the kinetic phase behavior reported for SSS/PSS mixtures [ 72 ]. In a similar manner, distinct β and β’ stable phases were found in PPP/PSP mixtures at each side of the Χ E , which, in this case, remained at a constant Χ PSP of 30%, regardless of the different thermal processing applied [ 107 ].…”

“…Research carried out in binary mixtures of monoacid and mixed-acid tri-saturated TAGs has shown the predisposition of these systems to display mixing phase behavior of eutectic nature, and a major influence of the mixed-acid component on the overall polymorphic behavior during crystallization, transformation, and melting events. The main features of binary mixtures of TAGs with such configuration will be illustrated through the kinetic phase behavior that was reported for SSS/PSS, PPP/PSS, and PPP/PSP binary systems [ 72 , 107 , 108 ].…”

“…As for PPP/PSS and PPP/PSP mixtures, the effect of small structural changes on the crystallization behavior was evident during cooling at 1 °C·min −1 . When this cooling rate was applied, PPP/PSS behaved similarly to mixtures which were crystallized at fast rates, exhibiting α-2L phases [ 107 ]. By contrast, in PPP/PSP mixtures, α polymorphs predominated at a Χ PSP below 30%, whereas mixtures at increased PSP content were characterized by the concurrent presence of β’ 2 and β’ 1 crystalline phases [ 108 ].…”

“…The kinetic phase diagrams of PPP/PSS mixtures that are displayed in Figure 4 may serve as an example of the mixing behavior in binary systems of monoacid and mixed-acid fully saturated TAGs. Overall, the sequence of polymorphic transformations starting from the crystallized α phase was not influenced by the thermal history of the mixtures or the heating rates applied [ 107 ]. For all of the mixtures under the different conditions, the melt-mediated transformation from α to β’ 2 phase was followed by its solid-sate transformation to β’ 1 phase, which showed to be the most stable in the mixtures, except for those that are rich in PPP, in which the additional recrystallization of a β phase after β’ 1 melting occurred.…”

“…In broad terms, these studies underlined how mixed-acid TAGs influence kinetics in such a way that, even at low concentrations, seem to dictate the overall polymorphic behavior during crystallization and transformation by easing the formation of β’ phases and hindering the β’→β transformation [ 59 , 72 , 107 , 108 ]. The apparent dominating role of mixed-acid TAGs in mixtures with monoacid TAGs is likely determined by the disturbing effect of chain-length mismatch over methyl-end group packing within interlamellar regions.…”