Crystallization and Transformation Behavior of Triacylglycerol Binary Mixtures Forming Molecular Compounds of POP/OPO, POP/rac-PPO, and POP/sn-PPO

Abstract

Fat-based soft materials generally contain different triacylglycerol (TAG) species; hence, TAG mixtures' phase behaviors determine their important physical properties. TAG binary mixtures typically exhibit eutectic, monotectic, solid solution, and molecular compound (MC) forming phases; among them, the molecular interactions and the crystallization kinetics controlling the formation processes of MC-phases remain unexplored. We used thermal analysis, X-ray diffraction, and optical microscopy to examine the crystallization and transformation kinetics of the three MC-forming 1:1 binary TAG mixtures of 1,3-dipalmitoyl-2-oleoyl glycerol (POP)/1,3-dioleoyl-2-palmitoyl glycerol (OPO), POP/1,2-dipalmitoyl-2-oleoyl-rac-glycerol (rac-PPO), and POP/1,2dipalmitoyl-3-oleoyl-sn-glycerol (sn-PPO). In the POP/OPO mixture, the MC POP/OPO crystals of the most stable β form with double chain length stacking (β-2L) were crystallized by simple cooling treatments. In the POP/rac-PPO mixture, the β-2L form of MC POP/rac-PPO was crystallized by only cooling at 0.1 °C/min, while metastable β′-2L formed at the cooling rates of 0.5 °C/min and 2.0 °C/min; then β′-2L melted and partially transformed into the β form during the heating processes. The POP/sn-PPO mixture showed different and significantly intricate polymorphic behavior. First, a single component of sn-PPO crystallized in β′ with triple chain length stacking (β′-3L) during cooling. This was followed by the crystallization of POP and MC POP/sn-PPO of the β′-2L forms at slow cooling rates of 0.1 °C/min and 0.5 °C/min, and the α-2L forms at 2.0 °C/min. During heating, the metastable β′ or α forms melted, and MC POP/sn-PPO transformed into the β form before melting. Observations of isothermal crystal growth rates by optical microscopy revealed that sn-PPO β-3L crystals grow faster than the other single TAGs or MCs, particularly above ∼18 °C. Differences in the crystallization kinetics between sn-PPO, POP, and MC POP/sn-PPO may cause the separate crystallization while the POP/sn-PPO mixture cools.

Formation mechanisms of molecular compound in saturated‐unsaturated mixed‐acid triacylglycerols mixture systems and its edible applications

“…To elucidate the effects of glycerol structures on the kinetic properties of MC‐forming mixture phases, thermal analysis, x‐ray diffraction and optical microscopy techniques were applied to the binary mixtures of POP/OPO, POP/ rac ‐PPO and POP/1,2‐dipalmitoyl‐3‐oleoyl‐ sn ‐glycerol ( sn ‐PPO) (Bayés‐García et al, 2023). All the three mixtures exhibited the MC‐forming mixture at a 1:1 ratio.…”

“…Summary of temperature‐dependent polymorphic crystallization in the binary mixtures of POP/OPO, POP/ rac ‐PPO and POP/ sn ‐PPO. Source : Reprinted with permission from Bayés‐García et al, 2023. Copyright 2023 American Chemical Society.…”

“…To understand the differences in the crystallization behavior of MC POP/OPO and MC POP/ sn ‐OPO , one can pay attention to their structural models, which are mainly based on the glycerol structures of POP, OPO and sn ‐PPO, as depicted in Figure 8 (Bayés‐García et al, 2023). It can be assumed that the tuning fork glycerol conformation is the most stable one for both POP and OPO β forms, since the palmitic and oleic acid chains are located on different leaflets, favoring the packing of TAG molecules in triple chain‐length structures that avoid the steric hindrance between straight palmitic and bent oleic acid chains.…”

“…Effects of chain packing and glycerol conformation on the formation of MC crystals. Source : Reprinted with permission from Bayés‐García et al, 2023. Copyright 2023 American Chemical Society.…”

“…As a fundamental study to elucidate the formation mechanisms of MC, precise observation was performed on the crystallization behavior of palmitic‐oleic‐based MC of 1,3‐dipalmitoyl‐2‐oleoyl‐glycerol (POP)/1,3‐dioleoyl‐2‐palmitoyl glycerol (OPO), POP/1,2‐dipalmitoyl‐3‐oleoyl‐ rac ‐glycerol ( rac ‐PPO) and POP/1,2‐dipalmitoyl‐3‐oleoyl‐ sn ‐glycerol ( sn ‐PPO) when subjected to varied cooling rates. By comparing the crystallization behavior of POP/ sn ‐PPO and POP/ rac ‐PPO systems, the effects of glycerol structures and optical isomerization on MC crystal formation were also briefly described (Bayés‐García et al, 2023). The analysis of the influence of the cis‐trans isomerization on MC formation was performed by replacing cis ‐oleoyl to trans ‐elaidoyl (E) chains in the binary system of 1,3‐dipalmitoyl‐2‐elaidoyl glycerol (PEP)/1,3‐dielaidoyl‐2‐palmitoyl glycerol (EPE) (Zhang et al, 2020).…”