Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the α-Phase (Part II)

A synchrotron X-ray scattering study of the crystallization behavior of mixtures of confectionary triacylglycerides: Effect of chemical composition and shear on polymorphism and kinetics

Cocoa butter equivalents (CBE) are mixtures of triglycerides from multiple sources (e.g., sunflower oil, mango kernel and sal), which resemble cocoa butter (CB) in both physical and chemical properties. Despite being widely used to replace CB in chocolate products, the crystallization behavior of many CBEs is still poorly understood. The aim of this work was to develop a fundamental understanding, at the molecular level, of the crystallization behavior of selected CBEs, and compare it with that of CB. Chromatography was used to determine the composition of CBEs, in terms of fatty acids and triacylglycerides (TAGs), while their thermodynamic behavior and crystallization kinetics were studied using polarized microscopy, differential calorimetry and three different synchrotron X-ray scattering setups. CBEs of different origin and chemical composition (e.g., different ratios of the main CB TAGs, namely POP, SOS and POS) crystallized in different polymorphs and with different kinetics of nucleation, growth and polymorphic transformation. SOS rich CBEs presented showed more polymorphs than CB and POP rich samples; whereas, CBEs with high concentration of POP showed slow kinetic of polymorphic transformation towards the stable β(3L) form. Additionally, it was observed that the presence of small amounts (<1% w/w) of specific TAGs, such as OOO, PPP or SSS, could significantly affect the crystallization behavior of CBEs and CBs in terms of kinetics of polymorphic transformation and number of phases detected (multiple high melting β(2L) polymorphs were identified in all samples studied). Finally, it was found that, regardless of the CBE composition, the presence of shear could promote the formation of stable β polymorphs over metastable β' and γ forms, and reduced the size of the crystal agglomerates formed due to increased secondary nucleation.

Polymorphism of a Highly Asymmetrical Triacylglycerol in Milk Fat: 1-Butyryl 2-Stearoyl 3-Palmitoyl-glycerol

Milk fat has more than 200 triacylglycerols (TAGs), which play a pivotal role in its crystallization behavior. Asymmetrical TAGs containing short butyryl chains contribute to a significant portion of milk fat TAGs. This work aims to elucidate the crystallization behavior of asymmetrical milk fat TAGs by employing the pure compound of 1-butyryl 2-stearoyl 3-palmitoyl-glycerol (BuSP). The structural evolution of BuSP after being cooled down to 20 °C from the melt is evaluated by small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry (DSC). The temporal structural observation shows that BuSP crystallizes into the α-form with short and long spacings of 4.10 and 56.9 Å, respectively, during the first hour of isothermal hold at 20 °C. The polymorphic transformation of the α to β' phase occurred after 4 h of isothermal hold, and the β'- to α-form fraction ratio was about 70:30 at the end of the isothermal experiment (18 h). Pure β'-form X-ray patterns are obtained from the BuSP powder with short spacings of 4.33, 4.14, and 3.80 Å, while the long spacing of 51.2 Å depicts a three-chain-length lamellar structure with a tilt angle of 32°. Corresponding DSC measurements display that BuSP crystallizes from the melt at 29.1 °C, whereas the melting of α- and β'-forms was recorded at 30.3 and 47.8 °C, respectively. In the absence of the β-form, the β'-polymorph is the most stable observed form in BuSP. This work exemplarily explains the crystallization behavior of asymmetrical milk fat TAGs and thus provides new insights into their role in overall milk fat crystallization.

Crystal Memory near Discontinuous Triacylglycerol Phase Transitions: Models, Metastable Regimes, and Critical Points

It is proposed that "crystal memory", observed in a discontinuous solid-liquid phase transition of saturated triacylglycerol (TAG) molecules, is due to the coexistence of solid TAG crystalline phases and a liquid TAG phase, in a superheated metastable regime. Such a coexistence has been detected. Solid crystals can act as heterogeneous nuclei onto which molecules can condense as the temperature is lowered. We outlined a mathematical model, with a single phase transition, that shows how the time-temperature observations can be explained, makes predictions, and relates them to recent experimental data. A modified Vogel-Fulcher-Tammann (VFT) equation is used to predict time-temperature relations for the observation of "crystal memory" and to show boundaries beyond which "crystal memory" is not observed. A plot of the lifetime of a metastable state versus temperature, using the modified VFT equation, agrees with recent time-temperature data. The model can be falsified through its predictions: the model possesses a critical point and we outline a procedure describing how it could be observed by changing the hydrocarbon chain length. We make predictions about how thermodynamic functions will change as the critical point is reached and as the system enters a crossover regime. The model predicts that the phenomenon of "crystal memory" will not be observed unless the system is cooled from a superheated metastable regime associated with a discontinuous phase transition.

Early stages of fat crystallisation evaluated by low-field NMR and small-angle X-ray scattering

Low-field time-domain nuclear magnetic resonance (NMR; 20 MHz) is commonly used in the studies of fats in the form of solid fat content (SFC) measurements. However, it has the disadvantage of low sensitivity to small amounts of crystalline material (0.5%), thus often incorrectly determining crystallisation induction times. From spin-lattice relaxation rate measurements (R₁ ) during the isothermal crystallisation measurements of cocoa butter between 0.01 and 10 MHz using fast field cycling NMR, we learnt previously that the most sensitive frequency region is below 1 MHz. Thus, we focused on analysing our 10-kHz data in detail, by observing the time dependence of R₁ and comparing it with standard SFC_NMR and SFC determinations from small-angle X-ray scattering (SFC_SAXS ). Although not reflecting directly the SFC, the R₁ at this low frequency is very sensitive to changes in molecular aggregation and hence potentially serving as an alternative for determination of crystallisation induction times. Alongside R₁ , we also show that SFC_SAXS is more sensitive to early stages of crystallisation, that is, standard SFC_NMR determinations become more relevant when crystal growth starts to dominate the crystallisation process but fail to pick up earlier crystallisation steps. This paper thus demonstrates the potential of studying triacylglycerols at frequencies below 1 MHz for obtaining further understanding of the early crystallisation stages of fats and presents an alternative and complementary method to estimate SFC by SAXS.

Self-assembled hexagonal liquid crystalline gels as novel ocular formulation with enhanced topical delivery of pilocarpine nitrate

The aim of this paper was to develop hexagonal liquid crystalline (H_II) gels that can be used as a novel ocular delivery system for pilocarpine nitrate (PN). H_II gels were prepared by a vortex method using phytantriol/triglyceride/water (71.15: 3.85: 26, w/w) ternary system. The gels were characterized by crossed polarized light microscopy, small-angle X-ray scattering, differential scanning calorimetry and rheology. And, in vitro drug release behavior and ex vivo corneal permeation were investigated. Finally, preocular residence time evaluation, eye irritation test, histological examination and miotic tests were studied in vivo and compared with carbopol gel. Based on various characterization techniques, the inner structure of the gels were H_II mesophase and exhibited a pseudoplastic fluid behaviour. In vitro release results revealed that PN could be released continuously from H_II gel over a period of 24 h. The ex vivo apparent permeability coefficient of H_II gel was 3.15-fold (P < 0.01) higher than that of the Carbopol gel. Compared with Carbopol gel, H_II gel displayed longer residence time on the eyeballs surface using fluorescent labeling technology. Furthermore, the H_II gel caused no ocular irritation was estimated by corneal hydration levels, Draize test and histological inspection. Additionally, in vivo miotic study showed that H_II gel had a remarkably long-lasting decrease in the pupil diameter of rabbits. In conclusion, H_II gels would be a promising sustained-release formulation for ocular drug delivery.