The Packing of Triacylglycerols from SAXS Measurements: Application to the Structure of 1,3-Distearoyl-2-oleoyl-sn-glycerol Crystal Phases

On the relevance of inhomogeneity in saturated fatty acid compositions for the crystallization kinetics of fat blends

In this contribution, the crystallization kinetics of model fat blends containing trisaturated and mono-unsaturated TAGs of different saturated fatty acid composition are investigated via differential scanning calorimetry (DSC) and time-resolved small and wide angle X-ray scattering (SAXS/WAXS). The effect of inhomogeneity in saturated fatty acid composition was studied by varying between palmitic and stearic sources for trisaturated (POSt - palm stearin and FHRO - fully hydrogenated rapeseed oil) and using palm mid fraction (PMF) and shea stearin (ShSt) sources for mimicking the mono-unsaturated TAGs (PMF - palm mid fraction and ShSt - shea stearin). The kinetics were followed via SAXS/WAXS upon cooling at different rates (1 and 10 °C min-1) and during the isothermal hold at 0 °C (273 K) for 60 min. The blends revealed a multiple step crystallization with co-existing polymorphic forms and a slow restructuring process. Generally, mono-unsaturated triglycerides tended to impede the polymorphic transition of the trisaturated fraction. The trisaturated triglyceride fraction formed an α phase at the beginning of the crystallization. Quickly after, co-existing β' and β phases were recorded. In each case, the subsequent crystallization events were dictated by the mono-unsaturated fraction with ShSt affecting the crystallization more drastically than PMF. Differences were diminished when a low cooling rate was applied. Further, the low cooling rate allowed the formation of a β phase in FHRO-containing samples before an additional β' phase was recorded. The differences in explicit fatty acid composition, TAG diversity and minor components are discussed as primary potential sources of differences in crystallization behavior. Calorimetric experiments capturing the crystallization and melting behavior of the industrial grade binary blends were compared to that of blends based on pure TAGs diluted in rapeseed oil, namely the combinations PPP-POP, SSS-SOS, PPP-SOS, and SSS-POP.

Crystallization Behavior of Plant-Based Fat Blends Formulated as an Alternative for Anhydrous Milk Fat in Milk Chocolate

Due to the increasing global demand for chocolate products and changes in consumer preferences, chocolate manufacturers have recently started to explore novel solutions to reformulate chocolate. Milk fat alternatives (MFA) are blends of triglycerides from different plant-based sources that resemble anhydrous milk fat in physical properties, particularly thermal behavior and solid fat content. However, in order to use MFA as potential ingredients for vegan milk chocolate formulations, it is necessary to understand their crystallization behavior, particularly in light of their chemical composition. Here, we applied synchrotron X-ray scattering, polarized light microscopy, and differential scanning calorimetry to investigate the crystallization behavior of four selected commercial MFAs (MF1, 2, 3, and 4), on their own and mixed with cocoa butter (CB). Chemical characterization revealed significant differences among samples and with both anhydrous milk fat (MF) and CB. POP-rich MF1 presented a specific polymorphic and thermal behavior, with the unstable β' form persisting for longer times than all other samples. Sample MF2 exhibited a polymorphic behavior more similar to CB in terms of number, type, and melting behavior due to the compositional similarities (e.g., prevalence of both SOS and POP). SOS-rich MF3 was characterized by metastable forms γ and β'(3L), whereas MF4 at ambient conditions showed only β(2L) forms due to its specific composition. Mixtures of CB and all MFAs behaved similarly to CB and MF mixtures, with good miscibility at ambient temperature and a lower melting point. Despite significant differences in chemical composition, MF4 presented similar solid fat content compared to MF; this is due to the high amount of relatively long chain, unsaturated fatty acids and the broad distribution of different TAGs, which all lower the melting point of this sample.

Unusual Structural Insights Revealed by Rheo-SAXS Studies of Nonaqueous Crystalline Gels

Glycerol is a nonaqueous polar solvent and is of interest in many industrial areas due to its beneficial properties, such as green production and biocompatibility. Our previous works have shown the presence of a fibrillar phase on the microscale that consists of lamellar sodium dodecyl sulfate (SDS) crystals containing interstitial glycerol on the nanoscale. The phase is gel-like at room temperature and demonstrates shear-thinning behavior upon application of a shear. Initially, small-angle X-ray scattering coupled with rheology (rheo-SAXS) measurements were performed to elucidate the structural transition of the gel phase under an applied shear, but it became clear that the aging process of the gel has a profound impact on both the gel nanostructure and also the mechanical properties. For younger gels, both the dissolution of SDS crystallites and the alignment of the fibrillar phase were seen. However, in the aged gels, an unexpected foam was formed at shear rates γ ˙ > 700 s-1. The microscopic structure of the foam phase was imaged using polarizing light microscopy and brightfield and darkfield optical microscopy. The nanostructure of the foam phase was investigated using rheo-SAXS. The foam phase was shown to be stabilized by the presence of SDS crystallites at the air-liquid interface, and the stability of the foam is high with foam persisting even t = 3 months after formation. These results highlight the importance of investigating green nonaqueous media and the gel aging process, both of which are interesting not only on a fundamental level but also for a range of industrial applications, from personal care products and cosmetics to food science.

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.

Rheo-SAXS characterization of lead-treated oils: Understanding the influence of lead driers on artistic oil paint's flow properties

From the 15^th century onwards, painters began to treat their oils with lead compounds before grinding them with pigments. Such a treatment induces the partial hydrolysis of the oil triglycerides and the formation of lead soaps, which significantly modify the rheological properties of the oil paint. Organization at the supramolecular scale is thus expected to explain these macroscopic changes. Synchrotron Rheo-SAXS (Small Angle X-ray Scattering) measurements were carried out on lead-treated oils, with different lead contents. We can now propose a full picture of the relationship between structure and rheological properties of historical saponified oils. At rest, lead soaps in oil are organized as lamellar phases with a characteristic period of 50 Å. Under shear, the loss of viscoelastic properties can be linked to the modification of this organization. Continuous shear resulted in a preferential and reversible orientation of the lamellar domains which increased with the concentration of lead soaps. The parallel orientation predominates over the entire shear range (0-1000 s^-1). Conversely, oscillatory shear coiled the lamellae into cylinders that oriented themselves vertically in the rheometer cell. This is the first report of such a vertical cylindrical structure obtained under shear from lamellae.

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.

Stabilisation of oleofoams by lauric acid and its glycerol esters

Four types of pure lipid, namely lauric acid (LA), glycerol monolaurate (MAG), diglycerol laurate (DAG) and triglyceride laurate (TAG) were used to prepare oleofoams. The relationship between crystal profiles and their performance in oleofoams was established. DAG formed small needle-like crystals while MAG formed large flake-like crystals in oleogels, and crystal shells around air bubbles were observed in LA-, MAG- and DAG-based oleofoams. LA and DAG displayed higher over-run whereas DAG-stabilised foam possessed smaller bubbles and higher physical stability due to the presence of small β and β' crystals. Upon heating, DAG and TAG-based foams showed varying extents of oil drainage indicating the crystals were distributed in a different manner. Therefore, DAG was shown to be an excellent gelator in the fabrication of ultra-stable oleofoams. This work extends the lipid varieties with nutritional features and allows a better understanding on the stabilization mechanisms of lauric acid lipids in oleofoams.

Co-assembly and Structure of Sodium Dodecylsulfate and other n-Alkyl Sulfates in Glycerol: n-Alkyl Sulfate-Glycerol Crystal Phase

HYPOTHESIS

Following the observation of a microfibrillar phase in sodium dodecylsulfate (SDS)-glycerol mixtures, it is hypothesized that this phase is a crystalline structure containing SDS and glycerol, where the interaction between sulfate and glycerol layers mediates the co-assembly, which also could be universal for similar systems formed by n-alkyl sulfate homologues. Experiment. n-alkyl sulfate glycerol solutions were studied using a combination of optical microscopy, small- and wide-angle X-ray scattering (SAXS/WAXS). Time-resolved SAXS was employed to determine the phase formation in SDS-glycerol-water mixtures.

FINDINGS

The microfibrillar crystalline phase was reproduced in even-chained n-alkyl sulfates with a chain length between 12 and 18 carbon atoms, where the phase lamellar period increased uniformly with the alkyl chain length. Reconstruction of electron density profiles from the diffraction patterns allowed the lamellar structural motif of the phase, the glycerol location and stoichiometry to be determined. When SDS-glycerol-water mixtures with water concentration below 6 wt% are isothermally solidified at 20 °C, SDS-glycerol crystals and/or anhydrous SDS form, where the former is inhibited by the latter at higher water concentrations. The learnings from the SDS-glycerol phase formation allows new gels to be created, utilising the glycerol-sulfate motif generating microfibrils. This expands the knowledge of the applicable formulation space for SDS-water containing mixtures.

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

The early-stage crystallization behavior in a triacylglycerol mixture has been investigated on the nanoscale with a novel global small-angle X-ray scattering analysis technique. This method has been tailored for the determination of the electron density profiles (EDPs) replicating both (i) the nanostructural texture of molten triacylglycerols (TAGs) (refer to "Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the Molten State (Part I)" of this publication series) and (ii) the lamellar structure of the metastable α-polymorph. In a first stage, the α-phase scattering contribution alone was examined by classical Fourier analysis as well as by globally fitting the data, leading to practically identical EDPs. On the basis of these findings, we extended our analysis to the entire X-ray scattering contribution arising from molten TAGs and the solid α-phase fraction. Remarkably, the experimental and theoretical data agree very well, providing for the first time a detailed nanostructural understanding about the coexisting molecular assemblies. This, in turn, also allowed us to quantitatively determine the solid fat content (SFC) with X-ray scattering data. Our new theoretical approach for measurement of SFC is based on the global analysis of small-angle scattering/diffraction patterns, and the SFC results are in good agreement with values obtained from other techniques such as NMR spectroscopy.

Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the Molten State (Part I)

The study of triacylglycerols (TAGs) in their molten state is of fundamental importance for a deeper understanding of the TAG crystallization processes, being highly relevant for both manufacturing and medical applications. Although different models have been proposed to explain the nanostructured nature of the fluid state of TAGs, none of them are fully satisfactory. In this paper, we propose a new model consisting of positionally uncorrelated lamellar TAG assemblies embedded in an isotropic medium that assist as prenucleating structures. This model was validated by applying a novel global fitting method, resulting in an excellent agreement with the small-angle X-ray scattering data. A deeper analysis of the scattering patterns at different temperatures, both in cooling and heating directions, allowed us further to detect the crystalline traces of TAGs even after heating to 40 °C and record, on cooling, the onset of crystallization at 30-25 °C. The application of the presented novel model not only explains the outstandingly structured fluid of molten TAGs, but also lays the basis for analyzing first the crystallization steps in greater detail, which is outlined in our follow-up paper "Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the α-Phase (Part II)".

Physicochemical Properties of α-Form Hydrated Crystalline Phase of 3-(10-Carboxydecyl)-1,1,1,3,5,5,5-heptamethyl Trisiloxane/Higher alcohol/Polyoxyethylene (5 mol) Glyceryl monostearate/Water System

The α-form hydrated crystalline phase (often called as an α-gel) is one of the hydrated crystalline phases which can be exhibited by surfactants and lipids. In this study, a novel system of an α-form hydrated crystal was developed, composed of 3-(10-carboxydecyl)-1,1,1,3,5,5,5-heptamethyl trisiloxane (CDTS), polyoxyethylene (5 mol) glyceryl monostearate (GMS-5), higher alcohol. This is the first report to indicate that a silicone surfactant can form an α-form hydrated crystal. The physicochemical properties of this system were characterized by small and wide angle X-ray scattering (SWAXS), differential scanning calorimetry (DSC), and diffusion-ordered NMR spectroscopy (DOSY) experiments. SWAXS and DSC measurements revealed that a plurality of crystalline phases coexist in the CDTS/higher alcohol/water ternary system. By adding GMS-5 to the ternary system, however, a wide region of a single α-form hydrated crystalline phase was obtained. The self-diffusion coefficients (D_sel) from the NMR measurements suggested that all of the CDTS, GMS-5, and higher alcohol molecules were incorporated into the same α-form hydrated crystals.

Self-Assembled Ag-MXA Superclusters with Structure-Dependent Mechanical Properties

The low elastic modulus and time-consuming formation process represent the major challenges that impede the penetration of nanoparticle superstructures into daily life applications. As observed in the molecular or atomic crystals, more effective interactions between adjacent nanoparticles would introduce beneficial features to assemblies enabling optimized mechanical properties. Here, a straightforward synthetic strategy is showed that allows fast and scalable fabrication of 2D Ag-mercaptoalkyl acid superclusters of either hexagonal or lamellar topology. Remarkably, these ordered superstructures exhibit a structure-dependent elastic modulus which is subject to the tether length of straight-chain mercaptoalkyl acids or the ratio between silver and tether molecules. These superclusters are plastic and moldable against arbitrarily shaped masters of macroscopic dimensions, thereby opening a wealth of possibilities to develop more nanocrystals with practically useful nanoscopic properties.

Production of sn-1,3-distearoyl-2-oleoyl-glycerol-rich fats from mango kernel fat by selective fractionation using 2-methylpentane based isohexane

High-purity isohexane containing 88.12% 2-methylpentane, which has a higher polarity than industrial hexane, was selected to selectively fractionate mango kernel fat to produce 1,3-distearoyl-2-oleoyl-glycerol (SOS)-rich fat. The three-stage fractionation process was optimized by considering the stearin yield and its SOS content to obtain a third stearin (TS). This fat contained a high percentage of SOS (69.2%) with only 0.8% diacylglycerol. Mass spectra and sn-2 fatty acid analyses further revealed that the TS was mainly composed of symmetrical monounsaturated triacylglycerols. Compared with cocoa butter (CB), the unique composition of the TS improved its thermal properties (35.6% and 0% solid fat content at 35°C for the TS and CB, respectively). In particular, tempered binary fat blends, which were composed of 20-50% TS and 50-80% CB, showed acceptable compatibility at 20-28°C. The TS could be utilized by chocolate manufacturers to make products suitable for sale and consumption in tropical conditions.

Structural Study on Polymorphism of Cis-Unsaturated Triacylglycerol: Triolein

To clarify the influence of cis-unsaturation on solid-state structures of triacylglycerols (TAGs), the crystal structures of three crystalline phases (alpha, beta' and beta) of triolein [C3H5(OCOC17H33)3] were investigated by powder X-ray diffractometry and IR and Raman spectroscopy. The influence on the structural change of the alpha phase in the course of cooling was also studied. With respect to the subcell structure and conformational order of hydrocarbon chains in the beta and beta' phases, triolein resembles saturated TAGs; trans-zigzag hydrocarbon chains are adopted in the T(parallel) subcell for the beta phase and in the O(perpendicular) subcell for the beta' phase. The influence of cis-unsaturation was most obvious in the structure of the alpha phase and its temperature dependence. The alpha phase of triolein does not form the ordinary hexagonal subcell but a rather loose distorted subcell, which hardly changes in cooling, forming a striking contrast to the hexagonal-->pseudohexagonal subcell transformation found in the alpha phase of saturated TAGs.

Hierarchical supramolecular fullerene architectures with controlled dimensionality

Self-assembly of a fullerene derivative with long alkyl chains in different solvents results in the formation of hierarchically-ordered supramolecular assemblies with well-defined 1, 2 and 3D architectures such as vesicles, fibers, discs and cones, whose fundamental structural sub-unit consists of bilayers.