Synchrotron radiation macrobeam and microbeam X-ray diffraction studies of interfacial crystallization of fats in water-in-oil emulsions

Recent advances in studying crystallisation of mono- and di-glycerides at oil-water interfaces

This review focuses on the current understanding regarding lipid crystallisation at oil-water interfaces. The main aspects of crystallisation in bulk lipids will be introduced, allowing for a more comprehensive overview of the crystallisation processes within emulsions. Additionally, the properties of an emulsion and the impact of lipid crystallisation on emulsion stability will be discussed. The effect of different emulsifiers on lipid crystallisation at oil-water interfaces will also be reviewed, however, this will be limited to their impact on the interfacial crystallisation of monoglycerides and diglycerides. The final part of the review highlights the recent methodologies used to study crystallisation at oil-water interfaces.

Polymorphic phase transitions in triglycerides and their mixtures studied by SAXS/WAXS techniques: In bulk and in emulsions

Triacylglycerols (TAGs) exhibit a monotropic polymorphism, forming three main polymorphic forms upon crystallization: α, β' and β. The distinct physicochemical properties of these polymorphs, such as melting temperature, subcell lattice structure, mass density, etc., significantly impact the appearance, texture, and long-term stability of a wide range products in the food and cosmetics industries. Additionally, TAGs are also of special interest in the field of controlled drug delivery and sustained release in pharmaceuticals, being a key material in the preparation of solid lipid nanoparticles. The present article outlines our current understanding of TAG phase behavior in both bulk and emulsified systems. While our primary focus are investigations involving monoacid TAGs and their mixtures, we also include illustrative examples with natural TAG oils, highlighting the knowledge transfer from simple to intricate systems. Special attention is given to recent discoveries via X-ray scattering techniques. The main factors influencing TAG polymorphism are discussed, revealing that a higher occurrence of structural defects in the TAG structure always accelerates the rate of the α → β polymorphic transformation. Diverse approaches can be employed based on the specific system: incorporating foreign molecules or solid particles into bulk TAGs, reducing drop size in dispersed systems, or using surfactants that remain fluid during TAG particle crystallization, ensuring the necessary molecular mobility for the polymorphic transformation. Furthermore, we showcase the role of TAG polymorphism on a recently discovered phenomenon: the creation of nanoparticles as small as 20 nm from initial coarse emulsions without any mechanical energy input. This analysis underscores how the broader understanding of the TAG polymorphism can be effectively applied to comprehend and control previously unexplored processes of notable practical importance.

Water Droplets Tailored as Wax Crystal Carriers to Mitigate Wax Deposition of Emulsion

This study explores how the micro-distribution change of wax crystals from the continuous oil phase to the oil-water interface mitigates the macro wax deposition of an emulsion. Two types of interfacial actions between wax crystals and water droplets, interfacial adsorption and interfacial crystallization, which were induced by two different emulsifiers, sorbitan monooleate (Span 80) and sorbitan monostearate (Span 60), respectively, were detected by differential scanning calorimetry and microscopy observation. The wax interfacial crystallization promoted by Span 60 resulted in the wax being nucleated directly at the oil-water interface prior to the continuous oil phase, conferring the nascent wax crystals and water droplets to be combined as coupled particles. The utilization of the wax interfacial crystallization behavior to hinder wax deposition of an emulsion was further explored. When the coupled wax crystal-water droplet particles were formed during the wax deposition process, water droplets acted as wax crystal carriers, entraining these nascent wax crystals to disperse in the emulsion, which significantly reduced the amount of wax crystals available to form the network of the deposit. In addition, this change also led to the basic structural units in the wax deposit evolving from wax crystal clusters/networks to water droplet flocs. The study elucidates that through adjusting the dispersion of wax crystals from the oil phase to the oil-water interface, water droplets could act as a functional component to tailor the properties of the emulsion or resolve related flow and deposition problems in pipeline transportation.

Crystallization of lipids and lipid emulsions treated by power ultrasound: A review

The actual food system with fat is always complex and fat crystal and fat crystal networks have important effects on the physical properties of food. Recently, power ultrasound (PU) had been widely recognized as an auxiliary technology of fat crystallization to modify food properties. This review expounded on the mechanism of ultrasonic crystallization, and summarized effects of various factors in the process of ultrasonic treatment on fat crystallization. Based on the above, combined with the application of ultrasound in emulsions, the ultrasonic fat crystallization effect in the emulsion system was judged and described. Research results indicated that PU could shorten the induction time of crystallization, accelerate the formation of crystal nuclei, and change the polymorphism of fat crystals. The product treated by PU formed smaller and more uniform crystals to produce a more viscoelastic fat crystal network. In emulsion systems, ultrasonic treatments showed the same effect, but the effect of ultrasonic crystallization on the emulsion stability was different due to fat crystals in different emulsion systems. Meanwhile, the importance of ultrasonic crystallization in lipid emulsions was emphasized, thus ultrasonic crystallization had great potential in emulsion systems.

A novel strategy to fabricate stable oil foams with sucrose ester surfactant

HYPOTHESIS

Can a mixture of sucrose ester surfactant in vegetable oil be aerated to yield stable oleofoams? Is foaming achievable from one-phase molecular solutions and/or two-phase crystal dispersions? Does cooling a foam after formation induce surfactant crystallisation and enhance foam stability?

EXPERIMENTS

Concentrating on extra virgin olive oil, we first study the effect of aeration temperature and surfactant concentration on foamability and foam stability of mixtures cooled from a one-phase oil solution. Based on this, we introduce a strategy to increase foam stability by rapidly cooling foam prepared at high temperature which induces surfactant crystallisation in situ. Differential scanning calorimetry, X-ray diffraction, infra-red spectroscopy, surface tension and rheology are used to elucidate the mechanisms.

FINDINGS

Unlike previous reports, both foamability and foam stability decrease upon decreasing the aeration temperature into the two-phase region containing surfactant crystals. At high temperature in the one-phase region, substantial foaming is achieved (over-run 170%) within minutes of whipping but foams ultimately collapse within a week. We show that surfactant molecules are surface-active at high temperature and that hydrogen bonds form between surfactant and oil molecules. Cooling these foams substantially increases foam stability due to both interfacial and bulk surfactant crystallisation. The generic nature of our findings is demonstrated for a range of vegetable oil foams with a maximum over-run of 330% and the absence of drainage, coalescence and disproportionation being achievable.

An Insight into the Solid-State Miscibility of Triacylglycerol Crystals

The crystallization properties of triacylglycerols (TAGs) strongly determine the functional properties of natural lipids. The polymorphic and mixing phase behavior of TAG molecules have long been, and still are, a hot topic of research with special relevance for the cosmetic, pharmaceutical, and food industry. To avoid the difficulties arising from the study of whole real fats, studies at the molecular level on mixtures of a limited number of TAGs has become an indispensable tool to identify the underlying causes of the physical properties in lipid systems. In particular, phase diagrams of binary mixtures of TAGs exhibiting a different degree of heterogeneity (monoacid or mixed fatty acids; molecular symmetry; the presence of cis or trans double bonds) have resulted in a significant breakthrough in our knowledge about structure-interaction-function relationships. The present work aims to provide an overview of the main reports regarding binary and ternary TAG systems, from the early studies to the most recent developments.

Effect of diacylglycerol interfacial crystallization on the physical stability of water-in-oil emulsions

The influence of diacylglycerol (DAG) combined with polyglycerol polyricinoleate (PGPR) on the stability of water-in-oil (W/O) emulsions containing hydrogenated palm oil (HPO) was studied. Polarized light microscope revealed that DAG promoted HPO to crystallize at the water-oil interface, providing the combination of Pickering and network stabilization effects. It was proposed that the molecular compatibility of fatty acids in DAG with HPO accounted for the promotional effect. The interfacial crystallization of DAG together with the surface activity of PGPR led to the formation of emulsions with uniform small droplets and high freeze-thaw stability. Further exploration of physical properties indicated that the combination of DAG and PGPR dramatically improved the emulsion's viscoelasticity and obtained a larger deformation yield. Water droplets in DAG-based emulsions acted as active fillers to improve the network rigidity. Therefore, DAG is a promising material to be used as emulsifier to enhance the physical stability of W/O emulsions.

Interfacial ordering of tristearin induced by glycerol monooleate and PGPR: A coarse-grained molecular dynamics study

We use coarse-grained molecular dynamics simulations to study the effect of surfactant structure on the ordering of bulk tristearin at an oil-water interface. In the absence of surfactant, tristearin acyl chains are marginally aligned normal to the interface. The surfactant glycerol monooleate (GMO), a common small-molecule monoacylglycerol (MW: 357 g/mol), preferentially adsorbs to the oil-water interface, displacing more of the tristearin as its concentration increases. The tristearin that remains at the interface is closely aligned normal to the interface. Adjacent to the interface, bulk tristearin increasingly aligns with its acyl chains entwined with the GMO acyl chain, which also preferentially aligns normal to the interface. In contrast, polyglycerol polyricinoleate (PGPR), a bulkier, polymeric surfactant (MW: 1398 g/mol for a molecule with five monomers), both displaces tristearin from the interface and reduces the alignment of the molecules that remain. We suggest that the similar fatty acid moieties of GMO (oleic acid) and tristearin (stearic acid) lead to liquid-state association and alignment, the latter of which can then serve as a template onto which tristearin crystals can nucleate. Conversely, by both displacing tristearin from the interface and reducing alignment below that of the surfactant-free system, PGPR eliminates the possibility of tristearin interfacial crystallisation.

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)".

Crystallization of Femtoliter Surface Droplet Arrays Revealed by Synchrotron Small-Angle X-ray Scattering

The crystallization of oil droplets is critical in the processing and storage of lipid-based food and pharmaceutical products. Arrays of femtoliter droplets on a surface offer a unique opportunity to study surfactant-free colloidlike systems. In this work, the crystal growth process in these confined droplets was followed by cooling a model lipid (trimyristin) from a liquid state utilizing synchrotron small-angle X-ray scattering (SAXS). The measurements by SAXS demonstrated a reduced crystallization rate and a greater degree of supercooling required to trigger lipid crystallization in droplets compared to those of bulk lipids. These results suggest that surface droplets crystallize in a stochastic manner. Interestingly, the crystallization rate is slower for larger femtoliter droplets, which may be explained by the onset of crystallization from the three-phase contact line. The larger surface nanodroplets exhibit a smaller ratio of droplet volume to the length of three-phase contact line and hence a slower crystallization rate.

Mass spectrometric imaging of localization of fat molecules in water-in-oil emulsions containing semi-solid fat

Firstly, we report the localization analysis of the lipid components of a water-in-oil (W/O) semi-solid emulsion by mass spectrometry imaging (MSI). Uniform emulsion droplets were prepared using microchannel emulsification devices with lecithin, stearic acid-binding monoglyceride (St-MAG), and polyglycerol polyricinoleate (PGPR) as emulsifiers. The mass image gives us the localization of phosphatidylcholine (PC) in lecithin, St-MAG, tripalmitin (PPP), medium-chain triglyceride (MCT), and high-melting-point triglyceride tristearin (C18-TAG). PC, St-MAG, and PPP were localized at the interface with the dispersed water droplets. PC and PPP took the same localized position, suggesting an interaction between PC and PPP at the interface. Conversely, PC existed in other regions with St-MAG. MSI revealed multiple target molecules in fat products in a single measurement, and it is expected to reveal fat crystallization at the emulsion interfaces, which will clarify the mechanisms related to the physical properties of high-fat products such as fat spread and butter.

Stability of the Metastable α-Polymorph in Solid Triglyceride Drug-Carrier Nanoparticles

Colloidal dispersions of crystalline nonpolar lipids are under intensive investigation as carrier systems in pharmaceutics and nutrition. In this context, the controlled preparation of particles in a metastable polymorphic state is of some interest for the delivery of active substances. In the present study, tristearin particles stabilized with three α-polymorph-preserving emulsifier regimes ((I) sodium glycocholate/saturated long-chain phospholipids, (II) sodium glycocholate, and (III) poly(vinyl alcohol) (PVA)) were investigated concerning the stability of the metastable α-polymorph after controlled crystallization of the particles from the melt. Upon long-term storage, the α-polymorph was preserved best in PVA-stabilized dispersions, followed by those stabilized with the glycocholate/phospholipid mixture and finally those stabilized solely with the bile salt. In particular for rapidly crystallized nanoparticles, the formation of an α-polymorph with highly reduced lamellarity was observed. According to time-/temperature-resolved synchrotron X-ray diffraction analysis with simultaneous DSC (differential scanning calorimetry) studies, this less-ordered α-polymorph transformed into the common, lamellar α-form upon heating. Although the presence of the less-ordered form is probably related to the extraordinarily high stability of the metastable α-polymorph observed in some of the dispersions, it could not completely prevent the transition into the stable β-polymorph. The higher the transition temperature of the less-ordered α-form to the ordered one, the slower was the polymorphic transition to the stable β-polymorph. To estimate the polymorphic stability of the differently stabilized particles upon isothermal long-term storage, standard DSC measurements on samples stored at 23 °C for 4 weeks seem to be of predictive value.

Promotional effects of new types of additives on fat crystallization

We examined the promotional effects of additives on fat crystallization, such as inorganic (talc, carbon nanotube (CNT), and graphite) and organic (theobromine, ellagic acid dihydrate (EAD), and terephthalic acid) materials. The triacylglycerols (TAGs) of trilauroylglycerol (LLL), trimyristoylglycerol (MMM), and tripalmitoylglycerol (PPP) were employed as the fats. The additives (1 wt%) were added to the molten TAGs, and then the mixtures were cooled at a rate of 1°C/min followed by heating at a rate of 5°C/min. The crystallization and melting properties were observed using differential scanning calorimetry, X-ray diffraction, and polarized optical microscope (POM). Consequently, we found that the above six additives remarkably increased the initial temperatures of crystallization (Ti) on cooling without changing the melting temperatures. For example, in the case of LLL, the increases in Ti were 2.6°C (talc), 3.9°C (CNT), 8.1°C (graphite), 1.1°C (theobromine), 2.0°C (EAD), and 6.8°C (terephthalic acid). Very similar effects were observed for the crystallization of MMM and PPP with the six additives. Furthermore, the polymorphs of the first occurring crystals were changed from metastable to more stable forms by many of these additives. The POM observation revealed that the crystallization was initiated at the surfaces of additive particles. This study has shown for the first time that the heterogeneous nucleation of fat crystals can be greatly promoted by new types of additives. Such additives have great potential to promote fat crystallization by not only hydrophobic but also hydrophilic molecular interactions between the fats and additives.

Fat crystallisation at oil-water interfaces

This review focuses on recent advances in the understanding of lipid crystallisation at or in the vicinity of an interface in emulsified systems and the consequences regarding stability, structure and thermal behaviour. Amphiphilic molecules such as emulsifiers are preferably adsorbed at the interface. Such molecules are known for their ability to interact with triglycerides under certain conditions. In the same manner that inorganic crystals grown on an organic matrix see their nucleation, morphology and structure controlled by the underlying matrix, recent studies report a templating effect linked to the presence of emulsifiers at the oil/water interface. Emulsifiers affect fat crystallisation and fat crystal behaviour in numerous ways, acting as impurities seeding nucleation and, in some cases, retarding or enhancing polymorphic transitions towards more stable forms. This understanding is of crucial importance for the design of stable structures within emulsions, regardless of whether the system is oil or water continuous. In this paper, crystallisation mechanisms are briefly described, as well as recent technical advances that allow the study of crystallisation and crystal forms. Indeed, the study of the interface and of its effect on lipid crystallisation in emulsions has been limited for a long time by the lack of in-situ investigative techniques. This review also highlights reported interfacial effects in food and pharmaceutical emulsion systems. These effects are strongly linked to the presence of emulsifiers at the interface and their effects on crystallisation kinetics, and crystal morphology and stability.

The Effective Factors on the Structure of Butter and Other Milk Fat-Based Products

Butter and other milk fat-based products are valuable products for the dairy industry due to their unique taste, their textural characteristics, and nutritional value. However, an increased consumer demand for low-fat-based products increases the need for an increased essential understanding of the effective factors governing the structure of milk fat-based products. Today, 2 manufacturing techniques are available: the churning method and the emulsification method. The first is typically used for production of butter with a globular structure, which has become increasingly popular to obtain low-fat-based products, typically without presence of milk fat globules. The microstructure of milk fat-based products is strongly related to their structural rheology, hence applications. Structural behavior is not determined by one single parameter, but by the interactions between many. This complexity is reviewed here. Parameters such as thermal treatment of cream prior to butter making, water content, and chemical composition influence not only crystal polymorphism, but also the number and sizes of fat crystals. The number of crystal-crystal interactions formed within the products is related to product hardness. During storage, however, postcrystallization increases the solid fat content and strengthens the fat crystal network. The fat crystal network is strengthened by the formation of more and stronger crystal-crystal interactions due to mechanically interlinking of fat crystals, which occurs during crystal growth. Postcrystallization is directly linked to chemical composition. The initially observed microstructural difference causing different rheological behavior will disappear during storage due to postcrystallization and formation of more crystal-crystal interactions.