Pickering Emulsions for Food Applications: Background, Trends, and Challenges

Particle-stabilized emulsions, also referred to as Pickering emulsions, have garnered exponentially increasing interest in recent years. This has also led to the first food applications, although the number of related publications is still rather low. The involved stabilization mechanisms are fundamentally different as compared to conventional emulsifiers, which can be an asset in terms of emulsion stability. Even though most of the research on Pickering emulsions has been conducted on model systems, with inorganic solid particles, recent progress has been made on the utilization of food-grade or food-compatible organic particles for this purpose. This review reports the latest advances in that respect, including technical challenges, and discusses the potential benefits and drawbacks of using Pickering emulsions for food applications, as an alternative to conventional emulsifier-based systems

Formation, Structural, and Functionality of Interfacial Layers in Food Emulsions

Emulsions, i.e., the dispersion of liquid droplets in a nonmiscible liquidphase, are overwhelmingly present in food products. In such systems, bothliquid phases (generally, oil and water) are separated by a narrow region, theoil-water interface. Despite the fact that this interface is very thin (in thenanometer range), it represents a large surface area and controls to a greatextent the physicochemical stability of emulsions. This review provides anoverview of the aspects that govern the composition, structure, and mechanicalproperties of interfaces in food emulsions, taking into account the complexityof such systems (presence of numerous surface-active molecules, influenceof processing steps, and dynamic evolution due to chemical changes).We also review methods that have conventionally, or recently, been used tostudy liquid-liquid interfaces at various scales. Finally, we focus on the linkbetween interfacial properties and the physical, chemical, and digestive stabilityof emulsions at different levels and point out trends to control stabilityvia interfacial engineering

Lipid oxidation in oil-in-water emulsions: Involvement of the interfacial layer

More polyunsaturated fats in processed foods and fewer additives are a huge demand of public health agencies and consumers. Consequently, although foods have an enhanced tendency to oxidize, the usage of antioxidants, especially synthetic antioxidants, is restrained. An alternate solution is to better control the localization of reactants inside the food matrix to limit oxidation. This review establishes the state-of-the-art on lipid oxidation in oil-in-water (O/W) emulsions, with an emphasis on the role of the interfacial region, a critical area in the system in that respect. We first provide a summary on the essential basic knowledge regarding (i) the structure of O/W emulsions and interfaces and (ii) the general mechanisms of lipid oxidation. Then, we discuss the factors involved in the development of lipid oxidation in O/W emulsions with a special focus on the role played by the interfacial region. The multiple effects that can be attributed to emulsifiers according to their chemical structure and their location, and the interrelationships between the parameters that define the physicochemistry and structure of emulsions are highlighted. This work sheds new light on the interpretation of reported results that are sometimes ambiguous or contradictory

Cross-flow microfluidic emulsification from a food perspective

Background: The use of microfluidics is a relatively new route to produce emulsions. Advantages of this method include high energy efficiency, high droplet monodispersity, and potential use for the production of high added-value and fragile products. However, the current productivity is still rather low compared to what would be needed in an industrial setting.Scope and Approach: In this review we discuss the mechanisms of emulsion droplet formation in crossflow microfluidics, and how microfluidic design, shear forces and interfacial tension forces influence droplet formation. These combined insights will be used to discuss the potential of cross-flow microfluidics for the production of food emulsions.Key Findings and Conclusions: In order to make emulsions at large scale, the current devices need to be parallelised even more than shown in the successful examples known from literature. Besides, the behaviour of ingredients used in emulsion formulation need to be tested in greater detail; e.g. the effectof interfacial tension is captured in scaling relations, but dynamic interfacial tension behaviour not. For this also microfluidic analytical tools have been suggested, and the first positive results were obtained. As soon as these two requirements are met, microfluidics become a promising option for the production of (high added-value) emulsion food products

The Importance of Interfacial Tension in Emulsification: Connecting Scaling Relations Used in Large Scale Preparation with Microfluidic Measurement Methods

This paper starts with short descriptions of emulsion preparation methods used at large and smaller scales. We give scaling relations as they are generally used, and focus on the central role that interfacial tension plays in these relations. The actual values of the interfacial tension are far from certain given the dynamic behavior of surface-active components, and the lack of measurementmethods that can be applied to conditions as they occur during large-scale preparation. Microfluidic techniques are expected to be very instrumental in closing this gap. Reduction of interfacial tension resulting from emulsifier adsorption at the oil-water interface is a complex process that consists ofvarious steps. We discuss them here, and present methods used to probe them. Specifically, methods based on microfluidic tools are of great interest to study short droplet formation times, and also coalescence behavior of droplets. We present the newest insights in this field, which are expected to bring interfacial tension observations to a level that is of direct relevance for the large-scale preparation of emulsions, and that of other multi-phase products.<br/

Effect of interfacial properties on the reactivity of a lipophilic ingredient in multilayered emulsions

The aim of this work was to investigate the location and reactivity of a lipophilic spin probe, 4-phenyl- 2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO) in multilayered, biopolymer-based emulsions stabilized with a primary anionic layer (sodium caseinate) and a secondary cationic layer (lysozyme or diethylaminoethyl (DEAE) dextran). A broad range of ¿-potential values, from ca. -55 mV to 35 mV, was achieved. Emulsions with good physical stability were achieved when the magnitude of the net charge on the droplets was sufficiently great, otherwise some physical destabilization (flocculation) could be observed, especially in the case of caseinate-lysozyme-stabilized emulsions. The analysis of electron paramagnetic resonance (EPR) spectra of PTMIO in emulsion systems showed that probe molecules partitioned between the oil droplet core (ca. 73%) and the aqueous phase (ca. 27%), independently of the interfacial composition. Surprisingly, the rate of reduction of the nitroxide group of PTMIO by ascorbate anions remained unchanged when secondary interfacial layers were added, showing that the droplet surface charge was not the prevalent factor controlling the interactions between lipophilic compounds and aqueous phase reagents. Instead we argue that the reduction of PTMIO occurs in the aqueous phase

Behavior of plant-dairy protein blends at air-water and oil-water interfaces

Recent work suggests that using blends of dairy and plant proteins could be a promising way to mitigate sustainability and functionality concerns. Many proteins form viscoelastic layers at fluid interfaces and provide physical stabilization to emulsion droplets; yet, the interfacial behavior of animal-plant protein blends is greatly underexplored. In the present work, we considered pea protein isolate (PPI) as a model legume protein, which was blended with well-studied dairy proteins (whey protein isolate (WPI) or sodium caseinate (SC)). We performed dilatational rheology at the air-water and oil-water interface using an automated drop tensiometer to chart the behavior and structure of the interfacial films, and to highlight differences between films made with either blends, or their constituting components only. The rheological response of the blend-stabilized interfaces deviated from what could be expected from averaging those of the individual proteins and depended on the proteins used; e.g. at the air-water interface, the response of the caseinate-pea protein blend was similar to that of PPI only. At the oil-water interface, the PPI and WPI-PPI interfaces gave comparable responses upon deformation and formed less elastic layers compared to the WPI-stabilized interface. Blending SC with PPI gave stronger interfacial layers compared to SC alone, but the layers were less stiff compared to the layers formed with WPI, PPI and WPI-PPI. In general, higher elastic moduli and more rigid interfacial layers were formed at the air-water interface, compared to the oil-water interface, except for PPI

Interfacial properties of whey protein and whey protein hydrolysates and their influence on O/W emulsion stability

Protein hydrolysates are commonly used in high-tolerance or hypoallergenic formulae. The relation between the physicochemical properties of hydrolysed proteins (i.e., size, molecular weight distribution, charge, hydrophobicity), and their emulsifying properties is not fully understood. In this work, the emulsion forming ability (i.e., the equilibrium between droplet formation and coalescence during emulsification), the gravitational stability, the adsorption kinetics and the interfacial dilatational rheology of whey proteins and whey protein hydrolysates were investigated. More extensive hydrolysis resulted in a progressive decrease of the surface hydrophobicity of the emulsifiers (i.e., whey protein or whey protein hydrolysates). Whey protein was able to form smaller emulsion droplets at low concentrations (2 wt%), similar minimum droplet sizes were obtained due to the adsorption of large peptides. Whey protein-stabilised interfaces showed the lowest interfacial tension and ζ-potential, which both increased with increasing degree of hydrolysis. Whey protein produced stronger oil-water interfacial layers (i.e., high dilatational moduli and non-linear behavior) and had higher protein surface coverage compared to WPH. Small whey protein peptides (5 kDa) were preferentially adsorbed over small peptides. Emulsion physical stability was strongly influenced by the oil droplet size, and by the formation of an inter-connected viscoelastic film at the oil droplet interface which was observed only for whey protein and peptides with high molecular weight (>5 kDa). These results should be considered when formulating specialized nutrition emulsions

Oxidative stability of emulsions fortified with iron: the role of liposomal phospholipids

Interest in supplementing food with iron to counteract dietary deficiencies has been on the rise in recent years. A major challenge is the pro‐oxidant activity of soluble iron, which compromises the chemical stability of the enriched food products. This problem could be mitigated by encapsulating iron, to physically keep it separated from oxidizable substrates, such as unsaturated fatty acids. In the present work, the physical and chemical stability of surfactant‐ or protein‐stabilized oil‐in‐water emulsions fortified with iron was investigated

Sequential adsorption and interfacial displacement in emulsions stabilized with plant-dairy protein blends

Hypothesis: Many traditional or emergent emulsion products contain mixtures of proteins, resulting in complex, non-equilibrated interfacial structures. It is expected that protein displacement at oil-water interfaces depends on the sequence in which proteins are introduced during emulsion preparation,and on its initial interfacial composition.Experiments: We produced emulsions with whey, pea or a whey-pea protein blend and added extra protein post-emulsification. The surface load was measured indirectly via the continuous phase, or directly via the creamed phase. The interfacial composition was monitored over a three-day period using SDSPAGEdensitometry. We compared these findings with results obtained using an automated drop tensiometer with bulk-phase exchange to highlight the effect of sequential protein adsorption on interfacial tension and dilatational rheology.Findings: Addition of a second protein increased the surface load; especially pea proteins adsorbed to pre-adsorbed whey proteins, leading to thick interfacial layers. The addition of whey proteins to a pe