Different interfaces for stabilizing liquid-liquid, liquid-gel and gel-gel emulsions: Design, comparison, and challenges.

Interfaces play essential roles in the stability and functions of emulsion systems. The quick development of novel emulsion systems (e.g., water-water emulsions, water-oleogel emulsions, hydrogel-oleogel emulsions) has brought great progress in interfacial engineering. These new interfaces, which are different from the traditional water-oil interfaces, and are also different from each other, have widened the applications of food emulsions, and also brought in challenges to stabilize the emulsions. We presented a comprehensive summary of various structured interfaces (stabilized by mixed-layers, multilayers, particles, nanodroplets, microgels etc.), and their characteristics, and designing strategies. We also discussed the applicability of these interfaces in stabilizing liquid-liquid (water-oil, water-water, oil-oil, alcohol-oil, etc.), liquid-gel, and gel-gel emulsion systems. Challenges and future research aspects were also proposed regarding interfacial engineering for different emulsions. Emulsions are interface-dominated materials, and the interfaces have dynamic natures, as the compositions and structures are not constant. Biopolymers, particles, nanodroplets, and microgels differed in their capacity to get absorbed onto the interface, to adjust their structures at the interface, to lower interfacial tension, and to stabilize different emulsions. The interactions between the interface and the bulk phases not only affected the properties of the interface, but also the two phases, leading to different functions of the emulsions. These structured interfaces have been used individually or cooperatively to achieve effective stabilization or better applications of different emulsion systems. However, dynamic changes of the interface during digestion are only poorly understood, and it is still challenging to fully characterize the interfaces.

Stabilization of oil-in-water high internal phase emulsions with octenyl succinic acid starch and beeswax oleogel.

High internal phase emulsions (HIPEs) based on beeswax (BW) oleogels and octenyl succinic acid starch (OSA starch) were prepared by a facile one-step method. Effects of the oleogelation of internal phase on the formation, stability and functionality of the HIPEs were investigated. OSA starch absorbed at the interface allowed high surface charge (|ζ| > 25 mV) of the droplets, and small droplet size (d ≈ 5 m). Microstructural observation suggested that the HIPEs were of O/W type with droplets packed tightly. With the increase in BW content (0-4 %), the particle size (4-7 µm) and ζ-potential (-25 ~ -30 mV) of the HIPEs were first decreased and then increased. Stability analysis revealed that the addition of BW effectively improved emulsion stability against centrifugation, freeze-thawing, changes in pH and ionic strength, and the HIPE with 2 % BW presented the best stability. Rheological tests indicated that the HIPEs with higher content of BW exhibited higher storage modulus, solid-like properties, and shear thinning behaviors. Creep-recovery results implied that the oleogelation enhanced the structure of HIPEs and improved the deformation resistance of the systems. When subjected to light and heat, oleogel-in-water HIPEs showed advantages in protecting ß-carotene from degradation, and ß-carotene in the HIPEs with 2 % BW had the lowest degradation rate. These findings suggested that gelation of oil phase could improve the stability of HIPEs and the encapsulation capability, which would be meaningful for the development of novel healthy food.

Enhanced freeze-thawing stability of water-in-oil Pickering emulsions stabilized by ethylcellulose nanoparticles and oleogels.

This study developed water-in-oil (W/O) Pickering emulsions stabilized by ethylcellulose (EC) nanoparticles and EC oleogels, which presented significantly improved freeze-thawing (F/T) stability. Microstructural observation suggested EC nanoparticles were distributed at the interface and within the water droplets, and the EC oleogel trapped oil in the continuous phase. Freezing and melting temperatures of water in the emulsions with more EC nanoparticles were lowered and the corresponding enthalpy values were reduced. F/T led to lower water binding capacity but higher oil binding capacity of the emulsions, compared to the initial emulsions. Low field-nuclear magnetic resonance confirmed the increased mobility of water but decreased mobility of oil in the emulsions after F/T. Both linear and nonlinear rheological properties proved that emulsions exhibited higher strength and higher viscosity after F/T. The widened area of the elastic and viscous Lissajous plots with more nanoparticles suggested the viscosity and elasticity of emulsions were increased.

Rheology and Tribology of Ethylcellulose-Based Oleogels and W/O Emulsions as Fat Substitutes: Role of Glycerol Monostearate.

Rheological and tribological properties of oleogels and water-in-oil (W/O) emulsions are important for application in fat substitutes. This study investigated the roles of glycerol monostearate (GMS) in tailoring the structural, rheological and tribological properties of ethylcellulose (EC)-based oleogels and W/O emulsions as potential fat substitutes. The addition of GMS contributed to more round and compact oil pores in oleogel networks. The oleogel with 5% GMS had higher crystallinity, leading to solid state (lower tanδ value), mechanical reversibility (higher thixotropic recovery), but a brittle (lower critical strain) structure in the samples. GMS gave the oleogels and emulsions higher oil binding capacity, storage modulus and yield stress. Under oral processing conditions, GMS addition contributed to higher textural attributes and viscosity. Friction coefficients in mixed and boundary regions of oleogels and emulsions were reduced with the increase in GMS content from 0~2%, but increased with 5% GMS. Rheological and tribological properties of lard, mayonnaise and cream cheese can be mimicked by EC oleogels with 5% GMS, or emulsions with 2% GMS and 2-5% GMS, respectively. The study showed the potentials of oleogel and W/O emulsions in designing low-fat products by tuning the structures for healthier and better sensory attributes.

W/O emulsions featuring ethylcellulose structuring in the water phase, interface and oil phase for multiple delivery.

This study reported a novel approach to stabilize W/O emulsions with nanoparticles and oleogel from ethylcellulose (EC), and the emulsions were applied for multiple delivery. Microstructural observations revealed that EC nanoparticles were distributed within the water droplets and at the interface, and EC oleogel was in the continuous phase. Storage modulus and viscosity were higher, and thixotropic structural recovery rates were lower, in the emulsions with more EC nanoparticles. Both EC nanoparticles and oleogels worked to stabilized the emulsions upon freeze-thawing, as only minor water or oil was released. Upon heating, some EC nanoparticles transited from the interface to the oil phase, and water droplets were aggregated. The interface was recreated when the samples were re-emulsified. When curcumin and anthocyanin were incorporated, both bioactives had improved light stability. During in vitro study, curcumin was mostly released at the early stage of intestinal digestion, while anthocyanin was released at later stage.