The Potential Application of Pickering Multiple Emulsions in Food

Influence of positively charged nanoparticles on the stability of oil-in-water emulsions stabilized by a cationic surfactant at extremely low concentration

HYPOTHESIS

Charged particles and like-charged ionic surfactant can co-stabilize oil-in-dispersion emulsions at very low concentrations. The surfactant molecules adsorb at the oil-water interface endowing droplets with charge whereas particles remain dispersed in the aqueous phase forming a thick lamella between droplets. The reduced van der Waals attraction between droplets together with the electric double layer repulsion between droplets and between droplets and particles prevents droplets from flocculation and coalescence.

EXPERIMENTS

n-Decane-in-water emulsions co-stabilized by positively charged alumina nanoparticles (0.0001-0.06 wt%) and cationic surfactant cetyltrimethyl-ammonium bromide (CTAB, 0.002-0.1 mM) were prepared and characterized. Specifically, the minimum particle concentration required for emulsion stabilization as a function of CTAB concentration was determined, which describes a V shaped boundary separating stable emulsions from unstable ones.

FINDINGS

A model based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was proposed to calculate the droplet-droplet and droplet-particle interactions. Along the V shaped boundary particles are attracted by droplets at large distance but may form a monolayer between droplets when they approach at small distance. Both the attraction and repulsion between droplets and particles are proportional to the particle number surrounding unit area of droplet interfaces and the repulsion is always larger than the attraction. This together with the droplet-droplet repulsion ensures emulsion stabilization. Increasing CTAB concentration results in high interfacial potential but also high counterion concentration; to compensate, the particle concentration required for emulsion stabilization is initially reduced and subsequently increased yielding the V shaped boundary.

Comprehensive Sensory Evaluation in Low-Fat Emulsions: A Systematic Review of Diverse Food Applications

The prevalence of diet-related health issues has driven the demand for healthier food options, particularly those with reduced fat content. This systematic review evaluates the integration of sensory analysis in low-fat emulsion research, highlighting a significant gap in current practices. From an initial pool of 400 articles, 227 unique studies were screened, but only 15 (6.6%) included sensory analysis, underscoring a major shortfall in evaluating consumer acceptance. The reviewed studies investigated various emulsion types, including simple emulsions, emulsion gels, and Pickering emulsions, utilizing a diverse range of fat replacers, such as plant-based oils, proteins, and modified starches. These fat replacers included natural and modified ingredients such as banana peel flour, lard-based diacylglycerols, cedar oil cake, microparticulated egg white proteins, Nigella sativa oil, avocado, whey protein, flaxseed oil, polyphenol extracts, okara, microcrystalline wax and cellulose, rapeseed cake, and polysaccharide nanoparticles. These innovative approaches aimed to improve the sensory attributes of meat products, dairy-type applications, salad dressings, and bakery products. The review highlights a disparity in the rigor and comprehensiveness of sensory evaluations among studies. While some studies have thoroughly assessed multiple attributes, others have been limited to general acceptability. This variability underscores the need for standardized, detailed sensory analysis in low-fat emulsion research to ensure a comprehensive understanding of consumer preferences and product quality.

Pickering emulsions stabilized by prolamin-based proteins as innovative carriers of bioactive compounds

Pickering emulsions (PEs) can be used as efficient carriers for encapsulation and controlled release of different bioactive compounds. Recent research has revealed the potential of prolamins in development of nanoparticle- and emulsion-based carriers which can improve the stability and bioavailability of bioactive compounds. Prolamin-based particles have been effectively used as stabilizers of various PEs including single PEs, high internal phase PEs, multiple PEs, novel triphasic PEs, and PE gels due to their tunable self-assembly behaviors. Prolamin particles can be fabricated via different techniques including anti-solvent precipitation, dissolution followed by pH adjustment, heating, and ion induced aggregation. Particles fabricated from prolamins alone or in combination with other hydrocolloids or polyphenols have also been used for stabilization of different PEs which were shown to be effective carriers for food bioactives, providing improved stability and functionality. This article covers the recent advances in various PEs stabilized by prolamin particles as innovative carriers for bioactive ingredients. Strategies applied for fabrication of prolamin particles and prolamin-based carriers are discussed. Emerging techno-functional applications of prolamin-based PEs and possible challenges are also highlighted.

W/O/W Pickering emulsions stabilized by complex modified phycocyanin

BACKGROUND

A water-in-oil-in-water (W/O/W) double emulsion can simultaneously load hydrophilic and hydrophobic substances due to its unique two-membrane, three-phase structure. However, thermodynamic instability greatly limits the application of double emulsions in food processing. Further development of Pickering emulsions based on proteins, etc., can improve the stability and loading capacity. It is of great significance to promote their practical application.

RESULTS

Herein, we prepared ultrasound pretreatment complex glycation-modified phycocyanin (UMPC) to stabilize a W/O/W Pickering emulsion for the codelivery of vitamin B12 (VB12) and vitamin E (VE). First, an inner water phase and oil phase containing polyglycerin polyricinoleate were homogenized to prepare a W/O emulsion. Subsequently, the W/O emulsion was homogenized with an outer water phase containing UMPC to obtain a W/O/W Pickering emulsion. A gel-like inner phase emulsion with excellent storage and thermal stabilities was obtained under the condition that the W/O emulsion volume ratio was 80% and the UMPC was stabilized by 10 g kg-1. The double emulsion after loading VB12 and VE showed good encapsulation effect during the storage period, the encapsulation rate could reach more than 90%, it also showed excellent protection effect under long-time storage and UV irradiation and the retention rate increased by more than 65%. In addition, the bioavailability of VB12 and VE significantly increased during simulated gastrointestinal digestion and reached 46.02% and 52.43%, respectively.

CONCLUSION

These results indicate that the UMPC-stabilized W/O/W Pickering emulsion is an effective carrier for the codelivery of hydrophilic and hydrophobic bioactive molecules and also provides a means for useful exploration of an efficient and stable emulsion system stabilized by biological macromolecules. © 2024 Society of Chemical Industry.

Physicochemical properties and in vitro digestive behavior of astaxanthin loaded Pickering emulsion gel regulated by konjac glucomannan and κ-carrageenan

This study aimed to elaborate the combination effect of polysaccharides on physicochemical properties and in vitro digestive behavior of astaxanthin (AST)-loaded Pickering emulsion gel. AST-loaded Pickering emulsion gel was prepared by heating Pickering emulsion with konjac glucomannan (KGM) and κ-carrageenan (CRG). The microstructure revealed that adding the two polysaccharides resulted in Pickering emulsion forming a network structure. It exhibited a denser and more uniform network structure, enhancing its mechanical properties four times and increasing its water-holding capacity by 20 %. In vitro digestion experiments demonstrated that the release of free fatty acids from the Pickering emulsion gel (4.25 %) was notably lower than that from conventional Pickering emulsion (17.19 %), whereas AST bioaccessibility was remarkably low at 0.003 %. It provided a feasible strategy to regulate the bioaccessibility in Pickering emulsion, which has theoretical significance to guide the current eutrophic diet people.

Pickering emulsion co-delivery system: Stimuli-responsive biomineralized particles act as particulate emulsifiers and bioactive carriers

Pickering emulsions provide a promising platform for the efficient delivery of bioactive. However, co-delivery of fragile bioactives with different physicochemical properties for comprehensive effects still faces practical challenges due to the limited protection for bioactives and the lack of stimuli-responsive property for on-demand release. Herein, a stimuli-responsive co-delivery system is developed based on biomineralized particles stabilized Pickering emulsions. In this tailor co-delivery system, hydrophilic bioactive (pepsin) with the fragile structure is encapsulated and immobilized by biomineralization, the obtained biomineralized particles (PPS@CaCO3) are further utilized as emulsifiers to form O/W Pickering emulsions, in which the hydrophobic oxidizable bioactive (curcumin) is stably trapped into the dispersed phase. The results show that two bioactives are successfully co-encapsulated in Pickering emulsions, and benefiting from the protection capacities of biomineralization and Pickering emulsions, the activity of pepsin and curcumin shows a 7.33-fold and 144.83-fold enhancement compared to the free state, respectively. Moreover, In vitro study demonstrates that Pickering emulsions enable to co-release of two bioactives with high activity retention by the acid-induced hydrolyzation of biomineralized particles. This work provides a powerful stimuli-responsive platform for the co-delivery of multiple bioactive compounds, enabling high activity of bioactives for the comprehensive health effects.

The Composition Optimization of Curcumin-Loaded Double Oil-Water-Oil Emulsions and Their Stability Evaluation

This study aimed to optimize the preparation of multiple oil-water-oil (O/W/O) emulsions using varying amounts of Tween 20 emulsifier, different homogenization methods, and optimal preparation temperatures as carriers for encapsulated curcumin. Following the optimization process, the optimal preparation temperature was found to be 25 °C, with a homogenization speed of 10,000 RPM and an emulsifier concentration of 0.5% Tween 20. Subsequently, the effects of physicochemical and viscoelastic properties on the different types of oils used in the outer phase, as well as the impact of storage time, were monitored. The novelty of this work lies in its comprehensive analysis of the stability and encapsulation efficiency of multiple emulsions using various oils, an area that has not been extensively explored before. After identifying the optimal preparation procedure, all samples with different edible oils demonstrated excellent stability and encapsulation efficiency, showing minimal variation in results. The most stable multiple emulsion was found to be the one with coconut oil in the outer phase, exhibiting half the particle size compared to other samples and the lowest encapsulation efficiency losses over 50 days of storage. This study provides new insights into the formulation of stable multiple emulsions for the effective delivery of curcumin and similar bioactive compounds.

Encapsulation of Probiotics within Double/Multiple Layer Beads/Carriers: A Concise Review

An increased demand for natural products nowadays most specifically probiotics (PROs) is evident since it comes in conjunction with beneficial health effects for consumers. In this regard, it is well known that encapsulation could positively affect the PROs' viability throughout food manufacturing and long-term storage. This paper aims to analyze and review various double/multilayer strategies for encapsulation of PROs. Double-layer encapsulation of PROs by electrohydrodynamic atomization or electrospraying technology has been reported along with layer-by-layer assembly and water-in-oil-in-water (W1/O/W2) double emulsions to produce multilayer PROs-loaded carriers. Finally, their applications in food products are presented. The resistance and viability of loaded PROs to mechanical damage, during gastrointestinal transit and shelf life of these trapping systems, are also described. The PROs encapsulation in double- and multiple-layer coatings combined with other technologies can be examined to increase the opportunities for new functional products with amended functionalities opening a novel horizon in food technology.

Fabricating Pea Protein Micro-Gel-Stabilized Pickering Emulsion as Saturated Fat Replacement in Ice Cream

Unsaturated fat replacement should be used to reduce the use of saturated fat and trans fatty acids in the diet. In this study, pea protein micro-gels (PPMs) with different structures were prepared by microparticulation at pH 4.0-7.0 and named as PPM (pH 4.0), PPM (pH 4.5), PPM (pH 5.0), PPM (pH 5.5), PPM (pH 6.0), PPM (pH 6.5), and PPM (pH 7.0). Pea protein was used as a control to evaluate the structure and interfacial properties of PPMs by particle size distribution, Fourier transform infrared spectroscopy (FTIR), free sulfhydryl group content, and emulsifying property. PPM (pH 7.0) was suitable for application in O/W emulsion stabilization because of its proper particle size, more flexible structure, high emulsifying activity index (EAI) and emulsifying stability index (ESI). The Pickering emulsion stabilized by PPM (pH 7.0) had a uniform oil droplet distribution and similar rheological properties to cream, so it can be used as a saturated fat replacement in the manufacture of ice cream. Saturated fat was partially replaced at different levels of 0%, 20%, 40%, 60%, 80%, and 100%, which were respectively named as PR0, PR20, PR40, PR60, PR80, and PR100. The rheological properties, physicochemical indexes, and sensory properties of low-saturated fat ice cream show that PPM (pH 7.0)-stabilized emulsion can be used to substitute 60% cream to manufacture low-saturated fat ice cream that has high structural stability and similar melting properties, overrun, and sensory properties to PR0. The article shows that it is feasible to prepare low-saturated fat ice cream with PPM (pH 7.0)-stabilized Pickering emulsion, which can not only maintain the fatty acid profile of the corn oil used, but also possess a solid-like structure. Its application is of positive significance for the development of nutritious and healthy foods and the reduction of chronic disease incidence.

Single-Step Formation of Pickering Double Emulsions by Exploiting Differential Wettability of Particles

We present a modular single-step strategy for the formation of single and Pickering double emulsions (DEs). To this end, we consider the role of surface modification of particles and their dispersibility in different phases in the context of the design of Pickering emulsions by varying the volume fraction of oil in the oil-water mixture (ϕoil) used for emulsification. In particular, the experiments are performed by considering (a) model spherical and nonspherical colloids of different wettabilities which are tailored by oleic acid treatment, (b) immiscible liquids with or without particles, and (c) varying ϕoil from 0.1 to 0.9. We show that it is possible to affect a transition from (i) oil-in-water (O/W) emulsion to water-in-oil (W/O) emulsion and (ii) oil-in-water (O/W) to oil-in-water-in-oil (O/W/O) to water-in-oil (W/O) as ϕoil is systematically varied. We elucidate that the range of ϕoil at which particle stabilized DEs of the O/W/O type form can be tuned by engineering surface modification of particles to different extents. Furthermore, the arrangement of particles on the surface of droplets in the Pickering DEs is discussed. Our results conclusively establish that the differential wettability of particles is the key for the design of Pickering DEs. The versatility of the proposed strategy is established by developing DEs using a number of model colloidal systems.

Interfacial Activity of Janus Particle: Unity of Molecular Surfactant and Homogeneous Particle

Janus particles with different compositions and properties segmented to different regions on the surface of one objector provide more opportunities for interfacial engineering. As a novel interfacial active material, Janus particles integrate the amphiphilic properties of molecular surfactants and the Pickering effect of homogeneous particles. In this research, the outstanding properties of Janus particles on various interfaces are examined from both theoretical and practical perspectives, and the advantages of Janus particles over molecular surfactants and homogeneous particle surfactants are analyzed. We believe that Janus particles are ideal tools for interface regulation and functionalization in the future.

Novel Pickering emulsion gels stabilized solely by phenylalanine amidated pectin: Characterization, stability and curcumin bioaccessibility

Pickering emulsion gels represent a novel class of non-toxic and biocompatible emulsions, offering extensive applications in the pharmaceutical and food additive sectors. This study delineates the synthesis of Pickering emulsion gels utilizing native and amidated pectin samples. Phenylalanine amidated pectin (AP) was procured via an ultra-low temperature enzyme method, while the control group (LP) adhered to an identical procedure without papain catalysis. Experimental outcomes revealed that the AP Pickering emulsion gel manifested superior stability compared to pectin emulsion samples (PE and LP). The Pickering emulsion gel from 5 % amidated pectin (5AP) retained stability throughout a 14-day emulsion stability assessment. Furthermore, all emulsion samples were evaluated for their capacity to deliver and sustain curcumin within an in vitro digestion simulation. Rheological properties and oil droplet size results indicated that the 5AP Pickering emulsion gel exhibited optimal cream index and emulsion stability, effectively inhibiting premature water-oil stratification within the emulsion and augmenting curcumin bioaccessibility. Within the in vitro digestion simulation, the 5AP Pickering emulsion gel demonstrated the highest curcumin bioaccessibility, measured at 17.96 %.

Preparation and Emulsifying Properties of Carbon-Based Pickering Emulsifier

Water is increasingly being used as a solvent in place of organic solvent in order to meet the demand for green chemical synthesis. Nevertheless, many of the reaction substrates are organic matter, which have low water solubility, resulting in a low reaction interface and limiting the development of organic-water biphasic systems. A surfactant is typically added to the two-phase system to form an emulsion to increase the contact area between the organic phase and the water. Compared to ordinary emulsion stabilized with the surfactant, Pickering emulsion offers better adhesion resistance, biocompatibility, and environmental friendliness. It possesses unrivaled benefits as an emulsifier and catalyst in a two-phase interfacial catalysis system (PIC). In this study, the amine group (NNDB) was employed to alter the surface of graphene oxide (GO). A stable Pickering emulsion was created by adsorbing GO-NNDB on the toluene–water interface. It was determined that the emulsion system had good stability by analyzing digital photographs and microscope images of droplets at various temperatures, and fluorescence microscopy images of emulsion droplets created by both newly added and recovered emulsifiers. This work provided the groundwork for future applications of Pickering emulsion in interfacial catalysis.

Preparation and Characterization of a Novel Natural Quercetin Self-Stabilizing Pickering Emulsion

In contrast to their well-known physiological properties, phytochemicals, such as flavonoids, have been less frequently examined for their physiochemical properties (e.g., surface activity). A natural quercetin self-stabilizing Pickering emulsion was fabricated and characterized in the present study. The antisolvent precipitation method was used to modify quercetin (in dihydrate form), and the obtained particles were characterized by light microscope, atom force microscope, XRD, and contact angle. The antisolvent treatment was found to reduce the particle size, crystallinity, and surface hydrophobicity of quercetin. We then examined the effects of the antisolvent ratio, particle concentration, and oil fraction on the properties of the quercetin particle-stabilized emulsions. In addition, increasing the antisolvent ratio (1:1~1:10) effectively improved the emulsification performance of the quercetin particles. The emulsion showed good storage stability, and the particle size of the emulsion decreased with the rising particle concentration and increased with the rising oil phase ratio. The findings indicate that natural quercetin treated with antisolvent method has a good ability to stabilize Pickering emulsion, and this emulsion may have good prospective application potential for the development of novel and functional emulsion foods.

PVA/Inulin-Based Sustainable Films Reinforced with Pickering Emulsion of Niaouli Essential Oil for Potential Wound Healing Applications

In this study, sustainable water-based films were produced via the solvent-casting method. Petroleum-free-based polyvinyl alcohol (PVA) and carbohydrate-based inulin (INL) were used as matrices. Vegetable-waste pumpkin powder was used in the study because of its sustainability and antibacterial properties. Pickering emulsions were prepared using β-cyclodextrin. The influence of the different ratios of the β-cyclodextrin/niaouli essential oil (β-CD/NEO) inclusion complex (such as 1:1, 1:3, and 1:5) on the morphological (SEM), thermal (TGA), physical (FT-IR), wettability (contact angle), and mechanical (tensile test) characteristics of PVA/inulin films were investigated. Moreover, the antibacterial activities against the Gram (-) (Escherichia coli and Pseudomonas aeruginosa) and Gram (+) (Staphylococcus aureus) bacteria of the obtained films were studied. From the morphological analysis, good emulsion stability and porosity were obtained in the Pickering films with the highest oil content, while instability was observed in the Pickering films with the lowest concentration of oil content. Thermal and spectroscopic analysis indicated there was no significant difference between the Pickering emulsion films and neat films. With the addition of Pickering emulsions, the tensile stress values decreased from 7.3 ± 1.9 MPa to 3.3 ± 0.2. According to the antibacterial efficiency results, films containing pumpkin powder and Pickering emulsion films containing both pumpkin powder and a ratio of 1:1 (β-CD/NEO) did not have an antibacterial effect, while Pickering emulsion films with a ratio of (β-CD/NEO) 1:3 and 1:5 showed an antibacterial effect against Escherichia coli, with a zone diameter of 12 cm and 17 cm, respectively. Among the samples, the films with ratio of (β-CD/NEO) 1:5 had the highest antioxidant capacity, as assessed by DPPH radical scavenging at 12 h intervals. Further, none of the samples showed any cytotoxic effects the according to LDH and WST-1 cytotoxicity analysis for the NIH3T3 cell line. Ultimately, it is expected that these films are completely bio-based and may be potential candidates for use in wound healing applications.

Hemp: A Sustainable Plant with High Industrial Value in Food Processing

In the era of SDGs, useful plants which provide valuable industrial outputs and at the same time pose less impact on the environment should be explored. Hemp seems one of the most relevant gluten-free crop plants to meet such requirements. Its high nutritional value is comparable to soy. Moreover, almost the whole body of the hemp plant has a wide array of utility: industrial production of food, fiber, and construction materials. In view of environmental sustainability, hemp requires less pesticides or water in cultivation compared to cotton, a representative fiber plant. This short review investigates hemp's sustainability as a plant as well as its utility value as a highly nutritional material in the food industry. Recent application research of hemp protein in food processing includes plant milk, emulsifiers, fortification of gluten-free bread, plant-based meat production, as well as membrane formation. These studies have revealed distinctive properties of hemp protein, especially in relation to disulfide (S-S)/sulfhydryl (-SH)-mediated interactions with protein from other sources. While its cultivation area and industrial use were limited for a while over confusion with marijuana, the market for industrial hemp is growing rapidly because it has been highly reevaluated in multiple areas of industry. Conclusively, with its sustainability as a plant as well as its distinctive useful property of the seed protein, hemp has promising value in the development of new foods.

Comparative Study of Food-Grade Pickering Stabilizers Obtained from Agri-Food Byproducts: Chemical Characterization and Emulsifying Capacity

Natural Pickering emulsions are gaining popularity in several industrial fields, especially in the food industry and plant-based alternative sector. Therefore, the objective of this study was to characterize and compare six agri-food wastes/byproducts (lupin hull, canola press-cake, lupin byproduct, camelina press-cake, linseed hull, and linseed press-cake) as potential sources of food-grade Pickering stabilizers. The results showed that all samples contained surface-active agents such as proteins (46.71-17.90 g/100 g) and dietary fiber (67.10-38.58 g/100 g). Canola press-cake, camelina press-cake, and linseed hull exhibited the highest concentrations of polyphenols: 2891, 2549, and 1672 mg GAE/100 g sample, respectively. Moreover, the agri-food byproduct particles presented a partial wettability with a water contact angle (WCA) between 77.5 and 42.2 degrees, and they were effective for stabilizing oil-in-water (O/W) emulsions. The emulsions stabilized by Camelina press-cake, lupin hull, and lupin by-product (≥3.5%, w/w) were highly stable against creaming during 45 days of storage. Furthermore, polarized and confocal microscopy revealed that the particles were anchored to the interfaces of oil droplets, which is a demonstration of the formation of a Pickering emulsion stabilized by solid particles. These results suggest that agri-food wastes/byproducts are good emulsifiers that can be applied to produce stable Pickering emulsions.

The Fabrication and Characterization of Pickering Emulsion Gels Stabilized by Sorghum Flour

Pickering emulsion gels have potential application as solid fat substitutes and nutraceutical carriers in foods, but a safe and easily available food-derived particle emulsifier is the bottleneck that limits their practical application. In this study, the function of sorghum flour as a particle emulsifier to stabilize the oil-in-water (O/W) Pickering emulsion gels with medium chain triglycerides (MCT) in the oil phase was introduced. Sorghum flour had suitable size distribution (median diameter, 21.47 μm) and wettability (contact angle, 38°) and could reduce the interfacial tension between MCT and water. The oil phase volume fraction (φ) and the addition amount of sorghum flour (c) had significant effects on the formation of Pickering emulsion gels. When c ≥ 5%, Pickering emulsion gels with φ = 70% could be obtained. Microstructure analysis indicated that sorghum flour not only played an emulsifying role at the O/W interface but also prevented oil droplets from coalescing through its viscous effect in the aqueous phase. With increases in c, the droplet size of the emulsion gel decreased, its mechanical properties gradually strengthened, and its protective effect on β-carotene against UV irradiation also improved.

Application of Pickering emulsions in probiotic encapsulation- A review

Probiotics are live microorganisms that confer health benefits to host organisms when consumed in adequate amounts and are often incorporated into foods for human consumption. However, this has negative implications on their viability as large numbers of these beneficial bacteria are deactivated when subjected to harsh conditions during processing, storage, and passage through the gastrointestinal tract. To address these issues, numerous studies on encapsulation techniques to protect probiotics have been conducted. This review focuses on emulsion technology for probiotic encapsulation, with a special focus on Pickering emulsions. Pickering emulsions are stabilized by solid particles, which adsorb strongly onto the liquid-liquid interfaces to prevent aggregation. Pickering emulsions have demonstrated enhanced stability, high encapsulation efficiency, and cost-effectiveness compared to other encapsulation techniques. Additionally, Pickering emulsions are regarded as safe and biocompatible and utilize natural materials, such as cellulose and chitosan derived from plants, shellfish, and fungi, which may also be viewed as more acceptable in food systems than common synthetic and natural molecular surfactants. This article reviews the current status of Pickering emulsion use for probiotic delivery and explores the potential of this technique for application in other fields, such as livestock farming, pet food, and aquaculture.

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Probiotics play an important role in maintaining the health of humans and animals.

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Encapsulation improves probiotic viability in harsh environments.

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Probiotics can be encapsulated by many techniques such as emulsification.

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Pickering emulsions use particles instead of molecules to stabilize emulsions.

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Natural particles are more acceptable to some consumers than synthetic emulsifiers.