Protein-Based Pickering High Internal Phase Emulsions as

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Perspective

Protein-based Pickering High Internal Phase Emulsions as Nutraceutical Vehicles of and the Template for Advanced Materials: A Perspective Paper Xiao-Nan Huang, Jing-Jing Zhu, Yongkang Xi, Shou-Wei Yin, To Ngai, and Xiaoquan Yang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.9b03356 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 10, 2019

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Journal of Agricultural and Food Chemistry

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Protein-based Pickering High Internal Phase Emulsions as Nutraceutical

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Vehicles of and the Template for Advanced Materials: A Perspective

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Paper

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Xiao-Nan Huang†, Jing-Jing Zhu†, Yong-Kang Xi†, Shou-Wei Yin†‡§ * To Ngai‡ *, Xiao-Quan Yang†

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Research and Development Center of Food Proteins, School of Food Science and Engineering and

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†

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Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety,

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South China University of Technology, Guangzhou 510640, PR China

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‡Department

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§Overseas

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Center), Guangzhou 510640, China

of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong

Expertise Introduction Center for Discipline Innovation of Food Nutrition and Human Health (111

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* Corresponding author

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Dr. S. W. Yin: [email protected]

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Prof. Dr. T Ngai: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT:

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Pickering high internal phase emulsions (HIPEs) are normally highly concentrated

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emulsions stabilized by colloidal particles with a minimum internal phase volume

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fraction of 0.74. They have received considerable attentions in many fields, including

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pharmaceuticals, tissue engineering, foods, as well as personal care products. The

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aim of this perspective is to update the current knowledge on the field of

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protein-based Pickering HIPEs, emphasizing those aspects that need to be explored

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and clarified. Research progresses in constructing HIPEs by protein-type colloid

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particles as well as promising research trends in basic research and potential

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applications were highlighted. Promising researches in this field include: (1) To clarify

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bioavailability and evolution of activity of active ingredients in Pickering HIPEs by oral

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administration, (2) To construct Pickering interfacial catalysis platform using protein

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colloidal particles, and (3) To expand the emerging applications of Pickering HIPEs in

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fields, such as partially hydrogenated oils (PHOs) replacers, probiotics encapsulation

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as well as the template for porous materials.

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KEYWORDS: Pickering HIPEs, HIPEs template, Nutraceutical delivery, Porous

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protein materials, Pickering interfacial catalysis, PHOs replacers.

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INTRODUCTION

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The medical community has reached an agreement on the linkage between the excess

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intake of trans fats and the risk of cardiovascular diseases, for example, coronary heart

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disease.

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trans fats raised the incidence of coronary heart disease by 23–29%.2 Most of trans fats in

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possessed foods come from partially hydrogenated oils (PHOs), and the major dietary

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sources of artificial trans fats include artificial margarines, shortening, bakery products,

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and deep-fried fast foods.3 The use of PHOs has been forbidden in processed foods by

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the U.S. FDA in 2018.4 Food industry is now facing challenges for seeking ideal

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alternatives for PHOs to maintain organoleptic qualities or flavor of the products.

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Previous works have assessed that an increase (2%) in energy intake from

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Constructing of clean-label HIPEs is one of promising strategies to formulate alternatives

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of PHOs by directly transforming liquid oil into solid-like fats and can also be served as

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delivery vehicle of active ingredients. Traditional HIPEs are generally stabilized by

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expensive surfactants with high concentrations. Pickering emulsion is an emulsion

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stabilized by solid particles, and the interfacial adsorption of particles in Pickering system

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is nearly irreversible. The major advantages of Pickering emulsion include the high

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resistance to coalescence and Ostwald ripening as well as surfactant-free attribute.

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Pickering HIPEs, in particular protein-based Pickering HIPEs have received considerable

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attentions in many fields, including pharmaceuticals, foods, personal care products as well

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as tissue engineering. The objective of this perspective is to update the current knowledge

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on the field of protein-based Pickering HIPEs. Research progresses in constructing HIPEs

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by protein-type colloid particles as along with research fronts in basic research and

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potential applications were highlighted. Finally, challenges and future directions about

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protein-based Pickering HIPEs will also be discussed. In particular, emerging research

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directions on protein-based Pickering interfacial catalysis platform will be highlighted.

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BASIC KNOWLEDGE OF PICKERING EMULSIONS

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Pickering emulsion is an emulsion which stabilized by solid particles of colloidal size

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instead of low molecular weight surfactants or polymers. The location of an individual

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particle with respect to the oil-water interface is defined as the three-phase contact angle

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(θ). In thermodynamic terms, the three-phase contact angle is related to the balance of

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surface free energies at the particle-water, particle-oil and oil-water interfaces.5 Unlike

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surfactants, once solid particles are adsorbed at the oil–water interface, they are effectively

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and irreversibly anchored as long as the three-phase contact angle (θ) is not too close to 0

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or 180º.6 The reason for the virtual permanence of the particle–surface binding is that the

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free energy of spontaneous desorption (∆E) is extremely high compared with the thermal

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energy.7

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∆E = π𝑟2𝛾(1 ― |cos 𝜃|)2

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Where 𝑟 is the radius of solid particle and 𝛾 is the interfacial tension between the two

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phases. The higher resistance to coalescence and Ostwald ripening is the major benefit of

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Pickering emulsions compared with the equivalents stabilized by surfactants.5, 7

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HIPEs are biphasic systems with the internal phase volume fractions over 74%.8

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Traditional HIPEs are commonly stabilized by a large number of expensive surfactants (5–

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50% v/v).9 Surfactant-free Pickering HIPEs provides additional and/or improved properties

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to final products10. The high desorption energy barrier of solid particles endows the

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Pickering HIPEs with higher resistance to Ostwald ripening and coalescence over the

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equivalents stabilized by surfactants. Unfortunately, phase inversion usually occurs when

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the internal fractions of a Pickering system increase gradually. Binks et al. reported that

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hydrophobic silica stabilized Pickering emulsions suffered catastrophic phase inversion

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when the internal phase fractions was in the range of 0.65 – 0.70 (v/v).11 Therefore, the

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particles usually must undergo surface functionalization for HIPEs development. Large

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numbers of studies reported that Pickering HIPEs were stabilized by inorganic colloidal

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particles, for example, silica,12 titania,13 and graphene oxide.14 A few works were available

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on Pickering HIPEs stabilized by food-grade particles. These Pickering HIPEs are usually

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stabilized by the particles derived from carbohydrates15,

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theme of "Pickering High Internal Phase Emulsion", 236 search results were retrieved on

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the Web of Science, of which 40 were stabilized by food-grade particles and 22 were

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protein-based Pickering HIPEs. Figure 1 present the publications related to Pickering

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HIPEs (Figure1A) and protein-based Pickering HIPEs (Figure 1B). In 1999, Pickering

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HIPEs was firstly reported, and substantial interests have been devoted to Pickering

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HIPEs since 2013. In contrast, Pickering HIPEs stabilized by food-grade particles, in

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particular protein-based colloid particles is still in its infancy (Figure 1B).

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PROTEIN-BASED PICKERING HIPEs

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or proteins.17-20 Based on the

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Recently, the application of colloidal particles with natural origin, from the delivery of

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bio-actives to bulk structuring and interfacial stabilization have attracted wide interests in

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the field of pharmaceuticals, functional foods, and personal care products, and agricultural

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formulations. The first work of Protein-based Pickering HIPEs was published in 2013 on

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pure BSA scaffolds from HIPEs template, and there were 22 papers on this theme since

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then (Figure 1B). These efforts mainly focused on finding or constructing protein colloid

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particles for HIPEs development, and characterizing their physical performance. In

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retrospect, Pickering HIPEs stabilized by kinds of protein-type particles were developed,

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including prolamine (i.e., gliadin, zein), gelatin, whey protein and native plant globular

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proteins. Among them, one part is pure protein particles coming from self-assemble,

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thermal or enzyme cross-linking, and the other part is proteins-polysaccharides complex

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particles formed by covalent bonding (e.g. Maillard reaction) or non-covalent interactions

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(e.g. hydrogen bonding, hydrophobic and electrostatic interactions, steric hindrance). The

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following items presented a brief overview of the recent advances in Pickering HIPEs

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stabilized by protein-type emulsifiers and discussed the challenges in this field.

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Prolamine. Prolamine, the major storage protein of cereals is water-insoluble but is soluble

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in aqueous alcohol solutions. Prolamine is usually amphiphilic and is able to form a variety

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of self-assembled nano-/micro-structures, which can find a wide range of emerging

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applications, e.g., Pickering emulsifiers. Three prolamines i.e., zein, gliadin and kafirin

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were used to construct the emulsifiers for Pickering emulsion. Among them, only zein and

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gliadin colloidal particles succeeded in stabilizing Pickering HIPEs.

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Zein contains sharply defined hydrophobic and hydrophilic domains at its surface, and it

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can self-assemble to form manifold nano-/micro-structures. The pristine zein colloid

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particles were too hydrophobic to adsorb and attach effectively on the oil-water interface.

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Surface coating (for example, chitosan and sodium stearate) is beneficial to regulate their

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wettability so as to develop stable Pickering emulsions. So far the oil fraction in zein-based

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colloidal particles stabilized Pickering emulsions was mainly limited to 50% or below. The

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oil droplets in Pickering emulsion (o/w) deformed to nearly merge together when the oil

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phase fractions were increased up to 70%, meaning that these emulsions were

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approaching the threshold of inversion or phase separation. In 2018, Zhou et al succeeded

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in constructing stable Pickering HIPEs by designing interfacial structure through the

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interaction between zein and pectin. Zein/pectin hybrid particles (ZPHPs) were irreversibly

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absorbed at the oil−water interface, forming ordered and robust interfacial structure. This

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situation helped shape a percolating 3D lipid droplet network, facilitating the formation of

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viscoelastic Pickering HIPEs with excellent storage stability and thixotropy (>91.0%). This

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HIPE endowed curcumin with ideal oxidant stability by protecting it from UV-induced

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degradation.20 In addition, other binary colloidal particles (e.g., zein/tannic acid particles),

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and ternary colloid particles (zein-propylene glycol alginate-rhamnolipid particles,

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zein−PGA-NaCas complexes) were constructed as particulate emulsifiers for HIPEs

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development.21-23 The main focus in this filed is constructing or seeking suitable particles

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for Pickering HIPEs, such as by zein-tannic acid interaction during anti-solvent process as

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well as the formation of ternary complex particles through pH-cycle protocol. Published

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works mainly focused on the physical performance of Pickering HIPEs, and investigating

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the rheological properties and microstructure and of the HIPEs under various external

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stresses (such as temperature, ionic strength, and pH).

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Gliadin is characterized by high content of glutamine and proline, but low level in basic

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amino acids. Gliadin is amphiphilic because its terminals are more hydrophobic than the

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repetitive domain. Gliadin can self-assemble to form a wide range of mesostructures. In

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2016, we demonstrated firstly that gliadin colloid particles (GCPs) can serve as an

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effective stabilizer for Pickering HIPEs development19. The surfactant-free Pickering HIPEs

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were prepared by a facile shearing approach, transforming physically liquid oils into

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solid-like viscoelastic emulsion gels, which is a promising substitute to solid fats. The

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compressed droplets of Pickering HIPEs formed a percolating 3D-network framework,

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endowing the emulsions self-sopporting and viscoelastic attribute.19 Chitosan was choosed

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to modify the surface wettability of gliadin colloidal particles and the resultant complex

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particles was used to construct antioxidant Pickering HIPEs, as a delivery vehicle of

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curcumin.24-26 Pickering HIPEs with core or shell curcumin were prepared, the lipid

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oxidation and curcumin bioaccessibility in the Pickering HIPEs extensively evaluated by

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the in vitro gastrointestinal digestion model. Interestingly, this HIPEs encapsulation

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strategy improved lipid oxidation stability during the storage and in vitro simulated

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digestion, and the curcumin bioaccessibility were raised from 2.13% (bulk oil) to 53.61%

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(core curcumin), and up to 76.82% (shell curcumin). In addition, Pickering HIPEs stabilized

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by gliadin/chitosan complex particles was used as a robust template to generate

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protein-based porous materials with high porosity and well-defined structures as efﬁcient

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oil absorbent.27

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Whey protein. Whey protein with numerous nutritional and functional properties, is a

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by-product of the cheese production. Whey proteins can self-assemble into various

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microgels (aggregates) under specific processing conditions such as temperature and pH.

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So far, several papers were available on Pickering HIPEs stabilized by whey protein-type

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emulsifiers, such as whey protein/polysaccharide complexes, heat-induced whey protein

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microgels and bovine serum albumin nanoparticles. Many studies have reported whey

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protein/polysaccharide complexes showed improved functionalities such as emulsifying

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properties and heat-stability.28 To date, there has been only one report on the preparation

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of Pickering HIPEs with whey protein-polysaccharide complexes where the colloidal

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complexes were produced through electrostatic interactions.29 The cooperation of

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Pickering principle and the inter-polyelectrolyte network in bulk phase accounted for the

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formation and stabilization of Pickering HIPEs.29 On the other hand, the globular proteins

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in WPI were irreversibly denatured into microgel colloids by heating at 80 °C, as Pickering

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emulsifiers for HIPEs.30,

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plantarum within Pickering HIPEs stabilized by WPI microgels successfully increased the

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cell viability under aqueous environment and food thermal processing (for example,

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pasteurization).30 In addition, hierarchical porous protein scaffold were obtained using

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HIPEs templates stabilized by bovine serum albumin colloidal particles, assisted by

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crosslinking the proteins in the continuous phase and the oil-water interface.32

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More interestingly, Su et al. reported the encapsulation of L.

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Gelatin. Gelatin is obtained by the controlled hydrolysis of the triple-helix structure of

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collagen, which is biodegradable and has low antigenicity.33 Although molecular gelatin

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can be successfully used as an emulsifier to form HIPE that always showed a weak

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long-term stability against coalescence.34 Thus, some studies have reported that the

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special interaction between gelatin and polyphenols (e.g. tannic acid) as well as the

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electrostatic interaction between gelation and polysaccharides (e.g. glucomannan,

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chitosan) could be used to form complexes that possessed promising potentials for

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developing stable Pickering HIPEs. In addition, gelatin is an amphiphilic biopolymer that

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can easily self-assembled into colloid particles (aggregates) under the specified

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temperature, pH, and solvent conditions.34,

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HIPEs possessed controlled-release and long-term retention of lipophilic compounds,

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which is conducive to the promotion of oral bioaccessibility.34,

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gelatin nanoparticles were used as co-stabilizer to stabilize Pickering HIPEs.34 The gelatin

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molecules in the aqueous solution tended to adsorb on the surface of gelatin nanoparticles

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to form sophisticated colloid nanostructure, which change the surface wettability of gelatin

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nanoparticles, facilitating the formation of stable Pickering HIPEs and the porous protein

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scaffold with interconnected porous morphology and high porosity. 34 Hierarchical porous

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protein scaffold from HIPEs stabilized by the co-stabilizer could be further reinforced by

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continuous cross-linking or polymerization for biomedical applications34.

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The gelatin particle-stabilized Pickering

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Moreover, gelatin and

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Native globular proteins as molecular Pickering stabilizers. It is generally recognized that

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globular proteins, once adsorbed at the interface, would undergo structural unfolding and

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rearrangement. Recently, Tang groups proposed the concept of molecular Pickering

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stabilizers, that is native globular proteins served as Pickering emulsifiers.17,18 They

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reported that ovalbumin or soy β-conglycinin exhibited an Pickering stabilization

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phenomenon for HIPEs development due to the strong structural integrity that could resist

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disruption from both interfacial Laplace pressure and repulsive force between two droplets,

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as well as high refolding ability.17,18 These results were deduced from secondary and

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tertiary structure of the adsorbed OVA (or soy β-conglycinin) and its native counterpart

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derived from the intrinsic fluorescence and circular dichroism (CD) spectra. These works

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opened a promising avenue for constructing Pickering HIPEs using native globular

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proteins. To consolidate this new concept, the application of direct visualization on

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interfacial configuration of adsorbed protein or powerful indirect strategies such as

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small-angle neutron scattering /X-ray (SANS/SAXS/) should be used in future works to

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obtain more robust and solid conformation data of the adsorbed globular protein and its

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native counterpart to explore the underlining mechanism of this HIPE.

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Overall, the present studies of protein-based Pickering HIPEs mainly focused on finding or

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constructing protein-based particles for HIPEs development, and characterizing their

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physical performance. The researches of this kind of HIPEs promised the potentials as

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PHOs replacers and/or nutraceuticals delivery vehicles, and further research practices are

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requisite to enrich our knowledge in this field. Besides, protein-based Pickering HIPEs

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were used as templates to yield biocompatible and biodegradable porous materials with

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emerging applications. In addition to the protein-based particles mentioned above,

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functional peptides based edible particles are also a promising alternative to explore

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constructing food-grade Pickering HIPEs with extra bioactive efficacy.35

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CHALLENGE AND RESEARCH TRENDS

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Inspired by the striking achievements in Pickering HIPEs stabilized by inorganic particles

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and/or biological original particles, as well as emerging research and application in edible

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particles stabilized emulsions, we proposed several future research directions in both

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basic research areas and application fields for Pickering HIPEs stabilized by protein-type

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particles.

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Filling gaps in basic research areas

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(1) To clarify bioavailability and evolution of activity of active ingredients in Pickering

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HIPEs by oral administration

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Nutraceuticals would suffer a series of complex physicochemical and physiological

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changes by oral administration. Oral bioavailability of nutrients depends on the digestion,

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release from food substrates, absorption by intestinal cells, and transportation to target

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body cells. The unit of the in vitro digestion and Caco-2 cell monolayer absorption,

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mimicked human digestion to assess absorption and transportation of nutraceuticals.

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Pickering HIPEs stabilized by protein-type emulsifers possessed promising potentials for

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using as texture modification and delivery vehicle of nutraceuticals. Regretfully, limited

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works were available on vehicle usage for this kind of Pickering HIPEs, and present works

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were mainly limited to the digestion fate of Pickering HIPEs loaded with curcumin or

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β-carotene through in vitro simulated gastrointestinal models.26,36,37 Zhou et al.

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characterized curcumin bioaccessibility of the ingested Pickering HIPEs with core or shell

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curcumin. This encapsulation strategy enhanced the curcumin bioaccessibility only 2.13%

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(bulk oil) to 53.61% (core curcumin), and up to 76.82% (shell curcumin)26. In order to

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enrich our knowledge in this field, further research practices are requisite. The

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bioavailability of an encapsulated component can be estimated using the unit of the in vitro

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digestion and Caco-2 cell monolayer absorption. In the process of simulated digestion and

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cell absorption, the chemical transformation and metabolism of nutraceuticals were should

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be clarified to explore the possible forms of nutraceuticals entering the systemic circulation.

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Then, the bio-efficacy of the bioavailable fractions of nutraceuticals can be assessed by

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the cellular antioxidant activity (CAA) assay and anti-inflammatory experiment procedure.

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Furthermore, animal feeding studies should be conducted to validate in vitro bioavailability

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and bioactivity of functional ingredients encapsulated in Pickering HIPEs (Figure 2).

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(2) To Construct Interfacial Catalysis Platform via Protein-based HIPEs.

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Biphasic interfacial catalysis platform which have the large interfacial area and facile mass

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transfer between aqueous/organic phases, attracted recently considerable attention.

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Pickering interfacial catalysis is typically realized by using catalytically active colloid

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articles as an emulsifier to stabilize emulsions. Recently, the concept of Pickering

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interfacial catalysis was evolved into Pickering interfacial biocatalysis, in which biocatalysts

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and carriers are integrated into biohybrid catalyst particles amenable to emulsion

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reactions.38 In brief, lipases were adsorbed into metal–organic frameworks (MOFs),39

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polymersomes,40 or alginate gel microparticles.41 Sun, et al. conjugated polymers onto

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enzymes (i.e., benzaldehyde lyase, glucose oxidase). The conjugates served as efficient

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interfacial biocatalysts which simultaneously stabilized Pickering emulsions and catalyzed

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reactions at water-oil interface.42 Protein as a natural macromolecule with abundant

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functional groups, provides a possibility for complexing (or conjugating) catalysts (or

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biocatalysts).

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protein-stabilized Pickering emulsions until now. Inspired with striking achievements in

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Pickering emulsions stabilized by inorganic particles and/or microgels, our team began to

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construct protein cages or porous proteins to immobilize the catalyst for catalyst recovery

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and recycling (Figure 3). Promising research in this filed include: (1) To construct

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interfacial catalysis platform of Pickering HIPEs stabilized by the conjugates of proteins

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and enzyme, (2) To form protein-based microgels (e.g., casein micelles, soy, protein and

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whey protein) as emulsifiers and interfacial vehicle of enzymes or metal nanoclusters to

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construct Pickering interfacial catalysis platform, (3) To construct or find protein nanocage

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to encapsulate enzymes to build Pickering HIPEs catalysis platform.

Regrettably,

no

works

were

available

on

interfacial

catalysis

in

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To exploit fields of applications

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(1)Protective storage and delivery vehicles for Probiotics

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Probiotics are popular in functional foods due to their positive physiological effects on

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intestinal microflora populations of the host. However, this application was generally

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limited, possibly due to the poor viability during prolonged storage and processing, and the

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low survival rate in acidic conditions and digestive juices, especially in the presence of

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biles.43 Temperature and relative humidity play a critical role in determining probiotic

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viability during food storage and processing. So far, only one paper was available on

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Pickering HIPEs as vehicle of probiotics to increase survive rate of Probiotics during the

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pasteurization processing. Su et al. reported that the encapsulation of L. plantarum within

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internal phase of Pickering HIPEs stabilized by WPI microgels increased successfully the

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cell viability after pasteurization, depending on the increase of the oil volume fractions or

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WPI microgel concentrations 30. More research practices need to be conducted to enrich

311

our knowledge in this field. Promising future research in this field include: (1) To clarify the

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stability of probiotics encapsulated by Pickering HIPEs during processing and prolonged

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storage, (2) To elucidate digestion fate of Pickering HIPEs and the related survival rate of

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probiotics throughout gastrointestinal tracts, and (3) To explore the effects of interfacial

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structure (protein types, charges, thichness) on the viability of probiotics encapsulated

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during the food processing, storage and transportation.

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(2)Promising formulation routine for PHOs replacers

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Most of trans fats in possessed foods come from partially hydrogenated oils (PHOs), and

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the major dietary sources of artificial trans fats include margarines, shortening, bakery

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products, and deep-fried fast foods.2 The USA FDA has banned the use of PHO in food

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production in 2018. The clean label HIPEs, consisting of edible oil, particles, and water,

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have a margarine-like appearance and can be of similar composition to margarine

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formulations, which makes food-grade HIPEs good alternatives for margarine (with PHOs).

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mono-dispersed gliadin/chitosan complexes particles as an emulsifier. The compressed

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droplets in Pickering HIPEs generated a percolating 3D-network framework, endowing the

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emulsions with self-standing and viscoelastic attribute. This work opens an attractive

329

means for the conversion of liquid oils to viscoelastic soft solids with zero trans fats, which

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can be served as a potential alternative for PHOs.

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in Pickering HIPEs stabilized by gliadin-based colloid particles,

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stabilized by kinds of protein colloid particles were gradually developed, including zein, soy

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protein, whey protein. Ongoing works should focus on the evaluation on performance of

334

Pickering HIPEs in real food products, and the influence on organoleptic properties of the

335

products when PHOs were replaced with Pickering HIPEs.

We reported for the first time the formation of edible Pickering HIPEs using the

25

Inspired by the striking achievements 25

Pickering HIPEs

336 337

(3) To construct porous material modulated by Pickering HIPEs for tissue engineering

338

and/or catalysis application.

339

HIPE templating is an attractive strategy to develop well-defined porous materials.44,

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Pickering HIPEs template possesses many advantages relative to the traditional ones, and

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the particle layers of interface endow porous materials with surface functionalization (e.g.

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magnetism, conductivity, pH and light responsiveness) and improved mechanical strength.

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Protein is a soft biopolymer with abundant functional groups, which is propitious to the

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porous materials with an interconnected-cell structure. The Pickering HIPEs are a facile

345

and robust template for fabricating porous material. Unfortunately, To date, only four

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studies were available on protein porous materials using protein-stabilized Pickering

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HIPEs the templets,27, 32, 34, 46

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and without any chemical reactions.27,

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gliadin-based porous materials using Pickering HIPE-template. The Pickering HIPE

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templates were constructed using gliadin−chitosan complex particles (GCCPs) as a sole

351

emulsifier, in which the internal phase fractions (90%) was the highest of all reported

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Pickering HIPEs stabilized by food-grade particles. The resultant porous materials

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possessed the high porosity (>90%) and the interconnected pore structure. The

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combination of the interfacial particle barrier and three-dimensional network generated by

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the GCCPs in the aqueous continuous phase played an important role in the development

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of porous materials with designed pore structure.27 Jiao et al. reported the formation of

357

porous

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protein-isolate-microgel-particle as the template, and this work mainly concentrated on the

359

preparation and microstructure of porous material.46 Based on present researches,

360

promising research trends in this field include: (1) To construct porous materials decorated

361

with abundant functional groups using HIPEs stabilized by protein-type emulsifiers, as

362

potential absorption materials, (2) To construct porous materials decorated with functional

363

bio-catalysts (enzyme) or metal nanoclusters (Au, Ag) using protein dual-functional

364

colloidal particles or microgels which act simultaneously as Pickering emulsifier and

365

interfacial delivery vehicles for emerging interfacial catalysis or antibacterial application, (3)

366

To form protein-based porous scaffolds for tissue engineering application by decorating

367

porous materials with bio-actives or drugs (Figure 4).

materials

using

and two of them are developed with all edible components 46

Recently, our group successfully developed

Pickering

HIPEs

18


stabilized

by

peanut

Page 19 of 34


368 369

CONCLUSIONS AND FUTURE PROSPECTS

370

Pickering HIPEs have received considerable attentions in many fields. In particular, they

371

have promising potentials for PHOs replacers in processed foods, with emerging

372

applications in food texture modification, delivery vehicle of active ingredients. The

373

objective of this perspective is to update the current knowledge on the field of

374

protein-based Pickering HIPEs, emphasizing those aspects that need to be explored and

375

clarified. Inspired by the striking achievements in Pickering HIPEs stabilized by inorganic

376

particles and/or biological original particles, as well as emerging research and application

377

in edible particles stabilized emulsions, we proposed promising future research directions

378

in both basic research areas and application fields for Pickering HIPEs stabilized by

379

protein-type particles. In particular, protein-based Pickering HIPEs can be used as

380

platform for advanced applications such as Pickering interfacial catalysis, Pickering

381

interfacial bio-catalysis (PIB), and porous materials for tissue engineering application.

382

These research directions will open a promising avenue for emerging applications of

383

protein-based Pickering HIPEs.

384 385

ACKNOWLEDGMENTS

386

This work was supported by the project granted by the National Key Research and

387

Development Program of China (Project No. 2017YFC1600405), and Sino-Singapore

388

International Joint Research Institute (Contract No: 201-A018003), and by the

389

Fundamental Research Funds for the Central Universities (SCUT, 2019ZD38).

19



Page 20 of 34

390 391

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Page 26 of 34

emulsion process. Langmuir 2018, 34, 10381−10388.

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stabilized solely by peanut-protein-isolate microgel particles with multiple potential

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applications. Angew. Chem. Int. Ed. 2018, 57, 9274−9278.

26


Page 27 of 34


526

Figure captions

527

Figure 1 Number of publications related to (A) Pickering High Internal Phase emulsions (HIPEs) and (B)

528

protein-based Pickering HIPEs from a search of the Web of Science database (with the most recent data

529

downloaded on 28th May 2019).

530

Figure 2. Schematic illustration of the in vitro digestion model and the in vitro cell models, in vivo

531

animal experiments proposed for clarifying the bioavailability and biotransformation of

532

nutraceuticals in Pickering HIPEs by oral administration.

533

Figure 3. Typical illustration of porous protein encapsulated enzyme for Pickering interface

534

bio-catalysis

535

Figure 4. Schematic representation of porous materials using protein-based Pickering HIPEs as the

536

template for emerging tissue engineering and catalysis applications

537

27



Page 28 of 34

Figure 1

Figure 1 Number of publications related to (A) Pickering High Internal Phase emulsions (HIPEs) and (B) protein-based Pickering HIPEs from a search of the Web of Science database (with the most recent data downloaded on 10th Apr. 2019).

28


Page 29 of 34


Figure 2

Figure 2. Schematic illustration of the in vitro digestion model, the in vitro cell models, and in vivo animal experiments proposed for clarifying the bioavailability and biotransformation of nutraceuticals in protein-Pickering HIPEs by oral administration.

29



Page 30 of 34

Figure 3

Particles or microgels

Catalyst

Substrate

Products

Figure 3. Typical illustration of porous protein encapsulated enzyme for Pickering interface bio-catalysis

30


Page 31 of 34


Figure 4. Schematic representation of porous materials using protein-based Pickering HIPEs as the template for emerging tissue engineering and catalysis applications

31



Page 32 of 34

Table 1 Overview of Pickering HIPEs stabilized by protein-based particles

type of protein

the form of

particles

particles

concentration

zein-pectin hybrid particles

(pectin :zein

(TA) complex

(zein : TA

particles

1:0.5)

complex particles

application

reference

oil phase Pickering HIPEs had viscoelasticity, thixotropy, corn oil 80%

and oxidation stability; encapsulated curcumin

0:10-5:10) zein 2wt %

ipid

fraction of

zein 2wt %

zein-tannic acid

zein-PGA-rhamnol

the type and

20

and provided UV protection sunflower oil 72-87%

the formed Pickering HIPEs had elastic gel-like structure and good stability across a range of pH,

21

temperatures and salt concentration

zein 1-2wt % ( zein:PGA: rhamnolipid

MCT 75%

enhance good stability against environmental stresses

22

1:0.1:2)

zein-sodium caseinate (NaCas)-propylene glycol prolamin

alginate(PGA) ternary

zein 1 wt % (zein: NaCas

soybean oil

1:1, PGA

80%

0.25-2wt %)

the ternary nanocomplexes formed by a pH-cycle method and Pickering HIPEs with mayonnaise-

23

like appearance and gel-like structures

nanocomplexes gliadin colloid particles

2wt %

corn oil 80%

exhibited stability against coalescence and inhibited creaming of the oil droplets

19

structured liquid oils into stable solid-like gliadin-chitosan hybrid particles

gliadin 2wt % chitosan

oil powders without hydrogenated oils, corn oil 80%

0.1wt %

encapsulated curcumin which act as shell/core

24 and 25

antioxidant retarded lipid oxidation in Pickering HIPEs

gliadin-chitosan hybrid particles

gliadin−chitosan hybrid particles

gliadin 2wt % chitosan

improve lipid oxidation stability and algal oil 75%

bioaccessibility of curcumin, reduce lipid

0.1wt %

digestion

0.5-3.0wt % gliadin:

26

hexane 80%

chitosan 20:1

as the template to fabricate porous materials with interconnected pore structure and high porosity

27

whey protein isolate (WPI)-low methoxyl pectin (LMP) colloidal whey protein

WPI : LMP

sunflower

1.0 : 0.5wt %

oil, 80%

the complex particles provided better stabilization to the HIPEs against coalescence at

29

the pH close to the pI of WPI

complexes thermal denaturation to form gel particles (WPM)

4wt %, 10wt %

grape seed

encapsulated L. plantarum and improve its

oil, 80%,

viability of probiotics during food thermal

90%

processing

32


30

Page 33 of 34


thermal denaturation to form gel particles

1wt %

corn oil, 75%

10wt %

hexane 80%

HIPE production with WPMs possessed thermal stability and improved viscoelastic properties

31

(WPM) bovine serum albumin (BSA)

hexane or gelatin particles

0.5-1.5wt %.

sunflower oil

hierarchical porous protein scaffolds obtained from BSA protein nanoparticles stabilized HIPEs controlled-release and long-term retention of lipophilic compounds

80%

gelatin

gelatin particles

0.5-2.0wt %

sunflower oil 80%

32

35

as bioactive compound carriers exhibited protective effect and improved bioaccessibility of

36

nutraceuticals utilized gelatin particles and gelatin as co-

gelatin particles

1.8wt %

hexane 80%

stabilizer to improve the properties of porous materials templated from gelatin particles-based

34

Pickering HIPEs soy β-conglycinin

globular protein

0.2-1wt %

dodecane

showed extremely stable against heating and

80%

possessed temperature-responsiveness

17

formed HIPEs exhibit coalescence stability

ovalbumin

globular protein

0.2-3.0wt %.

dodecane 80%

against storage and heating, susceptibility to freeze−thawing, enhanced heat stability of

18

encapsulated bioactives and vaporization inhibition of volatile oils

peanut-protein-isolate

microgel particles

1.5wt %

peanut oil or

the HIPEs with all edible components can be a

n-hexane

substitutes for PHOs and as a templet to produce

85%

nontoxic porous materials

33


45


TOC

34


Page 34 of 34

Protein-Based Pickering High Internal Phase Emulsions as

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