Cereal proteins in nanotechnology: formulation of encapsulation and delivery systems

Accepted Manuscript Title: Cereal proteins in nanotechnology: Formulation of encapsulation and delivery systems Authors: Liqiang Zou, Anqi Xie, Yuqing Zhu, David Julian McClements PII: DOI: Reference:

S2214-7993(18)30146-2 https://doi.org/10.1016/j.cofs.2019.02.004 COFS 433

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Please cite this article as: Zou L, Xie A, Zhu Y, McClements DJ, Cereal proteins in nanotechnology: Formulation of encapsulation and delivery systems, Current Opinion in Food Science (2019), https://doi.org/10.1016/j.cofs.2019.02.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Cereal

proteins

in

nanotechnology:

Formulation

of

encapsulation and delivery systems Liqiang Zou1*, Anqi Xie1, Yuqing Zhu1, and David Julian

State Key Laboratory of Food Science and Technology, Nanchang University, No. 235

Nanjing East Road, Nanchang 330047, Jiangxi, China

Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA

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McClements2*

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* Corresponding authors. These authors contributed equally to the manuscript.

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Graphical Abstract

Highlights 

A range of cereal proteins are available to fabricate nanoparticle-based delivery systems

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Cereal proteins can also be modified to improve their solubility and functionality.

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Cereal protein nanoparticles can improve the stability and bioavailability of bioactives. Cereal protein nanoparticles are suitable for fabricating Pickering emulsions.

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

Abstract

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Cereals are an abundant and relatively inexpensive source of plant-based proteins that play critical nutritional and functional roles in foods.

However, their poor

solubility in aqueous solutions limits their application in many food products. A

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number of strategies are therefore being developed to overcome this limitation.

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Physical, chemical, and enzymatic modification of cereals protein can be used to

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improve their solubility and functionality. For instance, modified cereal proteins can

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be used as emulsifiers to stabilize lipid nanodroplets. The low water-solubility of cereal proteins is an advantage in the fabrication of protein nanoparticles created using

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antisolvent precipitation methods. These nanoparticles can be used to encapsulate

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bioactive agents or to stabilize emulsions through a Pickering mechanism.

Keywords:

protein

nanoparticles;

nanoemulsions;

Pickering

emulsions;

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encapsulation; delivery systems

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Introduction

Cereals, such as rice, maize, and wheat represent some of the world’s most

important staple foods (Table 1) [1]. They are also an abundant source of inexpensive storage proteins that play critical nutritional and functional roles in foods [1]. The storage proteins extracted from different cereals have different common names: zein from maize; gliadins and glutenins from wheat; hordeins from barley; kafirins from

sorghum and millet; avenin from oats; and secalin from rye [2]. In general, the functional attributes of these proteins are determined by their unique molecular structures and physicochemical properties. Researchers have therefore worked for decades to elucidate the molecular features of cereal proteins, optimize their extraction from natural sources, modify their molecular structures to extend their functionality,

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establish the molecular basis of their functionality, and elucidate their contribution to the quality of specific foods [3-7]. More recently, cereal proteins have been examined for their application as building blocks to assemble nanostructured encapsulation

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systems for bioactive agents, such as vitamins and nutraceuticals. There have already

been numerous reviews on the use of food proteins for bioactive encapsulation [8-10], as well as some reviews on particular cereal proteins such as zein [11-13]. However,

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few have focused on the specific issues related to the use of cereal proteins. For this

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reason, we provide a critical survey of recent studies on the modification and utilization

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of cereal proteins, with an emphasis on their use to assemble nanoparticle-based

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delivery systems. This knowledge should encourage the more widespread use of cereal

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proteins as functional ingredients within the food industry. Extending cereal protein functionality in delivery systems using modification

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Native cereal proteins are not typically used as emulsifiers to form and stabilize emulsion-based delivery systems because of their low water-solubility. This problem

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can often be overcome by modifying their molecular structures and functional attributes using physical, chemical, or enzymatic approaches [8]. Physical modifications are often favored because they are relatively simple to

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perform and do not require relabeling of the final ingredient. Some of the commonest physical modifications are based on thermal treatment, high-pressure processing, irradiation, extrusion, or milling [14]. Physical treatments usually alter the non-covalent interactions between or within proteins, such as hydrogen bonds, hydrophobic interactions, and electrostatic interactions, without breaking the covalent bonds holding the proteins together. However, some physical treatments do alter disulfide bonds in

cereal proteins, for instance alkaline conditions or thermal processing [7]. Physical treatments of cereal proteins can enhance their performance in nano-enabled delivery systems. For instance, ultrasound treatment of millet protein has been shown to improve its emulsifying properties [15]. Chemical methods have also been used to modify cereal proteins. Deamidation

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converts amino groups on glutamine and asparagine into carboxyl groups, thereby altering the electrostatic repulsion between protein chains. As a result, deamidation

improves the solubility, emulsification, and digestion properties of cereal proteins [16].

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For instance deamidated gliadin has been successfully used as an emulsifier to produce

emulsions fortified with omega-3 fatty acids [17]. Cereal proteins can also be modified by adding carbohydrate groups through glycosylation or glycation.

In general,

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glycosylation is typically used to refer to enzyme-catalyzed processes whereas

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glycation (or non-enzymatic glycosylation) is used to refer to non-enzymatic processes.

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For the sake of convenience, we will use the term glycosylation to refer to both reactions

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in this article. Glycosylation, which is often carried out using the Maillard reaction, is based on a reaction between available amino groups on a protein and carbonyl-

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containing groups on other molecules, such as reducing sugars [18]. Glycosylation can be used to improve the solubility and emulsifying properties of cereal proteins. For

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instance, the conjugation of dextran to rice protein using the Maillard reaction greatly improves the water-solubility, emulsion formation, and emulsion stabilization

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properties of this cereal protein [19]. This effect was attributed to the strong steric repulsion generated by the neutral carbohydrate moieties attached to the rice proteins. Enzymatic modification of cereal proteins sometimes offers advantages over

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chemical modification due to its high selectivity, speed, and mild reaction conditions. Partial and selective hydrolysis using specific proteases is one of the most common enzymatic approaches used to modify cereal proteins. Hydrolysis splits specific peptide bonds and promotes partial protein unfolding, thereby increasing their water-solubility and functionality [20]. The enzymes and reaction conditions used have to be carefully

optimized to generate proteins with the required functional attributes. In some cases, multiple enzymes may be needed [21]. Partial hydrolysis of rice protein has recently been shown to improve its solubility and emulsifying properties [22, 23]. Hydrolysis using single [24] or multiple [25] enzymes has been shown to improve the antioxidant activity of cereal proteins. Combining partial hydrolysis with cross-linking

Formation of protein nanoparticles from cereal proteins

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enhances the rheological and thermal properties of wheat gluten [26].

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An innovative application of cereal proteins is the creation of functional protein

nanoparticles [27-29]. The formation of these nanoparticles relies on the poor watersolubility of these proteins [30].

Antisolvent precipitation is currently the most

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common method used to fabricate cereal protein nanoparticles [31]. In general, this

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method is based on changes in protein solubility when solution conditions, such as

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solvent quality, pH, ionic strength, or temperature, are altered. Currently, the most widely used approach involves the use of organic solvents [32]. Hydrophobic cereal

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proteins are first dissolved in a suitable organic solvent (such as 80% ethanol/20%

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water) and then the resulting solution is added dropwise into water with continuous stirring. When the proteins encounter water, their solubility decreases dramatically,

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causing them to spontaneously self-assemble into small protein particles [32]. Finally, the organic solvent is removed by dialysis or evaporation leaving a colloidal suspension

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containing protein nanoparticles suspended in water. This approach has been used to produce nanoparticles from gliadin, zein, and kafirin [33-35]. Cereal protein nanoparticles can also be produced by antisolvent precipitation by

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altering the pH of the surrounding solution, which has the advantage of not requiring an organic solvent [36]. The cereal proteins are dissolved in an aqueous alkaline solution (pH > 11), which is then acidified to neutral conditions (pH 7) while continuously stirring. The change from an alkaline to a neutral environment causes the proteins to precipitate, leading to spontaneous formation of protein nanoparticles [37]. One of the problems with cereal protein nanoparticles is they are often highly

susceptible to aggregation after formation because of their hydrophobic surfaces and low surface potential. This problem can be overcome by adsorbing biopolymers [3840] or surfactants [35, 41] to their surfaces during or after their fabrication. The creation of a biopolymer coating around the nanoparticles increases the steric and electrostatic repulsion between them, thereby improving their aggregation stability. The adsorption

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of surfactants to the surfaces of the protein nanoparticles also improves their stability by altering the colloidal interactions between them. The non-polar regions on the surfactants bind to non-polar patches on the protein nanoparticle surfaces, thereby

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reducing the hydrophobic attraction between them. Moreover, the hydrophilic regions on the surfactants increase the steric and electrostatic repulsion between the protein

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nanoparticles.

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Applications of cereal protein nanoparticles: Encapsulation

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Cereal protein nanoparticles are being designed to encapsulate, protect, and deliver a variety of bioactive agents. The nanoparticles protect the bioactive components from

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degradation within foods and then increase their bioavailability in the human gut [42-

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44]. For instance, the chemical stability and bioaccessibility of curcumin have been enhanced by encapsulating it within protein nanoparticles assembled from gliadin,

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kafirin, or zein [44-48]. The photochemical stability and antioxidant capacity of quercetin have been improved by trapping it inside zein [49] or gliadin [34]

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nanoparticles.

Applications of cereal protein nanoparticles: Pickering Stabilization Another interesting application of cereal protein nanoparticles is to form and

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stabilize emulsion-based delivery systems through a Pickering mechanism. This type of nanoparticle tends to strongly adsorb to oil-water interfaces because it is only partially wetted by both oil and water. Oil-in-water emulsions can be formed by blending a mixture of oil, water, and protein nanoparticles together. The protein nanoparticles adsorb to the surfaces of the oil droplets during homogenization, forming

a protective barrier [50]. Pickering emulsions have some advantages over conventional emulsions for certain applications due to their enhanced coalescence stability, novel textural attributes, and the fact no synthetic surfactants are needed [51, 52]. For this reason, there has been great interest in the development of this type of emulsion in the food, cosmetics, and pharmacy areas.

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Nanoparticles created from zein, gliadin, kafirin, and other cereal proteins have all been investigated for their potential to form and stabilize Pickering emulsions [32, 5355]. Cereal proteins are particularly suited for this purpose because they have poor

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water-solubility and so are able to persist as nanoparticles in aqueous solutions.

Moreover, their surfaces consist of both hydrophilic and hydrophobic groups, giving them suitable wetting characteristics. The wettability of nanoparticles determines the

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type of Pickering emulsion formed (O/W or W/O), as well as their stability, and so

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researchers developed methods to modify the surfaces of cereal protein nanoparticles

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to alter their surface characteristics and functionality, e.g., using polysaccharides,

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proteins, and phenolic acids [28, 54, 56, 57]. Optimization of nanoparticle surface characteristics can lead to Pickering emulsions with enhanced functional properties,

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such as better resistance to flocculation or coalescence, stronger antioxidant activity, and more controlled bioactive release [47, 54, 58].

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Protein nanoparticles can also be used to create a variety of more structurally complex Pickering emulsions, such as high internal phase emulsions (HIPEs), emulsion

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gels, and double emulsions [59, 60].

Pickering HIPEs stabilized by protein

nanoparticles have extremely high (>74%) droplet concentrations, which may be advantageous for some applications [61]. Pickering emulsion gels can be formed by

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adding substances, such as proteins, polysaccharides, or phenolic acids, to Pickering emulsions containing oil droplets stabilized by cereal protein nanoparticles (such as gliadin) to modulate the colloidal interactions acting between the oil droplets [55]. Pickering double emulsions, such as water-in-oil-in-water (W/O/W) systems, can be fabricated by replacing at least one of the regular surfactants with cereal protein

nanoparticles, such as kafirin [62]. These structurally-designed Pickering emulsions often have better physical and chemical stability, different textural attributes, and enhanced delivery characteristics than conventional emulsions [55]. A number of studies have shown that Pickering emulsions can be used to load and deliver bioactive substances. The poor water-solubility, chemical instability, and low

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bioavailability of curcumin, quercetin, β-carotene, and other hydrophobic bioactives often limits their application in foods, supplements and pharmaceuticals. Pickering

emulsions have been designed so the lipid phase is digested in the small intestine to

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produce free fatty acids and monoacylglycerols. These lipid digestion products interact

with phospholipids, bile salts, and cholesterol secreted by the human body to form mixed micelles. The mixed micelles then solubilize the hydrophobic bioactives in their

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non-polar interiors and transport them through the mucus layer to the intestinal walls

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where they can be absorbed [63]. Nevertheless, the Pickering emulsions have to be

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carefully designed so lipase can reach the surfaces of the oil droplets and digest the

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lipids, otherwise the bioactives will not be released. Encapsulation of bioactives in Pickering emulsions has also been shown to

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improve their stability to oxygen exposure and ultraviolet light, to enhance their antioxidant activity, and to improve their resistance to various environmental stresses

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[47, 64]. Pickering emulsions may therefore have considerable potential for creating

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novel foods, cosmetics, and other commercial products.

Conclusion and future directions There have been considerable advances in the utilization of cereal proteins to

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encapsulate and deliver bioactive food components recently. The utilization of cereal proteins for this purpose was previously limited by their low water-solubility. However, this limitation has been overcome using two approaches: (1) physical, chemical, or enzymatic modification of the cereal proteins: (2) formation of protein nanoparticles using antisolvent precipitation and other methods. Researchers are now trying to establish the relationship between protein structure and functionality so as to tailor

bioactive delivery systems for particular applications. As our knowledge of protein extraction and modification methods grow, and our understanding of structure-function relationships increases, we will be able to design food-grade delivery systems from cereal proteins with improved properties.

In

particular, we should be able to enhance the stability of bioactive components within

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foods and increase their bioavailability and bioactivity within the human gut. We may also be able to create foods and other products with a wide range of novel rheological

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properties.

Acknowledgements

The authors are grateful for the financial support of this study by the National

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Natural Science Foundation of China (31860452 and 31601468). This material was

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partly based upon work supported by the National Institute of Food and Agriculture,

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Grants (2016-25147 and 2016-08782).

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USDA, Massachusetts Agricultural Experiment Station (MAS00491) and USDA, AFRI

Conflict of Interest statement

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knowledge.

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All the authors declare that they have no conflict of interests to the best of their

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[63] Araiza-Calahorra A, Akhtar M, Sarkar A. Recent advances in emulsion-based delivery approaches for curcumin: From encapsulation to bioaccessibility. Trends in Food Science & Technology. 2018;71:155-69.

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[64] Wang LJ, Hu YQ, Yin SW, Yang XQ, Lai FR, Wang SQ. Fabrication and characterization of antioxidant pickering emulsions stabilized by zein/chitosan complex particles (ZCPs). Journal

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of agricultural and food chemistry. 2015;63:2514-24.

Figure 1. Cereal protein nanoparticles are commonly fabricated from native or modified proteins using the antisolvent precipitation method.

The proteins are

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dissolved in a solvent, then this solution is dropped into an antisolvent.

Figure 2. Cereal protein nanoparticles can be used to encapsulate bioactives themselves (top) or they can be used to form and stabilized Pickering emulsions that can be used as delivery systems or texture modifiers (bottom). In this latter case, the protein nanoparticles are homogenized with

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oil and water to form a particle-stabilized emulsion. The bioactives can be trapped in the oil

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droplets and/or protein nanoparticles

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Table 1. Global production of major kinds of cereal crops [1]

Global Production (Tons × 106)

Crops

1250

Rice

950

Wheat

855

Barley

146

Sorghum

72

Millets

31

Oats

23

Triticale

17

Rye

16

Buckwheat

2

Fonio

0.6

Quinoa

0.2

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A

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Maize

17