Nanostructures of zein for encapsulation of food ingredients

Nanostructures of zein for encapsulation of food ingredients

9

Niloufar Sharif*, Maria Jos
e Fabra†, Amparo Lo´pez-Rubio† *Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran, †Food Quality and Preservation Department, IATA-CSIC, Valencia, Spain

1

Introduction

Zein comprises a group of alcohol-soluble proteins (prolamins) present in corn endosperm (Wu, Luo, & Wang, 2012). It represents around 45%–65% of the total protein endosperm ( Jafari, 2017a, 2017b; Ren, Ma, Mao, & Zhou, 2014). Zein occurs as aggregates kinked by disulfide bonds in whole corn; however, some of those bonds may be cleaved by reducing agents during extraction or commercial wet-milling operations. Commercial zein is basically a by-product of the corn wet-milling industry (Shukla & Cheryan, 2001). Based on solubility and the differences on the amino acid sequences, zein is currently divided into four classes: α-zein (19 and 22 kDa, highest fraction of zein mass), β-zein (14 kDa), γ-zein (16 and 27 kDa), and δ-zein (10 kDa) (Zhang et al., 2015). α-Zein accounts for 75%–85% of the total protein and is formed by two groups of proteins, Z19 and Z22. Zein contains many nonpolar amino acids including alanine, phenylalanine, leucine, and proline that makes it a hydrophobic protein, being thus soluble in aqueous alcohol solutions (50%–95%). Although zein is extracted from food, it is known to be deficient in essential amino acids (tryptophan and lysine) and, thus has a negative dietary nitrogen balance. However, its unique organoleptic (odorless and tasteless) and physicochemical properties have made zein an excellent food biopolymer in the development of delivery systems for nutrients (Lawton, 2002; Shukla & Cheryan, 2001). In fact, zein is resistant to digestive enzymes causing a slower digestibility in the gastrointestinal fluid, which could be exploited to have a controlled release of functional molecules loaded in zein nanostructures (Patel & Velikov, 2014). All these characteristics have boosted research in this area, making zein an ideal candidate for the fabrication of functional nanostructures. In fact, since the 1990s, zein micro- and nanostructures have received increased research interest in the field of pharma, biomedicine, drug delivery, and functional foods (Patel & Velikov, 2014; Zhang et al., 2015, 2016), due to it GRAS (Generally Recognized as Safe) regulatory status (approved by the Food and Drug Administration) and easy preparation procedures. This chapter reviews the current developments of zein-based microand nanostructures as carriers of bioactive compounds as well as the existing fabricating methods.

Biopolymer Nanostructures for Food Encapsulation Purposes. https://doi.org/10.1016/B978-0-12-815663-6.00009-4 © 2019 Elsevier Inc. All rights reserved.

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Biopolymer Nanostructures for Food Encapsulation Purposes

Methods for obtaining zein nanostructures

2.1 Liquid-liquid dispersion (antisolvent precipitation) Antisolvent precipitation, also known as liquid-liquid dispersion or desolvation is a simple method to produce nanoparticles from proteins such as zein (Sadeghi, Mehryar, Karimi, & Kokini, 2017). Initially, zein and the bioactive compound have to be dissolved in a suitable solvent like ethanol. Subsequently, this solution is diluted with an antisolvent bulk solution such as water under stirring (Zhang et al., 2016). In other words, an antisolvent is a bulk solution that once it is added to the original polymer solution, decreases the solvent quality, resulting in supersaturation followed by solute precipitation ( Joye & McClements, 2014). In antisolvent precipitation, the driving force for the formation of nanoparticles comes from the imbalance of molecular interactions between a three-phase system: zein, solvent, and antisolvent ( Joye & McClements, 2014). Commonly applied solvents and antisolvents include organic solvents, ethanol, water, or supercritical CO2. Generally, ethanol serves as a solvent in which zein (and the bioactive compound) is dissolved. Then, this stock solution is added drop wised into an antisolvent solution (usually, water) ( Jin, Xia, & Zhao, 2012; Montes, Gordillo, Pereyra, & de la Ossa, 2011). The obtained zein nanoparticles can be used as a platform to encapsulate mainly nonpolar bioactive compounds if they have the property codissolving in the stock medium with zein (Zhong, Tian, & Zivanovic, 2009). The surface repulsion is the governing force involved in the stability of the obtained nanoparticles. Therefore they may lose their stability in different conditions such as varying the pH conditions or ionic strength among others, resulting in aggregation. Moreover, postprocessing including freeze drying (lyophilization) may induce the agglomeration of the nanoparticles (Patel & Velikov, 2014). To overcome these limitations, proteins such as sodium caseinate have been proposed. The approach is to precipitate zein in the presence of sodium caseinate (Patel, Bouwens, & Velikov, 2010). Sodium caseinate adsorbs onto the surface of the zein nanoparticles, reducing their surface hydrophobicity while improving electrostatic and steric repulsion between nanoparticles (Hu & McClements, 2014). The main advantage of the antisolvent precipitation is that it does not require to work at high temperatures, making it applicable to heat-sensitive bioactive ingredients (Malekzad et al., 2018).

2.2 Electrohydrodynamic processes Electrohydrodynamic processes, namely, electrospraying and electrospinning, have been mainly developed and utilized for the fabrication of functional nanoparticles and nanofibers (Assadpour & Jafari, 2018). Briefly, during electrohydrodynamic processing, a biopolymer solution draws from a syringe needle toward a grounded collector by the application of a high voltage electric field (Karthikeyan, Guhathakarta, Rajaram, & Korrapati, 2012; Neo, Ray, Easteal, Nikolaidis, & Quek, 2012). Both techniques are similar, mainly differing in the biopolymer solution concentration prepared and, as a result, different morphologies are obtained (Faridi Esfanjani & Jafari,

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2016; Jafari, 2017a, 2017b). Nanofibers are developed through electrospinning while nanocapsules are produced by electrospraying (Bhushani & Anandharamakrishnan, 2014). In addition, coaxial electrospinning has been developed as a novel approach in which the bioactive compound and zein are independently dissolved in their relevant solvents, and the core and the shell solutions are then ejected separately through two concentric nozzles (Yang, Zha, Yu, & Liu, 2013). The morphology of the nanofibers/nanocapsules is also affected by the processing parameters (e.g., applied voltage, solution feed rate, and the distance between needle tip and collector), solution characteristics (e.g., type and concentration of the polymer and solvent), and ambient conditions (e.g., temperature, humidity, and air velocity) (Greiner & Wendorff, 2007; Ghorani, Alehosseini, & Tucker, 2017; Neo et al., 2012; Tapia-Herna´ndez, Rodrı´guez-F
elix, & Katouzian, 2017). Zein has been widely used as a raw material in electrohydrodynamic processing to produce nanocapsules and nanofibers in ethanol or acetic acid as solvents (Antunes et al., 2017; Bilenler, Gokbulut, Sislioglu, & Karabulut, 2015; Chuacharoen & Sabliov, 2016a; Donsı`, Voudouris, Veen, & Velikov, 2017; Huang et al., 2013).

2.3 Coacervation (phase separation) Coacervation is another technique for the fabrication of nanoparticles. It relies on the phase separation of one (i.e., simple coacervation) or a mixture of two oppositely charged biopolymers (i.e., complex coacervation) in a solution (Ghasemi, Jafari, Assadpour, & Khomeiri, 2017, 2018). The biopolymers can be produced to precipitate around a core of bioactive compound to encapsulate it ( Jia, Dumont, & Orsat, 2016; McClements, 2017). Although the main driving force in forming coacervates is electrostatic interactions, other interactions such as hydrophobic interactions and hydrogen bonds are also involved in the formation of coacervates (Augustin & Hemar, 2009; Jia et al., 2016; Munin & Edwards-L
evy, 2011). Mostly, protein-polysaccharide complexes are used to produce coacervates. Therefore zein-based composite nanoparticles in combination with polysaccharides can be used to encapsulate food bioactives (Li & de Vries, 2018; Shishir, Xie, Sun, Zheng, & Chen, 2018).

3

Encapsulation and controlled release of bioactives using zein nanostructures

3.1 Carotenoids Carotenoids, also known as tetraterpenoids, are the category of fat-soluble pigments naturally found in plants (flowers, fruits, and vegetables) and photosynthetic microorganisms. Depending on the presence or absence of oxygen in the molecule, they can be classified into two general classes: carotenes (e.g., β-carotene and lycopene), which only contain carbon and hydrogen atoms, and xanthophylls (e.g., lutein), having oxygen besides carbon and hydrogen in their structure (Mercadante, Rodrigues, Petry, & Mariutti, 2017). Many health benefits have been attributed to carotenoids when

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Biopolymer Nanostructures for Food Encapsulation Purposes

consumed in sufficient levels (Qian, Decker, Xiao, & McClements, 2012) including antioxidant activity (Rodriguez-Amaya, 2018) as well as prevention of human diseases such as cancer and age-related disorders (Rao & Rao, 2007). In addition, they are vitamin A precursors (i.e., α-carotene and β-carotene), thus a vital dietary source of vitamin A (Leong, Show, Lim, Ooi, & Ling, 2018). Therefore these tetraterpenoids have been commonly incorporated into foods, not only due to their colorant properties, but also to their nutraceutical applications. Despite the health benefits of carotenoids, their low water solubility (Chuacharoen & Sabliov, 2016b), instability (susceptible to oxidants, light, and heat) (P
erez-Masia´, Lagaron, & Lopez-Rubio, 2015), and low bioaccessibility ( Jain et al., 2018) hinder their direct incorporation into food matrices intended to improve human health. To avoid these problems, micro/nanoencapsulation has been proposed as a plausible solution. Since zein is soluble only in aqueous ethanol, this solvent mixture may be used to dissolve both the lipophilic bioactive compounds (core material) and the shell material (zein). Ethanol can then be easily removed through evaporation. Therefore it is possible to prepare zein micro- and nanoparticles by phase separation in a nonsolvent followed by solvent removal by evaporation (liquid-liquid dispersion) or by means of the electrohydrodynamic process since the solvent evaporates during the formation of electrospun/electrosprayed micro-/nanostructures. Table 1 summarizes some works reported in the literature to protect carotenoidderived compounds in which zein has been postulated as a preferred shell material. The main objectives of these works were to improve the stability of carotenoids (in terms of photostability and thermal stability) and their bioaccessibility. Zein nanostructures containing β-carotene, lycopene, or lutein have been successfully developed by means of the electrospinning/electrospraying or liquid-liquid dispersion, with a wide range of encapsulation efficiencies and with improved thermal stability, light, or UV protection and with enhanced antioxidant properties (Chuacharoen & Sabliov, 2016a, 2016b; Fernandez et al., 2009; Hu et al., 2012; Kose & Bayraktar, 2016). More specifically, based on the results from confocal Raman imaging spectroscopy, β-carotene was stable in zein microstructures (ca. 1140 nm), produced by electrospraying, showing a significant enhancement in the light stability, with a remarkably good protection against oxidation when exposed to UV-vis irradiation for the encapsulated β-carotene in comparison with the nonencapsulated compound (Fernandez et al., 2009). Kose and Bayraktar (2016) optimized the conditions for producing uniform and nonaggregated electrosprayed lycopene-zein capsules (ca. 0.23 μm), showing that low flow rates were more adequate for the evaporation of the solvent (ethanol) during the electrospraying process. In addition, they observed that increasing the lycopene-zein ratio up to 1:20 (w/w) favored the formation of more uniform spherical structures with smoother surfaces. According to photostability analysis, encapsulation in electrosprayed zein lowered the degradation of the compound and enhanced its stability as compared to the pure lycopene (Kose & Bayraktar, 2016). However, to improve the bioaccessibility of carotenoids, which is somehow limited, some works have reported that, during digestion, they have to be previously incorporated into mixed micelles (Kaulmann, Andr
e, Schneider, Hoffmann, & Bohn, 2016) and, thus the consumption of carotenoids with digestible lipids enhances

Carotenoid

Preparation method

Size

Encapsulation efficiency

β-Carotene

Emulsion electrospraying

750–880 nm

Liquid-liquid dispersion Electrospinning Lycopene Lutein

Benefit

Reference

6%–34%

Increase in the bioaccessibility after in vitro digestion

168.17 nm

–

540–3580 nm

–

Improved stability and antioxidant activity Improved light stability

Electrospraying Liquid-liquid dispersion

0.1–0.75 μm 156.1–216.5 nm

– 69.1%–83%

Go´mez-Mascaraque, PerezMasia´, Gonza´lez-Barrio, Periago, and Lo´pez-Rubio (2017) Chuacharoen and Sabliov (2016a) Fernandez, Torres-Giner, and Lagaron (2009) Kose and Bayraktar (2016) Chuacharoen and Sabliov (2016b)

Liquid-liquid dispersion

198–355 nm

34.44%– 83.15%

Improved light stability Improved stability under different temperatures and UV irradiation Controlled release

Nanostructures of zein for encapsulation of food ingredients

Table 1 Zein-based nanostructures containing carotenoids

Hu, Lin, Liu, Li, and Zhao (2012)

221

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Biopolymer Nanostructures for Food Encapsulation Purposes

their bioavailability ( Jafari & McClements, 2017). As a strategy to improve the bioaccessibility of β-carotene after in vitro digestion, Go´mez-Mascaraque et al. (2017) successfully developed the emulsion electrospraying process using zein as a shell material and soybean oil as a carrier oil of β-carotene and using two different homogenization procedures (high-speed homogenization and ultrasonication). They found an increase in the bioaccessibility of β-carotene after in vitro digestion in zein electrosprayed capsules (ca. 088
0.39 μm and 0.75
0.30 μm) which was negligible in its free form. However, the bioaccessibility of encapsulated β-carotene was still considerably low probably due to the low encapsulation efficiency (34
7%) (Go´mezMascaraque et al., 2017). The liquid-liquid dispersion method has also been used to develop zein micro- and nanostructures containing carotenoids in order to evaluate their ability to enhance the physicochemical stability and antioxidant activity of β-carotene in the presence of milk under simulated gastrointestinal environments. Results from antioxidant activity based on 2,20 -azinobis-(3-ethyl benzthiazoline-6-sulphonic acid) (ABTS) assay showed that β-carotene in zein nanoparticles possessed an increased antioxidant activity which statistically differed from β-carotene in nanoemulsified form (which was studied as a control). However, in the presence of milk, this difference was not as pronounced as some milk components, mostly casein and its derivative peptides, having intrinsic antioxidant activity. β-carotene entrapped in zein nanoparticles had improved stability and enhanced antioxidant activity under simulated gastrointestinal conditions when compared to that of zein nanoemulsions (Chuacharoen & Sabliov, 2016a). Another carotenoid, namely, lutein, has also been encapsulated in zein nanoparticles (loaded with 7.5% lutein) using a combination of lecithin and pluronic F127 surfactants with the aim of improving stability in the presence of surfactants. Upon surfactant addition, particle size increased, while the polydispersity index and entrapment efficiency of lutein in zein nanoparticles improved due to the electrostatic affinity between the zein nanoparticles and surfactants. Surfactants protected lutein against chemical degradation when exposed to different temperatures and UV irradiation. The release of lutein from zein nanoparticles in phosphate buffered saline (PBS) exhibited an initial burst release at 24 h following a zero-order release (Chuacharoen & Sabliov, 2016b). Similarly, lutein-containing zein-based nanoparticles prepared by liquidliquid dispersion method with supercritical fluids such as CO2, resulted in high loading and high entrapment efficiency of lutein. The optimum process parameters in order to achieve smaller and more regular particles were lower processing temperature and solution flow rate coupled with high pressure. Moreover, the initial burst release of lutein was negligible after encapsulation in zein nanoparticles with the release profile having a near zero-order release which is a characteristic desired for controlled release delivery systems (Hu et al., 2012).

3.2 Essential oils and volatile compounds Essential oils, hydrophobic, and volatile plant-derived compounds (extracted from flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits, and roots) have found applications as flavoring agents in food, pharmaceutics, and cosmetics. They normally

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have an antioxidant activity useful for the inhibition of oxidative stress-related diseases. In addition, they can delay or hinder oxidative deterioration of lipids in foods by inhibiting the oxidative chain reactions (Bilenler et al., 2015; Del Nobile, Conte, Incoronato, & Panza, 2008). Many of them have also proved antimicrobial activity against a wide range of Gram-positive and Gram-negative bacteria, being a potential alternative to synthetic preservatives in various food-related applications, such as in active food packaging. They are sensitive compounds which readily undergo degradation in the presence of oxygen, light, and even at moderate temperatures (Zhang, Chen, & Pan, 2017). Therefore encapsulation has been widely explored as a feasible alternative to mitigate these problems. As it can be observed in Table 2, most of studies have been focused on the development of zein-based nanostructures containing essential oils prepared by a liquidliquid dispersion approach. Among these types of bioactive compounds, thymol (5-methyl-2-isopropylphenol) and carvacrol (5-isopropyl-2-methylphenol) (both monoterpenoid phenols) are, for instance, GRAS food additives approved by the Food and Drug Administration (FDA) list. Like other essential oils and volatile natural compounds, they have antioxidant (Camo, Lor
es, Djenane, Beltra´n, & Roncal
es, 2011) and antiinflammatory (Riella et al., 2012) properties. Moreover, they contain a phenolic hydroxyl group, which endues their antimicrobial properties, allowing disrupting the bacterial membrane and leading to cell death (Guarda, Rubilar, Miltz, & Galotto, 2011). Both thymol and carvacrol have been successfully encapsulated into zein-based matrices prepared by the liquid-liquid dispersion method and, subsequently, lyophilized and treated at different pH values (particle size ca. 800 nm), showing high antioxidant and antimicrobial properties against Escherichia coli growth (Wu et al., 2012). Interestingly, da Rosa et al. (2015) showed that particle size of thymol- and carvacrol-loaded zein nanoparticles were bigger than their counterparts prepared with pure zein, suggesting the surfactant role of both compounds which prevented the coalescence of the nanoparticles. The encapsulation of carvacrol or thymol into the zein matrices favors a controlled release of the bioactive compounds and, the antimicrobial activity was higher against Gram-positive bacteria than for Gram-negative bacteria. On the other hand, it has been reported that most of the particles prepared by the aforementioned technique are at microscale (Bilenler et al., 2015) and they are not soluble in water; therefore the aggregation and precipitation may occur when incorporating them into liquid food matrices (Li et al., 2013). To overcome these limitations, designing a core-shell structure, where zein serves as a core and a hydrophilic material (i.e., proteins, polysaccharides) acts as a shell, seems to be beneficial. For instance, a simple antisolvent method (liquid-liquid separation method) was developed to prepare thymol-zein-sodium caseinate nanoparticles in which sodium caseinate was directly poured into zein solutions. As the thymol-to-zein ratios increased from 0.1 to 0.4, the particle size of zein-sodium caseinate-thymol nanoparticles increased from 160.8 to 264.3 nm. After 8 months of storage, thymol was stable in the nanoparticles. Moreover, thymol-loaded zein-sodium caseinate nanoparticles had antimicrobial activity against E. coli and Salmonella as well as antioxidant activities which was dependent on the dose of thymol. Based on in vitro

224

Table 2 Zein-based nanostructures containing essential oil and volatile compounds Preparation method

Size

Encapsulation efficiency

Thymol

Liquid-liquid dispersion

<800 nm

>50%

Liquid-liquid dispersion

109.2–122.8 nm

89.75%–92.4%

Liquid-liquid dispersion

160.8–264.3 nm

67.2%–91.7%

Liquid-liquid dispersion

<800 nm

>50%

Liquid-liquid dispersion

108.9–119.9 nm

88.5%–99.9%

Emulsification (zein-sodium caseinatepectin complex) followed by nano spray drying Electrospinning

140 nm

–

333 and 389 nm

–

Carvacrol

Eugenol

Eucalyptus

Benefit

Reference

Antioxidant activity Antimicrobial property Storage stability Antimicrobial property Storage stability Antimicrobial property Initial burst release followed by slower release Antioxidant activity Antimicrobial property Storage stability Antimicrobial property Stability Redispersibility

Wu et al. (2012)

Antimicrobial activity

da Rosa et al. (2015) Li et al. (2013)

Wu et al. (2012) da Rosa et al. (2015) Veneranda et al. (2018) Antunes et al. (2017)

Biopolymer Nanostructures for Food Encapsulation Purposes

Essential oils

Nanostructures of zein for encapsulation of food ingredients

225

release profile, thymol showed an initial burst release followed by slower release from zein-sodium caseinate nanoparticles (Li et al., 2013). On the other hand, chitosan has also been evaluated as a shell material to stabilize thymol-zein-caseinate nanoparticles from coalescence, prepared by the liquid-liquid dispersion method. Coating with chitosan, the spherical shape and smooth surface of thymol-loaded zein and zeinsodium caseinate nanoparticles changed to rough surface with clumps in some areas. However, both sodium caseinate and chitosan enhanced the encapsulation efficiency (>80%) of thymol. Moreover, thymol-loaded zein-sodium caseinate-chitosan nanoparticles were more effective on Gram-positive bacteria, Staphylococcus aureus as compared to its free form for a long time (Zhang et al., 2014). Interestingly, pectin has also been used as outer shell for essential oils-zein-sodium caseinate nanoparticles in order to produce more stable nanostructures when they are spray-dried. Specifically, eugenol, a naturally occurring essential oil that is extracted from cloves was encapsulated in zein-caseinate-pectin complex nanoparticles. It was observed that the concentration of wall materials (zein and sodium caseinate) as well as heating and pH conditions during complexation process significantly affected the morphology and stability of the obtained nanoparticles with the optimum conditions around the isoelectric point of zein (pH 6.6). Based on the results from Fourier transform infrared (FTIR) spectroscopy, two interactions, namely, hydrophobic and electrostatic interactions were the driving forces to form the complexes. In addition, after nano spray drying, stable eugenol-loaded colloidal nanoparticles, in the presence and absence of sodium caseinate with good redispersibility in water were obtained, although in the presence of sodium caseinate the original nanoscale dimension of particles was maintained during redispersion (Veneranda et al., 2018). Therefore the combined use of other hydrocolloids like proteins (i.e., sodium caseinate) or polysaccharides (i.e., pectin, chitosan) in combination with zein can enhance the protection and stabilizing ability of encapsulated volatile compounds incorporated in zein nanoparticles, as these biopolymers are able to create an external shell layer, also having a positive impact on the encapsulation efficiency and on the sustained release. Finally, limited reports are available in the literature related to the production of zein nanostructures containing volatile compounds which have been produced by means of other methodologies and, most of them are related to the formation of zein nanofibers of interest in the development of antimicrobial food packaging. For instance, zein ultrafine fibers (ca. 333 nm of diameter) containing eucalyptus essential oil-β-cyclodextrin complexes have been successfully obtained by the electrospinning technique, with potential antimicrobial activity against a wide range of Gram-positive and Gram-negative bacteria (Antunes et al., 2017).

3.3 Phenolic compounds Recently, phenolic compounds, found in the plants kingdom as secondary metabolites, have gained a great interest in the area of functional foods and nutraceuticals, as they possess a high spectrum of health benefits such as antioxidant (Mohammadi, Jafari, Assadpour, & Faridi Esfanjani, 2016; Mohammadi, Jafari, Esfanjani, & Akhavan,

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Biopolymer Nanostructures for Food Encapsulation Purposes

2016; Tavakoli, Hosseini, Jafari, & Katouzian, 2018), antiinflammatory (Beara et al., 2012), anticarcinogenic ( Jeong et al., 2011), antibacterial (Silva, Rodrigues, Fea´s, & Estevinho, 2012), and antiviral activities (Fang & Bhandari, 2010). Moreover, they are responsible for the color of fruits and juices, for some flavor properties and also substrates of some enzymatic reactions including browning (Akhavan Mahdavi, Jafari, Assadpoor, & Dehnad, 2016; Cheynier, 2012; Mahdavi, Jafari, Ghorbani, & Assadpoor, 2014). Phenolic compounds, chemically structured as hydroxyl groups bound to aromatic rings include anthocyanins, flavonols, flavan3-ols, proanthocyanidins, and phenolic acid derivatives, among others (Assadpour, Jafari, & Esfanjani, 2017; Faridi Esfanjani, Assadpour, & Jafari, 2018). However, phenolic compounds are chemically unstable and prone to degradation due to exposure to temperature, certain pH conditions, and enzymatic attacks, fact which restricts their free form applications (Betz et al., 2012). It is well known that phenolic compounds can form complexes with proline-rich proteins via noncovalent interactions ( Jia et al., 2016; Ozdal, Capanoglu, & Altay, 2013). Therefore delivery systems such as zein which is rich in proline residues seem to be an ideal method to protect them as well as to improve their bioavailability after human consumption. Table 3 compiles different studies in which zein nanostructures have been used to protect phenolic compounds. Flavan-3-ols compounds are naturally found in plant food as aglycons of catechin and epicatechin without glycosylation. Furthermore, they can be esterified with gallic acid and ellagic acid, forming hydrolyzable tannins. Tea is a rich source of flavan3-ols being ()-epigallocatechin gallate (EGCG) the most abundant ( Jia et al., 2016). Encapsulation of EGCG in zein has been reported by various techniques. Donsı` et al. (2017) prepared EGCG-zein colloidal particles using an antisolvent precipitation method based on two different approaches: the fast addition of the ethanol/ water solvent (80:20) to water or to an aqueous solution containing sodium caseinate. The addition of sodium caseinate changed the pure zein mean particle size from 100 to 190 nm and caused a shift in positive surface ζ-potential to the negative surface ζ-potential (30 to 30 mV). Sodium caseinate significantly reduced the aggregation tendency of particles which could be attributed to an increase in viscosity in the presence of sodium caseinate, causing a reduction in coagulation and agglomeration of particles such as zein ( Joye & McClements, 2013). When loaded with EGCG, zein particles without sodium caseinate (<100 nm) were also smaller than zein particles in the presence of sodium caseinate (170 and 250 nm). The addition of EGCG had no influence on the particle morphology of zein-based particles. Differences in encapsulation efficiency (EE) were observed depending on the method of preparation and, thus while adding EGCG before precipitation, EE values were around 37% and 46% for the samples with and without sodium caseinate, respectively, only 17% of EGCG was incorporated if added after precipitation. This was ascribed to EGCG adsorption on the surface of the colloidal particles. Encapsulation of EGCG as a water-soluble bioactive is limited because the hydrophobic interactions between the aromatic rings of EGCG molecule and the hydrophobic residues of zein during the precipitation phase when using water as an antisolvent are weak. Interestingly, this challenge could be overcome by the addition of biopolymers dissolved in the antisolvent, including sodium caseinate by simply tuning the composition of the zein colloidal particles.

Phenolic compound

Preparation method

()Epigallocatechin gallate (EGCG)

Procyanidins Gallic acid Ferulic acid Quercetin

Curcumin

Size

Encapsulation efficiency

Liquid-liquid dispersion

<100 nm

17%–46.3%

Electrospinning

472 nm

–

Liquid-liquid dispersion (with chitosan)

155.5–225.4 nm

61.04%–80.7%

Liquid-liquid dispersion Electrospinning

392–447 nm

–

327–387 nm

100%

Coaxial electrospinning Liquid-liquid dispersion

760 nm

–

130–161 nm

–

Chemically stable under alkaline pH and UV

Liquid-liquid dispersion

<200 nm

–

Electrospraying

–

85%–90%

High solubility in simulated gastrointestinal track in the intestinal phase Stability after 3 months

Benefit

Reference

Tuning the release during in vitro digestion Modulation of fat digestion rate Improved stability in water A burst release followed by a slow release Higher antioxidant activity Reduced cytotoxicity

Donsı` et al. (2017)

Retained antioxidant activity Sustained release

Li, Lim, and Kakuda (2009) Liang et al. (2017)

Zou, Li, Percival, Bonard, and Gu (2012) Neo et al. (2013)

Nanostructures of zein for encapsulation of food ingredients

Table 3 Zein-based nanostructures containing phenolic compounds

Yang et al. (2013) Patel, Heussen, Hazekamp, Drost, and Velikov (2012) Zou et al. (2016b)

Continued

227

Gomez-Estaca, Balaguer, Gavara, and Hernandez-Munoz (2012)

228

Table 3 Continued Phenolic compound

Resveratrol

Size

Encapsulation efficiency

Liquid-liquid dispersion

191 nm

–

Coacervation (with pectin)

183.7–201.5 nm

61.6%–90.3%

Liquid-liquid dispersion Liquid-liquid dispersion

250 nm

>86%

215 nm

Liquid-liquid dispersion

Liquid-liquid dispersion (with ultrasound treatment) Liquid-liquid dispersion

Benefit

Reference

Increased bioaccessibility Good dispersion in semiskimmed milk matrix Stability improvement in gastrointestinal conditions Sustained release Enhanced antioxidant activity Good water dispersibility

Zou et al. (2016a)

–

Enhanced bioaccessibility Improved solubility

100–1000 nm

73%

469.47–632.15 nm

>80%

Relatively stable to aggregation and sedimentation in aqueous solution Increased encapsulation efficiency Thermal stability

Chen, Zheng, Decker, McClements, and Xiao (2015) Chen, Zheng, McClements, and Xiao (2014)

206.3 and 217.2 nm

85.3% and 86.5%

Thermal stability Photostability Higher antioxidant activity

Chang et al. (2017)

Hu et al. (2015)

Liang et al. (2018)

Fan, Liu, Gao, Zhang, and Yi (2018)

Biopolymer Nanostructures for Food Encapsulation Purposes

Tangeretin

Preparation method

Nanostructures of zein for encapsulation of food ingredients

229

Moreover, sodium caseinate modulates the functionality of EGCG, in terms of antioxidant activity and release during digestion by tuning particle composition (Donsı` et al., 2017). Besides nanoparticles, ultrafine electrospun zein fibers were fabricated by electrospinning using different zein ethanol-water solutions at working voltages of 15 and 20 kV to encapsulate EGCG, resulting in fibers with diameters ranging from 150 to 600 nm. The stability of EGCG when contacting water was dependent on the relative humidity and aging time after electrospinning. Based on the FTIR spectra, three types of interactions, namely, hydrogen bonding, hydrophobic interactions, and likely physical encapsulation occurred between EGCG and zein fibers, resulting in improvement in the retention of the bioactive compound after water immersion (Li et al., 2009). Moreover, zein, as a shell material for the encapsulation of EGCG, has been also investigated with biopolymers such as chitosan. EGCG-chitosan-loaded nanoparticles coated with zein had a burst release followed by slow release behavior in a fatty simulant medium with high antioxidant activity. Thus it was proposed that these nanoparticles might provide protection against oxidation for fatty foods during a long period (Liang et al., 2017). Procyanidins, the other member of polyphenols, are oligomers of ()-epicatechin and (+)-catechin. They have high binding affinity to proline-rich proteins such as zein. Procyanidins, extracted and purified from cranberry, were encapsulated in zein nanoparticles by the liquid-liquid dispersion technique. The particle size of the spherical cranberry procyanidin-loaded zein nanoparticles increased from 392 to 447 nm by increasing the cranberry procyanidin-zein ratio from 1:8 to 1:2. On the other hand, increasing the ratio led to a significant decrease in the loading efficiency from 86% to 10%. The main interactions between cranberry procyanidins and zein were through hydrogen bonding and hydrophobic interactions observed through infrared spectroscopy. Moreover, cell culture investigations using human promyelocytic leukemia HL-60 cells revealed that the entrapped cranberry procyanidins within the zein nanoparticles displayed reduced cytotoxicity in comparison with nonencapsulated cranberry procyanidins (Zou et al., 2012). Phenolic acids can be classified into two main groups: hydroxybenzoic acids such as gallic acid and derivatives of hydroxycinnamic acids such as ferulic acid. Chemically, these compounds have, at least, one aromatic ring in which at least one hydrogen is substituted by a hydroxyl group (Liu et al., 2016). These compounds have been encapsulated by electrospinning, forming ultrafine gallic acid-zein fibers with diameters ranging from 327 to 387 nm and reaching almost 100% of the loading efficiency. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy results proved that there were interactions between gallic acid and zein at the molecular level, likely leading to the formation of dimer/oligomeric structures. Moreover, the antioxidant activity of gallic acid was maintained after incorporation within the zein fibers (Neo et al., 2013). Another type of electrohydrodynamic process, namely, coaxial electrospinning (see Section 2.2) was applied to encapsulate ferulic acid, a type of hydroxycinnamic acid in zein fibers with acetic acid as shell fluid aiming to produce smooth fibers without clogging. The formed fibers had round, higher quality in terms of diameter and average size distribution compared to those formed by

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Biopolymer Nanostructures for Food Encapsulation Purposes

single-fluid electrospinning process. Results from ATR-FTIR revealed that the main interactions between ferulic acid and zein in the fibers were through hydrogen bonding. Additionally, the ferulic acid-loaded zein fibers obtained by the coaxial electrospinning exhibited better sustained release of ferulic acid during in vitro dissolution tests, indicating improved functional properties (Yang et al., 2013). Quercetin, found as a glucoside in fruits such as apples (Aceituno-Medina, Mendoza, Rodrı´guez, Lagaron, & Lo´pez-Rubio, 2015) belongs to the flavonols group (Ha, Kim, Lee, & Lee, 2013). For the antisolvent precipitation method, after dissolving quercetin and zein in an ethanol/water solution, a sodium caseinate aqueous solution was added. The size of the generated nanoparticles was in the range of 130–161 nm with the morphology of needle like or spherical depending on the zein concentration. Encapsulated quercetin in zein nanoparticles showed molecular stability enhancement in alkaline pH as well as photostability when exposed to UV irradiation (Patel et al., 2012). Curcumin, a natural polyphenol obtained from the rhizome of turmeric (Curcuma longa) is another phenolic compound encapsulated in zein prolamin. This compound has been encapsulated either by electrospraying, by the liquid-liquid dispersion, or by the coacervation method (Rafiee, Nejatian, Daeihamed, & Jafari, 2018). On the one hand, electrospraying has been widely explored for the fabrication of curcuminloaded zein nanoparticles. Incorporating curcumin to zein nanoparticles, with the encapsulation efficiency of 85%–90%, did not change the morphology of the obtained structures (particle size range between 175 and 900 nm). The homogeneously distributed curcumin in the zein nanoparticles was stable during a three-month storage period at 23°C and 43% RH. Moreover, the curcumin-loaded zein nanoparticles had good dispersion and coloring capacity in an aqueous food matrix (semiskimmed milk) in comparison to commercial curcumin (Gomez-Estaca et al., 2012). Other encapsulation methods have also been investigated. For instance, curcumin-zein nanoparticles produced by the antisolvent precipitation have been widely studied. The effect of the composition of curcumin-loaded nanoparticles on its degradation and bioaccessibility within a simulated gastrointestinal tract was investigated, comparing with phospholipid and lipid nanoparticles generated by emulsion. The mean diameter of nanoparticles was <200 nm for all three delivery systems. Interestingly, the loading capacity of curcumin was dependent on the system, being the zein nanoparticles the ones with the greatest load (11.7%). During the digestion in a simulated gastrointestinal tract (GIT) which included mouth, stomach, and small intestine phases, the curcumin-loaded zein nanoparticles showed the highest solubility characteristics in the intestinal phase. Based on the observation of this study, it was clear that curcumin release varied depending on the delivery system used and, thus depending on the intended application, a specific selection of the encapsulating matrix should be carried out (Zou et al., 2016b). In addition, in another research by Zou et al. (2016a), the zein nanoparticles were mixed with digestible lipid nanoparticles (through high-pressure homogenization to produce O/W emulsions) to fabricate a mixed colloidal dispersion with the aim of increasing curcumin bioaccessibility in the small intestine. The mean diameter of mixed colloidal dispersions varied in the range of the two individual colloidal systems

Nanostructures of zein for encapsulation of food ingredients

231

(between 120
12 and 191
2 nm for zein nanoparticles and digestible lipid nanoparticles, respectively). Moreover, the influence of gastrointestinal conditions, mouth, stomach, and small intestine regions on system properties were investigated. According to the results, the bioaccessibility of curcumin improved by increasing digestible lipid nanoparticle concentration in the mixed colloidal system, as the solubilization capacity of this hybrid system increased. Therefore zein and digestible lipid mixed colloidal dispersions could possess the relative advantages of each system, overcoming the different challenges such as low bioaccessibility in the case of zein nanoparticles and poor chemical stability in the case of digestible lipid nanoparticles. However, the authors suggested that a more comprehensive in vitro and in vivo study would be beneficial to investigate the bioaccessibility, absorption, and transformation characteristics of curcumin in this mixed colloidal system (Zou et al., 2016a). Combining matrices with zein and other biopolymers is also another approach to improve stability and release properties of curcumin. In this regard, a better water dispersibility has been observed for curcumin-zein-pectin nanoparticles (Hu et al., 2015). Moreover, it has been reported that incorporation of a pectin coating on zein-sodium caseinate nanoparticles would improve the stability of curcumin in gastrointestinal conditions with a sustained release behavior in this fluid (Chang et al., 2017). Tangeretin, a type of flavone found in citrus fruits (Xiao et al., 2009) has been encapsulated in zein nanoparticles using an antisolvent precipitation method. The objective of the research work was to study the influence of dietary lipids on the gastrointestinal fate of tangeretin-loaded zein nanoparticles. The bioaccessibility of tangeretin was enhanced with increasing the fat content of oil-in-water emulsions. This was explained by the improved solubilization within the mixed system in the intestinal fluids. Moreover, the presence of fat droplets resulted in 12 times increase in the tangeretin concentration when compared with tangeretin loaded in pure zein nanoparticles. As a result, it was concluded that the bioavailability of hydrophobic bioactive compounds such as tangeretin could be increased in the presence of lipid nanoparticles (Chen et al., 2015). Moreover, β-lactoglobulin was applied in the formation of stable zein colloidal suspensions to design a delivery system for tangeretin (Chen et al., 2014). Another naturally occurring polyphenol is resveratrol, largely found in grapes. The ultrasound had a positive effect on particle size and polydispersity index of resveratrolloaded zein particles produced by liquid-liquid dispersion method, causing a dramatical reduction of both; smaller and more homogeneous diameters with particles having smooth surfaces were obtained after the ultrasound treatment. As a result, there were molecular rearrangements possibly indicating the migration of some resveratrol to the inner part of zein particles with some interactions between resveratrol and zein. Moreover, analysis from infrared spectroscopy revealed that electrostatic interactions and hydrogen bonds were strengthened after the ultrasound treatment, fact that could be related to the changes in the secondary structure of zein, thus indirectly leading to improvements in the encapsulation efficiency and loading capacity of resveratrol in zein particles. In addition, differential scanning calorimetry (DSC) showed an improvement in the thermal stability of the resveratrol-loaded zein particles after sonication that again could be due to the change in the secondary structure of zein, inducing stronger hydrophobic attractions, hydrogen bonding, and electrostatic interactions between the

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two molecules (Liang et al., 2018). More recently, bovine serum albumin-caffeic acid conjugate has been proposed as a stabilizer in resveratrol-zein-loaded nanoparticles that had a positive effect on thermal properties and photostability of encapsulated resveratrol with a high antioxidant activity (Fan et al., 2018).

3.4 Essential fatty acids Oils rich in polyunsaturated fatty acids (PUFAs) such as omega-3 or omega-6 fatty acids can offer health benefits in terms of preventing a range of human diseases, as they can reduce the risk of coronary heart diseases, hypertension, and arthritis among others (Ghorbanzade, Jafari, Akhavan, & Hadavi, 2017; Rubio-Rodrı´guez et al., 2010; Timilsena, Wang, Adhikari, & Adhikari, 2017). On the other hand, as a consequence of having the unsaturated bonds, these compounds are very sensitive to oxygen, light, and heat, fact which hinders their direct application to food products (Torres-Giner et al., 2010). As an amphiphilic protein, zein can be used to encapsulate lipophilic unsaturated oil compounds. Fish oil is one of the major sources of omega-3 polyunsaturated fatty acids (PUFAs). Most of the attempts have been focused on enhancing the oxidative stability of fish oil ( Jafari, Assadpoor, Bhandari, & He, 2008). In this regard, electrohydrodynamic processes have been widely reported to fabricate fish oil-zein-based nanostructures. Moomand and Lim (2014) investigated encapsulation of fish oil (30%, w/w) in electrospun zein structures with two different solvents: ethanol and isopropanol. Electrospun fish oil-zein-loaded fibers were obtained from ethanol solutions, while using isopropanol led to the formation of beaded structures (electrosprayed particles). Both structures provided a better oxidative stability in comparison to nonencapsulated fish oil (Moomand & Lim, 2014). The release of fish oil from the electrospun and electrosprayed zein structures during in vitro gastrointestinal conditions revealed a rapid release followed by slower release in simulated gastric fluid conditions, with a greater release from electrosprayed zein particles (obtained from 10% zein solutions) than from the electrospun ones (from 20% zein solutions). This can be explained by the higher surface/volume ratio found in the electrosprayed zein capsules. A similar behavior was observed under simulated intestinal fluid conditions (Moomand & Lim, 2015). It is well known that core-shell structures can be better obtained by combining hydrophilic shell materials with hydrophobic bioactive compounds and vice versa. Thus in order to better protect the fish oil and to increase the encapsulation efficiency, a hydrophilic polymer (polyvinylpyrrolidone—PVP) has been used as a shell material and zein-fish oil as a core to produce electrospun fibers by means of the coaxial electrospinning setup and it was compared to the single electrospinning process. The average diameter of the core-shell structure was 560 nm with an encapsulation efficiency of 96.9% for fish oil. In addition, an enhancement in the oxidative stability of fish oil was observed for the coaxial nanofibers when compared to nanofibers obtained using the single needle configuration. On the other hand, a lower release of fish oil from the core-shell nanofibers was observed in comparison with the single nanofibers (83.5% vs. 91.3%) during in vitro digestion. Therefore the release behavior of fish oil changed after encapsulation in coaxial nanofibers (Yang, Wen, et al., 2017).

Nanostructures of zein for encapsulation of food ingredients

233

Therefore other strategies are needed to overcome this drawback. For instance, as a strategy to improve the oxidative stability of fish oil while maintaining its release behavior, composite zein nanofibers incorporating fish oil and ferulic acid (with antioxidant properties) were obtained by electrospinning. The incorporation of ferulic acid also resulted in enhanced nutritional value of the nanofibers. The mean diameter of nanofibers was 440 nm with an encapsulation efficiency of 94% for fish oil. Results from FTIR revealed that there were interactions among fish oil, ferulic acid, and zein altering the secondary structure of the zein protein. The addition of ferulic acid as an antioxidant into the zein nanofibers led to enhanced oxidative stability of encapsulated fish oil while it did not affect the release behavior of fish oil from the nanofiber mat (Yang et al., 2017). Moreover, other techniques, such as the liquid-liquid dispersion method, have been proposed as useful tools for the development of fish oil-zein nanostructures. In general, loaded zein capsules produced by means of the antisolvent method had smaller particle sizes than those obtained by using the electrospinning/ electrospraying process. For instance, Zhong et al. (2009) produced fish oil-zeinloaded particles (ca. 350–450 nm) by means of the liquid-liquid dispersion method with a good oxidative stability during 28-day storage at 37°C (Zhong et al., 2009). Recently, Soltani and Madadlou (2015) developed fish oil-loaded zein nanoparticles with the aim of inoculating them into the sugar beet pectin gels which would serve as the delivery vehicles of omega-3 fatty acids (Soltani & Madadlou, 2015). Other studies have focused on the encapsulation of pure fatty acids. Hereby again, electrohydrodynamic processes have been broadly used. Docosahexaenoic acid (DHA), one of the main PUFA ingredients of the omega-3 fatty acids in fish oil, was encapsulated in zein ultrafine capsules by electrospraying technique. The average diameter of the encapsulated DHA zein capsules was around 490
200 nm. The DHA-loaded zein capsules were more stable at high relative humidity and temperatures than the free compound. The zein shell matrix retarded the deteriorative oxidation reactions of DHA and increased the retention of volatiles, masking unwanted DHA flavor after encapsulation (Torres-Giner et al., 2010). Similarly, coaxial electrospraying was also employed to encapsulate α-linolenic acid (ALA), one of the essential omega-3 fatty acids in zein-zein or gelatin-zein core-shell matrices, reaching high encapsulation efficiencies due to the hydrophobic interactions between zein and ALA in the particles as confirmed by FTIR. In contrast to that abovementioned for fish oilzein PVP nanostructures, no significant differences were observed between the uniaxial and coaxial ALA-zein-loaded microparticles in terms of thermal stability while ALA-zein-gelatin-loaded particles contributed to a better stability of ALA (Go´mezMascaraque et al., 2018). A compilation of research works reporting about zein nanostructures containing essential fatty acids is present in Table 4.

3.5 Vitamins Vitamins naturally present in food are functional micronutrients with essential effects in the human body. However, they are sensitive compounds to degradation (Gonnet, Lethuaut, & Boury, 2010; Katouzian & Jafari, 2016). Therefore it is necessary to protect vitamins in order to prevent their deterioration during food processing as well as

234

Table 4 Zein-based nanostructures containing essential fatty acids Preparation method

Size

Encapsulation efficiency

Fish oil

Electrospinning

440 nm

94%

Coaxial electrospinning Electrospinning

560 nm

96.6%

300 and 190 nm <700 nm

Docosahexaenoic acid (DHA)

α-Linolenic acid (ALA)

Electrospinning/ electrospraying Liquid-liquid dispersion Liquid-liquid dispersion Electrospraying

Coaxial electrospinning

Benefit

Reference Yang et al. (2013)

91% and 96%

Oxidative stability Maintaining release behavior Oxidative stability Less release amount Oxidative stability

–

Release

Moomand and Lim (2015)

350–450 nm

–

Oxidative stability

Zhong et al. (2009)

69–83 nm

–

490 nm

–

submicron

>90% and >70%

Yang et al. (2017) Moomand and Lim (2014)

Soltani and Madadlou (2015) Stability (high relative humidity and temperatures) Masking unwanted DHA flavor Thermal stability

Torres-Giner, Martinez-Abad, Ocio, and Lagaron (2010)

Go´mez-Mascaraque, Tordera, Fabra, Martı´nez-Sanz, and LopezRubio (2018)

Biopolymer Nanostructures for Food Encapsulation Purposes

Fatty acid type

Nanostructures of zein for encapsulation of food ingredients

235

during consumption (Bochicchio, Barba, Grassi, & Lamberti, 2016). Depending on their solubility, vitamins are classified into two classes: water-soluble vitamins and fat-soluble vitamins (Tomas & Jafari, 2018). Zein nanoparticles have been intensively studied as fat-soluble vitamin delivery systems either alone or in mixed with other biopolymers to improve stability as well as other properties. Retinol (also known as vitamin A1) was encapsulated in zein nanoparticles with a chitosan coating with the purpose of investigating the effects of this coating on the physicochemical properties of zein nanoparticles. The retinol-loaded zein nanoparticles had an average particle size of around 300 nm, which increased to higher than 500 nm after coating with chitosan. Moreover, the encapsulation efficiency of retinol in zein nanoparticles improved from 64.9% to more than 80% after being coated with chitosan. Based on FTIR analysis, this might be attributed to crosslinking between the two biopolymers through electrostatic interactions, resulting in a thicker coating on the zein surface, thus improving encapsulation efficiency. Based on FTIR analyses, electrostatic interactions were established between retinol and zein in the nanoparticle structures. The release behavior of the retinol from zein nanoparticles in the absence of chitosan was 100% within 15 min, while after chitosan coating a slow release rate was obtained. In addition, improved photostability against UV exposure was observed after encapsulation of retinol in zein nanoparticles with or without chitosan (Park et al., 2015). Another fat-soluble vitamin, specifically vitamin D3, was encapsulated in zein nanoparticles prepared by the phase separation method. The nanoparticles were coated with carboxymethyl chitosan in the presence of calcium as a crosslinker. The particle size of spherical coated with carboxymethyl chitosan vitamin D3-zein-loaded nanoparticles was between 86 and 200 nm. An improvement in encapsulation efficiency was observed for coated nanoparticles in comparison with noncoated ones (87.9% versus 52.2%). In addition, a better control release of vitamin D3 was achieved for coated nanoparticles in PBS and simulated gastrointestinal fluid. Results from photostability demonstrated that there were no significant differences between coated and noncoated vitamin D3-zein-loaded nanoparticles (Luo et al., 2012). A form of vitamin E, α-tocopherol was incorporated in mucoadhesive composite zein-poly (ethylene oxide)-chitosan ultrafine fibers using electrospinning. Incorporation of α-tocopherol within composite fibers resulted in increased average fibers diameter from 363
120 to 449
126 nm. The mucoadhesivity of the composite ultrafine fibers was enhanced by incorporation of α-tocopherol at 20 wt% without any negative effects on the morphology of obtained fibers. Moreover, the release of α-tocopherol was triggered by swelling with subsequent diffusion in simulated gastric fluid (SGF) without pepsin at pH 2 and by degradation with pepsin at pH 1.2 (Wongsasulak et al., 2014). Luo et al. (2011) have also reported a better protection of α-tocopherol entrapped in zein-chitosan complexes in gastrointestinal conditions as well as improved release behavior in comparison with zein nanoparticles, explained by the chitosan coating (Luo et al., 2011). In a novel approach, Weissmuller and coworkers (2016) proposed a new zein nanoparticle fabrication method, named Flash Nanoprecipitation (FNP) using a Multi-inlet Vortex Mixer (MIVM) to encapsulate hydrophobic compounds such as vitamin E-acetate (a prodrug for the tocopherol)

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Biopolymer Nanostructures for Food Encapsulation Purposes

(Weissmueller et al., 2016). In the FNP process, through a rapid mixing, an insoluble low molecular weight compound can be encapsulated in a nano-sized, polymerstabilized delivery vehicle (Pustulka et al., 2013). Besides, sodium caseinate was used as a stabilizer. The size of nanoparticles was at sub-100 nm with a high loading of 45 wt% (Weissmueller et al., 2016). Furthermore, zein nanoparticles have also been developed with the aim of achieving controlled release of water-soluble vitamins by liquid-liquid dispersion method. For instance, zein nanoparticles have been explored as delivery systems for folic acid (vitamin B9) to improve its bioavailability when orally administered. The mean diameter of the nanoparticles was around 193
3 nm with an encapsulation efficiency of 57
6%. The oral bioavailability of folic acid was two times higher after encapsulation in comparison with its free form in aqueous solution, possibly due to mucoadhesive characteristics of folic acid-zein-loaded nanoparticles. It was found that the release behavior of folic acid from zein nanoparticles was dependent on the pH conditions. While there was no release of folic acid under simulated gastric conditions, approximately 70% of the folic acid content was released in a burst manner using simulated intestinal fluid (Pen˜alva et al., 2015). Another approach was to compare folic acid-zein-based nanoparticles with folic acid covalently linked to zein nanoparticles in order to have a sustained release of folic acid. The mean diameters of folic acid covalently linked and entrapped within zein nanoparticles were 67.2
6.6 and 96.8
5.9 nm, respectively. FTIR and H-NMR spectra confirmed the presence of the folic acid covalently linked to zein nanoparticles. For both studied nanoparticles, a sustained release in PBS at 37°C for 7 days was observed. To discuss it in more detail, the release of folic acid entrapped in zein nanoparticles had an initial burst release during the first 12 h which was completed after 6 days, while the release of folic acid covalently linked to zein nanoparticles consisted of a quicker initial release in the first 24 h and a slower release of the rest of the bioactive during the following 7 days. Therefore a sustained release of the folic acid could be also obtained by covalent attachment of folic acid onto surface of zein nanoparticles (Chuacharoen & Sabliov, 2017). In addition, the potential of zein blended with other biopolymers as carriers of riboflavin (vitamin B2) has also been explored. The hybrid nanofibers fabricated by electrospinning and combining zein and hordein in acetic acid solution with or without surface-modified cellulose nanowhiskers (SCN) were able to release riboflavin in PBS in a controlled manner (Wang & Chen, 2012, 2014). Moreover, this vitamin was stable in simulated gastric conditions but it was gradually released in a simulated intestinal environment. Interestingly, surface-modified cellulose nanowhiskers significantly enhanced the mechanical properties and water resistance of the fibers and resulted in better controlled release of riboflavin (Wang & Chen, 2014) (see Table 5).

3.6 Other compounds Zein has also been used as a platform to encapsulate other ingredients with various applications. For instance, it has been reported that coloring agents have interactions with proteins via various noncovalent interactions such as hydrogen bonding, hydrophobic, and ionic interactions (Sereikaite, Bumeliene, & Bumelis, 2005). In this

Table 5 Zein-based nanostructures containing vitamins Encapsulation efficiency

Benefit

Reference

80%

A slow release rate Improved photostability

Park, Park, and Kim (2015)

86–200 nm

52.2%–87.9%

A better controlled release

Electrospinning

449 nm

–

Increased mucoadhesivity

Alpha-tocopherol

Liquid-liquid precipitation

200–800 nm

81.3%–87.7%

Vitamin E-acetate (a prodrug for the tocopherol Riboflavin (vitamin B2)

Flash nanoprecipitation (FNP) Electrospinning

<100 nm

–

Protect against gastrointestinal conditions Enhanced release property Stability

Luo, Teng, and Wang (2012) Wongsasulak, Pathumban, and Yoovidhya (2014) Luo, Zhang, Whent, Yu, and Wang (2011)

–

–

Riboflavin (vitamin B2)

Electrospinning

<100 nm

–

Folic acid (vitamin B9)

Liquid-liquid dispersion

193 nm

57%

Folic acid (vitamin B9)

Liquid-liquid dispersion (and covalently linked)

96.8 and 67.2 nm

–

Vitamin type

Preparation method

Retinol (vitamin A1)

Liquid-liquid precipitation (coating with chitosan) Phase separation

500 nm

Alpha-tocopherol (vitamin E)

Vitamin D3

Wet stability in both water and ethanol Controlled release Reduced burst release Improved mechanical properties Increased oral bioavailability No release under simulated gastric conditions A sustained release

Weissmueller, Lu, Hurley, and Prud’homme (2016) Wang and Chen (2012)

Nanostructures of zein for encapsulation of food ingredients

Average nanostructure size

Wang and Chen (2014) Pen˜alva et al. (2015)

237

Chuacharoen and Sabliov (2017)

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Biopolymer Nanostructures for Food Encapsulation Purposes

aspect, curcumin (E100) and indigocarmine (E132), approved food colorants have been incorporated in zein colloidal particles. The mean diameter of the food colorant-loaded zein particles was in the range of 76–300 nm with spherical shape. Incorporation of different curcumin and indigocarmine amounts resulted in particles with various shades of color in the range of yellow-green-blue with a good stability against photodegradation (Patel, Heussen, Dorst, Hazekamp, & Velikov, 2013). Moreover, fortification of foods with minerals has gained much interest in the last decades due to their associated health benefits. For instance, insoluble mineral salts have been encapsulated in zein by the antisolvent precipitation method. A source of iron (Fe), namely, ferric pyrophosphate, was incorporated in zein particles. Fe-loaded zein particles with a size of around 150 nm were stable for months which would be helpful in the development of fortified foods with iron (Van Leeuwen, Velikov, & Kegel, 2014). Other compounds that have been incorporated in zein nanoparticles are antimicrobial compounds. For instance, thymol and nisin were encapsulated in spray-dried zein capsules for improvement of the antilisterial characteristics. Thymol-nisin-zein-loaded capsules showed a sustained release which significantly hindered the growth of L. monocytogenes at pH 6 and 30°C in the growth medium (Xiao, Davidson, & Zhong, 2011).

4

Future trends and concluding remarks

Zein, a biodegradable polymer and GRAS compound, with hydrophobic character is an excellent matrix to develop nanoparticles able to encapsulate a broad range of bioactive compounds, enhancing their stability during processing and storage conditions, as well as improving their bioavailability in simulated gastrointestinal systems, mimicking human consumption. Although there are many reports on the fabrication of zein nanostructures, further development may be necessary to understand the release mechanisms of zein nanostructures and how its combination with other biopolymers can tune the properties of the systems to allow a rational design of zein-based delivery systems in the future.

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