FO07CH11-Zhong
ARI
V I E W
Review in Advance first posted online on January 6, 2016. (Changes may still occur before final publication online and in print.)
N
I N A
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
13:0
C E
S
R
E
19 December 2015
D V A
Organic Nanoparticles in Foods: Fabrication, Characterization, and Utilization Kang Pan and Qixin Zhong Department of Food Science and Technology, University of Tennessee, Knoxville, Tennessee 37996; email: [email protected]
Annu. Rev. Food Sci. Technol. 2016. 7:11.1–11.22
Keywords
The Annual Review of Food Science and Technology is online at food.annualreviews.org
organic food nanoparticles, fabrication, structures, functions, potential applications
This article’s doi: 10.1146/annurev-food-041715-033215 c 2016 by Annual Reviews. Copyright All rights reserved
Abstract In the context of food systems, organic nanoparticles (ONPs) are fabricated from proteins, carbohydrates, lipids, and other organic compounds to a characteristic dimension, such as a radius smaller than 100 nm. ONPs can be fabricated with bottom-up and top-down approaches, or a combination of both, on the basis of the physicochemical properties of the source materials and the fundamental principles of physical chemistry, colloidal and polymer sciences, and materials science and engineering. ONPs are characterized for dimension, morphology, surface properties, internal structures, and biological properties to understand structure-function correlations and to explore their applications. These potential applications include modifying physical properties, improving sensory attributes and food quality, protecting labile compounds, and delivering encapsulated bioactive compounds for improved bioactivity and bioavailability. Because ONPs can have digestion and absorption properties different from conventional materials, the eventual applications of ONPs require in vitro and in vivo studies to guide the development of safe food products that utilize the unique functionalities of ONPs.
11.1
Changes may still occur before final publication online and in print
FO07CH11-Zhong
ARI
19 December 2015
13:0
INTRODUCTION
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
Nanoscale structures such as proteins and their aggregates are abundant in foods and are a portion of colloidal particles that were produced to a dimension between 1 and 1,000 nm before the term nanoparticles was coined (Walstra 2003). Food nanotechnology has been rapidly evolving in the past two decades because nanoscale structures have shown unique functionalities that improve sensorial, physical, chemical, biological, antimicrobial, nutritional, and healthfulness properties of food products. Notably, food scientists are adopting frameworks developed in disciplines such as condensed matter physics, colloidal and polymer sciences, materials science and engineering, and pharmaceutics to provide potential solutions to the food industry. Structures fabricated with engineering principles to a characteristic dimension smaller than approximately 100 nm fall in the group of engineered nanomaterials (ENMs) that can be one- (nanorods, nanofibrils, and nanotubes), two- (sheets and laminates), and three-dimensional (nanoparticles with a solid core and nanodroplets with a liquid core) substances. These nanoscale structures provide “unique phenomena enabling novel applications not feasible when working with bulk materials or even with single atoms or molecules” (Natl. Nanotechnol. Initiat. USA 2015). However, there is currently no consensus definition of nanoscale. In the literature, the term nano has been used for particles with a dimension ranging from below 100 nm to several hundred nanometers (Abd El-Salam & El-Shibiny 2012, Huang et al. 2010, McClements & Rao 2011). The imprecise definition of nanoscale causes ambiguity in regulating ENMs in different countries and regions (Chau et al. 2007, Joachim 2005). In the scientific community, the most commonly accepted nanoscale parameters are approximately 1 to 100 nm. These parameters are based on the recommendations of the National Nanotechnology Initiative of the United States (Natl. Nanotechnol. Initiat. USA 2015) and the European Commission (Bleeker et al. 2013, Kreyling et al. 2010), and the term nanotechnology is used only when unique phenomena are enabled by nanoscale structures (Natl. Nanotechnol. Initiat. USA 2015). ENMs relevant to food systems can be prepared from organic and inorganic matter. Inorganic nanoparticles such as silica, clay, and metals are applied in a variety of products such as packaging materials and sensors (Duncan 2011, Yada et al. 2014). In this article, the focus is on organic nanoparticles (ONPs) produced from organic compounds that have potential for use in foods to provide novel functionalities. The first section is dedicated to methods of producing ONPs. The second section presents techniques for the characterization of some physical, chemical, and biological properties of ONPs significant to food applications. The third section discusses potential applications of ONPs to improve quality, shelf stability, sensory, and bioactivity of foods and food ingredients. Given that there is insufficient scientific knowledge about the safety of ONPs produced from food ingredients, this article does not cover this important topic, nor does it address the regulation of ENMs, despite the pressing need (Szakal et al. 2014).
METHODS OF FABRICATING ORGANIC NANOPARTICLES ONPs can be produced through top-down and bottom-up procedures (Figure 1). In top-down methods, mechanical forces are applied to break down macro/microstructures to ONPs using various unit operations. In bottom-up methods, physical, chemical, and biological principles are used to build ONPs from particles or molecules. These two methods differ in the magnitude of mechanical energy used to produce ONPs; therefore, top-down and bottom-up approaches are also called high-energy and low-energy methods, respectively. The two methods can be used in combination by forming structures using a low-energy method and then reducing size using a high-energy method.
11.2
Pan
·
Zhong
Changes may still occur before final publication online and in print
FO07CH11-Zhong
ARI
19 December 2015
13:0
Top-down methods • Media milling • High-pressure homogenization • Microfluidization • High-shear homogenization • Colloidal mills • Membrane/microchannel emulsification • Ultrasonication
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
Bottom-up methods • Antisolvent precipitation • Self-assembly • Coacervation • Microemulsions • Self-nanoemulsification • Phase inversion • Template
Figure 1 An overview of some top-down and bottom-up methods used to fabricate organic nanoparticles.
Top-Down Methods Macro/microstructures in a solid or liquid form can be reduced in dimension by mechanical forces. In media milling, micrometer- and millimeter-sized particles can be ground to submicrometer and nanometer dimensions in a ball or bead milling unit with a liquid medium such as water or vegetable oil or without a liquid medium (i.e., dry milling) (Thies 2012). Finer particles are typically produced with smaller beads because of the enhanced contact between beads and the material; the rule of thumb in such production is that particles can be milled to a dimension of approximately one-thousandth of the grinding media (Thies 2012, Way 2008). Grinding cornstarch granules of 0.26–9.61 μm using 0.2 mm grinding media for 180 min resulted in particles smaller than 100 nm (Chen et al. 2010). Other materials being milled to nano- and submicrometer particles include chitosan (Zhang et al. 2012), wheat bran dietary fiber (Zhu et al. 2010), yam (Chiang et al. 2012), and Ganoderma tsugae (Chiang et al. 2014). In addition to the improved physicochemical properties, particles produced from media milling enhanced biological activities (Chiang et al. 2012, Chiang et al. 2014). Emulsions with two immiscible liquids or suspensions with soft particles are frequently reduced in dimension using various high-energy methods. High-pressure homogenization is one of the most popular methods to create fine dispersions in the food industry and has different mechanisms to generate high pressures to deform droplets/particles (Schultz et al. 2004). In high-speed or high-shear homogenization, droplets/particles are disrupted directly by shear forces in rotor-stator and disc setups (Urban et al. 2006). In colloidal mills, droplets/particles are deformed between a rotor and a stator (Urban et al. 2006). In membrane/microchannel emulsification techniques, the dispersed phase or coarse suspensions are forced through pores/microchannels in a membrane into a flowing continuous phase that can be predissolved with surfactants (McClements 2004). Because membranes can be fabricated with uniform pores or channels, it is possible to produce monodispersed droplets (Chuah et al. 2014, El-Abbassi et al. 2013). The uniformity in droplet dimension is important to eliminate the instability mechanism of Ostwald ripening (Walstra 2003). Ultrasonication is another convenient method to produce nanoparticles by applying high-intensity ultrasonic waves with a frequency higher than 20 kHz (McClements 2011). The principles, advantages, and limitations of these top-down methods are available in the books and review articles cited in this www.annualreviews.org • Organic Nanoparticles in Foods
Changes may still occur before final publication online and in print
11.3
FO07CH11-Zhong
ARI
19 December 2015
13:0
section and are therefore not discussed further in this article. In high-energy methods, the oil/water interfacial tension and the ratio of dispersed- to continuous-phase viscosity are two important parameters for reducing droplet dimension by mechanical forces, which can also be affected by the operating conditions of the devices ( Jafari et al. 2008, Vladisavljevi´c et al. 2006). Furthermore, mechanical forces have to overcome the restoring interfacial energy that can be understood by the Laplace pressure (P = 2 γ /R, with γ representing the interfacial tension and R representing the radius of curvature); therefore, there is a limit of size reduction in top-down methods (McClements 2011).
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
Bottom-Up Methods In bottom-up methods, dissolved molecules or dispersed particles are assembled to form larger structures by controlling solubility and aggregation properties. ONPs produced by this approach are appealing for many food applications because no specialized, costly equipment is required to produce the nanoparticles. Covalently cross-linking ONPs using thermal treatments or food enzymes can often enhance their stability. As such, numerous methods have been studied to prepare ONPs using low-energy methods; some of the more common ones are discussed below. Antisolvent precipitation. In antisolvent precipitation methods, the structure-forming molecules are dissolved in a good solvent, and the subsequent dispersion into another miscible solvent or changes in solution chemistry lower the quality of the final solvent to precipitate the molecules. Alcohol-soluble proteins (prolamins) are widely studied. For example, corn prolamins (i.e., zein) can be dissolved in ∼70–80% aqueous ethanol, which is a good solvent for zein (Li et al. 2012). Upon dispersion in water, ethanol in droplets with zein diffuses to the bulk phase, and the lowered ethanol concentration becomes a bad solvent to induce conformational changes of zein and the subsequent precipitation (Figure 2). The precipitated particles have a dimension from
ARI
V I E W
Review in Advance first posted online on January 6, 2016. (Changes may still occur before final publication online and in print.)
N
I N A
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
13:0
C E
S
R
E
19 December 2015
D V A
Organic Nanoparticles in Foods: Fabrication, Characterization, and Utilization Kang Pan and Qixin Zhong Department of Food Science and Technology, University of Tennessee, Knoxville, Tennessee 37996; email: [email protected]
Annu. Rev. Food Sci. Technol. 2016. 7:11.1–11.22
Keywords
The Annual Review of Food Science and Technology is online at food.annualreviews.org
organic food nanoparticles, fabrication, structures, functions, potential applications
This article’s doi: 10.1146/annurev-food-041715-033215 c 2016 by Annual Reviews. Copyright All rights reserved
Abstract In the context of food systems, organic nanoparticles (ONPs) are fabricated from proteins, carbohydrates, lipids, and other organic compounds to a characteristic dimension, such as a radius smaller than 100 nm. ONPs can be fabricated with bottom-up and top-down approaches, or a combination of both, on the basis of the physicochemical properties of the source materials and the fundamental principles of physical chemistry, colloidal and polymer sciences, and materials science and engineering. ONPs are characterized for dimension, morphology, surface properties, internal structures, and biological properties to understand structure-function correlations and to explore their applications. These potential applications include modifying physical properties, improving sensory attributes and food quality, protecting labile compounds, and delivering encapsulated bioactive compounds for improved bioactivity and bioavailability. Because ONPs can have digestion and absorption properties different from conventional materials, the eventual applications of ONPs require in vitro and in vivo studies to guide the development of safe food products that utilize the unique functionalities of ONPs.
11.1
Changes may still occur before final publication online and in print
FO07CH11-Zhong
ARI
19 December 2015
13:0
INTRODUCTION
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
Nanoscale structures such as proteins and their aggregates are abundant in foods and are a portion of colloidal particles that were produced to a dimension between 1 and 1,000 nm before the term nanoparticles was coined (Walstra 2003). Food nanotechnology has been rapidly evolving in the past two decades because nanoscale structures have shown unique functionalities that improve sensorial, physical, chemical, biological, antimicrobial, nutritional, and healthfulness properties of food products. Notably, food scientists are adopting frameworks developed in disciplines such as condensed matter physics, colloidal and polymer sciences, materials science and engineering, and pharmaceutics to provide potential solutions to the food industry. Structures fabricated with engineering principles to a characteristic dimension smaller than approximately 100 nm fall in the group of engineered nanomaterials (ENMs) that can be one- (nanorods, nanofibrils, and nanotubes), two- (sheets and laminates), and three-dimensional (nanoparticles with a solid core and nanodroplets with a liquid core) substances. These nanoscale structures provide “unique phenomena enabling novel applications not feasible when working with bulk materials or even with single atoms or molecules” (Natl. Nanotechnol. Initiat. USA 2015). However, there is currently no consensus definition of nanoscale. In the literature, the term nano has been used for particles with a dimension ranging from below 100 nm to several hundred nanometers (Abd El-Salam & El-Shibiny 2012, Huang et al. 2010, McClements & Rao 2011). The imprecise definition of nanoscale causes ambiguity in regulating ENMs in different countries and regions (Chau et al. 2007, Joachim 2005). In the scientific community, the most commonly accepted nanoscale parameters are approximately 1 to 100 nm. These parameters are based on the recommendations of the National Nanotechnology Initiative of the United States (Natl. Nanotechnol. Initiat. USA 2015) and the European Commission (Bleeker et al. 2013, Kreyling et al. 2010), and the term nanotechnology is used only when unique phenomena are enabled by nanoscale structures (Natl. Nanotechnol. Initiat. USA 2015). ENMs relevant to food systems can be prepared from organic and inorganic matter. Inorganic nanoparticles such as silica, clay, and metals are applied in a variety of products such as packaging materials and sensors (Duncan 2011, Yada et al. 2014). In this article, the focus is on organic nanoparticles (ONPs) produced from organic compounds that have potential for use in foods to provide novel functionalities. The first section is dedicated to methods of producing ONPs. The second section presents techniques for the characterization of some physical, chemical, and biological properties of ONPs significant to food applications. The third section discusses potential applications of ONPs to improve quality, shelf stability, sensory, and bioactivity of foods and food ingredients. Given that there is insufficient scientific knowledge about the safety of ONPs produced from food ingredients, this article does not cover this important topic, nor does it address the regulation of ENMs, despite the pressing need (Szakal et al. 2014).
METHODS OF FABRICATING ORGANIC NANOPARTICLES ONPs can be produced through top-down and bottom-up procedures (Figure 1). In top-down methods, mechanical forces are applied to break down macro/microstructures to ONPs using various unit operations. In bottom-up methods, physical, chemical, and biological principles are used to build ONPs from particles or molecules. These two methods differ in the magnitude of mechanical energy used to produce ONPs; therefore, top-down and bottom-up approaches are also called high-energy and low-energy methods, respectively. The two methods can be used in combination by forming structures using a low-energy method and then reducing size using a high-energy method.
11.2
Pan
·
Zhong
Changes may still occur before final publication online and in print
FO07CH11-Zhong
ARI
19 December 2015
13:0
Top-down methods • Media milling • High-pressure homogenization • Microfluidization • High-shear homogenization • Colloidal mills • Membrane/microchannel emulsification • Ultrasonication
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
Bottom-up methods • Antisolvent precipitation • Self-assembly • Coacervation • Microemulsions • Self-nanoemulsification • Phase inversion • Template
Figure 1 An overview of some top-down and bottom-up methods used to fabricate organic nanoparticles.
Top-Down Methods Macro/microstructures in a solid or liquid form can be reduced in dimension by mechanical forces. In media milling, micrometer- and millimeter-sized particles can be ground to submicrometer and nanometer dimensions in a ball or bead milling unit with a liquid medium such as water or vegetable oil or without a liquid medium (i.e., dry milling) (Thies 2012). Finer particles are typically produced with smaller beads because of the enhanced contact between beads and the material; the rule of thumb in such production is that particles can be milled to a dimension of approximately one-thousandth of the grinding media (Thies 2012, Way 2008). Grinding cornstarch granules of 0.26–9.61 μm using 0.2 mm grinding media for 180 min resulted in particles smaller than 100 nm (Chen et al. 2010). Other materials being milled to nano- and submicrometer particles include chitosan (Zhang et al. 2012), wheat bran dietary fiber (Zhu et al. 2010), yam (Chiang et al. 2012), and Ganoderma tsugae (Chiang et al. 2014). In addition to the improved physicochemical properties, particles produced from media milling enhanced biological activities (Chiang et al. 2012, Chiang et al. 2014). Emulsions with two immiscible liquids or suspensions with soft particles are frequently reduced in dimension using various high-energy methods. High-pressure homogenization is one of the most popular methods to create fine dispersions in the food industry and has different mechanisms to generate high pressures to deform droplets/particles (Schultz et al. 2004). In high-speed or high-shear homogenization, droplets/particles are disrupted directly by shear forces in rotor-stator and disc setups (Urban et al. 2006). In colloidal mills, droplets/particles are deformed between a rotor and a stator (Urban et al. 2006). In membrane/microchannel emulsification techniques, the dispersed phase or coarse suspensions are forced through pores/microchannels in a membrane into a flowing continuous phase that can be predissolved with surfactants (McClements 2004). Because membranes can be fabricated with uniform pores or channels, it is possible to produce monodispersed droplets (Chuah et al. 2014, El-Abbassi et al. 2013). The uniformity in droplet dimension is important to eliminate the instability mechanism of Ostwald ripening (Walstra 2003). Ultrasonication is another convenient method to produce nanoparticles by applying high-intensity ultrasonic waves with a frequency higher than 20 kHz (McClements 2011). The principles, advantages, and limitations of these top-down methods are available in the books and review articles cited in this www.annualreviews.org • Organic Nanoparticles in Foods
Changes may still occur before final publication online and in print
11.3
FO07CH11-Zhong
ARI
19 December 2015
13:0
section and are therefore not discussed further in this article. In high-energy methods, the oil/water interfacial tension and the ratio of dispersed- to continuous-phase viscosity are two important parameters for reducing droplet dimension by mechanical forces, which can also be affected by the operating conditions of the devices ( Jafari et al. 2008, Vladisavljevi´c et al. 2006). Furthermore, mechanical forces have to overcome the restoring interfacial energy that can be understood by the Laplace pressure (P = 2 γ /R, with γ representing the interfacial tension and R representing the radius of curvature); therefore, there is a limit of size reduction in top-down methods (McClements 2011).
Annu. Rev. Food Sci. Technol. 2016.7. Downloaded from www.annualreviews.org Access provided by Flinders University on 01/28/16. For personal use only.
Bottom-Up Methods In bottom-up methods, dissolved molecules or dispersed particles are assembled to form larger structures by controlling solubility and aggregation properties. ONPs produced by this approach are appealing for many food applications because no specialized, costly equipment is required to produce the nanoparticles. Covalently cross-linking ONPs using thermal treatments or food enzymes can often enhance their stability. As such, numerous methods have been studied to prepare ONPs using low-energy methods; some of the more common ones are discussed below. Antisolvent precipitation. In antisolvent precipitation methods, the structure-forming molecules are dissolved in a good solvent, and the subsequent dispersion into another miscible solvent or changes in solution chemistry lower the quality of the final solvent to precipitate the molecules. Alcohol-soluble proteins (prolamins) are widely studied. For example, corn prolamins (i.e., zein) can be dissolved in ∼70–80% aqueous ethanol, which is a good solvent for zein (Li et al. 2012). Upon dispersion in water, ethanol in droplets with zein diffuses to the bulk phase, and the lowered ethanol concentration becomes a bad solvent to induce conformational changes of zein and the subsequent precipitation (Figure 2). The precipitated particles have a dimension from
