R: Concise Reviews in Food Science

JFS R: Concise Reviews/Hypotheses in Food Science

Functional Materials in Food Nanotechnology

JOCHEN WEISS, PH.D., PAUL TAKHISTOV, PH.D., AND D. JULIAN MCCLEMENTS, PH.D.

The Institute of Food Technologists has issued this Scientific Status Summary to update readers on the applications
of nanotechnology in the food industry.
Keywords: colloids, delivery systems, nano-emulsions, nanolaminates, nanoparticles, nanoscale, nanotechno-
logy

N
anotechnology focuses on the characterization, fabrica- ical and physical phenomena in food systems. Since foods are com-
tion, and manipulation of biological and nonbiological plex biological systems that are governed by many of the same basic
structures smaller than 100 nm. Structures on this scale mechanisms and principles that biologists and biochemists study,
have been shown to have unique and novel functional proper- one would expect that the discoveries made in nanotechnology may
ties. Consequently, interest and activities in this research area have eventually also impact the food industry. However, foods undergo
greatly increased over the past years. According to the National Nan- a variety of postharvest and processing-induced modifications that
otechnology Initiative (2006), “Nanotechnology is the understand- affect the biological and biochemical functionality of the system.
ing and control of matter at dimensions of roughly 1 to 100 nanome- Nanotechnology allows scientists to measure, control, and manip-
ters, where unique phenomena enable novel applications. Encom- ulate matter at the nanoscale level to change those properties and
passing nanoscale science, engineering and technology, nanotech- functions in a beneficial way.
nology involves imaging, measuring, modeling, and manipulating This article provides an overview of some current development
matter at this length scale.” efforts in the area of nanotechnology as it applies to food systems. In
With the increased funding opportunities and interest in this particular, the article presents some of the morphologically different
field, the term “nano” is more frequently and often liberally used, structures and associated manufacturing technologies that could
which has led to some criticism within the scientific community. be used to build functional food systems. Moreover, the article fo-
Whether justified or not, it should be understood that the entire cuses on applications with which the authors have experience and
field of nanoscience is essentially an eclectic derivative of estab- is tailored specifically toward current and emerging technologies
lished disciplines such as chemistry, interface science, microfabri- that may be used for food formulations, processing, and storage. Al-
cation technologies, and so on. However, use of the term “nano” does though nanotechnology potentially has numerous applications in
allow researchers to highlight the fact that processes (for example, the food industry, this article does not delve into the areas of food
nanomanufacturing) or material structures (for example, nanoma- safety and security. These topics will be the subject of a separate ar-
terials) are designed and optimized to use specific properties and ticle. Interested readers seeking information about nanotechnology
behaviors at lengths of 10−7 to 10−9 m. in food and agricultural systems might additionally refer to a report
The potential benefits of nanotechnology have been recognized published by CSREES, USDA (2003).
by many industries, and commercial products are already being
manufactured, such as in the microelectronics, aerospace, and phar- Nanotechnology: drawing inspiration from nature
maceutical industries. Developments in these industries are driven Living organisms are not just a collection of nanoscale ob-
by fundamental and applied research in physics, chemistry, biology, jects: Atoms and molecules are organized in hierarchical struc-
engineering, and materials science. In contrast, applications of nan- tures and dynamic systems that are the results of millions of years
otechnology within the food industry are rather limited. However, of Mother Nature’s experiments. Tenth-nanometer diameter ions
achievements and discoveries in nanotechnology are beginning to such as potassium and sodium generate nerve impulses. The size
impact the food industry and associated industries; this affects im- of vital biomolecules—such as sugars, amino acids, hormones, and
portant aspects from food safety to the molecular synthesis of new DNA—is in the nanometer range. Membranes that separate 1 cell
food products and ingredients (Chen and others 2006). The fact that from another, or 1 subcellular organelle from another, are about
systems with structural features on the nanoscale have physical, 5 times bigger. Most protein and polysaccharide molecules have
chemical, and biological properties substantially different from their nanoscale dimensions. Every living organism on earth exists be-
macroscopic counterparts is changing the understanding of biolog- cause of the presence, absence, concentration, location, and inter-
action of these nanostructures.
.Authors Weiss and McClements are with Dept. of Food Science, Univ. of Nature is making extensive use of self-assembly principles to cre-
Massachusetts, Amherst MA 01003. Author Takhistov is with Dept. of Food ate nanoscale structures. Rather than requiring the expenditure of
Science, Rutgers Univ., New Brunswick, NJ 08901. Direct inquiries and
reprint requests to ttarver@ift.org.
large amounts of energy for assembly and creation, nanoscale struc-
tures are formed through a series of optimized processes that utilize


C 2006 Institute of Food Technologists Vol. 71, Nr. 9, 2006—JOURNAL OF FOOD SCIENCE R107
doi: 10.1111/j.1750-3841.2006.00195.x
Further reproduction without permission is prohibited
 Scientific Status Summary—Materials in food nanotechnology
R: Concise Reviews in Food Science

the tendency of a system to minimize its overall free energy, thereby lighted (Blundell and Thurlby 1987; Aguilera 2005). In addition, the
minimizing required activation energies. Production of nanoscale relationship between the morphology of food materials and their
structures for use in food science and technology therefore fre- bulk physicochemical properties has been investigated (Losche
quently relies on an in-depth understanding of thermodynamically 1997): for example, biopolymers in solutions, gels, and films
driven self-assembly processes. Areas of research that could prove (Chinnan and Park 1995; Janaswamy and Chandrasekaran 2005).
useful in the near future include molecular design of protective sur- Functional nanostructures can incorporate individual biological
face systems (Charpentier 2005), surface engineering (Krajewska molecules, which is useful in the development of biosensors that can
2004), and various methods of manufacturing, such as electrospin- use natural sugars or proteins as target-recognition groups (Charych
ning (Min and others 2004) and nanofiltration (van der Graaf and and others 1996).
others 2005). In summary, there are a large number of potential applications of
nanotechnology within the food industry; however, many of these
Potential Food Applications may be difficult to adopt commercially because they are either too

N anotechnology has the potential to impact many aspects

of food and agricultural systems. Food security, disease-
treatment delivery methods, new tools for molecular and cellular
expensive or too impractical to implement on an industrial scale.
The subsequent sections focus on a limited number of nanotech-
nology applications that may have commercial potential in the near
biology, new materials for pathogen detection, and protection of future. Most likely, the limited application of nanotechnology to the
the environment are examples of the important links of nanotech- food industry will change as nanofabrication technologies become
nology to the science and engineering of agriculture and food sys- more cost-effective.
tems. Examples of nanotechnology as a tool for achieving further
advancements in the food industry are as follows: Nanodispersions and Nanocapsules
r Increased security of manufacturing, processing, and shipping
of food products through sensors for pathogen and contaminant
detection.
F unctional ingredients (for example, drugs, vitamins, antimicro-
bials, antioxidants, flavorings, colorants, and preservatives) are
essential components of a wide range of industrial products, includ-
r Devices to maintain historical environmental records of a partic- ing pharmaceuticals, health-care products, cosmetics, agrochemi-
ular product and tracking of individual shipments. cals, and foods. These functional ingredients come in a variety of dif-
r Systems that provide integration of sensing, localization, report- ferent molecular and physical forms such as polarities (polar, non-
ing, and remote control of food products (smart/intelligent sys- polar, amphiphilic), molecular weights (low to high), and physical
tems) and that can increase efficacy and security of food process- states (solid, liquid, gas). Functional ingredients are rarely utilized
ing and transportation. directly in their pure form. Instead, they are often incorporated into
r Encapsulation and delivery systems that carry, protect, and de- some form of delivery system.
liver functional food ingredients to their specific site of action. A delivery system must perform a number of different roles. First,
Most nanotechnological research focuses on the development it serves as a vehicle for carrying the functional ingredient to the
of applications in biosciences and engineering. Strategies to ap- desired site of action. Second, it may have to protect the functional
ply nanoscience to the food industry are quite different from these ingredient from chemical or biological degradation (for example, ox-
more traditional applications of nanotechnology. Food processing is idation) during processing, storage, and utilization; this maintains
a multitechnological manufacturing industry involving a wide vari- the functional ingredient in its active state. Third, it may have to be
ety of raw materials, high biosafety requirements, and well-regulated capable of controlling the release of the functional ingredient, such
technological processes. Four major areas in food production may as the release rate or the specific environmental conditions that trig-
benefit from nanotechnology: development of new functional ma- ger release (for example, pH, ionic strength, or temperature). Fourth,
terials, microscale and nanoscale processing, product development, the delivery system has to be compatible with the other components
and methods and instrumentation design for improved food safety in the system, as well as being compatible with the physicochemi-
and biosecurity. Figure 1 depicts possible applications of nanotech- cal and qualitative attributes (that is, appearance, texture, taste, and
nology in the food industry. shelf-life) of the final product.
The influence of the material properties of foods at the nanoscale The characteristics of the delivery system are one of the most im-
level on their bioavailability and nutritional value has been high- portant factors influencing the efficacy of functional ingredients in
many industrial products. A wide variety of delivery systems has
been developed to encapsulate functional ingredients, including
simple solutions, association colloids, emulsions, biopolymer ma-
trices, and so on. Each type of delivery system has its own spe-
cific advantages and disadvantages for encapsulation, protection,
and delivery of functional ingredients, as well as cost, regula-
tory status, ease of use, biodegradability, and biocompatibility. A
number of potential delivery systems based on nanotechnology
follow.

Association colloids
Association colloids—such as surfactant micelles, vesicles, bi-
layers, reverse micelles, and liquid crystals—have been used for
many years to encapsulate and deliver polar, nonpolar, and/or
amphiphilic functional ingredients (Garti and others 2004, 2005;
Golding and Sein 2004; Flanagan and Singh 2006). For exam-
Figure 1 --- Application matrix of nanotechnology in food ple, a nonpolar functional ingredient may be solubilized within
science the hydrophobic core of a surfactant micelle or as part of the

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Scientific Status Summary—Materials in food nanotechnology

R: Concise Reviews in Food Science

micellar membrane structure; thus, it can be delivered in an aque- release an aqueous phase component trapped within the inner wa-
ous solution depending on the requirements of the specific ap- ter droplets (W 1 ) to a specific site such as the mouth, stomach, or
plication. Association colloids are thermodynamically favorable small intestine.
systems whose formation is normally driven by the hydropho-
bic effect—that is, the reduction of the contact area between the Nanostructured multilayer emulsions
nonpolar groups of the surfactant that comprise the association Recent studies have shown that the use of multilayer emulsions
colloid and water. The type of association colloid formed and can create novel delivery systems. These systems typically consist
the nature of the resultant structures depend on the concentra- of oil droplets (the core) surrounded by nanometer thick layers
tions and molecular characteristics of the surfactant and cosur- (the shell) comprised of different polyelectrolytes. These layers are
factant used as well as the prevailing environmental conditions formed using a layer-by-layer (LbL) electrostatic deposition method
(for example, temperature, ionic strength, and pH). It should be that involves sequential adsorption of polyelectrolytes onto the sur-
noted that the location of the encapsulated functional ingredient faces of oppositely charged colloidal particles. Figure 2 shows an
within the association colloid (for example, in the hydrophobic core example of the LbL approach to encapsulating oil droplets in an
or as part of the association colloidal membrane) is of particu- O/W emulsion. An ionic emulsifier that rapidly adsorbs to the sur-
lar importance to the functionality of the self-assembled system. face of lipid droplets during homogenization is used to produce
The dimensions of many association colloids are in the range of a primary emulsion containing small droplets; then an oppositely
5 to 100 nm, and these structures are therefore considered to be charged polyelectrolyte is added to the system, which adsorbs to
nanoparticles. the droplet surfaces and produces a secondary emulsion contain-
The major advantages of association colloid systems are that they ing droplets coated with a 2-layer interface. This procedure can be
form spontaneously, are thermodynamically favorable, and are typ- repeated to form oil droplets coated by interfaces containing 3 or
ically transparent solutions. On the other hand, the major disad- more layers. Under certain circumstances, emulsions containing oil
vantage is that a large quantity of surfactant (and in many cases, droplets surrounded by multilayer interfaces have been found to
cosurfactant) is required to form them, which may lead to prob- have better stability against environmental stresses than conven-
lems with flavor, cost, or legality. Moreover, because the formation tional oil-in-water emulsions with single-layer interfaces (Gu and
of association colloids is concentration-driven, diluting the solu- others 2005; Mun and others 2005; Guzey and McClements 2006).
tions containing the colloids can lead to their spontaneous disso- In addition, it is possible to develop smart delivery systems by en-
ciation. Thus, the choice of surfactants and cosurfactants to form gineering the properties of the nanostructured shell around the
colloids is critical in ensuring their functionality over a wide range droplets.
of environmental conditions. This interfacial engineering technology would utilize food-grade
ingredients (such as proteins, polysaccharides, and phospholipids)
Nano-emulsions and processing operations (such as homogenization and mixing)
The use of high-pressure valve homogenizers or microfluidizers that are already widely used in the manufacture of food emulsions.
often causes emulsions with droplet diameters of less than 100 to Therefore, this technology should be economically viable and could
500 nm. In modern literature such emulsions are often referred to as be easily implemented by the food industry.
“nano-emulsions.” Nano-emulsions have been produced and stud- A functional component trapped within the core of a multilayer
ied for many years, so a large body of literature dealing with their emulsion delivery system could be released in response to a specific
preparation, characterization, and utilization exists (McClements environmental trigger by designing the response of the shell to the
2004). Functional food components can be incorporated within the environment as in the following examples:
droplets, the interfacial region, or the continuous phase. Encapsu- 1. Complete Shell Dissociation: Weakening electrostatic interac-
lating functional components within the droplets often enables a tions can cause shells to completely dissociate under specific solu-
slowdown of chemical degradation processes by engineering the tion conditions (pH, ionic strength). For instance, changing the pH
properties of the interfacial layer surrounding them (McClements can cause one or more of the polyelectrolytes to lose its charge, or
and Decker 2000). While it is difficult to engineer the interfaces to increasing the ionic strength can weaken the electrostatic attraction
be completely impermeable to compounds in the bulk phase that of a polyelectrolyte to the next layer, thereby promoting desorption.
may interact with the encapsulated compounds, the rate of perme-
ation can often be significantly reduced, thus increasing the kinetic
Separate Oil Primary Secondary
stability of the bioactives.
and Water Emulsion Emulsion
Nanostructured multiple emulsions
The use of multiple emulsions can create delivery systems with
novel encapsulation and delivery properties. The most common
examples of this are oil-in-water-in-oil (O/W/O) and water-in-oil-
in-water (W/O/W) emulsions (Garti and Benichou 2001, 2004).
For example, a nanostructured W 1 OW 2 emulsion would consist of
nanometer-sized water droplets or reverse micelles (W 1 ) contained
Add Emulsifier Add Biopolymer
within larger oil droplets (O) that are dispersed within an aqueous
continuous phase (W 2 ). Functional food components could be en-
capsulated within the inner water phase, the oil phase, or the outer
water phase, thereby making it possible to develop a single delivery
system that contains multiple functional components. This tech-
Single-Layer Two Layers
nology could be used to separate 2 aqueous phase components that
might adversely react with each other if they were present in the Figure 2 --- Schematic for formation of a number of
same aqueous phase. Alternatively, it could be used to protect and nanolayers around particles

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 Scientific Status Summary—Materials in food nanotechnology
R: Concise Reviews in Food Science

2. Modulation of Shell Porosity: The thickness and porosity of nologies and may thus have a number of important applications
shells can change with exposure to pH and ionic strength. This de- within the food industry. Edible coatings and films are currently
termines the rate at which functional components trapped inside used on a wide variety of foods, including fruits, vegetables, meats,
the core will diffuse into the surrounding medium. By selecting the chocolate, candies, bakery products, and French fries (Morillon and
appropriate polyelectrolytes to use and the assembly conditions, others 2002; Cagri and others 2004; Cha and Chinnan 2004; Rhim
one could design systems to release, under specific environmen- 2004). These coatings or films could serve as moisture, lipid, and gas
tal triggers, functional components smaller than some particular barriers. Alternatively, they could improve the textural properties of
dimension. foods or serve as carriers of functional agents such as colors, flavors,
In principle, one could vary the release of 1 or more encapsulated antioxidants, nutrients, and antimicrobials.
materials using either of these release mechanisms, either individ- The basic functional properties of edible coatings and films de-
ually or in combination (simultaneously or sequentially). pend on the characteristics of the film-forming materials used for
their preparation. At present, the primary film-forming materials
Biopolymeric nanoparticles used to construct these edible coatings and films are polysaccha-
Particles in the nanometer-sized range can often be produced rides, proteins, and lipids. Generally, lipid-based films are good
using food-grade biopolymers such as proteins or polysaccharides moisture barriers, but they offer little resistance to gas transfer and
(Chang and Chen 2005; Gupta and Gupta 2005; Ritzoulis and others have poor mechanical strength. In contrast, biopolymer-based films
2005). These particles may be formed by promoting self-association are often good oxygen and carbon dioxide barriers, but they offer lit-
or aggregation of single biopolymers or by inducing phase separa- tle protection against moisture migration (Park 1999). Consequently,
tion in mixed biopolymer systems, for example, using aggregative there has been a great deal of research on identifying additives that
(net attraction) or segregative (net repulsion) interactions. Func- can be used to improve the functional properties of edible films
tional ingredients can be encapsulated in nanoparticles formed and and coatings (for example, polyols, emulsion droplets, surfactant
released in response to specific environmental triggers by altering micelles, fibers, or particulate matter). To date, most edible films
the solution conditions to induce complete particle dissolution or and coatings are formed with little consideration of the internal
changes in particle porosity. A more in-depth discussion on the pro- structure created. Moreover, specific design of nanoscale and mi-
duction and utilization of such particles follows. croscale internal structures to overcome problems associated with
edible films and coatings has not been pursued. Nanolaminates are
Nanolaminates more likely to be used as coatings that are attached to food surfaces,
rather than as self-standing films, because their extremely thin na-
N anotechnology provides food scientists with a number of ways
to create novel laminate films suitable for use in the food in-
dustry. A nanolaminate consists of 2 or more layers of material with
ture makes them very fragile (Kotov 2003).
Figure 4 shows an example of how nanolaminates could encase
nanometer dimensions that are physically or chemically bonded to food objects. The object to be coated with a nanolaminate would be
each other (Figure 3). One of the most powerful methods is based dipped into a series of solutions containing substances that would
on the LbL deposition technique, in which the charged surfaces are adsorb to the surface of the object (McClements and others 2005). Al-
coated with interfacial films consisting of multiple nanolayers of dif- ternatively, the solutions containing the adsorbing substances could
ferent materials (Decher and Schlenoff 2003). Similar to the prepara- be sprayed onto the surface of the object. The composition, thick-
tion of multiple emulsions, electrostatic attraction causes polyelec- ness, structure, and properties of the multilayered laminate formed
trolytes and other charged substances to be deposited onto oppo- around the object could be controlled in a number of ways, in-
sitely charged surfaces. This LbL technology allows precise control cluding (i) changing the type of adsorbing substances in the dip-
over the thickness and properties of the interfacial films, which in ping solutions; (ii) changing the total number of dipping steps used;
this case enables the creation of thin films (1 to 100 nm per layer). (iii) changing the order that the object is introduced into the various
Nanolaminates can give food scientists some advantages for the dipping solutions; or (iv) changing the solution and environmen-
preparation of edible coatings and films over conventional tech- tal conditions used, such as pH, ionic strength, dielectric constant,
temperature, and so on. The driving force for adsorption of a sub-
stance to a surface would depend on the nature of the surface and the
nature of the adsorbing substance. The force itself could be electro-
static, hydrogen-bonding, hydrophobic interactive, thermodynam-
ically incompatible, and so on, but it would usually be electrostatic
attraction of oppositely charged substances. The influence of the
properties of the substrate surface—such as topology and rough-
ness on the structure of the nanolaminates that are built on the
substrate surface—has not yet been established. It is possible that
nonuniform laminates could be formed that contain microscopic
and macroscopic pores that could negate the barrier function of
the laminate. Consequently, this would necessitate the formation
of a second base biopolymer layer on the food product to form a
more uniform substrate surface, followed by deposition of the layer
containing the functional ingredient.
A variety of different adsorbing substances could be used to
create the different layers (Figure 5), including natural polyelec-
trolytes (proteins, polysaccharides), charged lipids (phospholipids,
surfactants), and colloidal particles (micelles, vesicles, droplets).
Figure 3 --- Example of a possible nanolaminate material
formed from a globular protein and a polysaccharide. The choice of the type of adsorbing substances used to create each
Each layer is approximately 1 to 100 nm. layer, the total number of layers incorporated into the overall film,

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Scientific Status Summary—Materials in food nanotechnology

R: Concise Reviews in Food Science

the sequence of the different layers, and the preparation condi- increased production rate. These 2 developments led to polymers
tions used to prepare each layer will determine the functional- becoming the focus of many studies. Today, PLA is widely available
ity of the final films (for example, their permeability to gases, or- through a number of manufacturers. Biodegradable polymers have
ganic substances, minerals, or water; their mechanical properties, found wide applications in the field of biomedicine: for example,
such as rigidity, flexibility, or brittleness; their swelling and wet- eliminating the need for surgical implantation and removal, and
ting characteristics; and their environmental sensitivity to pH, ionic encapsulating and delivering a wide variety of compounds (includ-
strength, and temperature). In addition, the aforementioned proce- ing drugs, vaccines, and proteins). For example, chitosan—a natural
dure could be used to encapsulate various hydrophilic, amphiphilic, antimicrobial polymer obtained by deacetylating chitin extracted
or lipophilic substances within the films by incorporating them, for from crustacean shells—and synthetic polymers PLA, polyglycolic
example, in oil droplets or association colloids (such as micelles or acid (PGA), and polycaprolactic acid are used to encapsulate and
liposomes). As a result, it would be possible to incorporate active deliver compounds. Copolymers created using combinations of the
functional agents such as antimicrobials, antibrowning agents, an- monomers lactide, galactide, and caprolactone have also been ex-
tioxidants, enzymes, flavors, and colors into the films. These func- amined.
tional agents would increase the shelf life and quality of coated Because of its excellent encapsulation properties, PLA is one of the
foods. These nanolaminated coatings could be created entirely from principal building blocks of many biodegradable nanoparticles, but
food-grade ingredients (proteins, polysaccharides, lipids) by using it has its limitations. PLA is quickly removed from the bloodstream
simple processing operations such as dipping and washing. and sequestered in the liver and the kidneys. Although this is ideal
for the treatment of intracellular pathogens that are isolated in these
Biopolymeric Nanoparticles areas, it is less desirable for the delivery of active components to

R esearch into the production and use of biodegradable poly-

mers for use in the manufacturing of dispersed systems began
as early as 70 years ago. First developed in 1932, polylactic acid (PLA)
other areas of the body. PLA also breaks down in intestinal fluid,
which limits its use as a carrier for oral delivery. These problems
can be overcome by associating a hydrophilic compound—such as
is a key component of many biodegradable nanoparticles. However, polyethylene glycol (PEG)—with the hydrophobic PLA nanoparticle.
its high cost and susceptibility to hydrolytic breakdown supposedly Nanoparticles consisting of a PLA-PEG diblock copolymer form a
made it unsuitable for use in biomedical or agricultural applications; micellar-like assembly that can entrap a compound that is to be
thus, it was used only sparingly in research (Lunt 1998). However, delivered (Riley and others 1999).
in the 1970s, the use of PLA as an ideal material for sutures was dis- The molecular weight of PLA and the ratio of PLA to PEG are key
covered, and in the 1980s a process was developed to produce the factors in the formation of stable nanoparticles that can avoid the
polymer via bacterial fermentation, which greatly reduced costs and reticuloendothelial system and resist agglomeration and breakdown

Dip Wash Dip Wash Dip Wash Figure 4 --- Schematic

representation of coating
an object with multilayers
using a successive dipping
and washing procedure

Uncoated Coated
Object Object

Surface
Structure

Droplet Particulate Polyelectrolyte Figure 5 --- Possible

components that could be
used to assemble
multilayered edible films or
coatings

Micelle Lipid Polar

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 Scientific Status Summary—Materials in food nanotechnology
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in intestinal fluid. Compared to PLA nanoparticles made with sim- Salting Out (Ibrahim and others 1992). Salting out involves dis-
ilar amounts of PLA, copolymer nanoparticles are smaller, demon- solving a high concentration of a salt and a protective colloid in the
strating the strong influence of PEG on particle formation (Riley and aqueous phase, forming a viscous gel. The polymer, which forms
others 1999). A PEG block seemingly prevents agglomeration of PLA the bulk of the particle, and the drug to be encapsulated are dis-
chains, thus reducing the increase in particle size. Using this diblock solved in an organic, water miscible solvent (typically acetone). The
copolymer makes it possible to create particles in the sub-200 nm 2 solutions are combined with vigorous stirring to form an oil-in-
range (Tobio and others 2000). PEG also affects the zeta potential of water (O/W) emulsion. Water is added to this emulsion, causing
the particles; that is, it has a lower negative surface charge (approxi- the organic solvent to diffuse into the aqueous phase. The water
mately −6 mV for 30:5 PLA:PEG copolymer, approximately −50 mV insoluble polymer will simultaneously aggregate and encapsulate
for PLA alone) (Riley and others 1999; Tobio and others 2000). This the other compound present in the organic phase, thus forming
reduction is thought to be due to the capping of carboxy PLA end nanoparticles. Lastly, acetone and salting-out agents are eliminated
groups by PEG or by the shifting of the shear plane of the diffuse layer by cross-flow filtration. While salting out is associated with very high
to a greater distance from the nanoparticle. Changes in zeta poten- encapsulation efficiencies compared to the other methods, its use
tial are important because they influence the interaction of particles is typically limited to the encapsulation of lipophilic compounds.
with other compounds in the food system. The surface charge is of Nanoprecipitation (Fessi and others 1989). In contrast to the solid
particular importance if particles are to be used as building blocks nanoparticles that the salting-out method generates, the nanopre-
for more complex structures such as nanolaminates. cipitation method produces nanocapsules that consist of a central
One of the main criteria in using nanoparticles as a delivery sys- oily core surrounded by a thin polymer wall (Fessi and others 1989;
tem for bioactive compounds is that they are nontoxic. Because Guterres and others 1995). A polymer and a mixture of phospholipids
many early encapsulation procedures did not meet this criterion are dissolved in water-miscible organic solvents such as acetone or
(Oppenheim 1981), several new methods were developed using less- ethanol. The compound to be encapsulated or loaded is dissolved
harsh chemicals and chemicals that were easily removed from the in a lipophilic solvent and added to the organic solvent. The organic
final product. If a method requires using as processing aids organic solution is then added, via stirring, to an aqueous solution contain-
solvents that are associated with toxicity concerns, then the method ing a surfactant. Addition to the aqueous solution causes the water-
may not be suitable for production of nanoparticles that are to be miscible solvent to rapidly diffuse into the aqueous phase, which
used in foods. When selecting a method to produce nanoparticles, results in the formation of lipophilic nanodroplets that contain the
food manufacturers should thus carefully review the solvents re- compound to be encapsulated. The water-insoluble polymer now
quired. Ultimately, more development efforts are needed to adapt migrates to the O/W interface where it adsorbs to form an interfa-
these methods to strictly use only food-approved processing aids cial membrane around the lipophilic core. The resultant suspension
and components. Methods that may fulfill these requirements to is then concentrated by evaporating the organic solvent and water
produce nanoparticles include salting out, spontaneous emulsifica- under pressure.
tion/diffusion, solvent evaporation, polymerization, and nanopre- Solvent Evaporation (Beck and others 1979). In this method, the
cipitation (Ibrahim and others 1992). Figure 6 illustrates an overview polymer and compound to be encapsulated are dissolved into a
of these methods. In addition, electrospraying has shown to be capa- water-immiscible, volatile organic solvent. This solution is subse-
ble of producing uniform particles of less than 100 nm from polymer quently added to an aqueous solution containing a stabilizing com-
and biopolymer solutions. pound and then homogenized to form an emulsion. The formation

Bioactive Compound to Be Encapsulated Figure 6 ---

Overview of
nanoparticle
manufactur-
Water Miscible Partially Water Water Immiscible Water Immiscible ing methods
Organic Solvent Miscible Solvent Organic Solvent Organic Solvent
(Volatile)

Water Immiscible Water Water Water Water

Organic Solvent (+emulsifier+salt) (+emulsifier) (+emulsifier) (+emulsifier)

stirring stirring stirring stirring

Water o/w emulsion o/w emulsion o/w emulsion o/w emulsion

(+emulsifier)

o/w emulsion Water Water

(solvent (diffusion) (diffusion) (diffusion) (precipitation)

evaporation)

Nanoparticles Nanoparticles Nanoparticles Nanocapsules Nanoparticles

Spontaneous Emulsification/
Salting Out Emulsification-Diffusion Nanoprecipitation Solvent Evaporation
Solvent Diffusion
(Leroux and others 1995) (Fessi and others 1989) (Beck and others 1979)
(Murakami and others 2000) (Ibrahim and others 1992)

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Scientific Status Summary—Materials in food nanotechnology

R: Concise Reviews in Food Science

of microspheres is a phase separation process in which the organic must be improved. To increase mechanical and thermal properties,
solvent diffuses into the aqueous phase from the surface of the moisture stability, and flame and weather resistances, a small per-
droplets in the emulsion. This increases the polymer concentration centage, by weight, of clay in a polymer matrix can be included. Two
at the phase boundary and eventually causes the polymer to precip- types of hybrids—intercalation and exfoliation—are ideal nanoscale
itate, forming the particle (Bodmeier and others 1987a, 1987b). The composites (Figure 7). Intercalation is the state in which extended
volatile solvent is evaporated under vacuum and produces encap- polymer chains are present between the clay layers, resulting in a
sulated particles ranging in size from 10 to 250 nm (Beck and others multilayered structure with alternating polymer/inorganic layers at
1979). a repeated distance of a few nanometers. Exfoliation is a state in
Finally, depending on the choice of the base biopolymer used to which the silicate layers are completely separated and dispersed
manufacture the nanoparticles, particle surfaces may be hydropho- in a continuous polymer matrix. The structure and properties of
bic or hydrophilic. Thus, the type of solvent in which the parti- the resulting nanocomposites can be altered by controlling subtle
cles are dispersed for their application in food systems may lead to polymer-clay interactions (Oya and others 2000).
problematic particle aggregation. Aggregation of particles renders
them poorly dispersible, and this could negate some advantages of
these delivery systems. It is therefore critical to understand particle– Polysaccharide/clay nanocomposites
particle and particle–solvent interactions to ensure that particles Recently, the preparation of nano-clay containing carbohy-
remain dispersed in the solvent as individual particles. Approaches drate film has been reported (http://www.pre.wur.nl/UK/Research/
developed in the field of colloid science may be used to predict these Food+Structuring/Microstructured/). Here, carbohydrates are
interactions and help design systems that are dispersible in the food pumped together with clay layers through a high shear cell to pro-
application. duce a film that then contains the exfoliated clay layers. Since these
layers are impermeable to water, water can only migrate through the
Nanocomposites polysaccharide matrix following a torturous path. As a consequence,

I n the late 1980s, a car manufacturer’s researchers found that

adding 5%-by-weight nano-sized clays increased the mechan-
ical and thermal properties of nylons significantly (McGlashan
the nanocomposite carbohydrate film has substantially reduced
water-vapor permeability, solving one of the long-standing prob-
lems in the production of biopolymer films. Moreover, introduction
and Halley 2003). The most widely studied type of polymer-clay of the dispersed clay layers into the biopolymer matrix structure has
nanocomposites, a class of hybrid materials composed of organic been shown to greatly improve the overall mechanical strength of
polymer matrices and organophilic clay fillers (Kim and others the film, making the use of these films industrially practicable. For
2003), is montmorillonite (MMT). A hydrated alumina-silicate- example, Mathew and Dufresne (2002) examined nanocomposites
layered clay consisting of 2 silica tetrahedral sheets fused to an edge- from starch and amorphous poly (beta-hydroxyoctanoate) and from
shared octahedral sheet of aluminum hydroxide, MMT has several starch and tucinin whiskers. Nanocomposites have also been devel-
advantages. Its high surface area, large aspect ratio (50–1000), and oped using plant oil-clay hybrid materials (Uyama and others 2003).
platelet thickness of 10 Å make it suitable for reinforcement pur- Park and others (2003) used 2 clays—Cloisite 30B with ammonium
poses (Uyama and others 2003). However, it is not easily homo- cations located in the silicate gallery and one unmodified Cloisite
geneously dispersed in an organic polymer phase due to the hy- Na+ —to generate thermoplastic starch (TPS)/clay nanocomposites
drophilic nature of the MMT surface (Kim and others 2003). To be using a melt intercalation method. With 5% by weight inclusion of
compatible with the organic polymer, the organophilicity of MMT the clays, strong interactions between TPS and Cloisite Na+ led to

Inert Monolayers Protein Carbohydrates Figure 7 --- Principle of

formation of clay monolayer
containing nanocomposites
with enhanced mechanical
and barrier properties

Exfoliated
Microphase separated Intercalated

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 Scientific Status Summary—Materials in food nanotechnology
R: Concise Reviews in Food Science

higher tensile properties and a lower water vapor transmission rate troduced in the jet velocity vector, which substantially increases the
than the pristine TPS. length of the jet path prior to deposition on the grounded electrode.
Chemically derived by deacetylation of chitin, an abundant This elongation of the jet path due to bending instabilities is largely
polysaccharide found in shellfish, chitosan possesses a unique responsible for the submicron diameter of electrospun fibers, which
cationic nature relative to other neutral or negatively charged is the basis of their remarkable functionalities.
polysaccharides. In an acid environment, the amino group NH 2 in The majority of studies with composite nanofibers has been con-
chitosan can be protonated to yield NH 3 + , which yields antifungal cerned with the production and reinforcement of carbon nanotubes
or antimicrobial activities since cations can bind to anionic sites on for use in next-generation microprocessors. However, the majority
bacterial and fungal cell wall surfaces. Further, chitosan is a non- of filed patents has focused on applications in life sciences. For ex-
toxic natural polysaccharide and is compatible with living tissue. ample, biocompatible nanofibers have been used to produce porous
These appealing features make chitosan widely applicable in wound membranes for skin to aid in cleansing, healing, and dressing of
healing, production of artificial skin, food preservation, cosmetics, wounds; creating tubular fibers for blood vessel and nerve regener-
and wastewater treatment (Risbud and others 2000; Juang and Shao ation; 3-dimensional scaffolds for bones and cartilage regeneration;
2002). As discussed earlier, chitosan’s hydrophilic character and con- and drug-delivery matrices. In all these cases, the large surface-to-
sequently its poor mechanical properties in the presence of water volume ratio of electrospun fibers has been responsible for their
and humidity limit its application. In contrast, chitosan films con- superior functional properties. In the case of drug delivery, the dis-
taining exfoliated hydroxyapatite layers maintain functionality in solution rate of a particular drug increases with corresponding sur-
humid environments, providing good mechanical and barrier prop- face area. Hence, the smaller the dimension of the drug delivery
erties while having comparable antimicrobial efficacies to solution- vehicle, the better the drug is absorbed by the human body. Be-
cast chitosan films. cause the size is also smaller than that of a typical human cell, these
fibers provide ideal templates for cell deposition and proliferation.
Nanofibers Beyond biomedical applications, electrospinning has been applied

T he production of fibers with diameters of less than 100 nm is

now feasible with the invention of the electrospinning process.
Electrospinning is a manufacturing technology capable of produc-
to produce filter media for liquid and gas filtration and protective
clothing for the military that is capable of trapping aerosols and large
molecular-weight biocidal gases while minimally impeding air flow.
ing thin, solid polymer strands from solution by applying a strong Thermal, piezoelectric, and biochemical sensors made from elec-
electric field to a spinneret with a small capillary orifice. Generally, trospun fibers have demonstrated sensitivities that are 2 or 3 orders
electrospun polymer fibers can range in size from 10 to 1000 nm of magnitude higher than comparable thin films.
in diameter and may exhibit unusual functionalities with respect The food industry can use electrospun microfibers in several
to their mechanical, electrical, and thermal properties. Because of ways:
the large surface-to-volume ratio, the fibers have shown to be ideal r as a building/reinforcement element of composite green (that is,
materials to produce tissue templates, medical prostheses, pro- environmentally friendly) food packaging material,
tective clothing, and electronic devices (carbon nanotubes). This r as building elements of the food matrix for imitation/artificial
processing technology may have some novel applications within foods, and
the food industry for producing materials with new or improved r as nanostructured and microstructured scaffolding for bacterial
properties. cultures.
The general idea of fiber production from a polymer solution us- While the number of applications that make use of electrospun
ing an electrostatic force was first proposed in patents dating from fibers is increasing at an exponential rate, the applications for food
1934 to 1944. Polymer filaments are formed from solution between and agricultural systems are relatively few. This is probably because
2 electrodes that are oppositely charged, with 1 electrode being sub- fibers are not typically composed of biopolymers used in food and
merged in the polymer solution and the other electrode being con- agriculture; they are made primarily from synthetic polymers. As
nected to a collector. The application of a high-voltage electric field progress in the production of nanofibers from food biopolymers is
induces a charge on the surface of the liquid inside the capillary tube made, the use of biopolymeric nanofibers in the food industry will
used to eject the polymer solvent. Mutually charged polymer and likely increase.
solvent molecules subsequently repel each other and are attracted
to the oppositely charged electrode. As a consequence, the hemi- Nanotubes
spherical surface of the fluid at the tip of the capillary is distorted
to create a Taylor cone, which is “the cone observed in electrospray
and hydrodynamic spray processes from which a jet of charged par-
C arbon nanotubes have been widely used as a nonfood appli-
cation of nanotechnology. These structures have been used
as low-resistance conductors or catalytic reaction vessels among
ticles emanates above a threshold of voltage” (Wikipedia 2006). If the other uses. It has been shown that certain globular proteins from
molecular interaction forces become large enough to overcome the milk (such as hydrolyzed α-lactalbumin) can be made to self-
opposing surface tension, a charged polymer fluid jet will be ejected assemble into similarly structured nanotubes under appropriate en-
from the tip of the Taylor cone. Upon ejection of the charged solution vironmental conditions (Graveland-Bikker and de Kruif 2005, 2006;
from the small-diameter spinneret, the solvent rapidly evaporates, Graveland-Bikker and others 2005, 2006). This technique is applica-
forming 10 to 1000 nm diameter solid fibers that are deposited on ble to other proteins as well and has been explored to assist in the
the collector. Consequently, the mechanical and functional prop- immobilization of enzymes or to build analogues to muscle-fiber
erties of electrospun fine fibers are strongly influenced by the jet structures.
path and velocity. The fluid jet is subject to bending instabilities
that introduce normal and shear forces, which causes stretching of Food Product Innovation
the polymer jet. This reduces the effective fiber diameter and aligns
polymer molecules, thereby improving the mechanical properties
of fibers. Because of the tangential force, a rotational element is in-
A n important area where food nanotechnology is increasingly
used is in the design of functional food ingredients such as food
flavors (Imafidon and Spanier 1994) and antioxidants. Ultimately,

R114 JOURNAL OF FOOD SCIENCE—Vol. 71, Nr. 9, 2006 URLs and E-mail addresses are active links at www.ift.org
Scientific Status Summary—Materials in food nanotechnology

R: Concise Reviews in Food Science

the goal is to improve the functionality of these ingredients in food to improve safety and nutritional value of food products. In a re-
systems, which may minimize the concentrations needed. Deliv- cent study by the Helmut Kaiser Consultancy (2004) and the Royal
ery and controlled release systems for solubilization of nutraceuti- Academy of Engineering, it was estimated that the nanofood market
cals in foods have been previously mentioned (Lawrence and Rees will surge from $2.6 billion today to $20.4 billion in 2010. Currently,
2000). These new functional ingredients are increasingly integrated over 200 companies are pursuing research in the area of food nan-
into the food matrix development process (Haruyama 2003). Food otechnology, examining over 180 potential applications (IFST 2006).
ingredients such as nanoparticulate lycopene and carotenoids are Many of the principles, applications, and techniques that are in-
becoming commercially available. Bioavailability and the ability to cluded in the term “nanotechnology” are the same or fairly similar
disperse these compounds are typically higher than that of their to those that have already been widely understood and utilized. In
traditionally manufactured counterparts. particular, there are major areas of overlap between nanotechnology
and the more traditional disciplines of colloid, interfacial, and poly-
Regulations mer science. However, as this summary shows, one of the defining

T here are currently no special regulations for the application or

utilization of nanotechnology in foods in the United States, and
although recommendations for special regulations in the European
features of nanotechnology appears to be the emphasis on build-
ing structures on the nanoscale rather than on just understanding
their properties (which was a major focus of more traditional dis-
Union (EU) have been made, laws have yet to be changed. The U.S. ciplines). Nanotechnology should probably best be understood as
Food and Drug Administration (FDA) states that it regulates “prod- a conceptual and intellectual framework that enables the design of
ucts, not technologies,” and anticipates that many products of nan- more complex macroscopic structures using nanometer scale build-
otechnology will fall under the jurisdiction of multiple centers within ing blocks.
FDA and will therefore be regulated by the Office of Combination
Products. FDA regulates on a product-by-product basis and stresses Acknowledgments
that many products that are currently regulated produce particles Jennifer Cleveland McEntire, Ph.D., research scientist, IFT, and Toni
in the nano-size range. FDA says that “particle size is not the issue” Tarver, scientific and technical communications manager, IFT, con-
and stresses that new materials, regardless of the technology used tributed to the preparation and editing of this Scientific Status Sum-
to create them, will be subject to the standard battery of safety tests mary.
(http://www.fda.gov/nanotechnology/regulation.html).
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