Applications of nanotechnology for the food sector

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Introduction

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Nanotechnology has opened new avenues in the research and development of food technology. It is being used as a means to understand how physiochemical characteristics of nano-sized substances can change the structure, texture and quality of food. Application of this area already span development of improved tastes, color, flavor and consistency of foodstuffs, increased absorbtion of nutritions and health supplements, new food packaging with improved barrier, mechanical or antimicrobial properties, and nanosensors for traceability and monitoring the conditions of food during transport and storage[1]. The rapid profilation of nanotechnologies in a wide range of consumer products raises a number of safety, environmental, ethical and regulatory issues.

Even though the research of applications of nanotechnology in foods is going on strongly, there are currently few products available. According to the Woodrow Wilson database called "The Project on Emerging Nanotechnologies" there are currently only three available products using nanotechnology in the food sector.[2]

This text presents an overview of current trends in research on nanotechnology in food sector. The text consists of four large sections: market drivers, nano inside, nano outside and health risks. Market drivers present a quick overview of the economic background of nanotechnology in food today. Nano inside concerns the application of nanotechnology inside the food products. The focus in that section is delivery systems, how functional ingredients are delivered, and not so much how the delivered particles react when they arrive, since the latter field is still relatively unknown (also mentioned under the health risks section). Nano outside presents how different technologies can be used to make food packaging. The last section, health risks, presents research on the health risks concerning the applications of these new technologies.

Market drivers

Nanotechnology has in recent years developed into a wide-ranging global industry. The global marked of nanotechnology is widely expected to reach $1000,000,000,000 USD by 2015 with approximately two million workers[1]. Concerning the food sector, estimates of current global marked size are varied. Furthermore, a lot of the information available is aimed at projecting the "magical potential" of nanotechnology when applied to food, rather than real products and applications that are available in a few years time. A report from the consulting firm Cientifica estimates that the overall market value of food applications of nanotechnology will reach $5800000000 by 2012 (food processing $1303 millions, food ingredents $1475 millions, food safety $97 millions and food packaging $2930 millions).


A number of major food and beverage companies are reported to have an interest in nanotechnology. These include Nestlé, Kraft and Heinz. The food industry is ultimately driven by profitability, which is consequent on gaining consumer acceptance. This industry is constantly looking out for new technology to improve the nutritotional value, shelf life, taste and quality of their food in addition to new and innovative products. Nanotechnology processes and materials can provide answers to many of these needs, as it offers the ability to control and manipulate properties of substances close to molecular level.

The public acceptance of nanotechnology in foods

The introduction of new technology can be met with public skepticism. It remains unclear how public attitudes will impact the future of nanotechnology in the food sector. However, it is well known that lack of knowledge of potential risks, or lack of communication of risks and benefits can raise concerns amongst the public. A recent example is the negative reaction from the EU-consumers to genetically modified (GM) food. In a study taken place in Switzerland a random selected group (337 people) were given a mail survey to answer about the application of nanotechnology in food and food packaging. The survey indicated that people were less sceptic about nanotechnology food packaging than nanotechnology foods.[3]

Regulations

Questions have been raised regarding the regulation of new products on the marked. From a number of reports it becomes clear that there is no nano-specific regulation in the EU or other countries (in 2007). It as been made clear that companies making new products containing nanotechnology will have to obey current rules, and the Health Council of the Netherlands have considered that the best thing to do would be to modify allready existing rules to fit the new technology. However, such modifications can not be made with out the proper competance. As a start there is need for a strict definition of nanotechnology. SCNIHR (Scientific Committee on Emerging and Newly Identified Health Risk) has postulated, and it is to be found elsewhere, a definition of nanotechnology as "the order of 100nm of less". However, this definition will not cover all situations. Where characterization of chemicals normally is easily done (for instance by measuring purity and concentration), physio-chemical characterization of nanoparticles is difficult due to the technical complexity and general lack of knowledge in this area.[4]

Nano inside - Delivery systems

"Nano inside" is a term for applied nanotechnology in food ingredients.[3] This can either be applied as existing ingredients made into particles in nano scale (1-100nm) which would give the material new properties, or developing new types of ingredients. These ingredients could for instance give the food higher nutritional values and increased shelf life. As mentioned in the introduction, and also in the health risk section, the current research mainly concerns how the functional ingredients reach their destination, that is, delivery systems. The delivery system makes sure that the functional ingredient reaches its destination, that it is not degraded during the transport and that the release is done properly. Factors affecting the release rate can for example be pH, ionic strength or temperature. It is also important that the delivery system is compatible with other components in the system.[5]. While most reports published today mainly concerns the future possibilties of the techniques used, there are currently few commercial available products using these application methods listed under. One exception is a type of bread called Tip Top Up from the Australian company Tip Top. Using nanoencapsulation, tuna fish oil is used for getting Omega 3 acids into the bread. Omega 3 is well known for its health benefits, but fish oil is equally known for its unpleasent taste. The nanosize particles are not detected by the taste buds and therefore the bread will have no taste of fish oil.[6][1]


Emulsions

An oil-in-water (O/W) emulsion[7]
The delivery of nano particles applied in food technology is very often in form of emulsions (nanoemulsions)[4]An emulsion is a mixture of two or more liquids where one is distributed as droplets in the other. [8] One should notice that nano-emulsions have similar properties to emulsions (unlike microemulsions which have different thermodynamic properties than emulsions). Nano-emulsions have a droplet size of 20nm to 500nm. The formation is not spontaneous and their properties do not only depend on thermodynamic conditions, but also the way they are prepared.[9]
Illustration of fabrication of multilayer emulsion[5]
Encapsulating functional components in the droplets and modication of the interfacial layer between the droplet and the continuous have been shown to have a slowdown of the chemical degradation process.[5] Multiple emulsions, emulsions containing several layers can also be used (for instance a oil-in-water-in-oil emulsion (O/W/O). The advantage using the multiple emulsions is that phase components that are soluble in the same type of phase can be separated and still contained in the same colloidal particle. Multilayer emulsions, emulsion containg only one phase but has a multilayer interface has also been researched. Emulsions with oil droplets surrounded by multilayer interfaces have been shown to have better stability against enviromental stress than single layer emulsions. The multiple layers can be made by electrostatic deposition in which polyelectrolytes are adsorbed on surfaces of oppositely charged colloidal particles. [5][9]

Association colloids

A micelle[10]
Association colloids are aggregation of surface-active molecules. Examples of association colloids include microemulsions, micelles, vesicles, bilayers, reverse micelles and liquid crystals. They can be used for delivering polar, non-polar and amfiphiphilic functional ingredients. Advantages of association colloids are that they are thermodynamic favorable (reaction resulting in negative Gibbs' energy) and often form transparent solutions. The disadvantages are that a high concentration of surfactants is needed to form the association colloids and that they may dissociate if the solution is diluted.[5][11] Micelles are clusters of amphipathic molecules which are aggregating together. Considering a micelle in a polar solvent, the core of the micelle consists of the hydrophobic end (predominantly hydrocarbon)while the polar heads will point outwards to the water. The radius core of the micelle, available for functional ingredients, roughly equals the length of the fully extended hydrocarbon tail. A typical micelle will thus have a radius on a couple of nanometers. When the concentration of surfactants in the solution reach the critical micelle concentration, insoluble particles can interact with the nonpolar heads of the surfactants and become encapsulated in the micelle. Reversed micelles will occur when surfactants are added to a non-polar solvent. The polar head groups will then point inwards.[11]

Biopolymeric nanoparticles

Biopolymeric nanoparticles are made of one or several biodegradable polymers. PLA (polylactic acid) is a main component of many biopolymeric nanoparticles. PLA is already widely used in for instance biomedicine because it is biodegradable and can be used for surgery (for instance as sutures). The biodegradable property of PLA can be both an advantage and disadvantage. It will degrade quickly in the bloodstream and therefore it can be suitable for delivering functional ingredients there, but if the ingredient is supposed to be carried to another internal organ the encapsulation will most likely degrade before reaching its destination. PLA also degrades in intestinal fluid, but this can be overcome by adding a a hydrophilic compound. PEG (polyethylene glycol) is often used for this task. A PLA-PEG diblock compound can be able to form a micellar structure and entrap the compound that is to be delivered. PLA and PLA-PEG nanoparticles have very good encapsulation properties, but the main reason for using these particles for a delivery system is that they are non-toxic. PLA-PEG-particles will have a lower zeta potential (a lower surface charge) than PLA-particles, and this will influence the interaction of the particles with other compounds in the food.[5]

Nano outside - functional food packaging

Nanotechnology derived food packaging materials is the largest category of current nanotechnology applications for the food sector. Food packaging materials with improved mechanical, barrier and antimicrobial properties are of interest, and also nano-sensors for traceability and monitoring of conditions of food during transport and storage. Packaging materials for foodstuff, like any other short-term storage packaging material, represent a serious global environmental problem. A big effort to extend the shelf-life and enchange food quality while reducing packaging waste has encouraged the exploration of new, nanotechnology-based materials.


Nanocomposites for food packaging

Polymer composites are mixtures of polymers with inorganic or organic additives. Appropriately adding nanoparticles to a polymer matrix can enhance its performance, often in very dramatic degree, by simply capitalizing on the nature and properties of the nanoscale filler. With nanotechnology fillers, the composites can exhibit significant improvement in modulus, dimensional stability and resistance to humidity or gas.[12] Other advantages include low density, transparency, better surface properties, fire resistance and recyclability. Due to very large aspect ratios, a relatively low concentrations of nanoparticles is sufficient to change the properties of a material. These benefits have led to the development of a variety of nanoreinforced polymers, commonly referred to “nanocomposites”, which typically contains up to 5% nanoparticles.[1]

Nanocomposites: Schematic drawing of exfoliation and intercaltion states. [13]

Composites with nanoclay

The polymer composites incorporating clay nanoparticles are among the first nanocomposites to emerge on the market as improved materials for food packaging. The nanoclay mineral used in these materials, montimorillonite, is a widely available natural clay derived from volcanic ash and rocks[1]. The clay has a unique morphology that contains one dimension on the nanoscale. Nanoclay-composites have been developed for potential use in a variety of food packaging applications and some products are already available for the consumer. The montmorillonite is hydrated alumina-silicatelayered clay. This special clay has a structure that limits the permeation of gas due to the large aspect ratio and tutourity [14]. To obtain the polymer composite improvements mentioned above, a small percentage of clay can be included in the polymer matrix. This process is called solid layer dispersion in polymers and involves two major steps; intercalation and exfoliation[12]. In the intercalation step, the d-spacing in the crystal structure increases from its intrinsic value, as polymer chains or monomer molecules diffuse into the clay gallaries. In an intercalated state, the inorganic layers remain parallel to each other. In exfoliation, the clay particles are released from this system and are dispersed in the matrix polymer with no apparent particle interactions. The result is layers of nanoclay woven into the polymers structural matrix. Introduction of the dispersed clay layers into the polymer matrix structure has been shown to greatly improve the overall mechanical strength and barrier properties of the material, making the use of nanocomposites films industrially practicable[12]. The known industrial applications of nanoclay in multilayer film packaging include beer bottles and thermoformed containers. Miller Brewing Co. (USA) and Hite Brewery Co. (South Korea) have reported to be using this technology in their beer bottles.


Baby Dream: "Nano Silver Baby Milk Bottle" from Baby Dream Co. Ltd. South Korea. [15]

Composites with metalparticles

Polymer nanocomposites incorporating metal or metal oxide nanoparticles have been developed for packaging because of their antimicrobial or for example UV-resistant properties. Based on the antimicrobial action of nanosilver, a number of "active" food packaging materials has been claimed to be inhibiting the growth of microorganisms. Examples from the internet include "Nano Silver Food Containers" form A-DO Korea, and "Nano Silver Baby Dream Co. Ltd. from South Korea. Silver has been used in medical care for ages because of its antimicrobial properties. Silvernanoparticles are currently being used in a wide range of consumer products including antibacterial refrigerators and socks. When in contact with bacteria and fungus the silver nano particles will adversely affect cellular metabolism and inhibit cell growth. The silver suppresses respiration and transport of substrate in the microbial cell membrane due to the chemical properties of its ionized form, Ag+. Ag+ forms strong molecular bonds with substances used by bacteria, such as molecules containing sulfur, nitrogen, and oxygen. Once the Ag+ ion complexes with these molecules, they are rendered unusable by the bacteria, depriving it of necessary compounds and eventually leading to the bacteria's death.

Chitosan

Chitosan: Haworth projection of chitosan[16].

Nanocomposites have also been developed using for example chitosan. Chemically derived by deacetylation of chitin, an abundant polysaccharide found in shellfish, chitosan is a linear polysaccharide composed of randomly distributed β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It has a number of commercial and possible biomedical applications[16]. Chitosan possesses a unique cationic nature relative to other neutral or negatively charged polysaccharides. In an acid environment, the amino group NH2 in chitosan can be protonated to NH3 +, which yields antifungal or antimicrobial activities since cations can bind to anionic sites on bacterial and fungal cell wall surfaces. Further, chitosan is a nontoxic natural polysaccharide and is compatible with living tissue. These appealing features make chitosan widely applicable in wound healing, production of artificial skin, food preservation, and cosmetics.


Biodegredable polymers

New research on nanocomposites based on biodegredable polymers is increasing. Biodegradable plastics are polymeric materials of which at least one step in the degradation process is through metabolism in the presence of natural occurring organisms. Biodegradation leads to fragmentation or disintegration of the plastic with no toxic or environmentally harmful residue [12]. However, current biodegradable films exhibit poor barrier and mechanical properties and these properties need to be improved considerably before they could replace traditional plastics [23,24] and thus help to manage the world’s waste problem. Introduction of nanoparticles as described above can dramatically improve the properties of biodegredable polymers [14].

Edible food packaging

Bio-based packaging materials, such as edible and biodegradable films from renewable resources are interesting because they could, at least to some extent, solve the waste problem. Edible films and coatings are defined as thin, continous layers of edible material used as coating or as a film placed on food components. The difference between a coating and a film is that a coating is an integral part of the food product formed directly on it by spaying, dipping etc, while a film is in contrast a freestanding structure formed and then applied to the food product.[17].

Nanolaminates

Films and coatings can be manufactured in many ways. One option is to use nanolaminates. A nanolaminate consists of two or more chemically bonded layers of polymers. The LbL-technology provides a way to create the nanolaminates. In this process a charged surface is coated with interfacial films consisting of nanolayers of different materials. The electrostatic forces hold these layers together. Very thin films (1-100nm) can be created with this technology. These films can give many properties to the food applied on. This can be shelf-life improving properties, for instance that it can prevent moisture from escaping. The films can also serve as encapsulation of functional ingredients, giving flavour, vitamins etc to the food. Because nanolaminates are very thin they are more often used as coatings than as films. Application of nanolaminate coating can be done by dipping the food into several solutions where substances will be attracted to the surface most often by electrostatic forces. Another way of coating the food is to spray the different solutions on the food product. Examples of substances that can be used for creating the laminates are proteins, polysaccharides (natural polyelectrolytes), charged lipids (phospholipids, surfactants) and colloidal particles.
Illustration of a laminate formed from a protein and a polysaccharide[5]

Changing the properties of the applied layer can be done in several ways.

There are still many uncertainties for this technique. For instance, the influence of the topography of the surface on the coating has still not been properly established.[5]

Biopolymer nanofibres

In addition to manufacturing nanoparticles the new option of electrospinning of materials allows fibres of biopolymers to be made in addition to the nanoparticles refered to earlier. These fibres can be used both as food packaging and as encapsulation of functional ingredients in the material. Electrospinning is a technique which has been applied to synthetic polymers for a while, but as the technology gets better it is also possible to electrospin biopolymers. What makes electrospinning of biopolymers difficult is that they in general can not handle as much mechanical stress as synthetic polymers.
Schematic image showing a setup for electrospinning [18]

The principle of electrospinning is based on deposition of a solution containing the polymer on a stationary plate or a rotating drum or disk. The solution (often organic) containing the polymer is put into a syringe with a thin metallic capillary needle at the end. At the end of the needle, ordinarily, a droplet would form, balancing the surface tension and the gravitational forces. Applying an electrical field will make a more complex situation causing charge repulsion between the polymer and the solvent molecules and there will be an attraction to the target electrode. A cone shape structure is formed (Taylor cone) pointing towards the grounded deposition plate (or drum/disk). At a critical point a thin jet of the solution would burst from the tip of the Taylor cone and this jet will be accelerated towards the grounded plate (or drum/disk). In this process the solvent can evaporate causing only the polymer to be deposited. Also differences in charge on the surface of the jet may cause it to bend.

Research has been done and is still going on in the field of using this technology. Many biological compounds, for instance polysaccharides and proteins, can be suitable for electrospinning, making very interesting possibilities for delivery systems and food packaging. Even though there are very interesting possibilities in this technology and research going on, there are currently very few applications of this in the food sector.[19]

Health risks

It is known that materials at nano-scale exhibit different properties than found at macro-scale. An extreme example concerning different properties at different scales is aluminum oxide. Aluminum oxide is used in dentistry because it is inert at macro-scale, but when manufactured as nano-scale particles it can spontaneously explode and has been tested as a potential rocket fuel. Examining new nano-materials properties as particle size, mass, chemical composition, aggregation, and surface charge of the nanoparticles must be done. There are currently available data that indicates that properties of nanoparticles, such as surface charge and functional groups can influence absorption, metabolism, distribution and excretion. Still, very little is known how these physio-chemical properties will affect the behavior of the particles in the body.

Entrance of nanoparticles in the human body

It is three possible ways a nanoparticle, or any particle, could enter the human body. That is through the skin, through inhalation or through ingestion.[20][4]

Dermal exposure

Nanoparticles can have an impact if they penetrate the outer layers of the skin. Some studies show that a healthy epidermis (the outermost layer of the skin) could protect against nanoparticles. There are findings that fluorecent microspheres or dextran beads (>1μm) can, when in motion, penetrate the stratum corneum (the uppermost sublayer of the epidermis), go down in the epidermis and occasionally reach the dermis. If nanoparticles penetrate the dermis they could translocate via lymph to regional lymph nodes. Some researches claim that small titanium dioxide particles (about 20nm in diameter) could even interact with the immune system when they penetrate the skin. It should of course be noticed that possible health consequences still remain rather speculative.[20]

Inhalation

If a particle has a aerodynamic diameter less than 10μm it can pass through the nasal cavity to the lungs. The aerodynamic diameter describes the particles aerodynamical behaviour as if it was a perfect sphere with density 1.[21] The particles can get deep into the lungs, and when inhaled, some nanoparticles may accumulate in the lungs and cause cronic diseases such as pulmonary (lung) inflammation. There are also speculations if the particles when in the blood stream, would be able to cross the blood brain barrier. However, it is again hard to draw conclusions and individual nanomaterials needs to be evaluated.[20]

Ingestion

Nanoparticles can avoid intestinal clearance mechanisms and penetrate deeply into tissues and fine capillaries. It seems that small particles can penetrate the mucus layer faster than larger particles. In the mucus layer, it also seems that the diffusion is dependent of the charge of the particles. Some experiments show that anions can reach the epithelial surface under the mucus layer, while cations stays trapped in the mucus. The gastrointestinal epithelium is the second barrier the particles reach.[22] The cells in the gastrointestinal epithelium are connected with tight junctions and as bacterias, toxins and immunogens can enter here it is to be considered a risk that nanoparticles can enter through this way. In addition to those particles directly ingested, particles inhaled and cleared by the mucociliary apparatus can end up in the gastrointestional tract.[20][4]

Distribution of entered nanoparticles

Even though little is known about the nutrional effect of having functional ingredients as nanoparticles there have been done some research on the distribution of nanoparticles in the human body.

There are tests indicating that nanoparticles will have a widespread distribution inside an animal body, where the smallest particles even can reach the brain and bone marrow. Also on a cellular level there are indications that cell membranes can be penetrated by nanoparticles. Gold and titanium-oxide nanoparticles have been identified inside human red blood cells. The blood-brain barrier restricts permeability to the molecules which are lipophilic, but there exist eveidence that some nanoparticles can cross this barrier.[4]

References

  1. ↑ 1,0 1,1 1,2 1,3 1,4 Chaudry, Scotter, Blackburn, Ross, Boxall, Castle, Aitken & Watkins, "Applications and implicatons of nanotechnologies for the food sector", Food Additives and Contaminants, March 2008
  2. ↑ Website: "The Project on Emerging Nanotechnologies" (http://www.nanotechproject.org/about/mission/) Visited 2009.03.21
  3. ↑ 3,0 3,1 Siegrist M, Stampfli N, Kastenholz H, Keller C, "Perceived risks and perceived benefits of different nanotechonology foods and nanotechnology food packaging", Appetite (2008); 51: 283-290.
  4. ↑ 4,0 4,1 4,2 4,3 4,4 Bouwmeester H, Dekkers S, Noordam MY, Hagens WI, Bulder A, Heer C, ten Voorde SECG, Wijnhoven S, Marvin HJP, Sips AJAM, "Review of health safety aspects of nanotechnologies in food production", (2009), Regulatory Toxicology and Pharmacology 53 (2009); 52–62
  5. ↑ 5,0 5,1 5,2 5,3 5,4 5,5 5,6 5,7 Weiss J, Takhistov P, McClements DJ, "Functional Materials in Food Nanotechnology", Journal of Food Science (2006); 71(9): 107-116.
  6. ↑ Website: Tip-Top (http://www.tiptop.com.au/), visited 2009.03.21
  7. ↑ Website: http://www.ifr.ac.uk/Materials/fractures/emulsions.html Visited: 2009.03.22
  8. ↑ Website: Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 14. Mar. 2009 (http://www.britannica.com/EBchecked/topic/186307/emulsion)
  9. ↑ 9,0 9,1 M. Porrasa, C. Solansb, C. Gonzáleza, A. Martíneza, A. Guinarta and J.M. Gutiérre, "Studies of formation of W/O nano-emulsions", Colloids and Surfaces A: Physicochemical and Engineering Aspects (2004); 115-118
  10. ↑ Website: http://en.wikipedia.org/wiki/Micelle Visited: 2009.03.22
  11. ↑ 11,0 11,1 Hiemenz PC, Rajagopalan R, "Principles of Colloid and Surface Chemistry", Taylor & Francis Group (1997)
  12. ↑ 12,0 12,1 12,2 12,3 Sorrentino, Gorrasi & Vittora, "Potential perpectives of bio-nanocomposites for food packaging applications, Trends in Foof Science and Technology 18, 2007
  13. ↑ Website: http://imi.cnrc-nrc.gc.ca/Carrefour_d_informations/Factsheets/pnc_tech_e.html Visited: 2009.03.22
  14. ↑ 14,0 14,1 Sozer & Kokini, Nanotechnology and its applications in the food sector, Trends in Biotechnology Vol.27 No.2
  15. ↑ Website: http://babydream.en.ec21.com/product_detail.jsp?group_id=GC00887651&product_id=CA00895940&product_nm=Nurser Visited: 2009.03.22
  16. ↑ 16,0 16,1 Website: Chitosan (http://en.wikipedia.org/wiki/Chitosan)Visited 22.03.2009
  17. ↑ Balasubramaniam, Chinnan, Mallikarjunan & Phillips, 1997; Guilbert et al., 1997
  18. ↑ Website: Electrospinning (http://en.wikipedia.org/wiki/Electrospinning) Visited: 2009.03.22
  19. ↑ Kriegel C, Arrechi A, Kit K, McClements DJ, Weiss J, "Fabrication, Functionalization, and Application of Electrospun Biopolymer Nanofibers", Critical Reviews in Food Science and Nutrition, 48:775–797 (2008)
  20. ↑ 20,0 20,1 20,2 20,3 Chau C, Wu S, Yen G, "The development of regulations for food nanotechnology", Trends in Food Science & Technology (2007) 18: 269-270
  21. ↑ Website: http://en.wikipedia.org/wiki/Aerodynamic_diameter, visited 2009.03.17
  22. ↑ Website: http://en.wikipedia.org/wiki/Epithelium visited 2009.03.23
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