Natural and Designed Self-assembled Nanostructures in Foods

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Natural and Designed Self-assembled Nanostructures in Foods

VJ Morris & R Parker

Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK

E-mail: vic.morris@bbsrc.ac.uk

ABSTRACT

Nanotechnology in food is an area of emerging interest in the UK and Europe. The use of nanoscience tools to understand the structure of naturally occurring nanostructures in raw materials and the nanostructures induced on processing will lead to rational selection of improved raw materials, enhanced processing and design of novel foods. This is unlikely to be widely perceived as food nanotechnology. There is more interest and concern in the use of ‘inert’ nanoparticles in foods and food contact materials. In the latter case, provided there is no release of nanoparticles into food, it is considered by regulatory bodies that there is no need at present to revise current procedures on clearance and use. In the case of the use of nanoparticles in foods then the route to clearance will be easier for products which are metabolised and where metabolism is not drastically affected by particle size. For nanoparticles which are not metabolised then there is a much greater need for new information on their bio-accumulation and potential toxicity, and how such factors are affected by particle size. The general perception appears to be that food materials and foods produced through the use of nanotechnology will require clearance as novel foods, thus ensuring the safety and quality of the products. However, the general acceptance of nanotechnology may depend on the availability of generic information on the safety of nanotechnology in food and the opportunity for the consumer to exercise choice in the use of such products. This may require modification to certain aspects of the approval, regulation and labelling requirements for this new technology.

Introduction

Nanoscience is defined as the study of phenomena and the manipulation of materials at atomic, molecular and macromolecular scales, where the resultant properties differ significantly from those at the larger scale. Effectively nanotechnology involves exploiting nanoscience to deliver improved or novel functional properties. Nanoscience and nanotechnology are emerging areas and it is important in the food sector that consumers are well informed and have choice in the use of this technology if it is to be commercially successful.

The of nanoscience tools such as the atomic force microscope (AFM) allow imaging of nanostructures in raw materials and of the new nanostructures introduced through food processing. Such images provide insights which can be used to select or design structures that improve the quality of food products. In this form of nanotechnology the selection and processing methods are conventional and well accepted. Nanotechnology will also influence food quality through the introduction of nanoparticles into foods and food contact materials. In this case the technology is new, the particles show unusual properties, and there are opportunities for the production of novel functional foods. At present in the UK and EU there are no approved nanofoods and the advantages and concerns over the use of this technology need to be considered in reviewing mechanisms for clearance, regulation and labelling.

Natural self-assembled nanostructures

There are many examples of self-assembled structures that determine the function of raw materials. Many food processing operations produce new self-assembled molecular nanostructures. Examples include starch and molecular structures at interfaces.

Starch – a self-assembled structure

Starch is the major carbohydrate consumed by mankind. Plants store energy by assembling partially-crystalline granular structures. The detailed physical basis for the biosynthesis of starch is still obscure. However, it is likely that the production of the granules involves some form of self-assembly modified by enzymatic modification of the assembled structures. Methodology developed for AFM imaging of the internal structure of starch granules can be used to image isolated starches, or the structure of starch granules directly within seeds (1). Using AFM it is possible to image the crystalline structure within the granule responsible for gelatinisation. By studying isogenic mutations in the biosynthesis of pea starches it has been possible to identify mutations which produce high-amylose starches that alter the crystalline structure within the granule. Starches possessing these structures only gelatinise at temperatures above 100oC and are thus likely to be intrinsically resistant starch. Screening of pea varieties using microscopic methods has identified commercial varieties, such as Greenshaft, that contain blends of normal starches and these ‘naturally resistant’ novel starches. Preliminary human studies suggest that consumption of such products leads to a reduced glycaemic index (GI). Normally the GI and resistance (RS) of starches is determined by the processing of foods since the granular structure is lost on cooking. Through use of nanoscience it has been possible to identify starches with ‘intrinsic resistance’ and improved nutritional value.

Interfacial protein films, emulsified fats and fat

A major source of fats such as triglycerides in the diet is processed food emulsions. Production of emulsions involves creation and stabilisation of an interface between liquid fat and water. Proteins are commonly used as emulsifiers. The proteins are considered to adsorb at the interface, partially unfold and interact to form elastic layers. New methodology for imaging interfacial structures confirmed the existence of elastic protein networks and allowed discovery of a new, unexpected orogenic mechanism by which surfactants, can displace protein networks from the interface (2). Such information allows the production of stable protein networks which enhance the stability and shelf-life of food emulsions. However, on consumption, the presence of stable protein films would inhibit adsorption of co-lipases and lipases in the duodenum and inhibit fat metabolism. The nanoscience methods developed to image interfacial films can be used to assess disruption of these films under simulated digestion conditions and the effect on eventual protein displacement by bile salts in the duodenum. Such information provides a route to the design of interfacial structure to control and regulate rates of fat metabolism. The protein content of natural polysaccharide emulsifiers such as sugar beet pectin, arabinoxylans and gum Arabic is considered essential to their function. Use of AFM (3) to characterise such materials and the structures they form at interfaces will allow improved selection or modification of these materials, or the creation of new emulsifiers through the formation of protein-polysaccharide complexes.

Designer interfaces, coatings and barriers

In addition to visualising and manipulating naturally occurring nanostructures it is also possible to design new interfacial structures, or coatings and barriers on surfaces, through layer-by-layer self-assembly of biopolymers. AFM is one of a number of physical techniques that can be used to probe and understand these self-assembly processes. At our institute we are investigating the generic aspects of the assembly of multilayer structures and how to control the interaction between molecules in different layers. These interactions will determine basic properties such as elasticity or porosity. The structures can be designed to respond to different stimuli such as pH, ionic strength or different enzymes during digestion. Applications of this type of assembly process are just starting to appear in the food area (4). These types of nanostructures can be assembled by simple, conventional processing operations such as sequential dipping of samples into polymer solutions alternating with washing stages. The incorporation of nanoparticles into such structures is one example of the use of nanoparticles in food contact materials.

Nanoparticles in foods

The areas described above are examples of nanotechnology which use nanoscience to rationally enhance food quality through conventional technologies. An area that offers new products, but has also raised concerns, is the direct use of nanoparticles in foods or food contact materials. Information on this emerging area of nanotechnology in food is available through our website (5). This provides links to other websites discussing new products, social aspects of nanotechnology, regulation and funding for nanotechnology. The website provides access to a consumer products directory (6) that lists most nanotechnology products currently available worldwide.

Nanoparticles in food contact materials

In the UK and EU there is real interest in the use of nanotechnology in food packaging and containers and in the development of anti-microbial coatings and surfaces to prevent bacterial and fungal growth. The latter can be simple surfaces containing anti-microbial agents or ‘smart’ surfaces designed to detect bacteria and then release agents to combat any infection. The concern over use of ‘inert’ nanoparticles in food contact materials centres on whether nanoparticles can partition into the food and then be ingested. If there is no release or partioning of nanoparticles into the food then there is no additional concern about their safety with regard to the food industry. However, there may be concerns about the ultimate fate of these products and the release of nanoparticles into the environment on disposal. This raises issues about clearance, regulation and labelling, and the effects on consumer awareness and choice. Such factors may ultimately prove to be important in consumer perception and acceptance of the introduction of nanotechnology in food.

Nanoparticles in foods

The most likely use of nanoparticles in foods in the EU and UK is in the grey area of functional foods, health foods and food supplements. This is at present the biggest growth area in nanofoods worldwide (6). Most of the products appearing on the market are concerned with improving the delivery, uptake and bioavailability of various food components. In the UK and Europe it is generally considered that the use of nanoparticles in foods or food supplements would constitute a novel food and require clearance by the appropriate regulatory bodies. EU law requires that all food products must be safe for consumption. The requirements for approval of such new products will depend on whether or not the nanoparticles are materials that are normally metabolised or digested within the body. If metabolism or digestion is considered to be unaltered through the nanofabrication process then such products are likely to be considered as generally safe for use in foods. At present there appear to be no foods or food supplements cleared for use in the UK and Europe. However, examples of such products available worldwide are nanoencapsulated omega 3 fatty acids, vitamins, minerals and carotenoids. The BASF nanoparticulate synthetic carotenoid (‘lycopene’) dispersions claimed for use in a wide diversity of colouring properties associated with improved bioavailability is an example of a European product which has GRAS (generally regarded as safe) status in the USA (7).

If the nanoparticles are not normally metabolised within the body then the clearance procedures may be more complex. In the UK and Europe there are no examples of products of this kind that have been approved for use. A perusal of the patent suggests areas that may ultimately be submitted for approval. Examples include suggested use of nanoparticles in coatings on foods to control moisture and oxygen diffusion to enhance shelf-life and flavour perception or nanosilver as an anti-bacterial agent in wheat flours. For these types of products there is need for information on the bio-accumulation and potential toxicity of the added nanoparticles, particularly where this is related to the reduced size of the particles.

Nanoparticles in food – advantages and concerns

The main advantages claimed for the use of nanoparticles in foods is availability. Oil-soluble products can be made water-dispersible and water-soluble products dispersed in oils using inverted micelles or nanosomes. The use of nanoparticles or nano emulsions as carriers offers the advantage of clear rather than opaque products and improved perception. Use of micelles or nanodrops suggests a form of targeted delivery to mucus membranes or micellar structures within the body. There are opportunities for the design of systems for specific delivery of materials to targeted sites. This type of nanoceutical or functional food appears to be the growth area at present and fits between foods and pharmaceuticals.

The main concerns are about inadvertent or deliberate introduction of nanoparticles into foods where there is a general lack of knowledge on the effects of reducing size on toxicity and bioaccumulation. This concern is mostly about inert materials not normally metabolised within the body. Nanoparticles can penetrate into structures that normal particles cannot enter. This has advantages in the design of novel foods but could cause unexpected problems with accumulation and toxicity. Many nanoparticles are claimed to show anti-microbial action yet there is a dearth of knowledge available in the public domain on their effect on the microbial ecology of the mouth or gut.

If we are to learn from the lesson of the GM debate in the UK and Europe then it is important that the consumer and the general public are kept aware of developments of the use of nanotechnology in food. It is also important that the products are seen to have significant benefits, rather than appearing as trivial offshoots of nanotechnology.

There are several problems with consumer awareness for products appearing worldwide which may require addressing within the UK and EU. Firstly, there is no clear definition of the term ‘nano’ as applied to foods. There is no consistency in the labelling of products making it difficult for consumers to exercise choice. There are also problems associated with labelling. If a food product contains nanoparticles then how should it be labelled? In the UK or in Europe if a product contains nanoparticles of a food-approved additive or ingredient then should it be called a nanofood? Should it specify which additives or ingredients are in the nanoform or should it use component names or E-numbers? The latter would imply safety data and acceptable daily intake values (ADIs) deduced for the ‘native’ component. Are these applicable for the nano-component? Should a modified E-number be used as suggested by the [UK] IFST (8) in their information statement on nanotechnology. Without some of these problems being resolved it will become increasingly difficult for consumers to exercise choice in the use of nanotechnology applied to foods and food contact materials. This may be detrimental to the widespread use and acceptance of nanotechnology in food.

References

Dr Vic Morris is a Fellow of the International Academy of Food Science and Technology


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