Nanotechnologies in Food: What, Why and How?

QASIM CHAUDHRY, RICHARD WATKINSb AND LAURENCE CASTLE

1.1 Background

The possibility of controlling and manipulating certain properties of materials and substances by reducing their particle size to very small scales has been hypothesized since 1959. The term 'nanotechnology' has been coined in the past few decades to encompass different processes, materials and applications derived from a wide range of fields in the physical, chemical and biological sciences and in electronics and engineering with the common theme of the manufacture and use of materials on a nanometre size scale. One nanometre is one-billionth of a metre (1 x 10-9 m). The advent of modern analytical tools that can detect and characterize the various physicochemical aspects of materials at the nanoscale has further boosted developments in this field and nanotechnology has started to provide a systematic method for the study and fine-tuning of material properties to suit specific applications. This has inevitably attracted much interest from virtually all industrial sectors for the development of new or improved products and applications based on nanomaterials. Of particular interest in this regard are engineered nanoparticles (ENPs), which are manufactured specifically to achieve a certain composition or material property or composition for a particular purpose.

Like any new emergent field of science and technology, nanotechnology has brought both the promise of a number of new prospects and applications and new challenges. For example, it has not been easy to provide an exact definition of a nanomaterial. Despite several proposals, an internationally agreed definition is not yet on the horizon (see Chapter 2). Nevertheless, as a result of the commonality between different facets of nanotechnology in terms of a nanoscale particle size, there is a broad understanding that a typical nanomaterial could be characterized as having one or more external dimensions in the size range 1-100 nm. Nanomaterials can be in the form of nanoparticles, where all three external dimensions are at the nanoscale; nanorods or nanotubes, where two dimensions are at the nanoscale; and coatings or sheets, where only one dimension is at the nanoscale (Figure 1.1).

The chemical nature of the substances that can be used to manufacture ENPs is very diverse. In theory, any particulate material can be produced in nanoform either by a top-down (i.e. grinding down larger materials to the nanoscale) or a bottom-up (i.e. the upwards assembly of atoms/molecules to build nanoscale particles) approach. A nanomaterial can therefore be inorganic, organic or hybrid in nature. In addition to manufactured nanoparticles, some nanomaterials can be derived from natural sources — for example, montmorillonite is a clay obtained from volcanic ash or rocks. It has a natural nanoplate structure and has been used as a nanofiller in food packaging applications.

Nanomaterials are generally produced in the form of primary particles with nanoscale dimensions. However, most nanoparticles have a tendency to stick together to form larger agglomerates and/or aggregates during subsequent processing, formulation or storage. Unlike aggregates in which the primary particles are strongly bound together, these agglomerates only hold the primary particles together through weak van der Waals forces. The agglomerates can therefore de-agglomerate with changes in certain conditions, such as pH or ionic strength. Nanomaterials may be present as free particles in some applications, such as cosmetics and personal care products, but in other applications they are present as fixed, bound or embedded forms in a matrix, such as food packaging materials. Thus a nanomaterial may be present in a product in the form of free (separate from each other) nanoparticles and as larger sized clusters depending on the type of product or application.

To help visualize nanomaterials in context, organic life is carbon-based and the C-C bond length is about 0.15 nm. Thus, when placed in a food context, most ENPs are larger than molecules such as lipids, are a similar size to many proteins, but are smaller than the intact cells in plant- and animal-based foods (Figure 1.2).

The fundamental drivers at the heart of most nanotechnology applications are the potential for improvement in material properties, the development of new functionalities and/or a reduction in the amount of (chemical) substances needed for a function. This is because, on an equivalent weight basis, the nanoforms of a material will have a much larger surface to mass ratio than their conventional bulk equivalents. Thus a much smaller amount of an ENP could, in theory, provide the level of functionality that would otherwise require a much greater amount of the same material in conventional form. The notion 'a little goes a long way' is probably the single most powerful reasoning behind many nanotechnology applications. Other benefits have also been attributed to the very small dimensions of ENPs — for example, nanosizing may allow the conversion of water-insoluble substances to forms that are dispersible in aqueous formulations. This may make it possible to reduce the need for solvents in certain consumer products, such as cosmetics, paints and coatings. The similar processing of water-insoluble food additives, such as colours, flavours and preservatives, may improve their dispersion in low-fat products. Nanoforms of various nutrients and supplements have also been claimed to have a greater uptake, absorption and bioavailability in the body than their bulk equivalents. This aspect alone has attracted a lot of interest in the use of nanosized ingredients in supplements, nutraceuticals and (health) food...