ReviewNanocomposites for food packaging applications
Introduction
‘‘Nano-’’ denotes nanometer (10−9 m). The concept of nanotechnology was introduced by Richard Feynman in 1959 at a meeting of the American Physical Society (Khademhosseini & Langer, 2006). Since then, nanotechnology has developed into a multidisciplinary field of applied science and technology. Nanotechnology is the ability to work on a scale of about 1–100 nm in order to understand, create, characterize and use material structures, devices and systems with new properties derived from their nanostructures (Roco, 2003). Because of their size, nanoparticles have proportionally larger surface area and consequently more surface atoms than their microscale counterpart. In the nanoscale range, materials may present different electronic properties, which in turn affects its optical, catalytic and other reactive properties (Boccuni et al., 2008, Kahn, 2006).
All biological and man-made systems have the first level of organization at the nanoscale. By using nanotechnology techniques, it is possible to assemble molecules into objects, along several length scales, and to disassemble objects into molecules, as nature already does (Roco, 2003).
Two building strategies are currently used in nanotechnology: a “top-down” approach and the “bottom-up” approach. The commercial scale production of nanomaterials currently involves basically the “top-down” approach, in which nanometric structures are obtained by size reduction of bulk materials, by using milling, nanolithography, or precision engineering. Size usually relates to functionality of food materials, smaller sizes meaning a bigger surface area, desirable for several purposes. The newer “bottom-up” approach, on the other hand, allows nanostructures to be built from individual atoms or molecules capable of self-assembling (Föster and Konrad, 2003, Moraru et al., 2003). Self-assembly relies on balancing attraction and repulsion forces between a pair of molecules as building blocks to form more functional supramolecular structures (Sanguansri & Augustin, 2006).
Nowadays, most materials used for food packaging are practically undegradable, representing a serious global environmental problem. New bio-based materials have been exploited to develop edible and biodegradable films as a big effort to extend shelf life and improve quality of food while reducing packaging waste (Tharanathan, 2003). However, the use of edible and biodegradable polymers has been limited because of problems related to performance (such as brittleness, poor gas and moisture barrier), processing (such as low heat distortion temperature), and cost. Starch, as an example, has received considerable attention as a biodegradable thermoplastic polymer. However, it has a poor performance by itself because of its water sensitivity and limited mechanical properties (Vaidya & Bhattacharya, 1994) with high brittleness, which is related to the anarchical growth of amylose crystals with time (Dufresne & Vignon, 1998). The application of nanotechnology to these polymers may open new possibilities for improving not only the properties but also the cost-price-efficiency (Sorrentino, Gorrasi, & Vittoria, 2007).
Several composites have been developed by adding reinforcing compounds to polymers to enhance their thermal, mechanical and barrier properties. Most of these reinforced materials present poor interactions at the interface of both components. Macroscopic reinforcing components usually contain defects, which become less important as the particles of the reinforcing component are smaller (Ludueña, Alvarez, & Vasquez, 2007).
Polymer composites are mixtures of polymers with inorganic or organic fillers with certain geometries (fibers, flakes, spheres, particulates). The use of fillers which have at least one dimension in the nanometric range (nanoparticles) produces polymer nanocomposites (Alexandre & Dubois, 2000). Three types of fillers can be distinguished, depending on how many dimensions are in the nanometric range. Isodimensional nanoparticles, such as spherical silica nanoparticles or semiconductor nanoclusters, have three nanometric dimensions. Nanotubes or whiskers are elongated structures in which two dimensions are in the nanometer scale and the third is larger. When only one dimension is in the nanometer range, the composites are known as polymer-layered crystal nanocomposites, almost exclusively obtained by the intercalation of the polymer (or a monomer subsequently polymerized) inside the galleries of layered host crystals (Alexandre & Dubois, 2000).
A uniform dispersion of nanoparticles leads to a very large matrix/filler interfacial area, which changes the molecular mobility, the relaxation behavior and the consequent thermal and mechanical properties of the material. Fillers with a high ratio of the largest to the smallest dimension (i.e., aspect ratio) are particularly interesting because of their high specific surface area, providing better reinforcing effects (Azizi Samir et al., 2005, Dalmas et al., 2007, Dubief et al., 1999). In addition to the effects of the nanoreinforcements themselves, an interphase region of altered mobility surrounding each nanoparticle is induced by well dispersed nanoparticles, resulting in a percolating interphase network in the composite and playing an important role in improving the nanocomposite properties (Qiao & Brinson, 2009). According to Jordan, Jacob, Tannenbaum, Sharaf, and Jasiuk (2005), for a constant filler content, a reduction in particle size increases the number of filler particles, bringing them closer to one another; thus, the interface layers from adjacent particles overlap, altering the bulk properties significantly.
Besides reinforcing nanoparticles, whose main role is to improve mechanical and barrier properties of the packaging materials, there are several types of nanostructures responsible for other functions, sometimes providing active or “smart” properties to the packaging system such as antimicrobial activity, enzyme immobilization, biosensing, etc. The most studied nanoparticles will be presented according to their primary functions/applications in food packaging systems. Some particles can have multiple applications, and sometimes the applications can overlap, such as some immobilized enzymes which can act as antimicrobial components, oxygen scavengers and/or biosensors.
Section snippets
Structure, properties and types of composites
Although several nanoparticles have been recognized as possible additives to enhance polymer performance, the packaging industry has focused its attention mainly on layered inorganic solids like clays and silicates, due to their availability, low cost, significant enhancements and relative simple processability. The concept of polymer–clay nanocomposites (PCN) was developed in the late 1980s, and firstly commercialized by Toyota (Collister, 2002), but only since the late 1990s researches have
Antimicrobial nanocomposites
The incorporation of antimicrobial compounds into food packaging materials has received considerable attention. Films with antimicrobial activity could help control the growth of pathogenic and spoilage microorganisms. An antimicrobial nanocomposite film is particularly desirable due to its acceptable structural integrity and barrier properties imparted by the nanocomposite matrix, and the antimicrobial properties contributed by the natural antimicrobial agents impregnated within (Rhim & Ng,
Oxygen scavenging films
Oxygen (O2) is responsible for the deterioration of many foods either directly or indirectly. Direct oxidation reactions result in browning of fruits and rancidity of vegetable oils, to name a few examples. Food deterioration by indirect action of O2 includes food spoilage by aerobic microorganisms. The incorporation of O2 scavengers into food package can maintain very low O2 levels, which is useful for several applications. Attention has particularly focused on the photocatalytic activity of
Nanoscale enzyme immobilization systems
Enzymes are widely used by food industry for several applications. In some cases, the direct use of enzymes can be restricted by their sensitivity to processing conditions and/or to compounds which can inhibit their action, resulting in short operational life or inactivation. When immobilized in different tailored carriers, the enzymes can have improved stability to pH and temperature, resistance to proteases and other denaturing compounds, as well as an adequate environment for their repeated
Final considerations
The use of biopolymers by the food industry has faced feasibility problems related mainly to their relatively high cost and poor overall performance when compared to those of synthetic polymers. However, since industries are concerned with sustainable development, the production cost of biopolymers has decreased, allowing biopolymer-based materials to be increasingly developed. More important, nanocomposites promise to expand the use of edible and biodegradable films, since the addition of
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