ANTIMICROBIAL NANOMATERIALS FOR FOOD PACKAGING APPLICATIONS

Food packaging industry presents one of the fastest growing industries nowadays. New trends in this industry, which include reducing food as well as packaging waste, improved preservation of food and prolonged shelf-life together with substitution of petrochemical sources with renewable ones are leading to development of this industrial area in diverse directions. This multidisciplinary challenge is set up both in front of food and material scientists. Nanotechnology is recently answering to these challenges, with different solutions-from improvements in materials properties to active packaging solutions, or both at the same time. Incorporation of nanoparticles into polymer matrix and preparation of hybrid materials is one of the methods of modification of polymer properties. Nano scaled materials with antimicrobial properties can act as active components when added into polymer, thereby leading to prolonged protective function of pristine food packaging material. This paper presents a review in the field of antimicrobial nanomaterials for food packaging in turn of technology, application and regulatory issues.


INTRODUCTION
As human population is growing, changes in all spheres of life are inducing innovations through different solutions.Concept of bio economy as Europe's response to key environmental challenges to reduce the dependence on natural resources, transform manufacturing and promote sustainable production of renewable resources (land, fisheries and aquaculture) and their conversion into food, feed, fiber, biobased products and bio-energy (McCormick and Kautto, 2013).Food packaging is one of the most challenging research topics, concerning the balance between designing materials with specific properties and the demands of the packed food as well as satisfying four basic functions (protection and preservation, containment, convenience and marketing and communication) of food packaging (Sorrentino et.al., 2007;Petersen et.al, 1999).Adequate selection of food packaging materials as well as packaging conditions can contribute to a better sustainability of pa-cked food and reduce the total food waste (Marsh and Bugussu, 2007;Duncan, 2011).However, novel technologies are pushing forward new trends, and opening new chapters in food packaging research: active packaging as extension of protection and preservation, and intelligent packaging as extension of communication and marketing (Figure 1.) Development of nanotechnology has been growing in the last decade, opening the opportunity for innovation in many industrial sectors including food packaging.Nanotechnology presents the impeller of advanced food packaging technologies and gives optimal solutions that were not possible on micro-or macro-scales (Arora and Padua, 2010;Lagaron et.al. 2005, De Azeredo, 2009).Polymer nanocomposites present new class of composite materials with at least one component which is on nanodimension (between 1-100 nm).Nano technology is relying on incorporation of organic or inorganic nanoparticles into polymer matrix for improvement of material properties (mechanical, barrier, optical, thermal) (Duncan, 2011;De Azeredo, 2009;Silvestre et al., 2011).Small particle size and extremely high surface area of nanoparticles create large interfaces between nanofillers and polymer matrix and enhance the effect of particle-particle and/or polymer-particle interaction.This interaction promotes strong physical contact between the particles and polymer matrix causing the improvement in material properties.Depending on the shape, there are three types of nanoparticles: one-dimensional (layered), two-dimensional (nanotubes) or three-dimensional (spherical nanoparticles) (Silvestre et al., 2011).
Material properties are highly dependent on dispersion and distribution of nanoparticles in polymer matrix, and besides the particles behavior in different systems, preparation of nanocomposite films is also of great importance (Mihindukulasu-riya and Lim, 2014).
Besides nanofillers used to improve materials properties, nanoparticles also find application in nanocomposite packaging materials with antimicrobial properties.Nanocomposite packaging materials with antimicrobial functions have a high potential in active food packaging.Taking in consideration the sensitivity of food products to growth of divers' microorganisms, antimicrobial packaging systems present optimal solution for prolonged shelf-life influencing product quality and safety.Antimicrobial nanomaterials presents a part of active packaging concept designed to carry the active nanoparticles that can be integrated into a food package (Mihindukulasuriya and Lim, 2014).These antimicrobial systems are particularly effective, because of the high surface-to-volume ratio and enhanced surface reactivity of the nanosized antimicrobial agents, making them able to inactivate microorganisms more effectively than their micro-or macro-scale features.Commonly used or tested antimicrobial nanocomposite materials include metal ions (silver, copper, gold, platinum), metal oxide (titanium dioxide, zinc oxide, magnesium oxide), and organically modified nanoclays.In real systems different combinations of antimicrobials are used by incorporating into packaging materials thus giving the best results in combination of their activity (de Azeredo, 2013).

ANTIMICROBIAL NANOCOMPOSITE SYSTEMS
Metal ions are usually used as nanoparticles incorporated into different polymer systems.Antimicrobial activity of metal ions when they are on nanodimension is accelerated and can be more effective against pathogen microorganisms.Antimicrobial activity of silver ions can be assigned to their ability to disrupt both inner and outer cell membranes.They can also inhibit respiratory chain enzymes and reduce the ATP levels (Li et al., 2017;Mihindukulasuriya and Lim, 2014).Recent researches on antimicrobial activity of different nanoparticles are summarised in Table 1.
Turalija et al. ( 2016) analysed the influence of silver nanoparticles as well as chitosan as antimicrobial agents in composite structure with PLA, however, this group also used plasma treatment for surface modification of polymer matrix as activation of polymer surface and antimicrobial components.This system is a very good example of synergetic approach where more than two influences are included for improvement of material properties and also creating active packaging solution.EFSA Nanonetwork is a network established for risk assessment of nanotechnology in food and feed sector responsible for harmonisation of methodlogies and practise (practical recommendations on how to assess applications of engineered nanomaterials (ENMs) in industry as food additives, enzymes, flavorings, food contact materials, novel foods, food supplements, feed additives and pesticides) (EFSA, n. d.).The latest priority topics of EFSA (EFSA committee in 2016-2018) are considering nanomaterials in the section 3.2.3 and food contact materials are also included in this section.The guidance update should take into account the general extensions needed to cover also nanopesticides and nanoformulations, food contact materials, food and feed additives and novel foods; as well an update of the physico-chemical property measurements and the other data needed for food/feed assessment.In addition, a second guidance document should be produced on the environmental risk assessment for nanoparticles used in the food chain (EFSA, 2016).
European Chemical agency (ECHA) established a specific working group "Nanomaterials" for scientific and technical discussion answering the question related to nanomaterials under REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) and CLP (Classification, Labeling and Packaging of Substances and Mixtures).
Regulatory issues on nanotechnologies in diverse sectors are also changing and developing in other countries then EU (USA, Canada, Australia and New Zealand, Russia, Africa, South Africa), and are regulated by their national legislations (Amenta et al., 2015).General frameworks in the EU are applicable to the antimicrobial systems as well.
The growth of nanotechnology will produce new products on the global market, so the evolution of the legislation is also expected.Further increase in this area will require more detailed legislation, but until then, every system will be studied on case-by-case scenarios.This approach is not the most efficient one from the safety and risk assessment point of view.For the future perspective legislation in food contact nanomaterials will require also harmonization of national with international legislative and more detailed analysis of various systems.

CONCLUSION
Antimicrobial food packaging is in focus of multidisciplinary scientific networks be-cause it can answer more easily on various challenges that are placed in front food scientists and material scientist as well.Development of these systems requires multidisciplinary approach thus teams of scientist are working on research in this area.There are different studies on different nanoparticles incorporated into various polymeric matrices, and results are indicating a great potential of antimicrobial nanocomposite system for prolonging the shelf-life and preservation of different food stuff.However, it is clear that this part of active packaging solution is still in the developing phase, because there are not so many studies on the real food systems.It is expected that the future research will provide more in vivo studies and real products on the markets world-wide.
Besides the great potential, antimicrobial nanocomposite systems need public acceptance.Therefore legislation that regulates nanomaterials for food contact materials is in focus of global international bodies responsible for improvements in legislative, aiming to provide sufficient documents and public acceptance on nanocomposite materials for food packaging application.

АCKNOWLEDGEMENTS
This work is based upon work from COST Action FP1405 ActInPak, supported by COST (European Cooperation in Science and Technology).

Table 1 .
Summary table of antimicrobial packaging systems with different nanoparticles