report
2.4.3.2 Antimicrobial and Antimycotic Packaging

In addition to acting as a passive barrier, packaging can contribute to the control of microbial growth in foodstuffs that leads to spoiling.  Most activities to combat this, have centred around nanoparticulates of silver, and zinc oxide, however there is also research into the antimicrobial effects of natural biological compounds[i].

Silver nanoparticles have been incorporated in a wide variety of consumer goods including clothing, electrical goods, kitchenware, and wound dressings[ii].  Nanoparticulate silver releases ions more efficiently than bulk metal, and it is the silver ions that have a bactericidal due to the inhibition of a wide variety of biological processes within the bacteria[iii].  As the levels of silver ions liberated are too low to have toxic effects in humans; it is likely that nanoparticulate silver will be included in further composite materials.  However, there is some concern over the effects of large amounts of silver ions being discharged into the environment and accumulating in ecosystems, as silver ions are known to be toxic to aquatic life.

Zinc oxide exhibits antibacterial activity that increases with decreasing particle size[iv].  This activity does not require the presence of UV light (unlike titanium dioxide) but is stimulated by visible light[v].  The exact mechanism(s) of action is still unknown.  Zinc oxide nanoparticles have been incorporated in a number of different polymers including polypropylene[vi].  In addition zinc oxide effectively absorbs UV light, without re-emitting as heat, and therefore improves the stability of polymer composites.

Chitosan is a biopolymer derived from chitin (a polysaccharide constituent of crustacean shells).  It has seen much interest in recent years as a material for the encapsulation of nutriceuticals (see report ‘Food Processing and Functional Food’).  In addition, to its utility as a packaging material, it also exhibits antimicrobial properties[vii].  This has led a number of groups to investigate its incorporation into different composite materials which could have applications in healthcare and food packaging, including using it as a ‘green’ reagent to reduce and stabilise silver ions[viii], in combination with clays such as rectorite which could then be used in polymer composites[ix]^,[x].