Low levels of silver in food packaging materials may have no functional advantage, instead enhance microbial spoilage of food through hormetic effect

Recently there has been an influx of silver infused food packaging materials that claim to enhance the shelf-life of stored food. We tested commercially available normal plastic packing material (S1) and two packing materials (S2 and S3) which claimed to contain silver to enhance shelf-life of stored food for the presence of silver and its antibacterial performance. Microwave assisted acid digestion of the plastic material and ICP-OES analysis confirmed the presence of silver in S2 (103 mu g/g) and S3 (74 mu g/g) but not in S1. Migration studies showed that 12-14% of silver incorporated into the plastic could leach into milk (used as a food simulant). Shelf-life studies conducted using milk showed that in comparison to normal plastic materials (S1), those containing silver (S2 and S3) had no functional advantage but slightly enhanced the microbial growth. Further studies showed that sublethal concentration of silver generates reactive oxygen species (ROS) within bacterial cells (Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923) accompanied by increased cell proliferation and biofilm formation suggestive of a hormetic effect. In short, these studies showed that the incorporation of silver at low concentration in packaging materials may not provide a functional advantage, but mitigate the migration of silver into the stored food products at sub-lethal concentration and may induce bacterial hormesis and accelerate food spoilage.