Development of TiO₂ powder-coated food packaging film and its ability to inactivate Escherichia coli in vitro and in actual tests

Titanium dioxide (TiO₂) has attracted a great deal of attention as a photocatalytic disinfecting material in the food and environmental industry. TiO₂ has been used to inactivate a wide variety of microorganisms in many applications. In the present study, we aimed to develop a TiO₂ powder-coated packaging film and clarify its ability to inactivate Escherichia coli both in vitro and in actual tests, using two different particle sizes and two types of illumination at different intensities. No inhibition effect of the testing method itself on the growth of E. coli was observed. The cells of E. coli were found to have decreased 3 log CFU/ml after 180 min of illumination by two 20 W black-light bulbs (wavelength of 300-400 nm) on TiO₂-coated oriented-polypropylene (OPP) film, while E. coli decreased 1 log CFU/m with black-light illumination of uncoated OPP film. The results showed that both ultraviolet A (UVA; wavelength of 315-400 nm) alone and TiO₂-coated OPP film combined with UVA reduced the number of E. coli cell in vitro, but that the reduction of E. coli cell numbers was greater by TiO₂-coated OPP film combined with UVA. The antimicrobial effect of TiO₂-coated film is dependent on the UVA light intensity (0, <0.05 and 1 mW/cm²) and the kind of artificial light (black-light and daylight fluorescent bulbs), but it is independent of the particle size of TiO₂ coating on the surface of OPP film. The surviving cell numbers of E. coli on TiO₂-coated film decreased 3 log and 0.35 log CFU/ml after 180 min of illumination by two 20 W black bulbs and two 20 W daylight fluorescent bulbs, respectively. Despite the lesser efficacy of the photocatalytic method with fluorescent lights, the survival of E. coli cells using this method was 50% of that using fluorescent lights alone. In the actual test, the number of E. coli cells from cut lettuce stored in a TiO₂-coated film bag irradiated with UVA light decreased from 6.4 on Day 0 to 4.9 log CFU/g on Day 1, while that of an uncoated film bag irradiated with UVA light decreased from 6.4 to 6.1 log CFU/g after 1 day of storage. The result shows that the TiO₂-coated film could reduce the microbial contamination on the surface of solid food products and thus reduce the risks of microbial growth on fresh-cut produce.