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

https://doi.org/10.1016/j.ijfoodmicro.2007.12.017Get rights and content

Abstract

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.

Introduction

Titanium dioxide (TiO2) is a photocatalyst and widely utilized as a self-cleaning and self-disinfecting material for surface coatings in many applications (Fujishima et al., 1999, Fujishima et al., 2000). The photocatalytic reaction of TiO2 has been used to inactivate a wide spectrum of microorganisms (Matsunaga et al., 1988, Fujishima et al., 1999, Kim et al., 2003, Duffy et al., 2004, Maneerat and Hayata, 2006). The first work on the microbiocidal effect of TiO2 photocatalyst was carried out with Escherichia coli in water (Matsunaga et al., 1985). These authors reported that E. coli was killed by contact with a TiO2 photocatalyst upon illumination with light. Hydroxyl radicals (•OH) and reactive oxygen species (ROS) generated on the illuminated TiO2 surface play a role in inactivating microorganisms by oxidizing the polyunsaturated phospholipid component of the cell membrane of microbes (Saito et al., 1992, Fujishima et al., 1999, Maness et al., 1999, Huang et al., 2000, Kuhn et al., 2003, Cho et al., 2004). OH radicals are approximately one thousand or possibly ten thousand times more effective for E. coli inactivation than common disinfectants such as chlorine, ozone and chlorine dioxide (Cho et al., 2004).

TiO2 is non-toxic and has been approved by the American Food and Drug Administration (FDA) for use in human food, drugs, cosmetics and food contact materials. Currently there is considerable interest in the self-disinfecting property of TiO2 for meeting hygienic design requirements in food processing and packaging surfaces. Bactericidal and fungicidal effects of TiO2 on E. coli, Salmonella choleraesuis, Vibrio parahaemolyticus, Listeria monocytogenes, Pseudomonas aeruginosa, Stayphylococcus aureus, Diaporthe actinidiae and Penicillium expansum have been reported (Matsunaga et al., 1985, Matsunaga et al., 1988, Wei et al., 1994, Kikuchi et al., 1997, Horie et al., 1998, Sunada et al., 1998, Maness et al., 1999, Choi and Kim, 2000, Wist et al., 2002, Kim et al., 2003, Cho et al., 2004, Hur et al., 2005, Maneerat and Hayata, 2006). Application of TiO2 photocatalytic disinfection for drinking water production was investigated in Wist et al. (2002). The development of TiO2-coated or -incorporated food packaging and food preparing equipment has also received attention.

The aim of the present study was to develop TiO2-coated oriented-polypropylene (OPP) film, clarify its antimicrobial activity and causative factors and assess its potential use in food packaging. The TiO2-coated OPP packaging films were developed with two different particle sizes of TiO2 powder. The antimicrobial capacity of TiO2-coated packaging film against E. coli was examined both in vitro and in actual tests under two kinds of artificial light.

Section snippets

Preparation of TiO2-coated packaging film

Nanoparticle TiO2 powder (anatase-phase crystal structure with a 7-nm particle size) from Ishihara Sangyo Company, Ltd., Tokyo, Japan, and microparticle TiO2 powder (anatase-phase crystal structure with a 5-μm particle size) from Wako Pure Chemical Industries, Ltd., Tokyo, Japan, were used. An oriented-polypropyrene (OPP) plastic film with a thickness of 30 μm was obtained from Utsunomiya Pack Co., Ltd, Hiroshima, Japan. TiO2 powder was mixed with organic solvents (normally ethyl methyl ketone,

Results and discussion

The experiment on the antimicrobial activity of the TiO2-coated OPP film was carried out as shown in Fig. 1. The surviving number of E. coli cells on the surface of uncoated and TiO2-coated OPP film in the dark condition did not decrease after 3 h of experimental period (data not shown). This suggests that the antimicrobial testing method itself did not inactivate E. coli during the experiment. When exposed to black-light illumination, the TiO2-coated OPP film exhibited photocatalytic

Acknowledgments

This work was fully supported by the JSPS postdoctoral fellowship program for foreign researchers of the Japan Society for the Promotion of Science (JSPS). We are also grateful to the Utsunomiya Pack Company, Hiroshima, Japan, for the assistance in preparation of a TiO2-coated plastic film.

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