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Despite significant advances made towards a better understanding of bacterial transmission and pathogenicity in foods, the incidence of reported food-borne illnesses associated with bacterial enteropathogens continues to be a major problem in the United Kingdom (5) and North America (7). It is generally accepted that contamination of both unprocessed and uncooked foods with pathogenic bacteria is a major source of concern, and any method of either reducing or eliminating food contamination will have a significant effect on the incidence of food-borne disease (5). A possible approach to reducing the level of microbial contamination on food surfaces in slaughterhouses, and in other food preparation environments, is UV irradiation (1-3, 5, 8, 9). Stermer et al. (8) indicated that the bacterial load on fresh meat can be effectively reduced by UV irradiation, while Wallner-Pendleton and coworkers (9) showed that this method of disinfection reduced Salmonella surface contamination without adversely affecting poultry carcass color or increasing meat rancidity. These studies indicate that if an effective and economic method of UV generation can be developed, then UV irradiation may have a practical application in the disinfection of food and contact surfaces.

While conventional alternating-current systems produce light with a power dissipation in the range of 100 to 1,000 W per device, a pulse power energization technique (PPET) can dissipate many megawatts of electrical power in the light source (4). PPET also tends to produce a greater intensity of the shorter bactericidal wavelengths of light, and, by using this approach, it is possible to design an extremely short energization time of the light source (~100 ns). For modest energy input (e.g., 3 J), this results in high peak power dissipation (~35 MW). Here we present evidence that PPET may lend itself to surface disinfection since it significantly reduces large populations of various food-related microorganisms on laboratory-based media.

The effectiveness of PPET, with two different light sources, in reducing predetermined microbial numbers on agar surfaces was determined by using a variety of proven bacterial pathogens, namely, Escherichia coli NCTC 12079 (serotype O157:H7), Listeria monocytogenes NCTC 11994 (serotype 4b), diarrheagenic Bacillus cereus NCTC 11145, Salmonella enteritidis NCTC 4444, Staphylococcus aureus NCTC 4135, Pseudomonas aeruginosa NCTC 8203, and the yeast Saccharomyces cerevisiae NCTC 10716, obtained from the National Collection of Type Cultures, Colindale, London, United Kingdom. Bacterial cultures were grown at 30°C and maintained on tryptone soya agar supplemented with 0.6% yeast extract (TSYEA). The yeast culture was grown at 25°C and maintained on malt extract agar supplemented with 0.3% yeast extract, 1% glucose, and 0.3% mycological peptone (MYGPA). Analysis of variance, balanced model (Minitab software release 11; Minitab Inc., State College, Pa.), was used to compare the effects of pulsed-light irradiation, number of pulses applied, and the type of microorganism treated. The studies were performed in quadruplicate, and all significant differences are reported at 95% (P < 0.05) and 99.9% (P < 0.001) confidence intervals.

Pulsed-light inactivation of food-related microorganisms. The bacterial test strains were inoculated into 100 ml of tryptone soya broth supplemented with 0.6% yeast extract (TSYEB) and cultivated on a shaker at 125 oscillations per minute to a population density of ~10⁹ cells ml¹ (confirmed via plate counts). A 0.1-ml aliquot of a 10⁵ dilution of this culture (the diluent used was 0.01 M sodium phosphate [pH 7.2]-0.15 M NaCl) was transferred to 100 ml of fresh TSYEB. The bacterial test strains were again grown to a density of ~10⁹ cells ml¹. S. cerevisiae was grown to a cell density of ~10⁹ cells ml¹ (confirmed via plate counts) in 100 ml of malt extract broth supplemented with 0.3% yeast extract, 1% glucose, and 0.3% mycological peptone (MYGPB) at 25°C. Ten samples, each containing 20 µl of a bacterial or yeast test culture, were surface inoculated on separate TSYEA or MYGPA plates, respectively, by using the Miles and Misra method (6).

View larger version (50K): [in this window] [in a new window]   FIG. 1.   Schematic layout of experimental facility for microbial inactivation with a pulsed-light source.

View larger version (18K): [in this window] [in a new window]   FIG. 2.   Emission spectra (range, 200 to 900 nm) from two different light sources; one shows a low UV content (A), and the other shows a higher UV content (B).

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View larger version (12K): [in this window] [in a new window]   FIG. 3.   Pulsed-light inactivation of surface-inoculated E. coli using two light sources which contained either a low- or high-UV content.