Department of

Architectural Engineering

 


Pulsed Light & PEF

Pulsed White Light (PWL), also called Pulsed Light or Pulsed UV Light, involves the pulsing of a high-power xenon lamp for about 0.1-3 milliseconds per some sources (Dunn 1990, Rowan 1999, Johnson 1982), or about 100 microseconds to 10 milliseconds per other sources (Wekhof 2000). The spectrum of light produced resembles the spectrum of sunlight but is momentarily 20,000 times as intense (Bushnell et al. 1997). Figure 1 compares the spectrum of a single pulse of PWL with that of continuous sunlight at the earth's surface, however, since only broad spectrum UV light between 200-400 nm contributes to the disinfection effect, the comparison of solar and PWL spectra has only illustrative value. The spectrum of PWL includes a large component of ultraviolet light.

These high intensity flashes of broad spectrum white light pulsed several times a second can inactivate microbes with remarkable rapidity and effectiveness. The germicidal effect appears to be due to both the high ultraviolet content and the brief heating effects (Wekhof 2000), however, these systems can be tuned to produce pulsed light with different compositions. The Figure below compares two different pulses in which the frequency spectra have been shifted (Wekhof 2000). The brevity of the pulse assures no heating effects will occur on a macroscopic level.

Figure 2. (above) Spectra of a xenon flash lamp: 1- at a high current density of 6.500 kA/cm2, 2- at a low current density of approx. 1000 kA/cm2.

This technology is currently being applied in the pharmaceutical packaging industry where translucent aseptically manufactured bottles and containers are sterilized in a once-through light treatment chamber. The chamber generates a light intensity at the surface of the exposed containers of about 1.7 J/sq.cm., or 1.7 x E06 microWatt-s/sq.cm. Sunlight produces about 1359 Watts/sq.cm.

Only two or three pulses are sufficient to completely eradicate bacteria and fungal spores. Two pulses at 0.75 J/cm2 each were sufficient to sterilize plate cultures of Staphylococcus aureus from more than 7 logs of CFU (Dunn et al. 1997). Spores of Bacillus subtilis, Bacillus pumilus, Bacillus stearothermophilus, and Aspergillus niger were inactivated completely from 6-8 logs of CFU with 1-3 pulses (Bushnell et al. 1998). These results are depicted in Figure 3. One of the surprising aspects of PWL exposed cultures is that they exhibit no tailing to their survival curves (Dunn et al. 1997). In other words, there seems to be no innate capacity for resistance among segments of the microbial populations, unlike other inactivation mechanisms.

The exact mechanism by which PWL kills bacteria and spores appears to be due to the effects of UV combined with a new disinfection mechanism -- disintegration of the cell wall (Wekhof 2000). While UV causes damage to the nucleic acid and other components of the cell, the instantaneous heating of the cell results in the rupture of the cell wall, or lysing. This disintegrating effect has been demonstrated to occur in the absence of UV (Wekhof 2000, Dunn 2000).

A comparison of the disinfection rates due to PWL with the disinfection rates under UVGI exposure suggests that doses for sterilization by PWL are an order of magnitude lower than that for UV exposure (Wekhof 1991, Rowan et al 1999, Dunn 2000). Bacillus subtilis, for example is sterilized (99.999% disinfection) by about 42,600 microW-s/cm2 of UV while requiring a dose of only 4500 microW-s/cm2 under pulsed light. PWL clearly results in an apparent synergy of the pulsed energy quanta as compared to the relatively continuous stream of lower density UVGI quanta.

In terms of the dose for sterilization, PWL may represent the most efficient energy delivery mechanism to date. However, the generation of the pulse requires a considerable amount of energy, and some units requires external cooling. The power consumption for a typical pulsed light system is about 1000 W while similar results can be achieved with a UVGI system drawing only 10 W of total power. Applications are therefore limited to situations where the benefits of rapid sterilization outweigh the costs of pulse generation, as in the pharmaceuticals and health care industries.

Limited data on energy consumption is currently available for pulsed light technology, but one production unit uses four 14-inch Xenon gas lamps powered by a pulsing unit. An economics of use analysis for PWL in food applications estimates a cost of 0.1-0.4 cents/sq.ft. of irradiated surface area (Dunn et al. 1997).

This technology has also been applied to water systems, such as for the eradication of Cryptosporidium, and systems are currently available for such applications. Water may attenuate the effects to some degree, and PEF may more suitable for this application as it suffers less attenuation.

PEF involves the pulsing of an electric fields of about 4-14 kV/cm through a liquid medium. The result of this momentary field is a membrane potential across the bacterial cell wall of more than 1.0 V, which is sufficient to lyse or damage the cell irreparably. The inactivation of various microbes, including Escherichia coli, Lactobacillus brevis, Pseudomonas fluorescens, Bacillus cereus spores, and S. cerevisiae has been found to be dependent on field strength and treatment times that are unique to each species. Since this method has little effect on proteins, enzymes, or vitamins, it is perfectly suited for food processing where the liquid medium may be anything from boullion soup to milk.

PWL is a variation of pulsed electric field technology. Electric fields and light are both electromagnetic radiation, however, the mechanism of inactivation due to electric fields appears to be distinctly different. In addition, spores do not appear to be inactivated by pulsed electric fields.

PEF sterilization requires an electric fields of no less than 8 kV/cm. PEF exposure exhibits the characteristic survival tail and conforms to the standard logarithmic decay rate (death curve/survival curve) of microbes subjected to lethal mechanisms such as radiation, biocides, and heating.

There are currently two manufacturers of pulsed light technologies, PurePulse Technologies, Inc. of San Diego and Wek-Tec of Heilbronn, Germany.

References

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