Pulsed (UV) Light
From Top Wiki
Pulsed light (PL) is a technique to decontaminate surfaces by killing MO’s using pulses of an intense broad spectrum, rich in UV-C light. UV-C is the portion of the electromagnetic spectrum corresponding to the band between 200 and 280 nm. PL is produced by storing electricity in a capacitor over relatively long times (fraction of second) and releasing it in a short time (millionths or thousandths of a second). The emitted light flash has a high peak power and consists of wavelengths from 200 to 1100 nm. This technique has, besides the high peak power, a greater relative production of light with shorter bactericidal wavelengths. The technique of using UV-C to preserve foods was discovered in the 1930s. PL is a modified and claimed improved version of delivering UV-C to bodies. PL works with Xenon lamps that can produce flashes several times per second. The following parameters characterize the PL treatment:
- Fluency rate (W/m2): the energy received from the lamp by the sample per unit area per second
- Fluency (J/M2): the energy received from the lamp by the sample per unit area during treatment
- Dose: synonym for Fluency
- Pulse width (s): Time interval during which energy is delivered
- Pulse-repetition-rate (Hz): number of pules per second, a.k.a. pps (pulses per second)
- Peak power (W): pulse energy divided by the pulse duration
Xenon flash lamps have an emission spectrum ranging from ultraviolet to infrared light. The UV-C part of the spectrum is the most important for microbial inactivation. Rowan et al. (1999) reported that the inactivation of food related MO´s was 5-6 log CFU/plate using a high UV flash (whereas with low UV-light only 1-2 log CFU/plate was achieved). The authors concluded that the rich UV content from 220 to 290 nm in the UV spectrum provides the major contribution to inactivation.
The lethal action of PL can be due to a photo thermal and/or a photochemical mechanism. It is possible that both mechanisms coexist, and the relative importance of each on would depend on the influence and target MO’s. But most of the inactivation of MOs is probably due to the photochemical mechanism.
The mechanism of inactivation by PL is explained based on studies using CW (continuous) UV, in which the inactivation is photochemical. The germicidal effect (so killing of MO’s) of UV light on bacteria is primarily due to the formation of pyrimidine dimers, mainly thymine dimers. The dimers inhibits the formation of new DNA chains in the process of cell replication, thus resulting in the inactivation of affected MO by UV. On spores, UV-C treatment results mainly in the formation of the “spore photoproduct” 5-thyminyl-5, 6-dihydrothymine, and in single-strand breaks, double-strand breaks and cyclobutane pyrimidine dimers. There is also evidence that the photo thermal effect can occur. Hiramoto (1944) proposed that the rays absorbed into A.niger are expected to heat the mold instantaneously, providing a sort of thermal sterilization. Not much research is done on the photo thermal mechanism.
The shape of inactivation curve is sigmoid. The initial plateau occurs due to injury phase, when max injury is reached, minimal addition of UV would be lethal for MO’s, and so a quick decrease in amount of living cells (exponential phase). The end of curve has a tailing phase that has received several explanations:
- Lack of homogeneous population
- Multi-hit phenomena
- Presence of suspended solids
- Use of multiple strains
- Varying abilities of cells to repair DNA mutations
- Shading effects (places were no UV can reach the MO)
The Bunson-Roscoe reciprocity law is valid. It says that the effectiveness of radiation does not matter whether the fluency is reached with high fluency rate and short exposure time, or with low fluency rate and long exposure time.
Most important factor determining the effect of PL is the fluency incident on the sample. The energy emitted by the xenon flash lamp is different from the energy incident on the sample.
- Propagation vehicle (air/water/fruit juice), affect level of energy that ultimately reaches the target. Inactivation is better when samples are closer to the lamp.
- Sample thickness is another limiting factor for microbial inactivation with PL.
- Decontamination efficacy decrease at high contamination levels, which is also related to light. attenuation.
- With regard to the long term applicability of PL, the possible development of resistant strains should be taken into account.
Currently, the only known industrial application is the use of PL in the sterilization of packaging material.
Ozone might have the potential to extent the storage life of fruits and vegetables. The effect on shelf-life and quality of the product depends however on the kind of fruit or vegetable. A study of Keutgen and Pawelzik 2007 shows that exposure to ozone decreases the level of vitamin C, increases lipid peroxidation and lowers sweetness in strawberries. The advantage however is that no residue is left on the product. The advantages and disadvantages per product therefore have to be investigated.
Pros and cons
Pros Pulsed UV source may provide practical advantages over CW UV sources in situations where rapid disinfection is required. Rice and Ewell (2001) needed 3 h to deliver 10^4 J/m2 using a CW UV lamp and 40 s to deliver the same total fluency using a laser with a repetition rate of 10 Hz. Other advantages of PL treatment are:
- The lack of residual compounds
- Absence of chemicals that can cause ecological problems (like chloride) and/or are potentially harmful
- Xenon flash lamps are also more environment friendly then CW EV lamps, because they do not use mercury
- Sample heating is perhaps the most important limiting factor of PL for practical applications
- Another disadvantage of PL is the possibility of shadowing. (In order for a PL treatment to inactivate MOs contact between protons and MOs)
- Possibility that MOs repair themselves (photo reactivation, dark repair)
- Regarding fluid foods, and MO suspensions, the liquid will absorb light depending on its absorption coefficient and depth
Examples of companies that are selling Pulsed Light equipment are Claranor (FR) and Loehrke GmbH (DE).