This technote compares the bacterial disinfection efficacy of UV-LED lamps with that of UV discharge lamps, specifically mercury lamps.
UV discharge lamps have been widely used for disinfecting fruit, vegetables, processed food, and equipment surfaces. The UVC region of the spectrum (200-280 nm) is the most effective for killing bacteria, as the bacteria’s DNA has a peak absorption at 265 nm. The most commonly used UV discharge lamp is the Mercury Discharge Lamp, which has a high-intensity UVC radiation at 254 nm. Although the peak emission of a mercury lamp does not precisely coincide with the peak absorption of the bacteria's DNA, it is sufficiently close that it could inflict severe damage and destroy them. However, there are some limitations for the mercury lamp, which are listed as follows:
· The mercury lamp envelope is fragile and could break if a shock is inflicted. In that case, the toxic mercury would be released into the environment.
· The mercury lamp does not emit well under cold temperature conditions, and therefore, the irradiance decreases at lower temperatures.
· There is a long warm-up time for the mercury lamp, and it does not immediately reach maximum power as soon as it is turned on.
· As stated earlier, the wavelength of emission is limited to 254 nm, which is not at the peak absorption of the bacteria’s DNA as determined by the germicidal effectiveness curve.
A suitable alternative to a mercury discharge lamp for germicidal disinfection is a UV LED. The UV LED is a p-n junction device, and the recombination of electron-hole pairs at the junction emits photons. Depending on the p-n junction design, different wavelengths can be emitted. The UV LED will have the following advantages over the mercury discharge lamp:
· UV LEDs could be designed to have a peak emission closer to the germicidal absorption peak. They will be more effective for disinfection purposes.
· UV LEDs’ output is not temperature-dependent
· UV LEDs have tiny sizes and are not fragile.
· UV LEDs do not contain toxic mercury
· UV LEDs emit as soon as they turn on and don’t have a warm-up time.
· For equal power, the UV LED produces more irradiance at the same distance as compared to a Mercury lamp.
In one research study, three different types of bacteria, namely E. coli, Salmonella, and Listeria, were grown in agar media and were subjected to illumination by four UV LEDs at various wavelengths and one mercury discharge lamp at fluence levels of 0.1, 0.2, 0.5, and 0.7 mJ/cm2. The UV LED irradiance was 4.5 mW/cm2, so the intense mercury lamp needed to be attenuated using 52 polypropylene thin sheets by a factor of nearly 200 to bring its irradiance down to 4 mW/cm2. The three bacterial cultures, grown in petri dish agar media, were illuminated with these LEDs and the mercury lamp at four different fluences (as mentioned above), and the reduction in bacterial concentration was measured on a log scale.
E. coli and Salmonella both show an OD6 reduction at 0.7 mJ/cm2, and Listeria shows an OD5 reduction at the exact dosage, as observed in the graphs. The attenuated mercury lamp in each case has a much lower reduction. Both E. coli and Salmonella are gram-negative types of bacteria, whereas Listeria is a gram-positive bacterium. Studies have shown that gram-positive bacteria require a higher dosage to be killed than gram-negative bacteria, since they are more resistant to UV irradiation2. The main reason the mercury lamp is not showing the same level of efficacy in germicidal disinfection is the inverse-square law attenuation of the radiation, compared to the more directional nature and greater concentration on the target area of the UV LED.
The same research work was also conducted on sliced cheese measuring 85mm x 85mm x 2mm. It was found that under a dosage of 3 mJ/cm2, both e-coli and salmonella show a reduction of 4 to 5 log scale and the listeria shows a decrease of 3-4 log scale. Comparison was also made between the 266 nm LED and the 279 nm LED and it was found that for all three species of bacteria, the log kill was slightly higher for the 266 nm UV LED (E-coli: 4.88, Salmonella: 4.72, Listeria: 3.52) than the 279 nm LED ( (E-coli: 4.04, Salmonella: 3.91, Listeria 3.24).
Allied Scientific Pro is on a mission to use photonics technology to combat the pandemic and offers various disinfection lamps in its product line.
Several peak wavelengths are available, ranging from 222nm, 230nm, 254nm, 265nm, 275 nm, and 280nm. For more information, refer to the following link:
https://www.alliedscientificpro.com/shop/high-power-uv-led-365nm-uvee-portable-light-17575
References:
1- Using UVC Light-Emitting Diodes at Wavelengths of 266 to 279 Nanometers To Inactivate Foodborne Pathogens and Pasteurize Sliced Cheese, S.J.Kim et.al, Applied and Environmental Microbiology, Volume 82, Number 1, 2016.
2- Resistance of the genome of Escherichia coli and Listeria monocytogenes to irradiation evaluated by the induction of cyclo-butane pyrimidine dimers and 6-4 photoproducts using gamma and UVC radiations, S Beauchamp et.al, Radiat Physics Chem 81: 1193-1197, 2012