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Title: Laser sterilisation of bacterial and fungal spores
Author: Armstrong, Graham N.
ISNI:       0000 0001 3428 2948
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2000
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This work investigated the sterilisation efficiency of the Nd:YAG and CO2 laser. This work centred on micro-organisms that were important to the food industry and resistant to sterilisation. The efficiency of laser sterilisation was ascertained and quantified. The efficiency of incorporating laser, UV irradiation and heating of bactericidal chemicals was also investigated. These results will aid the future development of novel sterilisation systems. The Nd;YAG laser was found capable of sterilising stainless steel. The addition of UV gave an added effect. Nd:YAG laser heating of hydrogen peroxide gave a marked improvement in the chemical's killing efficiency. Nd:YAG irradiation completely sterilised B. cereus spores on discs of stainless steel (mass 5g) with an energy density of 1800Jcm2. By decreasing the beam diameter, the percentage of the beam in contact with the spores increased from 43% to 72% but killing remained the same. Calorimetry showed that after exposing steel discs to 1800Jcm2 with a beam diameter of 16 and 8mm the temperature of the steel was the same. Possible killing mechanisms were looked into and it was found that for the Nd:YAG laser it was primarily thermal, and not due to direct photonic interaction. This hypothesis was supported by the killing of spores on 2mm thin steel discs (mass 0.8g) with an energy density 120Jcm2. This was due to the thin steel heating up rapidly with the relatively low energy density. Electron microscopy showed that the spores responded cytologically in the same way after heating by Nd:YAG laser irradiation and water-bath heating. It was attempted to increase the laser's killing effect by combining the laser with UV irradiation from 2, 8W UV lamps (254nm wavelength). Lawns of B. cereus spores were made and exposed to Nd:YAG laser irradiation (ED 3000Jcm2). No clearing was observed. By adding UV irradiation (sequentially) the diameter of clearance caused by the laser irradiation increased. To quantify the killing, aliquots of spore suspensions were raised in temperature with Nd:YAG laser irradiation to 90°C. There was little effect on the viability of the spores. When the treatment was preceded or followed by 10s of UV irradiation an increased killing effect was shown. To investigate the effect further, a multifactorial experiment was designed, incorporating laser followed by UV irradiation of spores on stainless steel discs. These results were analysed by the analysis of variance method. It was found that the laser and UV had a significant effect on the spores, and the effect was additive. Spores were submerged in sub-lethal concentrations of lactic acid and H2O2 and heated with Nd:YAG laser irradiation. There was a definite increased killing effect, and the rapid heating by the laser irradiation allowed low chemical concentrations and low temperatures to have a sporicidal effect. These experiments have shown that laser irradiation alone does have an effect on bacterial endospores, although at a reduced rate compared with vegetative cells. This killing affect can be increased using UV. A more efficient method to improve the laser killing effect was to combine the laser treatment with H2O2. The spores were then killed by laser irradiation at relatively low temperatures and with low chemical concentrations. The treatment of spores of fungal species was also examined. These bodies are more resistant than the vegetative cells of bacteria. The aim of this section was to investigate scanning of spores with CO2 laser irradiation. Lawns of spores on agar plates were scanned with IkW irradiation at various speeds, good clearing was obtained, but there was a significant difference between the clearance of different fungal species. Artificially contaminated fruit was scanned with the same laser parameters, but no killing was observed, there was no clearing from the surface of contaminated cheese. A low power CO2 laser was used and clearing of spores on agar with power as low as 6W (also species dependent) was achieved. The effects of this laser were also shown on cheese and 2W scans completely cleared its surface. In conclusion, spores of both fungal and bacterial origin were inactivated with laser irradiation, and increased killing of bacterial endospores was achieved after laser irradiation was combined with UV irradiation, lactic acid and H2O2. Promising aspects of this work include the success of the small scanning 13W CO2 laser system, especially the spiral system that could lead into many branches of experimentation e.g. surfaces and food, and the combination of laser heated H2O2 treatment. This combination facilitates not only sterilisation at lower energy densities, but also overcome any problems associated with shadowing. These techniques could be elaborated.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: UV irradiation