Effects of shading and growth phase on the microbial inactivation by pulsed light

Abstract

Cudemos E., Izquier A., Medina-Martínez M.S., Gómez-López V.M. (2013): Effects of shading and growth phase on the microbial inactivation by pulsed light. Czech J. Food Sci., 31: 189-193. Pulsed light is an emerging technology that kills microorganisms using pulses of an intense broad-spectrum light. This work aimed to determine the effect of population density and microbial growth phase on its microbicidal efficacy. To this, pseudomonas fluorescens cultures were grown, diluted to different population densities, flashed, plated, incubated, and enumerated, Also, cultures of p. fluorescens, Bacillus cereus, and saccharomyces cerevisiae were taken at different growth phases, diluted to the same population density, flashed, plated, incubated, and enumerated. Microbial inactivation was lower at high densities, probably as the consequence of the shading effect, and higher at the exponential phase. This study sets the background information useful for scientists and industrial implementation. The population density and growth phase must be taken into account in the planning experiments and comparing the literature. On the industrial scale, heavily contaminated solids are not suitable for pulsed light (PL) treatment; while liquids should receive several PL flashes under the flow conditions that assure that all microorganisms receive a PL treatment, that should be also designed in function of the growth phase of the microbial contaminant.Keywords: UV-C light; non-thermal methods; shading effect; decontamination; inactivation Pulsed light (PL) is a non-thermal emerging technology to decontaminate surfaces and transparent liquids by killing microorganisms using pulses of an intense broad-spectrum light where its UV-C part is the most lethal (Gómez-López et al. 2007).The shading effect is usually cited as a limiting factor of PL efficacy. It occurs when microorganisms are present one upon another, which causes that those at the top can be killed by PL but protect in turn the underlying microorganisms (Gómez-López et al. 2007). Its existence is claimed from the beginning of the use of PL as a germicidal method, however, just for specific microorganisms that readily absorb UV rays such as Aspergillus niger and not as a general limiting factor (Hiramoto 1984). Furthermore, the patent of Hiramoto (1984) also claimed that this phenomenon occurs with the conventional UV lamps and is overcome by the photothermal effect of PL. On the other side, Farrell et al. (2010) reported a population density effect in the inactivation curves of staphylococcus aureus and pseudomonas aeruginosa, where tailing was only observed at very high initial sizes.The susceptibility of microorganisms to inactivation by physical methods depends on several factors, which depend in turn on the specific method. The effect of the growth phase of the microor- 190Vol. 31, 2013, No. 2: 189-193 Czech J. Food Sci. PL equipment. PL processing was performed using a XeMaticA-1XL system (SteriBeam GmbH, Kehl, Germany), which had been described previously (I...

Inactivation of Polyphenoloxidase by Pulsed Light

“…In fact, the effect of light is known to be reduced by anything between target object and light source. For instance, microorganisms can be protected from the germicidal effect of light by other cells (Cudemos and others ). Analogously, proteins could protect each other from light according to local shadowing effects.…”

Recent findings in pulsed light disinfection

“…Farrell et al (2010) showed that the size of S. aureus and P. aeruginosa populations on an agar surface affect the inactivation efficiency, leading to lower reductions at a higher cell density due to shadow effects caused by cell clusters and multilayers. Similarly, G omez-L opez et al (2005a) and Cudemos et al (2013) reported that higher degrees of contamination reduce the PL efficiency. The importance of the distribution of micro-organism on a surface for PL applications has also been demonstrated by Levy et al (2011) for B. subtilis endospores, which were inoculated by airbrush spraying and drop inoculation respectively.…”

“…Cudemos et al . () likewise reported that Pseudomonas fluorescence , B. cereus and Saccharomyces cerevisiae are more resistant to PL in the stationary than in the latency or exponential growth phase. Besides the impact of the microbial growth stage, it is also important to take into account the sample handling after PL exposure.…”

“…() and Cudemos et al . () reported that higher degrees of contamination reduce the PL efficiency. The importance of the distribution of micro‐organism on a surface for PL applications has also been demonstrated by Levy et al .…”

Post-packaging application of pulsed light for microbial decontamination of solid foods: A review

“…Microbial contamination influences the efficacy of a PL treatment in respect of the microorganism, its physiological constitution, population density and the growth parameters (growth rate and lag time) (Augustin et al, 2011;Cudemos, Izquier, Medina-Martínez, & Gómez-López, 2013;Dunn et al, 1989). Some distinctions regarding the susceptibility of microorganisms to PL can be observed.…”