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

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.