Aim: To determine the influence of several factors on the inactivation of micro-organisms by intense light pulses (ILP). Methods and Results: Micro-organisms on agar media were flashed 50 times under different conditions and their inactivation measured. Micro-organisms differed in sensitivity to ILP but no pattern was observed among different groups. Several enumeration methods to quantify the effect of ILP were investigated and showed relevant differences, shading effect and photoreactivation accounted for them, the strike method yielded the most reliable results. Higher decontamination efficiencies were obtained for Petri dishes located close to the strobe and inside the illumination cone. Decontamination efficacy decreased significantly at contamination levels >6·85 log₁₀. After 13 successive treatments, no resistance to ILP could be demonstrated. Media warming up depended on the distance from the strobe and the number of flashes. Conclusions: For an industrial implementation: the position and orientation of strobes in an unit will determine the lethality, products should be flashed as soon as possible after contamination occurs, a cooling system should be used for heat-sensitive products and flashed products should be light protected. No resistant flora is expected to develop. Significance and Impact of the Study: Conclusions derived from this work will allow a better implementation of this decontamination technique at industrial level.

The two major fumonisin-producing Fusarium species are Fusarium verticillioides and Fusarium proliferatum. The growth and fumonisin production of these two isolates on corn was studied at water act...

Aims: The effect of temperature (2–30°C), pH (4·8–7·4) and water activity (0·946–0·995) on the relationship between optical density (OD) at 600 nm and the plate count (CFU ml⁻¹) was investigated for Listeria monocytogenes. Methods and Results: Calibration curves, relating OD with plate counts, were collected by measuring the OD of consecutive one-half dilution series, before determining the cell density by classic plate count methods. The calibration curves were observed to be shifting in a parallel way, with increasing stress levels. Especially pH influenced the curve in a great extent, while the other variables were showing more synergetic effects. The reason for the shift was investigated by a microscopic viability test, showing a viability decrease with increasing stress levels, causing the shift of the calibration curve. In a last step a model was made describing the effect of environmental factors on the calibration curve, with different data transformations being tested. A polynomial equation was fitted to the data, taking into account a set of constraints to incorporate microbiological knowledge in the black box model. Hence, illogical interpolation results and overfitting of the data could be avoided. Conclusions: Different stress factors are affecting the relationship between the OD and the cell count of L. monocytogenes by lowering the cell viability. These effects could be modelled using a constrained polynomial model. Significance and Impact of the Study: The observed phenomena are important when calculating growth parameters, like growth rate and lag phase, based on OD data.