Heat inactivation of Listeria monocytogenes Scott A on potato surfaces

The aim of this work was to determine the death kinetics of Listeria monocytogenes Scott A, on potato surfaces (with and without skin), when treated with a surface pasteurisation method. The inoculation was done before any heat treatment was applied, to ensure that any death due to the treatment was recorded. In each case, some reduction in cell numbers was seen during the time to come-up to temperature, before the start of the timed phase of heating at the set temperature. This reduction was considerable at the higher temperatures reaching a maximum of 4.7 log reductions during slow (5.2 min) heat up to 100 degrees C. This is a relevant simulation of an industrial situation where natural contaminants would be exposed to increasing temperatures for various lengths of time during the heating up phase of a process, before the set temperature was achieved. In addition to this initial reduction in numbers, in all cases the number of cells remaining were reduced by all treatments as the temperature increased. It was apparent that moisture loss affected cell reduction in potatoes with the skin removed. For example, the z values calculated to achieve 6 log reductions against temperature were 18.7 degrees C and 17.2 degrees C for fast and slow heat up with skin on, however, they were 28.4 degrees C and 28.3 degrees C for fast and slow heat up respectively with skin off. The higher rate is indicative of lower moisture content, as would be expected without the presence of the skin to retain more moisture. As most contaminants are likely to be present on the skin of intact vegetables, it is encouraging that the rate of cell reduction on potato with skin on is of a manageable size relative to the typical temperature ranges likely to be used during processing. It is evident that over this temperature range effective decontamination to levels of greater than 6 log reductions of L. monocytogenes, can be achieved in 105, 16, 0.9 and 0.75 min dry heating at 60, 75, 90 and 100 degrees C respectively. If the lethal effect of the come-up time is included, a level of greater than 6 log reductions can be achieved in 61, 10, 0.23 and 0.2 min at 60, 75, 90, and 100 degrees C respectively.