Investigating effect and efficiency of short term heat stress applied in food stuff decontamination

Background. Foodborne pathogenic bacteria such as Listeria, Salmonella and Campylobacter represent an increasing health risk in foodstuffs as shelf life is increased and preparation temperatures and time are decreased. SonoSteam® is a thermal treatment, in which the effect of hot steam (> 90°C) and ultrasound are combined in order to rapidly and effectively reduce the amount of bacteria on the surfaces of foodstuffs. The technology is uncontroversial, environmentally friendly and non-invasive. However, the precise molecular effects on foodstuff and bacteria are largely unknown.

Objectives. Due to the very short treatment times employed (1-2 seconds), we hypothesize that surviving microorganisms may not perceive the applied stress as heat shock. We wish to identify the nature of the damage imposed on bacteria subjected to this type of treatment and investigate the bacterial stress recovery responses of surviving organisms. In addition the nature of the changes induced in the foodstuff model is investigated in order to reveal possible influence on microbial growth subsequent to treatment.

Methods. Because of their relevance in food industry, we employ Listeria and Salmonella as model bacteria and chicken broiler skin as food surface model. We employ advanced fluorescence based visualization methods including 2-photon excitation microscopy and second harmonic imaging microscopy for in-depth characterization of our surface model and of the damages and stress recovery strategies of our model bacteria.

Results. Preliminary investigations conclude that collagen denaturation quantified by SHG and calorimetry is extensive in the surface model subsequent to SonoSteam treatment, although visible changes in the surface are limited. In the bacteria model our results suggest that the response to the high temperature, short duration stress is not a typical heat shock response in bacteria but may involve the more general SOS response.