PhD thesis of Noura Elmnasser (2004-2007)

Listeria monocytogenes is an ubiquitous bacterial species known to be a major hazard in food safety. At first, we have assessed the behaviour of 3 strains of L. monocytogenes in relation to cold, saline and acid stress in food system and in laboratory conditions. Results of this study indicate that L. monocytogenes is capable of survival but not growth on the surface of frozen raw salmon for a period of at least 10 months. Nevertheless, freezing is a way of reducing the linked risk Listeria for the seafood products industry. These 3 strains are able to grow in the presence of high concentrations in salt (10 % NaCl) in neutral and acid pH at 37, 25 and 4°C. Then, the effects of broad spectrum pulsed light on the survival of L. monocytogenes, Ps. fluorescens and Ph. phosphoreum populations on agar, in a liquid medium and on food were investigated. The present study clearly demonstrates the potential of pulsed light for bacterial inactivation. Complete inactivation of these bacteria can be achieved within a few microseconds. The microbicidal effect depends on the position of the microbes, so that any shadowing of target cells must be avoided. A solution of DNA was used to demonstrate DNA strand-breaking after pulsed light treatment. The influence of pulsed light treatment on protein components of milk was evaluated by means of spectroscopy ultraviolet, spectrofluorimetry, electrophoresis and HPLC. The pulsed light treatment did not produce any significant changes of protein components.