Relationship between inactivation kinetics of a Listeria monocytogenes suspension by chlorine and its chlorine demand R. Virto, D. Sanz, I. A ´ lvarez, S. Condon and J. Raso Tecnologı´a de los Alimentos, Facultad de Veterinaria, Universidad de Zaragoza, Zaragoza, Spain 2004/0351: received 29 March 2004, revised 28 June 2004 and accepted 6 July 2004 ABSTRACT R. VIRTO, D. SANZ, I. A ´ LVAREZ, S. CONDON AND J. RASO. 2004. Aims: Chlorine demand by Listeria monocytogenes cells and inactivation of L. monocytogenes by chlorine (0Æ6– 1Æ0 mg l )1 ) at different temperatures (4, 20 and 30°C) have been investigated in a batch reactor. Methods and Results: Chlorine demand depended on the microbial concentration and was independent on the initial chlorine concentration and temperature. Chlorine decay was modelled by the addition of two first-order decay equations. Inactivation of L. monocytogenes by chlorine depended on the initial microbial concentration, initial chlorine concentration and temperature. A mathematical model based on a biphasic inactivation properly described survival curves of L. monocytogenes and a tertiary model was developed that satisfactorily predicted the inactivation of L. monocytogenes by different concentrations of initial chlorine at different temperatures. Conclusions: Both available chlorine decay and inactivation of L. monocytogenes by chlorine were biphasic and can be modelled by a two-term exponential model. Significance and Impact of the Study: The biphasic nature of survival curves of L. monocytogenes did not reflect the effect of a change of available chlorine concentration during the treatment. The microbial inactivation was caused by successive reactions that occur after the consumption of the chlorine by the bacterial cell components. Keywords: chlorine, Listeria monocytogenes, inactivation kinetics, sanitation. INTRODUCTION Chlorine is widely used for microbial control in water and wastewater as well as to sanitize surfaces in food-processing environments. In addition, aqueous chlorine is used to reduce microbial population on raw fruits and vegetables and on carcasses of slaughter animals after evisceration (Kotula et al. 1997; Davidson and Harrison 2002). The mechanisms of action of chlorine on micro-organisms have been widely investigated (Dukan and Touati 1996). Chlorine is considered to be a nonselective oxidant that reacts avidly with a variety of microbial compounds and affects metabolic processes of micro-organisms. Chlorine acts on microbial membranes altering their permeability, which leads to leakage of intracellular contents such as proteins and nucleic acids (Venkobachar et al. 1997). However, chlorine inactivates enzymes, inhibits membrane transport and causes lethal DNA damage (Thomas et al. 1987; Barrette et al. 1989). The bactericidal effect of chlorine is well recognized and widely exploited. However, several drawbacks of using chlorine as a sanitizer have been identified including production of off-tastes and odours in water and foods, formation of undesirable chloro-organic compounds with carcinogenic potential or variability on the resistance of microbial pathogens to chlorine inactivation (Richardson Correspondence to: Javier Raso, Tecnologı ´a de los Alimentos, Facultad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, 50.013 Zaragoza, Spain (e-mail: jraso@unizar.es). ª 2004 The Society for Applied Microbiology Journal of Applied Microbiology 2004, 97, 1281–1288 doi:10.1111/j.1365-2672.2004.02414.x