Abstract—Variations in the growth rate constant of the Listeria monocytogenes bacterial species were determined at 37°C in irradiated environments and compared to the situation of a non- irradiated environment. The bacteria cells, contained in a suspension made of a nutrient solution of Brain Heart Infusion, were made to grow at different frequency (2.30÷2.60 GHz) and power (0÷400 mW) values, in a plug flow reactor positioned in the irradiated environment. Then the reacting suspension was made to pass into a cylindrical cuvette where its optical density was read every 2.5 minutes at a wavelength of 600 nm. The obtained experimental data of optical density vs. time allowed the bacterial growth rate constant to be derived; this was found to be slightly influenced by microwave power, but not by microwave frequency; in particular, a minimum value was found for powers in the 50÷150 mW field. Keywords—Growth rate constant, irradiated environment, Listeria monocytogenes, microwaves, plug flow reactor. I. INTRODUCTION LECTROMAGNETIC radiation of frequency (ν) in the field between 2.30 and 2.60 GHz, although lacking a photon energy E (E=h·ν) high enough to be able to modify the atomic structure of components that are present in the irradiated environment, can still interact with supramolecular structures, such as cells, modifying their developmental characteristics. It can also interact with bacterial populations in the same way [1], [2]. Furthermore, since the existence of interactions between microwave (MW) radiation and the human body have been ascertained [3], [4], it may be thought that if microwaves can create an interference with extremely developed organisms and with the natural protection of the human body, they may interfere even more with microorganism populations which have far less natural protection than more evolved organisms. Therefore, the object of the present work was the study of the possibility of reducing the growth rate of the Listeria monocytogenes bacterial species, a common cause of food- borne infections, in a nutrient aqueous solution of Brain Heart Infusion at a temperature of 37°C; the microorganisms were made to grow in a PFR reactor irradiated with very low-power R. Carta is with the University of Cagliari, Department of Mechanical Chemical and Materials Engineering, Cagliari, Italy (phone: +39-070-675- 5068; fax: +39-070-675-5067; e-mail: renzo.carta@dimcm.unica.it). F. Desogus is with the University of Cagliari, Department of Mechanical Chemical and Materials Engineering, Cagliari, Italy (phone: +39-070-675- 5070; fax: +39-070-675-5067; e-mail: f.desogus@dimcm.unica.it). (P<400 mW) microwaves and at frequencies ranging between 2.30 and 2.60 GHz; similar studies have already been conducted with other bacterial species in similar experimental conditions [5]–[8]. Any reduction in the bacterial growth rate due to the use of low power MWs would have considerable importance, not only from a scientific point of view, as it would indicate the presence of the non-thermal effects of this type of radiation, but also from a technological one, since it would show the possibility of using low power MWs in food industry processes, thereby increasing the shelf-life of finished products while contemporaneously preserving their nutritional and organoleptic characteristics; in fact, the use of low power allows operation at low temperatures, both during production and in the storage phase, which preserves the afore-mentioned characteristics in the treated substances. This latter consideration has stimulated the authors’ interest towards the study of the variation of the bacterial growth rate constant in environments that are irradiated with low power MWs and at a temperature of 37°C: in fact, at this temperature it is reasonable to assume that all the nutritional and organoleptic properties of interest for humans are preserved. II. EXPERIMENTAL The nutrient solution was prepared by dissolving 3.7 g of Brain Heart Infusion (BHI) in 100 ml of distilled water; this aqueous solution, previously sterilized in an autoclave, was inoculated with 1 ml of a suspension of Listeria monocytogenes containing about 2 . 10 7 cells. The obtained reagent suspension was made to pass through a PFR reactor where it could react both in an irradiated environment and in a non-irradiated one. In order to avoid any external contamination of the reacting suspension at any time, the process was made to evolve in a specially designed system and completely isolated from the external environment. The bacterial concentration in the reacting suspension exiting the reactor was continuously determined spectrophotometrically, without taking samples; in fact withdrawal operations could be the cause of contamination. The experimental structure in use is schematically shown in Fig. 1. The Pyrex bottle (5) was filled with a nutrient solution containing 37 g/l of BHI and then hermetically sealed by means of the special cap (4) and connected to the circuit (part A in Fig. 1); all parts, together with the plate with the reactor, Renzo Carta, and Francesco Desogus The Influence of Low Power Microwave Radiation on the Growth Rate of Listeria Monocytogenes E World Academy of Science, Engineering and Technology International Journal of Environmental and Ecological Engineering Vol:6, No:12, 2012 796 International Scholarly and Scientific Research & Innovation 6(12) 2012 scholar.waset.org/1307-6892/5137 International Science Index, Environmental and Ecological Engineering Vol:6, No:12, 2012 waset.org/Publication/5137