M: Food Microbiology & Safety Cold Plasma Rapid Decontamination of Food Contact Surfaces Contaminated with Salmonella Biofilms Brendan A. Niemira, Glenn Boyd, and Joseph Sites Abstract: Cross-contamination of foods from persistent pathogen reservoirs is a known risk factor in processing environments. Industry requires a rapid, waterless, zero-contact, chemical-free method for removing pathogens from food contact surfaces. Cold plasma was tested for its ability to inactivate Salmonella biofilms. A 3-strain Salmonella culture was grown to form adherent biofilms for 24, 48, or 72 h on a test surface (glass slides). These were placed on a conveyor belt and passed at various line speeds to provide exposure times of 5, 10, or 15 s. The test plate was either 5 or 7.5 cm under a plasma jet emitter operating at 1 atm using filtered air as the feed gas. The frequency of high-voltage electricity was varied from 23 to 48 kHz. At the closer spacing (5 cm), cold plasma reduced Salmonella biofilms by up to 1.57 log CFU/mL (5 s), 1.82 log CFU/mL (10 s), and 2.13 log CFU/mL (15 s). Increasing the distance to 7.5 cm generally reduced the efficacy of the 15 s treatment, but had variable effects on the 5 and 10 s treatments. Variation of the high-voltage electricity had a greater effect on 10 and 15 s treatments, particularly at the 7.5 cm spacing. For each combination of time, distance, and frequency, Salmonella biofilms of 24, 48, and 72 h growth responded consistently with each other. The results show that short treatments with cold plasma yielded up to a 2.13 log reduction of a durable form of Salmonella contamination on a model food contact surface. This technology shows promise as a possible tool for rapid disinfection of materials associated with food processing. Keywords: biofilm, cold plasma, food safety, Salmonella, sanitizer Practical Application: Pathogens such as Salmonella can form chemical-resistant biofilms, making them difficult to remove from food contact surfaces. A 15 s treatment with cold plasma reduced mature Salmonella biofilms by up to 2.13 log CFU/mL (99.3%). This contact-free, waterless method uses no chemical sanitizers. Cold plasma may therefore have a practical application for conveyor belts, equipment, and other food contact surfaces where a rapid, dry antimicrobial process is required. Introduction Foodborne contamination by bacterial pathogens is an ongo- ing concern for producers and consumers (Sivapalasingam and others 2004; Mandrell 2009). Contamination of foods may occur through direct contact with food contact surfaces such as contain- ers, conveyor belts, knives, and so on (Doyle and Erickson 2008). Pathogens such as Salmonella are known to persist on these sur- faces, often as durable biofilms (Ryu and Beuchat 2005; Reisner and others 2006). These close conglomerations of cells are more resistant to removal and inactivation than free living planktonic cells. This limits the effectiveness of conventional antimicrobial chemical treatments in treating biofilm contamination (Niemira and Solomon 2005; Ryu and Beuchat 2005). Compounding the significance of biofilms for impact on human health, the native mi- croflora that are capable of forming biofilms are not only widely distributed in fresh-produce processing environments, but can sup- port communal biofilms that include human pathogens (Jahid and Ha 2012; Liu and others 2013). MS 20131340 Submitted 9/20/2013, Accepted 12/23/2013. Authors are with Food Safety and Intervention Technologies Research Unit, Eastern Re- gional Research Center, U.S. Dept. of Agriculture, Agricultural Research Ser- vice, Wyndmoor, PA 19038, U.S.A. Direct inquiries to author Niemira (E-mail: brendan.Niemira@ars.usda.gov). Cold plasma (also known as cool plasma or nonthermal plasma) is a relatively new antimicrobial process being developed for ap- plications in the food industry (Niemira 2012a). The terminology used in the literature (cold, cool, nonthermal, and so on) refers to the physical parameter of operation at or near room tempera- ture, rather than the thermal plasmas found in electric arc welders, combustion tools or other high-temperature applications (Niemira and Gutsol 2010). This technology has shown promise as a direct treatment for fresh and fresh-cut fruits and vegetables, as well as for nuts and other foods. Generally, high-voltage electricity or other energy inputs are used to ionize gas molecules, thereby imparting reactive properties. Cold plasma is waterless, uses no antiseptic chemicals, and is contact-free. Given the reactive nature of cold plasma chemistry, gas plasmas have long been used for sur- face treatment of such materials as electronics, textiles, polymers, and print surfaces (Niemira and Gutsol 2010). Therefore, cold plasma may be applicable to food contact surfaces susceptible to contamination with human pathogens. Little data are available on the antimicrobial efficacy of cold plasma applied directly to active mature biofilms. The objectives of this study are to determine the efficacy of very rapid cold plasma treatments for inactivating Salmonella biofilms, and to determine the impact of (a) distance from cold plasma emit- ter head, (b) frequency of cold plasma generation, and (c) biofilm growth time on antimicrobial efficacy. C  2014 Institute of Food Technologists R  doi: 10.1111/1750-3841.12379 Vol. 00, Nr. 0, 2014  Journal of Food Science M1 Further reproduction without permission is prohibited