ORIGINAL ARTICLE Reduction in Listeria monocytogenes and spoilage bacteria on smoked catfish using X-ray treatments B.S.M. Mahmoud 1 , R. Coker 1 and Y.-C. Su 2 1 Coastal Research & Extension Center, Mississippi State University, Pascagoula, MS, USA 2 Seafood Research and Education Center, Oregon State University, Astoria, OR, USA Introduction Mississippi is the leading state for catfish production in the United States (USDA, 2012). Ready-to-eat smoked fish is one of the most popular fish products worldwide (Medina et al. 2009). Smoked catfish has not yet been commercially produced in the United States (the seafood industry is looking for ways to commercially produce smoked catfish). Also, people may make smoked catfish at home. Catfish can be contaminated with Listeria mono- cytogenes during smoking or packaging process owing to unhygienic conditions. L. monocytogenes can grow well at refrigerated temperatures (5°C) which makes L. monocyt- ogenes a threat to the health of consumers who eat smoked catfish (Listeriosis accounts for approximately 2500 cases of illness in the United States annually) (Huss et al. 2000; Vogel et al. 2001; Hof 2003). There are many nonthermal techniques and processes can be used to prevent L. monocytogenes growth in smoked fish including carbon dioxide in modified atmo- sphere packaging (Nilsson et al. 1997), lactic acid bacteria (Nilsson et al. 1999), bacteriocins (Duffes et al. 1999), a combination of sodium diacetate with potassium lactate (Vogel et al. 2006), high-pressure (HP) treatment (Lakshmanan and Dalgaard 2004) and E-beam irradiation (Su et al. 2004). Applications of X-ray radiation for inactivating food- borne pathogens have been studied in a variety of seafood including oysters (Mahmoud 2009a; Mahmoud and Bur- rage 2009) and ready-to-eat shrimp (Mahmoud 2009b). Currently, no study has reported inactivation of L. mono- cytogenes on smoked catfish using X-ray irradiation. The objectives of this study were to (i) determine the efficacy of X-ray on the inactivation of L. monocytogenes on Keywords inactivation, Listeria monocytogenes, smoked catfish, X-ray radiation. Correspondence Barakat S.M. Mahmoud, Coastal Research & Extension Center, Mississippi State University, 3411 Frederic St., Pascagoula, MS 39567, USA. E-mail: bm547@msstate.edu This paper is a journal article of the Mississippi Agriculture and Forestry Experiment Station. 2012 ⁄ 0044: received 10 January 2012, revised 12 March 2012 and accepted 13 March 2012 doi:10.1111/j.1472-765X.2012.03242.x Abstract Aims: To determine the efficacy of X-ray processes in inactivating L. monocyt- ogenes levels in smoked catfish during storage at 5°C and to determine the effects of X-ray doses on controlling the growth of spoilage bacteria on smoked catfish during storage at 5°C for up to 5 weeks. Methods and Results: Smoked catfish fillets inoculated with L. monocytogenes were treated with 0Æ0–2Æ0 kGy X-ray and stored at 5°C for 5 weeks. The nega- tive controls (uninoculated ⁄ untreated) and uninoculated samples treated with the lowest (0Æ1 kGy) and highest (2Æ0 kGy) doses were stored at 5°C and tested for psychrotrophs count during the 5 weeks of storage. The initial L. monocyt- ogenes population on smoked catfish was significantly (P <0Æ05) reduced to undetectable level by a treatment of 1Æ0 kGy or higher. The initial psychro- trophs count on smoked catfish was significantly reduced from 4Æ7 CFU g )1 to below the detectable level by a treatment with 2Æ0 kGy. Conclusions: Smoked catfish treated with 2Æ0 kGy X-ray had no detectable L. monocytogenes throughout 35 days of storage at 5°C. A treatment with 2Æ0 kGy X-ray also kept the levels of psychrotrophs in the smoked catfish within the acceptable level until 35 days. Significance and Impact of the Study: The results of this investigation indicate that X-ray at 2Æ0 kGy can eliminate L. monocytogenes and extend the shelf life of smoked catfish stored at refrigeration temperature. Letters in Applied Microbiology ISSN 0266-8254 524 Letters in Applied Microbiology 54, 524–529 ª 2012 The Society for Applied Microbiology ª 2012 The Authors