ORIGINAL ARTICLE Antimicrobial effect and shelf-life extension by combined thermal and pulsed electric field treatment of milk M. Walkling-Ribeiro, F. Noci, D.A. Cronin, J.G. Lyng and D.J. Morgan School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, UCD Dublin, Dublin 4, Ireland Introduction Nonthermal processing technologies applied to food pres- ervation have attracted increasing research interest in the past two decades as they may provide a valid alternative to conventional thermal processing and lead to improved quality in various food products. In the case of milk and dairy products the antimicrobial efficacy of novel technol- ogies such as high hydrostatic pressure (De Ancos et al. 2000; Guan et al. 2005; Lo ´ pez-Pedemonte et al. 2007) and microfiltration (Fritsch and Moraru 2006; Hoffmann et al. 2006; Kaufmann and Kulozik 2006) have been demonstrated. Another approach with considerable anti- microbial potential involves the use of pulsed electric fields (PEF) and the impact of the latter on a range of pathogenic and spoilage micro-organisms inoculated into milk substrates has been examined. The organisms stud- ied in milk include Escherichia coli (Martı´n et al. 1996), Salmonella subsp. (Sensoy et al. 1997), Listeria monocytog- enes (Reina et al. 1998), Staphylococcus aureus (Sobrino- Lo ´ pez et al. 2006), Lactobacillus subsp. (Grahl and Ma ¨rkl 1996) and Pseudomonas subsp. (Ferna ´ndez-Molina et al. 2006). The majority of research published with regard to PEF treatment of milk, as summarized in a recent review by Sampedro et al. (2005), looked at either one or more individual bacterial species inoculated in milk or looked at total counts of native microbiota in raw milk. In addition, despite the availability of PEF shelf-life studies on bovine milk (Qin et al. 1995; Evrendilek et al. 2001; Ferna ´ndez-Molina et al. 2005; Sepulveda et al. 2005; Keywords hurdle technology, milk, native microbiota, nonthermal preservation, pulsed electric fields, shelf-life. Correspondence D.J. Morgan, School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, UCD Dublin, Dublin 4, Ireland. E-mail: desmond.morgan@ucd.ie 2008 ⁄ 0539: received 29 March 2008, revised and accepted 10 June 2008 doi:10.1111/j.1365-2672.2008.03997.x Abstract Aims: The impact of a combined hurdle treatment of heat and pulsed electric fields (PEF) was studied on native microbiota used for the inoculation of low- fat ultra-high temperature (UHT) milk and whole raw milk. Microbiological shelf-life of the latter following hurdle treatment or thermal pasteurization was also investigated. Methods and Results: UHT milk was preheated to 30°C, 40°C or 50°C over a 60-s period, pulsed for 50 ls or 60 ls at a field strength of 40 kV cm )1 or for 33 ls at 50 kV cm )1 . Heat and PEF reduced the microbial count by a maxi- mum of 6Æ4 log in UHT milk (50°C; 50 kV cm )1 , 33 ls) compared to 6Æ0 log (P ‡ 0Æ05) obtained by thermal pasteurization (26 s, 72°C). When raw milk was treated with a combination of hurdles (50°C; 40 kV cm )1 , 60 ls) a 6Æ0 log inactivation of microbiota was achieved and microbiological milk shelf-life was extended to 21 days under refrigeration (4°C) vs 14 days in thermally pasteu- rized milk. Native microbiota was decreased by 6Æ7 log following conventional pasteurization. Conclusions: The findings suggest that heat and PEF achieved similar inactiva- tion of native microbiota in milk and longer stabilization of microbiological shelf-life than thermal pasteurization. Significance and Impact of the Study: A hurdle approach of heat and PEF could represent a valid milk processing alternative to conventional pasteuriza- tion. Hurdle treatment might also preserve native milk quality better due to less thermal exposure. Journal of Applied Microbiology ISSN 1364-5072 ª 2008 The Authors Journal compilation ª 2008 The Society for Applied Microbiology, Journal of Applied Microbiology 106 (2009) 241–248 241