Biosystems Engineering (2004) 87 (2), 137–144 doi:10.1016/j.biosystemseng.2003.11.005 Available online at www.sciencedirect.com AE}Automation and Emerging Technologies Application of Pulsed Electric Fields to Skim Milk inoculated with Staphylococcus aureus G.A. Evrendilek 1 ; Q.H. Zhang 2 ; E.R. Richter 3 1 Department of Food Engineering, Faculty of Agriculture, Tayfur Sokmen Campus, Mustafa Kemal University, 31034 Alahan-Hatay, Turkey; e-mail of corresponding author: gevrendilek@mku.edu.tr 2 The Ohio State University, Department of Food Science and Technology, 2015 Fyffe Court, Columbus, OH 43210, USA; e-mail: zhang.138@osu.edu 3 Richter International Inc., 505 King Avenue, Columbus, OH 43201, USA; e-mail: Richter-C@battelle.org (Received 15 February 2003; accepted in revised form 18 November 2003; published online 13 January 2004) Application of pulsed electric fields (PEF) to skim milk inoculated with Staphylococcus aureus was conducted in the study. Skim milk samples were treated by PEF, using fluid handling systems of both circulation and stepwise modes. Parameters for the PEF were 37 ms pulse duration time, 250 Hz pulse repetition rate, 1 ml s 1 flow rate, 460 ms total treatment time, and 35kVmm 1 electric field strength. After the PEF-treatment, the samples were plated onto both tryptic soya agar and Baird Parker agar with egg yolk tellurite enrichment to enumerate the number of inactivated and injured cells. Survivals of PEF-treated S. aureus cells were also enumerated at refrigeration temperature for 2 weeks. There was a significant decrease in S. aureus cells caused by the two fluid handling systems (probability P5005). The difference in the inactivation due to the PEF treatment with the fluid handling systems was not significant (P>005). After the PEF treatment, there was no significant injury of the PEF-treated cells (P>005). Relative to the control samples, there was a significant reduction in the number of the PEF-treated cells at 48C for 2 weeks (P5005). # 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd 1. Introduction Increasing consumer demands on fresh-like quality food products have stimulated research on innovative non-thermal food preservation methods. Application of pulsed electric fields (PEF) is a non-thermal preservation method applied to liquid foods to inactivate both spoilage and pathogenic microorganisms, with minimal losses of nutrients and flavour (Hamilton & Sale, 1967; Mertens & Knorr, 1992; Qin et al., 1995). Pulsed electric field processing has been successful in a variety of liquid products with low viscosity and electrical conductivity such as orange, apple, and cranberry juice. Recent studies revealed more than a 3 log 10 cfu ml 1 reduction in orange juice (Qiu et al., 1998) and apple juice (Evrendilek et al., 1999 & 2000), and a 2 log 10 cfu ml 1 reduction in cranberry juice (Jin & Zhang, 1999), where cfu denotes colony forming units. Moreover, no significant difference in the colour and flavour of PEF- treated juices was reported (Evrendilek et al., 2000 & 2001; Jin & Zhang, 1999). Pulsed electric field treatment involves application of high-voltage pulses (typically 20–80 kV cm 1 ) to foods placed between two electrodes. With the application of electric fields as short duration pulses, minimal heat is generated and process remains non-thermal. When a food contains ions, and it usually does, a current will flow through the food. It has been very well established that PEF causes microbial inactivation by dielectrical breakdown and electroporation of cell membrane. In fact, PEF is more effective and energy efficient when applied to media with low electrical conductivity than to regular liquid foods (Zhang et al., 1995). Milk is very susceptible to both spoilage and pathogenic microorganisms. Current regulations require the thermal pasteurisation of milk for the inactivation of microorganisms such as Staphylococcus aureus. How- ever, the thermal pasteurisation causes undesirable changes in flavour. Generation of cooked flavour is the most obvious organoleptic change in milk processed by heat (Lucisano et al., 1994; Wirjantoro & Lewis, 1997). ARTICLE IN PRESS 1537-5110/$30.00 137 # 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd