Alkaline phosphatase and microbial inactivation by pulsed electric field in bovine milk Kambiz Shamsi a , Cornelis Versteeg b , Frank Sherkat a , Jason Wan b, ⁎ a RMIT University, School of Applied Sciences, Food Science, GPO Box 2476V, Melbourne, VIC 3000, Australia b Food Science Australia, 671 Sneydes Road, Private Bag 16, Werribee, VIC 3030, Australia Abstract The effects of pulsed electric field (PEF) treatments at field intensities of 25–37 kV cm - 1 and final PEF treatment temperatures of 15 °C and 60 °C on the inactivation of alkaline phosphatase (ALP), Total Plate Count (TPC), Pseudomonas and Enterobacteriaceae counts were determined in raw skim milk. At 15 °C, PEF treatments of 28 to 37 kV cm - 1 resulted in 24–42% inactivation in ALP activity and b 1 log reduction in TPC and Pseudomonas count, while the Enterobacteriaceae count was reduced by at least 2.1 log units to below the detection limit of 1 CFU mL - 1 . PEF treatments of 25 to 35 kV cm - 1 at 60 °C resulted in 29–67% inactivation in ALP activity and up to 2.4 log reduction in TPC, while the Pseudomonas and Enterobacteriaceae counts were reduced by at least 5.9 and 2.1 logs, respectively, to below the detection limit of 1 CFU mL - 1 . Kinetic studies suggested that the effect of field intensity on ALP inactivation at the final PEF treatment temperature of 60 °C was more than twice that at 15 °C. A combined effect was observed between the field intensity and temperature in the inactivation of both ALP enzyme and the natural microbial flora in raw skim milk. © 2007 Elsevier Ltd. All rights reserved. Keywords: Pulsed electric field (PEF); Alkaline phosphatase; Enzyme inactivation; Microbial inactivation; Enterobacteriaceae; Pseudomonas; Total Plate Count Industrial relevance: Milk has been pasteurised to ensure its safety and extend its shelf life. However, the need for retaining heat-sensitive nutrient and sensory properties of milk has resulted in interest in the application of alternative technologies. The results of the current study suggest that PEF as a non-thermal process can be employed for the treatment of raw milk in mild temperature to achieve adequate safety and shelf life while preserving the heat-sensitive enzymes, nutrients and bioactive compounds. 1. Introduction Pulsed electric field (PEF) technology is an emerging non- thermal preservation method that uses short high voltage pulses to inactivate spoilage organisms and pathogens in liquid foods such as milk or juices (Barbosa-Canovas, Gongora- Nieto, Pothkamury, & Swanson, 1999). The aim is to produce safe foods with better nutritional and sensory properties than that produced by heat treatment alone (Bendicho, Espachs, Arantegui, & Martin, 2002a). The interest in PEF technology is increasing for processing foods as a means of avoiding the negative effects of thermal treatments such as colour alteration, flavour damage and nutrient losses (Giner et al., 2000). The mechanisms of microbial inac- tivation by PEF are not completely elucidated. It is postulated that the membrane permeability of cells exposed to high electric field increases and causes electroporation and once a critical trans-membrane voltage is applied for a sufficient time, a non- reversible pore of given diameter is formed on the cell membrane which leads to cell death (Zimmermann, 1986; Schoenbach, Joshi, Stark, Dobbs, & Beebe, 2000). Over the last two decades, various studies on milk treatment by PEF have shown that PEF is very effective for the inactivation of vegetative moulds, yeasts and bacterial cells (Bendicho, Barbosa-Canovas, & Martin, 2002b). In general, Gram-positive bacteria are more resistant to PEF treatment than Gram-negative bacteria (Hülsheger, Potel, & Niemann, 1983). Yeasts are more sensitive to electric field strength than bacteria due to their larger size (Sale & Hamilton, 1967; Qin et al., 1995). Available online at www.sciencedirect.com Innovative Food Science and Emerging Technologies 9 (2008) 217 – 223 www.elsevier.com/locate/ifset ⁎ Corresponding author. Tel.: +61 3 9731 3320; fax: +61 3 9731 3250. E-mail address: jason.wan@csiro.au (J. Wan). 1466-8564/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2007.06.012