Effect of processing parameters on inactivation of Bacillus cereus spores in milk using pulsed electric fields Daniela Bermúdez-Aguirre a, * , C. Patrick Dunne b , Gustavo V. Barbosa-Cánovas a a Center for Nonthermal Processing of Food, Washington State University, Pullman, WA 99164-6120, USA b US Army Natick Soldier RDEC Natick, MA 01760-5018, USA article info Article history: Received 8 August 2011 Received in revised form 7 October 2011 Accepted 4 November 2011 abstract The inactivation of Bacillus cereus spores in milk by treatment using pulsed electric fields (PEF) was studied. The effects of the electric field, temperature, number of pulses, pulse width, frequency and nisin concentration were tested in skim and whole milk. Results showed high resistance of spores to PEF alone; mild thermal treatment (40  C) applied together with PEF had an antagonistic effect, and treat- ment at 50  C enhanced spore death. Skim milk was a better medium than whole milk for inactivation; stronger thermal treatment (65  C) together with PEF achieved a slight increase in spore reduction. The effect of pulse width was not consistent. Addition of nisin (50 IU mL 1 ) was synergistically effective (40 kV cm 1 , 144 pulses, 65  C), reducing spore count (by 3.6 log) in skim milk. Regardless of the severity of the treatment, spores showed resistance to the different processing variables. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Bacillus cereus is a spore-forming, Gram-positive bacterium. Its optimum growth temperature is between 28 and 35  C; the minimum growth temperature is 4e5  C and the maximum is 48  C. B. cereus can grow at pH values from 4.9 to 9.3; spores are heat-resistant and the strains of this Bacillus present in milk are responsible for the production of diarrheagenic enterotoxin (Batt, 2000; Jay, Loessner, & Golden, 2005). Food intoxication caused by B. cereus is produced when the cell concentration is around 10 5 cfu g 1 (Salustiano et al., 2009). B. cereus can be found in dairy products such as ice cream, milk powders, fermented and pasteurized milk (Røssland, Langsrud, & Sørhaug, 2005). Some food products subjected to drying or heating processes can contain spores of this microorganism; it can grow in low-acid foods after pasteurization during refrigerated storage because of its characteristics (Van Opstal, Bagamboula, Vanmuysen, Wuytack, & Michiels, 2004). Often, the presence of B. cereus in milk is attributed to cross-contamination from raw milk, for instance, if good cleaning practices are not applied to the processing equip- ment. However, studies have shown that raw milk is the main source of B. cereus in pasteurized milk (Bartoszewicz, Hansen, & Swiecicka, 2008; Larsen & Jørgensen, 1997; Lin, Schraft, Odumeru, & Griffiths, 1998). In addition, spores can survive thermal pro- cessing and promote the so-called “broken cream” defect, which is related to proteolytic activity in milk (Batt, 2000). This quality defect is also called sweet curdling (Batt, 2000; Hanson, Wendorff, & Houck, 2005), which is the formation of a sweet curd that has a bitter off-flavor due to the precipitation of casein. The presence of different species of Bacillus is one contributing factor in the reduction of the shelf-life of milk (Hanson et al., 2005) because many of these species are psychrotolerant and find a good level of nutrients in milk, which supports their growth even at low temperature (Bartoszewicz et al., 2008; Walkling-Ribeiro, Rodríguez- González, Jayaram, & Griffiths, 2011). Pulsed electric field (PEF) treatment is a novel technology that has shown positive results in the inactivation of pathogenic and spoilage microorganisms in liquid foods (Amiali, Ngadi, Smith, & Raghavan, 2007; We˛ sierska & Trziska, 2007), in addition to having low energy requirements (Vega-Mercado et al., 1997). Synergistic effects between PEF and thermal treatment have been reported with regard to microbial inactivation (Amiali et al., 2007; Evrendilek & Zhang, 2003; Walkling-Ribeiro, Noci, Cronin, Lyng, & Morgan, 2010). Nisin is an antibiotic that was discovered in 1920, which is comprised of 34 amino and is acids produced by Lactococcus lactis subspecies lactis during fermentation of a modified milk medium (Barbosa-Cánovas, Pothakamury, Palou, & Swanson,1998; Guinane, Cotter, Hill, & Ross, 2005; Thomas & Delves-Broughton, 2005). Nisin is a non-toxic compound that can be produced naturally, is heat * Corresponding author. Tel.: þ1 509 335 8109; fax: þ1 509 335 2722. E-mail address: daniela@wsu.edu (D. Bermúdez-Aguirre). Contents lists available at SciVerse ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj 0958-6946/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2011.11.003 International Dairy Journal 24 (2012) 13e21