M: Food Microbiology & Safety JFS M: Food Microbiology and Safety Inactivation of Staphylococcus aureus in Milk Using Flow-Through Pulsed UV-Light Treatment System K. KRISHNAMURTHY, A. DEMIRCI, AND J.M. IRUDAYARAJ ABSTRACT: This study investigated the efficacy of pulsed UV-light for continuous-flow milk treatment for the in- activation of Staphylococcus aureus, a pathogenic microorganism frequently associated with milk safety concerns. Pulsed UV light is an emerging technology, which can be used for the inactivation of this pathogen in milk in a rela- tively short time. Pulsed UV light damages the DNA of the bacteria by forming thymine dimers that lead to bacterial death. The effect of sample distance from the quartz window of the UV-light source, number of passes, and flow rate was investigated. A response surface methodology was used for the design and analysis of experiments. Milk was treated at 5-, 8-, or 11-cm distance from a UV-light strobe at 20, 30, or 40 mL/min flow rate and treated up to 3 times by recirculation of milk to assess the effect of the number of passes on inactivation efficiency. Log 10 reductions var- ied from 0.55- to 7.26-log 10 CFU/mL. Complete inactivation was obtained in 2 cases and no growth was observed following an enrichment protocol. Predicted results were in agreement with the experimental data. Overall, this work demonstrates that pulsed UV-light has a potential for inactivation of milk pathogens. Keywords: milk pasteurization, pathogen inactivation, pulsed UV light, Staphylococcus aureus Introduction T he Center for Disease Control and Prevention (CDC) estimates that there have been 17248 and 1413 cases of Staphylococ- cus aureus reported during 1973–1987 and 1983–1987, respectively, which accounted for 14% and 1.6%, respectively, of all the cases (Bean and Griffin 1990; Bean and others 1990; Olsen and others 2000). There have been several outbreaks associated with milk or milk products. Several milk products, including cheese (Bergdoll 1989) and chocolate milk (Bennet 1992; Lori and others 2003), con- tribute to staphylococcal food poisoning. In the United Kingdom, staphylococcal food poisoning due to milk products accounted for 8% between 1969 and 1990 (Wieneke and others 1993; Lori and others 2003). However, in France, 32% of staphylococcal food poi- soning occurred due to consumption of milk products, especially cheese products between 1999 and 2000 (Haeghebaert and oth- ers 2002; Lori and others 2003). Similarly, in the United States, be- tween 1975 and 1982, 1.4% of staphylococcal food poisoning was attributed to consumption of milk products and seafood (Genige- orgis 1989; Lori and others 2003). Milk may contain other spoilage and pathogenic microorganisms because of improper handling and contamination from both inside and outside the udder (Bram- ley and McKinnon 1990). Therefore, it is necessary to process milk to inactivate these microorganisms. Conventionally, milk is pas- teurized using heat exchangers. However, pasteurization is a high- energy demand process with high capital and operational costs in- volved. Pulsed UV-light treatment is an emerging technology and MS 20060629 Submitted 11/15/2006, Accepted 6/6/2007. Author Krishna- murthy is with Dept. of Food and Animal Sciences, Alabama A&M Univ., Normal AL 35762, U.S.A. Author Demirci is with Dept. of Agricultural and Biological Engineering & The Huck Inst. of the Life Sciences, The Penn- sylvania State Univ., Univ. Park, PA 16802, U.S.A. Author Irudayaraj is with Dept. of Agricultural and Biological Engineering, Purdue Univ., West Lafayette, IN 47907, U.S.A. Direct inquiries to author Demirci (E-mail: demirci@psu.edu). Authors Krishnamurthy and Irudayaraj were affiliated with Dept. of Agri- cultural and Biological Engineering, Pennsylvania State Univ., Univ. Park, PA 16802, U.S.A. when this study was performed. has the potential to be used as a pasteurization technology in spe- cific processing situations, such as small-scale milk processing op- erations. Pulsed UV light has been proven to be effective against pathogenic microorganisms and can serve as a potential method for the inactivation of S. aureus in milk. There are 2 modes of ap- plication of UV light: continuous or pulsed UV-light mode. Con- tinuous UV light is the conventional one, which delivers the UV light in a continuous mode. In the pulsed UV-light mode, the UV light is stored in a capacitor and released as intermittent short pulses, thus increasing the instantaneous energy intensity. Thus, pulsed UV light has more instantaneous energy than continuous UV light for the same total energy supplied. Therefore, pulsed UV- light treatment is a more effective and rapid way of inactivating mi- croorganisms than continuous UV-light sources since the energy is multiplied manyfold (Dunn 1995; CFSAN-FDA 2000). Furthermore, typically pulsed UV-light sources generate a broad wavelength spectrum ranging from 100 to 1100 nm, as opposed to 100 to 400 nm in the case of continuous UV light. Therefore, pulsed UV light refers to a broadband spectrum consisting of ultraviolet, visi- ble, and infrared radiation. McDonald and others (2000) reported that the almost identical level of inactivation of Bacillus subtilis spores was obtained with 40 J/m 2 of pulsed UV-light source and 80 J/m 2 continuous UV-light sources. Several parameters such as opacity of the liquid, presence of par- ticulate materials, treatment time, distance of sample from the light source, and volume of the sample affect the sample–light interac- tion. Furthermore, the number of lamps, orientation, and design of lamps have a direct relevance to the energy delivered and dose effectiveness. Bank and others (1990) reported that a 60-s pulsed UV-light treatment at 31-cm distance from the light source (ap- proximately 4 J/m 2 ) resulted in a 6- to 7-log 10 reduction of viable bacterial populations of Staphylococcus epidermidis, Pseudomonas aeruginosa, Escherichia coli, S. aureus, or Serratia marcescens on Trypticase soy agar plates. Jun and others (2003) investigated the effect of pulsed UV light on inactivating Aspergillus niger spores in corn meal and reported a 4.95-log 10 reduction in 100 s when the distance between the quartz C  2007 Institute of Food Technologists Vol. 72, Nr. 7, 2007—JOURNAL OF FOOD SCIENCE M233 doi: 10.1111/j.1750-3841.2007.00438.x Further reproduction without permission is prohibited