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FEMS Immunology and Medical Microbiology 40 (2004) 243^247

www.fems-microbiology.org

Injury recovery of foodborne pathogens in high hydrostatic pressure treated milk during storage
Faruk Bozoglu a , Hami Alpas
b

a;Ã

, Gonul Kaletunc ° « «

b

a Food Engineering Department, Middle East Technical University, Ankara 06531, Turkey Department of Food Agricultural and BiologicalEngineering, The Ohio State University, Columbus, OH 43210-1057, USA

Received 6 November 2003 ; received in revised form 16 December 2003 ; accepted 24 December 2003 First published online 24 January 2004

Abstract
Bacteria are expected to be injured or killed by high hydrostatic pressure (HHP). This depends on pressure levels, species and strain of the microorganism and subsequent storage.Injured bacteria may be repaired which could affect the microbiological quality of foodstuffs with an important safety consideration especially in low acid food products. In this study two Gram-positive (Listeria monocytogenes CA and Staphylococcus aureus 485) and two Gram-negative (Escherichia coli O157:H7 933 and Salmonella enteritidis FDA) relatively pressure resistant strains of foodborne pathogenswere pressurized at 350, 450 and 550 MPa in milk (pH 6.65) and stored at 4, 22 and 30‡C. The results of shelf life studies indicated two types of injury, I1 and I2, for all the pathogens studied. It is obvious that I2 type injury is a major injury and after its repair (I2 to I1), the cells can form colonies on non-selective but not on selective agar. The formation of colonies on both selectiveand non-selective agar occurs only after full recovery of injury (I1 to AC). The results presented in this study show that even if injured cells are not detected immediately after HHP treatment, I2 type injury could be potentially present in the food system. Therefore, it is imperative that shelf life studies must be conducted over a period of time for potential repair of I2 type injury either todetectable injury (I1) or to active cells (AC) to ascertain microbiological safety of low acid food products. ß 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.
Keywords : High pressure ; Injury recovery ; Foodborne pathogen; Milk

1. Introduction Bacterial cells exposed to di¡erent physical and chemical treatments su¡er injury that could bereversible in food materials during storage. Injury has been observed for many bacterial cells. The injured cells can repair in a medium containing the necessary nutrients under conditions of optimum pH and temperature leading to outbreaks of foodborne disease and food spoilage. Speci¢c studies have shown that the metabolic processes during injury repair vary with the nature of stress and involvethe synthesis of ATP, RNA, DNA and mycopeptides [13,14,21]. The structural and functional components known to be damaged by sublethal stresses are the cell wall, cytoplasmic membrane, ribosomal RNA and DNA, as well as some enzymes [9,12,20,21]. High hydrostatic

* Corresponding author. Tel. : +90 (312) 210 5634 ; Fax : +90 (312) 210 1270. E-mail address : imah@metu.edu.tr (H. Alpas).

pressure(HHP) is one of the emerging technologies proposed as an alternative to thermal processing and has been investigated to enhance safety and shelf life of many perishable foods [11]. The e¡ect of HHP on foodborne pathogens has been extensively studied [1,2,7,8]. Magnitude of pressure, pressurization time and temperature, type of microorganism, cell growth phase, suspending media and the presence ofantimicrobial substances have been shown to in£uence the e⁄cacy of HHP [5,20]. The ability to inactivate foodborne pathogens at pressures between 300 and 600 MPa without detrimental effects on important quality characteristics of foods has increased the interest in HHP applications on milk and dairy products [19]. Although HHP treated cell membranes show altered permeability, it is reported that...