Food Microbiology and Safety Survival of Escherichia Coli 0157:H7 on Vacuum-Packaged Raw Beef Treated with Polylactic Acid, Lactic Acid, and Nisin A. MUSTAPHA, T. ARIYAPITIPUN, AND A.D. CLARKE ABSTRACT: The antimicrobial effect of 2% low molecular weight polylactic acid (LMW-PLA), 2% lactic acid (LA), 200 IU nisin (Nisaplin TM )/mL, and combinations of nisin and each acid on Escherichia coli O157:H7 on raw beef was investigated. Fresh beef, inoculated with E. coli O157:H7 and treated with each solution, was vacuum packaged and stored at 4°C for 28 d. At day 28, counts of samples treated with LMW-PLA, NPLA, LA, and NLA were significantly reduced to 3.13, 2.91, 2.99, and 2.83 log 10 CFU/cm 2 , respectively. However, the antimicrobial effect of LMW-PLA and LA was not significantly different from each other and Nisaplin did not enhance the effects of either acid against this organism. Keywords: lactic acid, polylactic [sp??] acid, nisin, beef, Escherichia coli O157:H7 Introduction S INCE 1982, FOODBORNE OUTBREAKS OF ESCHERICHIA COLI O157:H7 have emerged in multiple states in the U.S. (Bean and others 1996; CDC 1993). Recently, it was found that E. coli O157:H7 can survive and be protected in acidic environ- ments, such as apple cider (pH of approximately 4.0) (Miller and Kaspar 1994; Uljas and Ingham 1998) or fermented dry sausages (Glass and others 1992). An in vitro study of the survival of E. coli O157:H7 in apple juice and tryptic soy broth acidified with hydrochloric or organic acids at 4 and 21 °C, indicated that low temperature alone or combined with pH 3.5 was able to enhance the acid tolerance of E. coli O157:H7 (Przybylski and Witter 1979; Uljas and Ingham 1998). It was also reported that lactic acid (LA) (pH 3.5) was effective at reducing populations of E. coli O157:H7 from ap- proximately 5 log 10 MPN/mL to 2.6 or < 1.5 log 10 MPN/mL during storage for 7 d at 4 or 21 °C, respectively (Uljas and Ingham 1998). Efficacies of LA against microorganisms on fresh meat surfaces have been reported in many studies. Warm or hot LA was used alone to sanitize meat surfaces or combined with other treatments, such as trimming, water washing, hot water spraying, or mixing with other organic acids and applying by spraying or dipping (Anderson and Marshall 1990; Castillo and others 1998; Dorsa and others 1997; Podolak and others 1996; Smulders and Woolthuis 1985; Smulders and others 1986; Woolthuis and Smulders 1985). At 25 °C, dipping in 2% LA reduced the numbers of E. coli C5 by approximately 0.3 log 10 CFU/cm 2 (Anderson and Marshall 1990). Hot 2% LA (55 or 70 °C) increased the lethality of LA to almost 1 log reduction (Anderson and Marshall 1990). Spray- ing with LA reduced E. coli O157:H7 numbers by more than 4 log 10 from an initial number of 5 log 10 CFU/cm 2 (Castillo and others 1998). Low molecular weight-PLA has been found to be effective at controlling microorganisms of both spoilage and pathogenic types on raw beef (Ariyapitipun and others 1997, 1998, 1999; Chellappa 1997; Constantinescu and others 1998; Jin 1995; Xing and Iannotti 1997). It is suggested that LMW- PLA releases free LA for an extended time following its application and that a low pH can be maintained longer than is possible with conventional free LA. Compared with LA, LMW-PLA has a prolonged antimicrobial activity, both in broth culture and on beef surfaces (Ariyapitipun and others 1997; Chellappa 1997; Jin 1995). One percent LMW-PLA was more effective at reducing numbers of E. coli O157:H7 than was 1% LA at 24 to 72 h after dipping (Chellappa 1997). The bactericidal activity of 1% LMW-PLA was maximum at 48 h after dipping (Chellappa 1997). Moreover, application of 2.5% hot (60 °C) LMW-PLA on beef carcasses lowered total aerobic and Enterobacteriaceae counts by 2.0 to 3.5 log 10 CFU/cm 2 and by 3.5 to 4.6 log 10 CFU/cm 2 , respectively (Ariyapitipun and others 1997). In this study, nisin was included, in addition to the organic acids, to determine if any multiple antimicrobial effect would occur. Inhibition by nisin is based on its adsorption to vegetative cell membranes and disruption of membrane activity via membrane insertion, pore formation, and simultaneous depolarization (De Vuyst and Vandamme 1994). The increase in membrane permeability disturbs membrane transport, and inhibits energy production and biosynthesis of proteins or nucleic acids. Moreover, nisin induces autolysis of sensitive bacterial cells (De Vuyst and Vandamme 1994). Nisin is effective against gram positive bacteria such as Brochothrix thermosphacta, Clostridium botulinum, Staphylococcus aureus, Listeria innocua or Listeria monocytogenes (Chung and others 1989; Cutter and Siragusa 1994a, 1996; De Vuyst and Vandamme 1994; El-Khateib and others 1993). Effectiveness of nisin against sublethally acid-injured gram-negative bacteria, such as Yersinia enterocolitica and Pseudomonas fluorescens, was observed in vitro (Kalchayanand and others 1992). Two percent of LMW- PLA, 2% LA, or the combinations of 200 IU Nisaplin/mL and each acid significantly reduced growth of mesophilic and psychrotrophic Enterobacteriaceae and Pseudomonas on vacuum-packaged raw beef kept at 4 °C up to 56 d (Ariyapitipun and others 1998). Moreover, it was found that 2% LMW-PLA, 2% LA, 400 IU Nisaplin/mL, 2% NPLA, and 2% NLA eliminated L. monocytogenes Scott A after dipping and suppressed its growth during storage up to 42 d (Ariyapitipun and others 1999). Based on results of our