Effects of Salt, Sodium Pyrophosphate, and Sodium Lactate on the Probability of Growth of <i>Escherichia</i> <i>coli</i> O157:H7 in Ground Beef

Ground beef products are susceptible to contamination with Escherichia coli O157:H7. The objective of this study was to examine the effect of salt, sodium pyrophosphate (SPP), and sodium lactate on the probability of growth of E. coli O157:H7 in ground beef under a temperature abuse condition. Ground beef containing 0 to 2.25% salt, 0 to 0.5% SPP, and 0 to 3% lactate was inoculated with a four-strain mixture of E. coli O157:H7, vacuum packaged, and stored at 10uC for 15 days. A total of 25 combinations of the three additives, each with 20 samples, were tested. A logistic regression was used to model the probability of growth of E. coli O157:H7 (with a 1.0-log CFU/g increase during storage) as a function of salt, SPP, and lactate. The resultant probability model indicated that lactate at higher concentrations decreased the probability of growth of E. coli O157:H7 in ground beef, and the effect was more pronounced at higher salt concentrations. At salt concentrations below 1.3%, the increase of SPP concentration marginally increased the growth probabilities of E. coli O157:H7. The model illustrated the effect of salt, SPP, and lactate on the growth probabilities and growth or no-growth behavior of E. coli O157:H7 in ground beef and can be used to improve the microbial food safety of ground beef products. Escherichia coli O157:H7 is a foodborne pathogen capable of producing large quantities of verotoxin in the intestine, causing hemorrhagic colitis illness and hemolytic uremic syndrome. This pathogen has been associated with alfalfa sprouts, unpasteurized fruit juices, dry-cured salami, lettuce, game meat, cheese curds, and raw beef products, particularly ground beef and beef patties (21). Ground beef is produced by grinding beef trimmings of various parts of raw beef. The beef trimmings are often contaminated with low levels of E. coli O157:H7, and ground beef processing does not have a kill step to eliminate this pathogen (10, 14). Rhoades et al. (18) reviewed published literature reporting the prevalence of verocytotoxigenic E. coli in minced beef, beef burgers, and beef trimmings for making ground beef products in several counties. They reported that the prevalence of E. coli O157:H7 in raw beef products ranged from 0 to as high as 17%, with a mean prevalence of 1.2%. E. coli O157:H7 has long been implicated in outbreaks of foodborne illness associated with the consumption of undercooked beef patties (3, 19, 23). Recently, this pathogen was implicated in two multistate outbreaks in the United States linked to undercooked ground beef (4, 5) and nonintact beef (6). Studies have examined the addition of antimicrobials, such as sodium lactate, to reduce or inactivate E. coli O157:H7 in ground beef products during storage and cooking. Byrne et al. (2) reported that adding 4% sodium lactate in beef burger patties reduced E. coli O157:H7 survival during frozen storage and increased the susceptibility of the pathogen to cooking at 50 to 60uC. In addition to beef patties, ground beef has been increasingly used in refrigerated prepared meals and as pre-prepared cook-and-serve food items. Salt and food-grade phosphates are common ingredients used in ground beef products to improve organoleptic quality, moisture retention, and cooking yield (11). The effectiveness of salt, phosphates, and lactate on the growth of E. coli O157:H7 in ground beef has not been examined. Accordingly, the objective of this study was to examine the probability of growth of E. coli O157:H7 in ground beef as affected by salt, sodium pyrophosphate (SPP), and lactate at 10uC, to represent temperature abuse refrigeration conditions. MATERIALS AND METHODS E. coli O157:H7 strains and inoculum preparation. Four strains of E. coli O157:H7 (933, 45753-35, C9490, and A9218-C1) were used in this study. These strains were obtained from the Microbial Food Safety Research Unit, Eastern Regional Research Center, U.S. Department of Agriculture, Wyndmoor, PA. Frozen cultures (280uC) were thawed, and 0.1 ml of each strain was inoculated separately into 10 ml of brain heart infusion broth * Author for correspondence. Tel: 215-233-6416; Fax: 215-233-6581; E-mail: andy.hwang@ars.usda. gov. { Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. 622 Journal of Food Protection, Vol. 74, No. 4, 2011, Pages 622–626 doi:10.4315/0362-028X.JFP-10-325 D ow naded rom hp://m eridianenpress.com /doi/pdf/10.462-028X.JFP-10-325 by gest on 11 M arch 2020 (Difco, BD, Detroit, MI) and incubated at 37uC. After 24 h, 0.1 ml of cell suspension was subcultured again in 10 ml of brain heart infusion broth at 37uC for 24 h. One day before the experiment, 0.1 ml of cell suspension was inoculated into 50 ml of brain heart infusion broth in a 250-ml flask and incubated at 37uC for 18 h. A cell pellet was collected by centrifugation at 2,800 | g for 15 min at 4uC. The collected pellet was resuspended in 10 ml of sterile 0.1% peptone water by vortexing and collected again by centrifugation at 2,800 | g for 15 min at 4uC. The cell pellet was resuspended in 2 ml of peptone water. All five strains were combined and diluted to 10 CFU/ml for use as inoculum. Experimental design. A precision uniform central composite design (SAS 9.1, SAS Institute Inc., Cary, NC) was used to select the combinations (treatments) of levels of salt, SPP, and sodium lactate for testing. The ranges of concentration were 0 to 2.25% (wt/wt) for salt, 0 to 0.5% (wt/wt) for SPP, and 0 to 3% (wt/wt) for lactate. A total of 16 treatments were selected by the design, and an additional nine treatments were added after the initial 16 treatments were evaluated (Table 1). The additional treatments were selected to fill the data gap observed after the data from the 16 treatments were analyzed. Two trials with 10 replicate samples in each trial