An estimated 9.4 million cases of human illness occur annually in the United States as a result of consumption of undercooked or improperly handled foods contaminated with 31 known pathogens (Scallan et al., 2011). Nontyphoidal Salmonella spp. are estimated to be the leading cause of foodborne illness from a bacterial agent and are responsible for about 35% of hospitalizations and 28% of deaths related to foodborne illness (Scallan et al., 2011). Additionally, Escherichia coli O157:H7 and non-O157 Shiga toxin–producing E. coli (STEC) are estimated to cause approximately 175,000 illnesses and 20 deaths each year (Scallan et al., 2011). In the US, E. coli O157:H7 and 6 non-O157 STEC serogroups, including O26, O45, O103, O111, O121, and O145, are considered adulterants in raw nonintact beef products (USDA-FSIS, 2012; Wheeler et al., 2014).

Cattle are known reservoirs for STEC and Salmonella, and it has been reported that fecal contamination of the hide is likely the primary source of contamination of beef carcasses with these pathogens (Barkocy-Gallagher et al., 2002; Yang et al., 2017). In an effort to ultimately provide consumers with microbiologically safe products, the beef industry utilizes a number of sequential decontamination interventions during processing to reduce pathogen contamination levels on carcasses and beef products (Pohlman et al., 2002; Geornaras et al., 2012a; Buncic et al., 2014; Schmidt et al., 2014; Wheeler et al., 2014; Scott et al., 2015; Yang et al., 2017). These decontamination processes are physical and chemical in nature and include, among others, knife trimming, steam vacuuming, water washes, and application of antimicrobial treatments (Pohlman et al., 2002; Wheeler et al., 2014). The efficacy of various chemical interventions—such as organic acids (e.g., lactic acid) and peroxyacetic acid (PAA)—for reducing STEC and Salmonella have been extensively evaluated (Bacon et al., 2000; Berry and Cutter, 2000; Ransom et al., 2003; Gill and Badoni, 2004; Koohmaraie et al., 2005; Mohan and Pohlman, 2016; Kocharunchitt et al., 2020). Still, the beef industry continues to seek new chemical interventions for use in a multiple hurdle system to reduce pathogens on beef carcasses.

Chemical interventions should be inexpensive and easily implemented into existing systems while meeting regulatory standards (Scott et al., 2015). Use of PAA has been shown to effectively reduce STEC and Salmonella contamination on various beef products (Kitis, 2004; King et al., 2005; Geornaras et al., 2012a, b; Mohan and Pohlman, 2016), utilizing an oxidative mechanism for killing bacteria (Kitis, 2004). However, to our knowledge, there are no published studies reporting on the efficacy of PAA solutions that are pH adjusted with organic or inorganic acids (hereafter referred to as “acidified PAA”) for reducing bacterial contamination on beef. Therefore, the objective of two studies that were conducted was to evaluate the antimicrobial effects of PAA acidified with different acids against inoculated surrogates for STEC and Salmonella on prerigor beef carcass surface tissue.

Levels of naturally occurring Enterobacteriaceae populations on the uninoculated beef tissue samples analyzed in the first study were ≤1.9 log CFU/cm², while in the second study, Enterobacteriaceae populations were not detected (<0.2 log CFU/cm²) in any of the samples analyzed. Therefore, Enterobacteriaceae levels naturally associated with the prerigor beef samples used in the studies were lower than the Enterobacteriaceae populations recovered from any of the inoculated samples (Tables 1–2). As such, it can be concluded that the bacterial populations recovered from inoculated control (untreated) and immersion- or spray-treated beef samples were those of the E. coli inoculum.

Enterobacteriaceae populations recovered from inoculated untreated and immersion-treated beef tissue samples are shown in Table 1. Least-squares means are presented for the main effect of antimicrobial treatment. When compared to the untreated control, all treatments were effective (P < 0.05) in reducing inoculated E. coli populations (Table 1). Specifically, all 6 immersion treatments reduced (P < 0.05) initial inoculated bacterial populations (6.2 log CFU/cm²) by 2.3 to 2.8 log CFU/cm². No (P ≥ 0.05) differences in antimicrobial effects against the surrogates were observed between the 400-ppm PAA acidified with acetic acid, lactic acid, or pH 1.2 SSS treatments. Furthermore, antimicrobial effects against the E. coli populations were similar (P ≥ 0.05) for samples immersion-treated with 400-ppm PAA acidified with acetic acid, citric acid, or pH 1.8 SSS, as well as 300-ppm PAA acidified with pH 1.2 SSS. Overall, although statistical differences (P < 0.05) were noted between some of the treatments, the difference in log reductions between these treatments was small (0.5 log CFU/cm²).

Results obtained for the untreated and spray-treated beef samples are shown in Table 2. Least-squares means are presented for the main effect of antimicrobial treatment. Application of the water treatment was not effective (P ≥ 0.05) in reducing inoculated bacterial populations. However, all of the tested acid treatments effectively (P < 0.05) lowered the inoculated E. coli populations, when compared to the untreated control and water treatments. Specifically, the PAA and acidified PAA treatments reduced (P < 0.05) initial surrogate populations (6.4 log CFU/cm²) by 1.7 to 1.9 log CFU/cm². Under the experimental conditions of the study, no differences (P ≥ 0.05) in decontamination efficacy were obtained between the acidified and nonacidified PAA treatments.

Various chemical interventions used within the beef industry have been reported to have differing efficacies against bacterial populations on different beef tissues (Podolak et al., 1996; King et al., 2005; Geornaras et al., 2012a, 2012b; Schmidt et al., 2014; Yang et al., 2017; Cap et al., 2019; Thomas et al., 2020). In the US, PAA is commonly applied as a carcass wash intervention in beef processing facilities at a maximum concentration of 400 ppm (USDA-FSIS, 2019). In the current study, PAA (300 ppm or 400 ppm) solutions that were pH adjusted with one of 3 organic acids or SSS were initially screened, using an immersion application method, for their decontamination efficacy against the surrogate populations. In the second study, a spray application method was used, which is more representative of how the industry would apply antimicrobial interventions to beef carcasses, to compare the antimicrobial effects of 3 acidified PAA treatments to those of PAA solutions (350 ppm and 400 ppm) that were not acidified. Overall, the results showed that all acidified PAA immersion and spray treatments were effective (P < 0.05) in reducing bacterial contamination on prerigor beef carcass surface tissue. Initial surrogate populations were lowered by 2.3 to 2.8 log CFU/cm² when samples were immersion treated and by 1.7 to 1.9 log CFU/cm² when spray treated (Tables 1–2). However, as seen, antimicrobial effects of the acidified PAA spray treatments were not (P ≥ 0.05) different from those of PAA that was not acidified (Table 2). It should be noted that since a selective-differential plating medium, namely Enterobacteriaceae Petrifilm, was used for recovery of surviving populations, selective ingredients present in the culture medium could have hindered resuscitation of bacterial cells sublethally injured from exposure to the treatments (Bosilevac et al., 2004). As such, it is possible that surviving bacterial populations could actually be greater than those reported here.

To our knowledge, this is the first report on the antimicrobial effects of pH-adjusted solutions of PAA against inoculated bacterial populations on prerigor beef carcass surface tissue. In comparison, there are numerous published reports on the decontamination efficacy of the individual chemicals used in the current studies, including PAA (nonacidified), lactic acid (LA), and SSS, against pathogen populations on various beef tissue types (Berry and Cutter, 2000; King et al., 2005; Geornaras et al., 2012b; Schmidt et al., 2014; Scott-Bullard et al., 2017; Yang et al., 2017; Signorini et al., 2018). Due to differences in experimental parameters (e.g., antimicrobial concentrations tested, treatment application parameters, target populations, beef tissue type), direct comparisons of the results of these studies with those obtained in the current work cannot be made; however, the findings of these studies can be discussed as alternative intervention methods. In one study (Schmidt et al., 2014), researchers evaluated the efficacy of LA (2.5% and 5%), SSS (1%), and PAA (220 ppm), applied as immersion treatments, for reducing levels of pathogen populations inoculated on the adipose side of beef cheek meat. Cheek meat samples were inoculated with either E. coli O157:H7 (3.9 log CFU/cm²), non-O157 STEC (4.0 log CFU/cm²), or Salmonella (4.1 log CFU/cm²) and were immersed in the treatment solutions for 1 min, 2.5 min, or 5 min. Schmidt et al. (2014) reported that, regardless of pathogen type and immersion treatment time, LA (2.5% and 5%), PAA, and SSS reduced inoculated populations by 1.4 to 2.1, 0.8 to 1.3, and 1.1 to 1.5 log CFU/cm², respectively. Geornaras et al. (2012b) evaluated 30-s immersion treatments of PAA (200 ppm) or SSS (pH 1.2) for their antimicrobial effects against inoculated populations of E. coli O157:H7 and non-O157 STEC on beef trimmings. The authors reported that treatment with 200 ppm PAA reduced inoculated populations (3.4–3.9 log CFU/cm²) by 0.6 to 1.0 log CFU/cm² and treatment with SSS (pH 1.2) reduced pathogen populations by 0.3 to 0.4 log CFU/cm² (Geornaras et al., 2012b). In another study (King et al., 2005), PAA (200 ppm, 43°C)—applied as a 15-s spray treatment to prerigor tissue obtained from the outside round, plate, clod, and brisket regions of beef carcasses—reduced inoculated STEC and Salmonella populations by 0.7 log CFU/cm².

Overall, under the experimental conditions of the studies conducted, PAA and pH-adjusted PAA treatments effectively reduced populations of the E. coli surrogates for STEC and Salmonella on prerigor beef carcass surface tissue. However, our results showed that acidification of PAA (350 ppm or 400 ppm) with 2% acetic acid or pH 1.2 SSS did not enhance the antimicrobial effects of PAA. Chemical spray washes are commonly applied to beef carcasses at one or more points throughout the harvesting process (e.g., after hide removal, after evisceration, during carcass chilling) as part of a multiple-hurdle approach to reduce pathogen levels during slaughter. To better elucidate the decontamination efficacy, future studies should consider testing the pH-adjusted PAA in such a multi-hurdle system.