**4. Discussion**

Multiple intervention strategies, including the use of chemical antimicrobial treatments, are used by the U.S. poultry processing industry to reduce the prevalence of *Campylobacter* and *Salmonella* on whole carcasses and parts [11]. These chemical decontamination treatments are applied as sprays and/or immersion (dip) treatments at pre- and post-chill stages of processing [11,22]. In the current study, SSS, formic acid, PAA, and two acidified PAA treatments (SSS-aPAA and FA-aPAA) were evaluated for their antimicrobial effects against *C. jejuni* populations on chicken wings. Overall, all of the chemical treatments were effective (*p* < 0.05) in reducing initial pathogen levels, and under the experimental conditions of the study, greater reductions were obtained when the wings received the treatment by immersion (5 s) than as a spray (4 s) (Tables 2 and 3).

The antimicrobial effects of SSS against various foodborne pathogens have been previously evaluated, mostly on beef products [23–30] but also on poultry carcasses and parts by a few investigators [18,31,32]. Scott et al. [18] reported a 1.2 log CFU/mL reduction of

inoculated (5.5 log CFU/mL) *Salmonella* populations on chicken wings that were immersed for 20 s in a pH 1.1 solution of SSS. In another study [31], immersion of turkey drumsticks in SSS (pH 1.3) for 30 s lowered inoculated (7–8 log CFU/g) *Salmonella* Reading and *Salmonella* Typhimurium populations by 2.2 and 2.4 log CFU/g, respectively. In the current study, *C. jejuni* levels on wings were reduced (*p* < 0.05) by 1.7 and 0.5 log CFU/mL immediately following immersion or spray treatment with SSS (pH 1.2), respectively (Tables 2 and 3). To our knowledge, there has only been one other published study that has investigated the antimicrobial efficacy of SSS against *Campylobacter* on poultry. In this particular study [32], a 1.5 log CFU/chicken reduction of naturally occurring *Campylobacter* spp. populations was reported when post-chilled whole carcasses were immersed in SSS (pH 1.4) for 15 s.

Published reports on the use of formic acid as a decontamination treatment of poultry are limited. Riedel et al. [33] observed a 1.6 log CFU/mL reduction of *C. jejuni* inoculated on chicken skin that was immersed for 1 min in 2% formic acid. In the present study, the antimicrobial efficacy of 1.5% formic acid against initial populations of *C. jejuni* was similar (*p* ≥ 0.05) to that of SSS, regardless of the application method (Tables 2 and 3). Specifically, reductions of 1.8 and 0.7 log CFU/mL were obtained for wings immersion- or spray-treated with formic acid, respectively.

As previously mentioned, PAA is currently one of the most commonly used antimicrobials in U.S. poultry slaughter and processing facilities, and its effectiveness in reducing pathogen contamination on poultry-associated products has been extensively reported [11,14,16,18,31,32,34–39]. Naturally occurring *Campylobacter* spp. levels were reduced by 2.2 log CFU/chicken when post-chilled whole carcasses were subjected to a 15 s dip in 750 ppm PAA [32]. Nagel et al. [35] also evaluated PAA as a post-chill immersion (20 s) treatment of whole carcasses and reported 1.9 and 2.0 log CFU/mL reductions of inoculated (ca. 5 log CFU/mL) *C. jejuni* populations with 400 ppm and 1000 ppm PAA, respectively. In another study [39], 200 ppm PAA applied as an immersion (60 s) or spray (62 s) treatment lowered *C. jejuni* levels of chicken carcasses by 1.4 and 0.6 log CFU/mL, respectively. PAA was also recently evaluated as a decontamination treatment of skinless, boneless chicken breast fillets [38]. Specifically, breast fillets inoculated with *Campylobacter coli* populations (4.9 log CFU/mL) were reduced by 0.9 and 0.8 log CFU/mL when they were immersed (3.5 L, 4 s) or sprayed (15 mL/s, 5 s) with 500 ppm PAA [38].

While the antimicrobial effects of PAA have been extensively investigated, there are only a few recently published studies on the use of pH-adjusted (acidified) PAA as a decontamination treatment of meat and poultry products [30,31]. In our study, no differences (*p* ≥ 0.05) were obtained between PAA and the acidified PAA treatments (SSSaPAA and FA-aPAA) with regard to reducing initial (0 h) levels of *C. jejuni* contamination, irrespective of whether the treatments were applied by immersion or in the spray cabinet. Specifically, the three PAA-containing treatments reduced 0 h pathogen populations by 2.1 to 2.2 log CFU/mL in immersion-treated samples, and 0.9 to 1.2 log CFU/mL in spray-treated samples (Tables 2 and 3). After refrigerated storage (4 ◦C, 24 h), however, differences (*p* < 0.05) were noted between recovered pathogen populations from wings that had been treated (immersion or spray) with PAA and those that received one of the acidified PAA treatments. While 0 h *C. jejuni* populations of PAA-treated wings remained relatively unchanged (*p* ≥ 0.05) following the 24-h storage period, pathogen levels of 24 h samples that had received either of the acidified PAA treatments were lower (*p* < 0.05; by 0.8 to >1.2 log CFU/mL for immersion-treated samples, and 0.4 to 0.6 log CFU/mL for spray-treated samples) than the populations recovered from the corresponding treatments at 0 h. Acidification of PAA, regardless of the acidifier (i.e., SSS or formic acid), combines two mechanisms of action. PAA is an oxidizing agen<sup>t</sup> that disrupts bacterial cell walls and essential enzyme functions [40,41], and formic acid and SSS cause cytoplasmic acidification which results in the accumulation of protons that leads to the cell using its energy to try to re-establish the intracellular pH [42–44]. Therefore, the combination of hurdles of the acidified PAA coupled with the subsequent low-temperature storage conditions probably impeded recovery of sub-lethally injured cells and likely explains the further reduction

of *C. jejuni* levels in the 24-h acidified PAA-treated samples. Evidence of sub-lethal cell injury was also observed for wings that were immersed in SSS or formic acid (i.e., without PAA) (Table 2). Scott et al. [18] and Riedel et al. [33] also reported further reductions of pathogen populations following refrigerated storage of SSS- and formic acid-treated samples, respectively.

Two previous studies have evaluated the antimicrobial effects of acidified PAA treatments [30,31]. Similar to the0hresults of our study, Olson et al. [31] reported no differences between *Salmonella* reductions obtained immediately following treatment (30 s immersion) of turkey drumsticks with 500 ppm PAA or PAA (500 ppm) acidified with SSS (pH 1.3). In contrast to the findings of our study, subsequent storage (4 ◦C, 24 h) did not result in further reductions of *Salmonella* populations on samples treated with SSS-acidified PAA [31]. Acidified PAA solutions have also been evaluated as spray treatments (10 s, 103 kPa) of prerigor beef carcass surface tissue for reduction of nonpathogenic *Escherichia coli* surrogates for Shiga toxin-producing *E. coli* and *Salmonella* [30]. The authors of this study reported that acidification of PAA (350 ppm or 400 ppm) with 2% acetic acid or pH 1.2 SSS did not (*p* ≥ 0.05) enhance the immediate antimicrobial effects of non-acidified PAA (350 ppm or 400 ppm) [30].
