Oxidants as an antimicrobial intervention on Escherichia coli during carcase chilling

The red meat industry is a multi-billion-dollar business for Australia and many other countries around the world. Ensuring meat quality and safety is paramount to the survival and economic success of the industry. Interventions used for hygiene and safety start from on-farm animal husbandry practices, to physical and chemical applications during processing, right up until consumption with the use of effective packaging and storage technologies. Processing-based interventions including knife trimming, organic acid washes, steam vacuums and UV light have been shown to improve the microbial shelf-life and safety of fresh meat. However, no single intervention can ensure complete safety of red meat. Pathogenic Escherichia coli, especially the O157 serotype, have been linked to serval large food-borne outbreaks associated with consumption of bovine origin foods. This is despite control measures, referred to here as ‘antimicrobial interventions’, already in use during carcase processing. Currently, no single method is completely effective in controlling these pathogens. Zero tolerance policies for pathogenic E. coli serotypes require Australian exporters to have confidence the meat products have minimal risk of E. coli and other pathogen contamination. Thus, there is the opportunity and desire for additional novel antimicrobial interventions within the processing system that can reliably achieve high quality, safe meat for export. As carcase chilling is an existing step in the processing chain of meat, it would be an advantage to couple this process with an additional intervention that would enhance the inactivation of contaminant bacteria, especially pathogenic E. coli and Salmonella spp. In implementing an additional practise during chilling in plants already set up for spray chilling or spray washing of some sort, this approach should be relatively inexpensive and easy to put in place. In 2 deciding on any new intervention, consideration needs to be taken for the specific industry environment, ability to implement additional infrastructure, cost effectiveness for site specific issues and the types of products produced. This study investigated the potential for oxidants as an additional intervention strategy when applied during carcase chilling. Initially broth-based studies were done to test King et al. (2016) hypothesis of increased susceptibility of gram-negative bacterial pathogens to oxidants during typical carcase chilling conditions. An oxidant (hydrogen peroxide at 75 ppm or chlorine dioxide at 7.5 ppm), at a non-lethal level under optimal (growth-permitting) conditions, was applied during dynamic changes in growth kinetics of E. coli O157:H7 Sakai, induced by abrupt downshifts in temperature and water (35°C a\(_w\) 0.993 to 14°C a\(_w\) 0.967). The addition of hydrogen peroxide at 1.5, 7.5, and 22.5 h after the downshift caused a reduction in viable bacterial counts (>3 log reduction) only at the 1.5 h time point. Similarly, chlorine dioxide when added to cultures under chilling conditions caused inactivation of E. coli O157:H7 Sakai though at all time points. Comparisons were also made with other E. coli and Salmonella enterica strains. All additions of chlorine dioxide at the early stages of the spray chilling conditions induced faster inactivation rates in culture-based trials. The results highlighted the potential application of oxidants under chilling conditions and further studies on meat tissue were warranted. To determine further the potential utility of oxidants during carcase chilling, a series of laboratory-scale studies were undertaken to evaluate the efficacy of chlorine dioxide (ClO\(_2\)) or peroxyacetic acid (PAA) in eliminating E. coli O157:H7 Sakai on beef meat during the simulated process of spray chilling (4 sec every 15 min for 36 cycles) or when applied continuously prior to the spray chilling process (144 sec). In all cases, the effect of the oxidant was most evident on fat surfaces, rather than on lean surfaces. ClO\(_2\) at 15 ppm, a non-lethal level under optimal growth conditions, when applied during spray chilling, caused higher 3 reductions in E. coli O157:H7 Sakai numbers (~3 log reduction) than when applied before the same spray chilling (~1 log reduction). This reinforces the increased susceptibility of E. coli O157:H7 Sakai to oxidative stress during spray chilling. In subsequent studies, both ClO\(_2\) and PAA at levels of 20 and 200 ppm, respectively, produced more pronounced lethal effects on E. coli, achieving ≥4 log reduction at the end of chilling. These results indicate the potential for further developing an oxidant-based application during spray chilling as an antimicrobial intervention, to minimise the problems associated with enteric pathogens on beef meat. The addition of an oxidant, in the early stages of chilling, warranted further studies on carcase chilling in commercial settings. Therefore, an in-plant trial was conducted to evaluate and compare the performance of two different oxidants (ClO\(_2\) and PAA), as antimicrobial interventions for E. coli on beef. These trials involved the use of beef carcase sides and a chiller allowing for an industrial spray regime. Natural populations of E. coli were too low and infrequent to assess the effect of the oxidants. Therefore, a cocktail of non-pathogenic E. coli strains was deliberately applied to meat surfaces. Both oxidants reduced the total microbial count and E. coli significantly, although a greater effect was seen on the E. coli populations. A 1 - 3 log CFU/cm\(^2\) reduction was found on the carcase sides depending on the site sampled. Sites located at the top of the suspended carcase, such as the hind leg and flank-brisket had the highest log reductions, most likely due to greater exposure to the spray. The results showed adding an oxidant to a spray chilling regime is an attractive potential antimicrobial intervention for industry, with little to no structural changes in the plant. Shelf-life studies were also conducted following the in-plant trials with non-inoculated vacuum-packed (VP) striploins. VP striploins were stored at -1°C for up to 194 days and were assessed periodically for pH, viable count, and sensory attributes (colour, odour, and visual preference). No detrimental effects were detected for either oxidant treatment. However, some striploins had significant (P < 0.5) odour differences between PAA treated and non-treated 4 samples. There were also significant differences in visual preferences observed by sensory panellists for PAA treatments. Expansion of the sensory panel accompanied by training would make to make the results more reliable, because much variation was observed in the results of the small, untrained, panel used in this study. Also, different cuts and higher grades of meat would assist in the evaluation of the treatments from a sensory perspective. Overall, PAA was found to be a more effective antimicrobial intervention compared to ClO\(_2\) during chilling, with greater log reductions of enteric pathogens, greater compatibility with existing industry practices, and more positive outcomes from sensorial analysis. The current studies indicate that adding an oxidant to spray chilling systems in abattoirs can increase consumer safety and realise economic benefits for the industry