Decontamination of pork carcasses during scalding and the prevention of Salmonella cross-contamination

Modeling nonlinear inactivation of hygiene indicator bacteria in pig carcasses during scalding at different pHs

This study aimed to investigate the influence of adding an alkalizing agent to the scalding water of a slaughterhouse in Brazil to inactivate hygiene indicator bacteria in pig carcasses. Scalding is critical during carcass processing because slaughterhouses' scalding water is constantly renewed; therefore, it is usually contaminated with organic matter, such as faeces and dirt from the previous carcasses. The treatments evaluated consisted of counting Enterobacteriaceae and mesophilic bacteria in pork jowls at 62 °C, 65 °C, 68 C, and 72 °C after 0.0, 1.5, 3.0, 4.5, 6.0, and 7.5 min of simulated scalding at the pHs of 7.0 (control) and 11.0 (after addition of alkalizing agent). Decimal reduction times of hygiene indicator bacteria for all treatments were estimated with different nonlinear bacterial inactivation models. As a result, adding the alkalizing agent did not significantly inactivate most of the bacteria in the studied samples. However, it contributed to the inactivation of some bacteria, mostly belonging to the mesophilic group, at some specific temperatures. The results obtained in the current study can provide useful insights into dealing with pig carcass contamination in a real-world scenario and applying the obtained information in the industrial environment.

The porcine respiratory microbiome: recent insights and future challenges

Understanding the structure of the respiratory microbiome and its complex interactions with opportunistic pathogenic bacteria has become a topic of great scientific and economic interest in livestock production, given the severe consequences of respiratory disease on animal health and welfare. The present review focuses on the microbial structures of the porcine upper and lower airways, and the factors that influence microbiome development and onset of respiratory disease. Following a literature search on PubMed and Scopus, 21 articles were selected based on defined exclusion criteria (20 studies performed by 16S rRNA gene sequencing and one by shotgun metagenomics). Analysis of the selected literature indicated that the microbial structure of the upper respiratory tract undergoes a remarkable evolution after birth and tends to stabilise around weaning. Antimicrobial treatment, gaseous ammonia concentration, diet and floor type are amongst the recognized environmental factors influencing microbiome structure. The predominant phyla of the upper respiratory tract are Proteobacteria and Firmicutes with significant differences at the genus level between the nasal and the oropharyngeal cavity. Only five studies investigated the lower respiratory tract and their results diverged in relation to the relative abundance of these two phyla and even more in the composition of the lung microbiome at the genus level, likely because of methodological differences. Reduced diversity and imbalanced microbial composition are associated with an increased risk of respiratory disease. However, most studies presented methodological pitfalls concerning specimen collection, sequencing target and depth, and lack of quality control. Standardization of sampling and sequencing procedures would contribute to a better understanding of the structure of the microbiota inhabiting the lower respiratory tract and its relationship with pig health and disease.

Increased thermal sensitivity of Listeria monocytogenes in sous-vide salmon by oregano essential oil and citric acid

Inadequate cooking during sous-vide processing may cause foodborne diseases in case the food is contaminated with pathogens such as Listeria monocytogenes. In this study, thermal inactivation of L. monocytogenes in sous-vide processed salmon was investigated. Oregano oil and citric acid were used alone or in combination to determine the probability of increasing the efficiency of heat treatment. Control (C); 0.5% citric acid added (S); 1% oregano essential oil added (O); and citric acid and oregano essential oil combined (OS) groups were prepared. Samples were inoculated with L. monocytogenes, vacuum packed, then sous-vide cooked at 55, 57.5, 60, or 62.5 °C for predetermined times. The D-values of all treated samples were significantly lower than control. The use of oregano oil (O), citric acid (S) and their combination (OS) significantly reduced the time required to inactivate L. monocytogenes. The z-values of L. monocytogenes in C, O, S and OS groups were 5.50, 5.62, 6.54, and 6.92 °C, respectively. It was determined that effective results could be achieved by adding natural antimicrobials to provide safety in sous-vide fish.

Thermal Inactivation Kinetics of Three Heat-Resistant Salmonella Strains in Whole Liquid Egg

The heat resistance of three heat-resistant strains of Salmonella was determined in whole liquid egg (WLE). Inoculated samples in glass capillary tubes were completely immersed in a circulating water bath and held at 56, 58, 60, 62, and 64°C for predetermined lengths of time. The recovery medium was tryptic soy agar with 0.1% sodium pyruvate and 50 ppm of nalidixic acid. Survival data were fitted using log-linear, log-linear with shoulder, and Weibull models using GInaFiT version 1.7. Based on the R² and mean square error, the log-linear with shoulder and Weibull models consistently produced a better fit to Salmonella survival curves obtained at these temperatures. Contaminated WLE must be heated at 56, 60, and 64°C for at least 33.2, 2.7, and 0.31 min, respectively, to achieve a 4-log reduction of Salmonella; 39.0, 3.1, and 0.34 min, respectively, for a 5-log reduction; and 45.0, 3.5, and 0.39 min, respectively, for a 6-log reduction. The z-values calculated from the D-values were 3.67 and 4.18°C for the log-linear with shoulder and Weibull models, respectively. Thermal death times presented in this study will be beneficial for WLE distributors and regulatory agencies when designing pasteurization processes to effectively eliminate Salmonella in WLE, thereby ensuring the microbiological safety of the product.

Validation of a Sequential Hide-On Bob Veal Carcass Antimicrobial Intervention Composed of a Hot Water Wash and Lactic Acid Spray in Combination with Scalding To Control Shiga Toxin-Producing Escherichia coli Surrogates†

Scalding of hide-on bob veal carcasses with or without standard scalding chemical agents typically used for hogs, followed by an 82.2°C hot water wash and lactic acid spray (applied at ambient temperature) before chilling, was evaluated to determine its effectiveness in reducing Shiga toxin-producing Escherichia coli surrogate populations. A five-strain cocktail of rifampin-resistant, nonpathogenic E. coli surrogates was used to inoculate hides of veal carcasses immediately after exsanguination (target inoculation level of 7.0 log CFU/100 cm²). For carcasses receiving no scalding treatments, spraying with 82.2°C water as a final wash resulted in a 4.5-log CFU/100 cm² surrogate reduction, and an additional 1.2-log CFU/100 cm² reduction was achieved by spraying with 4.5% lactic acid before chilling. Scalding hide-on carcasses in 60°C water (no chemicals added) for 4 min in a traditional hog scalding tank resulted in a 2.1-log CFU/100 cm² reduction in surrogate levels, and a subsequent preevisceration 82.2°C water wash provided an additional 2.9-log CFU/100 cm² reduction. Spraying a 4.5% solution of lactic acid onto scalded, hide-on carcasses (after the 82.2°C water wash) resulted in a minimal additional reduction of 0.4 log CFU/100 cm². Incorporation of scalding chemicals into the scald water resulted in a 4.1-log CFU/100 cm² reduction (1.9 log CFU/100 cm² greater than scalding without chemicals) in the surrogate population, and the first 82.2°C wash provided an additional 2.5-log CFU/100 cm² reduction. Application of antimicrobial interventions did not affect the carcass temperature decline during chilling, the pH decline, or the color characteristics of the ribeye or the flank of the bob veal carcasses.

Contamination of Carcasses and Utensils in Small Swine Slaughterhouses by Salmonella in the Northwestern Region of the State of Rio de Janeiro, Brazil

Salmonella is a major foodborne pathogen that constantly threatens food safety in developed countries and underdeveloped countries such as Brazil, where it is responsible for 38% of notified cases of foodborne illness. Swine are one of the main meat-producing species that may asymptomatically carry Salmonella, periodically shedding the bacteria through feces. The state of Rio de Janeiro is not a major producer of swine meat, but small slaughterhouses are operational and produce meat for consumption within the state, although few studies have been conducted in the region. In this context, this study was designed to evaluate Salmonella contamination in carcasses, lymph nodes, feces, utensils, and the environment of three small slaughterhouses in the state of Rio de Janeiro. A total of 344 samples from carcasses, utensils, equipment, water, and the environment were collected from these slaughterhouses in the northwestern region of Rio de Janeiro. Salmonella was isolated from 36 (10.5%) samples: 10 of 48 carcasses, 19 of 96 lymph nodes, 4 of 48 fecal samples, 2 of 6 water samples, and 1 of 6 bleeding knives; 55 isolates were recovered. Serotyping revealed the predominance of Salmonella Typhimurium (20 isolates) followed by Salmonella Abony (10 isolates), Salmonella Give (7 isolates), Salmonella Heidelberg (4 isolates), and Salmonella Infantis (1 isolate). Ten isolates were only partially typeable, with only their O antigen identified, and three isolates had rough, nontypeable colonies. Despite the overall low prevalence of Salmonella in this study, all three slaughterhouses had poor hygienic and sanitary conditions, providing easy routes for carcass and, consequently, meat contamination. Thus, it is imperative to enforce sanitary inspections in these establishments and to apply good manufacture practices to assure the safety of the produced pork.

Modeling of Salmonella Contamination in the Pig Slaughterhouse

In this article we present a model for Salmonella contamination of pig carcasses in the slaughterhouse. This model forms part of a larger QMRA (quantitative microbial risk assessment) on Salmonella in slaughter and breeder pigs, which uses a generic model framework that can be parameterized for European member states, to describe the entire chain from farm-to-consumption and the resultant human illness. We focus on model construction, giving mathematical formulae to describe Salmonella concentrations on individual pigs and slaughter equipment at different stages of the slaughter process. Variability among individual pigs and over slaughterhouses is incorporated using statistical distributions, and simulated by Monte Carlo iteration. We present the results over the various slaughter stages and show that such a framework is especially suitable to investigate the effect of various interventions. In this article we present the results of the slaughterhouse module for two case study member states. The model outcome represents an increase in average prevalence of Salmonella contamination and Salmonella numbers at dehairing and a decrease of Salmonella numbers at scalding. These results show good agreement when compared to several other QMRAs and microbiological studies.

The commercial impact of pig Salmonella spp. infections in border-free markets during an economic recession

The genus Salmonella, a group of important zoonotic pathogens, is having global economic and political importance. Its main political importance results from the pathogenicity of many of its serovars for man. Serovars Salmonella Enteritidis and Salmonella Typhimurium are currently the most frequently associated to foodborne infections, but they are not the only ones. Animal food products contaminated from subclinically infected animals are a risk to consumers. In border free markets, an example is the EU, these consumers at risk are international. This is why, economic competition could use the risk of consumer infection either to restrict or promote free border trade in animals and their products. Such use of public health threats increases during economic recessions in nations economically weak to effectively enforce surveillance. In free trade conditions, those unable to pay the costs of pathogen control are unable to effectively implement agreed regulations, centrally decided, but leaving their enforcement to individual states. Free trade of animal food products depends largely on the promotion of safety, included in "quality," when traders target foreign markets. They will overtake eventually the markets of those ineffectively implementing agreed safety regulations, if their offered prices are also attractive for recession hit consumers. Nations unable to effectively enforce safety regulations become disadvantaged partners unequally competing with producers of economically robust states when it comes to public health. Thus, surveillance and control of pathogens like Salmonella are not only quantitative. They are also political issues upon which states base national trade decisions. Hence, the quantitative calculation of costs incurring from surveillance and control of animal salmonelloses, should not only include the cost for public health protection, but also the long term international economic and political costs for an individual state. These qualitative and qualitative costs of man and animal Salmonella infections should be calculated in the light of free trade and open borders. Understandably, accurate calculation of the economic and political costs requires knowledge of the many factors influencing nationally the quality and safety of pork products and internationally free trade. Thus, how Salmonella pig infections affect commerce and public health across open borders depends on a state's ability to accurately calculate costs for the surveillance and control of animal salmonelloses in general, and pig infections as a particular example.

Effectiveness of simulated interventions in reducing the estimated prevalence of Salmonella in UK pig herds

Salmonella spp are a major foodborne zoonotic cause of human illness. Consumption of pork products is believed to be a major source of human salmonellosis and Salmonella control throughout the food-chain is recommended. A number of on-farm interventions have been proposed, and some have been implemented in order to try to achieve Salmonella control. In this study we utilize previously developed models describing Salmonella dynamics to investigate the potential effects of a range of these on-farm interventions. As the models indicated that the number of bacteria shed in the faeces of an infectious animal was a key factor, interventions applied within a high-shedding scenario were also analysed. From simulation of the model, the probability of infection after Salmonella exposure was found to be a key driver of Salmonella transmission. The model also highlighted that minimising physiological stress can have a large effect but only when shedding levels are not excessive. When shedding was high, weekly cleaning and disinfection was not effective in Salmonella control. However it is possible that cleaning may have an effect if conducted more often. Furthermore, separating infectious animals, shedding bacteria at a high rate, from the rest of the population was found to be able to minimise the spread of Salmonella.

Role of slaughtering in Salmonella spreading and control in pork production

Salmonella is one of the major foodborne pathogens worldwide. Pork products are among the main sources of Salmonella infection in humans, and several countries have established Salmonella surveillance and control programs. The role of slaughtering in carcass contamination has been indicated by studies focused on the slaughterhouse environment. In this review, we examine and discuss the information available regarding the influence that farm status, pig transport, and lairage have on the carriage of Salmonella by pigs entering the slaughter line. The evolution of carcass contamination throughout the slaughtering process, the main sources of contamination in the dirty and clean zones of the slaughter line, and previously reported prevalence of Salmonella on carcasses and factors affecting this prevalence also are discussed. The importance of implementing interventions at the slaughter level is discussed briefly. Consistent with the information available, pigs from infected farms and newly acquired or recrudescent infections in pigs at the subsequent stages of transport and lairage are important sources of Salmonella at the slaughtering plant. The continuous introduction of Salmonella into the slaughterhouse and the potential for resident flora constitute a risk for carcass contamination. At the slaughterhouse, some dressing activities can reduce carcass contamination, but others are critical control points that jeopardize carcass hygiene. This information indicates the importance of considering slaughter and previous stages in the pork production chain for controlling Salmonella in swine production.

Thermal inactivation of Yersinia enterocolitica in pork slaughter plant scald tank water

The objective of this study was to establish the time-temperature combinations required to ensure the thermal inactivation of Yersinia enterocolitica during scalding of pork carcasses. A 2 strain cocktail of Y. enterocolitica (bioserotypes 2/O:5,27 and 1A/O:6,30) was heat treated at 50, 55 and 60°C in samples of scald tank water obtained from a commercial pork slaughter plant. Samples were removed at regular intervals and surviving cells enumerated using (i) Cefsulodin-Irgasan-Novobiocin Agar (CIN) supplemented with ampicillin and arabinose and (ii) Tryptone Soya Agar (TSA), overlaid with CIN agar with ampicillin and arabinose. The data generated was used to estimate D- and z-values and the formula Dx=log(-1)(log D60-((t2-t1)/z)) was applied to calculate thermal death time-temperature combinations from 55 to 65°C. D50, D55 and D60-values of 45.9, 10.6 and 2.7min were calculated from the cell counts obtained on CIN agar, respectively. The corresponding D-values calculated from the TSA/CIN counts were 45.1, 11 and 2.5min, respectively. The z-value was 7.8. It was concluded that a time-temperature combination of 2.7min at 60°C is required to achieve a 1 log reduction in Y. enterocolitica in pork scald tank water. The predicted equivalent at 65°C was 0.6min. This study provides data and a model to enable pork processors to identify and apply parameters to limit the risk of carcass cross-contamination with Y. enterocolitica in pork carcass scald tanks.

Longitudinal Dissemination of Salmonella enterica Clonal Groups through the Slaughter Process of Salmonella-Positive Pig Batches

This study was conducted to assess the dissemination of Salmonella clonal groups in slaughterhouses that received batches of Salmonella-positive pigs and used different routine processing procedures. Eight serial sampling sessions were conducted in three slaughterhouses (A, B, and C). Blood was collected randomly (n = 25) from each batch of pigs and processed for serology. Carcasses (n = 12) were identified and sampled after dehairing, after singeing, after evisceration, and before chilling. A section of cecum also was collected. Salmonella isolates were submitted to pulsed-field gel electrophoresis. The overall seroprevalence of Salmonella was 80.6% (316 of 392 samples), and cecal contents were positive for Salmonella in 23.8% (26 of 109) of the pigs sampled. Carcasses after dehairing had a significantly higher prevalence of Salmonella (P = 0.004) and the highest Salmonella levels (median = 0.26 log CFU/300 cm2). The singeing step significantly affected the Salmonella status of the carcasses (P < 0.001); however, the efficacy of singeing differed among slaughterhouses. In the prechilling step, 14.7% (16 of 109) of the carcasses were positive for Salmonella. Salmonella pulsotypes found on the prechill carcasses were also found in the lairage, in the cecal contents, and on carcasses after dehairing, suggesting that the main source of contamination was the slaughter process before singeing. Slaughterhouse C was the most likely (odds ration [OR] = 6.51) to have pigs carrying Salmonella in the gut, and slaughterhouse B was the most likely (OR = 14.66) to have contaminated carcasses at the prechilling step. These findings indicate that the procedures adopted in slaughterhouse B contributed to the spread of Salmonella strains. In contrast, in slaughterhouse C the Salmonella strains carried by the pigs or found in the lairage were not recovered from prechilled carcasses, validating the effectiveness of the slaughterhouse interventions. These results indicate that an effective slaughter process can help decrease the number of Salmonella-positive carcasses in slaughterhouses that receive Salmonella-positive pig batches.

Risk factors for Salmonella and hygiene indicators in the 10 largest Belgian pig slaughterhouses

A survey was conducted to collect data on Salmonella prevalence, Escherichia coli counts (ECCs), and aerobic bacteria colony counts (ACCs) on pig carcasses after chilling at the 10 largest Belgian pig slaughterhouses during 2000 through 2004. Potential risk factors of contamination associated with production parameters, technical descriptions of the installations, and cleaning and disinfection methods were assessed during investigations in the slaughterhouses. These variables were used first in a univariate analysis and then were extended to a multivariate analysis with a logistic mixed regression model for Salmonella and a linear mixed model for ECCs and ACCs with slaughterhouses as the random effect. The results indicated high variability concerning Salmonella contamination among the 10 slaughterhouses, with prevalence ranging from 2.6 to 34.3% according to the area of origin. The median ECC and median ACC ranged from -0.43 to 1.11 log CFU/cm2 and from 2.37 to 3.65 log CFU/cm2, respectively. The results of the logistic and linear regressions revealed that some working practices such as scalding with steam, second flaming after polishing, and complete cleaning and disinfection of the splitting machine several times a day were beneficial for reducing Salmonella prevalence, ECCs, and ACCs. Changing the carcass hooks just before chilling, using water as the cleaning method, and a higher frequency of disinfection of the lairage seemed to be protective against E. coli in the multivariate mixed linear model. The monitoring of critical points, slaughterhouse equipment, good slaughtering practices, and effective washing and disinfection are the keys to obtaining good microbiological results.

Thermal inactivation of Bacillus cereus and Clostridium perfringens vegetative cells and spores in pork luncheon roll

The aim of this study was to design a thermal treatment(s) for pork luncheon roll, which would destroy Bacillus cereus and Clostridium perfringens vegetative cells and spores. B. cereus and C. perfringens vegetative and spore cocktails were used to inoculate luncheon meat. Samples were subjected to different temperatures and removal times. The decimal-reduction times (D-values) were calculated by linear regression analysis (D = -1/slope of a plot of log surviving cells versus time). The log(10) of the resulting D-values were plotted against their corresponding temperatures to calculate (-1/slope of the curve) the thermal resistance (z-values) of each cocktail. The D-values for vegetative cells ranged from 1 min (60 degrees C) to 33.2 min (50 degrees C) for B. cereus and from 0.9 min (65 degrees C) to 16.3 min (55 degrees C) for C. perfringens. The D-values for B. cereus spores ranged from 2.0 min (95 degrees C) to 32.1 min (85 degrees C) and from 2.2 min (100 degrees C) to 34.2 min (90 degrees C) for C. perfringens. The z-values were calculated to be 6.6 and 8.5 degrees C for B. cereus vegetative and spores, respectively, and 7.8 and 8.4 degrees C for C. perfringens vegetative cells and spores, respectively. The D-values of B. cereus and C. perfringens suggest that a mild cook of 70 degrees C for 12s and 1.3 min would achieve a 6 log reduction of B. cereus and C. perfringens vegetative cells, respectively. The equivalent reduction of B. cereus and C. perfringens spores would require the pork luncheon meat to be heated for 36 s at 105 and 110 degrees C, respectively. The results of this study provide the thermal inactivation data necessary to design a cooking protocol for pork luncheon roll that would inactivate B. cereus and C. perfringens vegetative cells and spores. The data may also be used in future risk assessment studies.

Band-like opacity in the corneas of abattoir-acquired pig eyes

In the course of an investigation into the distribution of immune cells in the porcine cornea, a band-like lesion on the cornea of two-thirds of the eyes acquired from the local abattoir was noted. Histological investigations revealed an area of corneal epithelium debridement with no obvious other pathological changes. Discussions with abattoir staff soon revealed the cause of the lesions to be the scalding process that all pig carcasses undergo immediately post-mortem that serves to remove unwanted hair and reduce skin-dwelling bacterial contamination. We concluded that the band-like opacity was the result of thermal injury to the anterior surface of the cornea between the open eyelids. It is hoped that this short communication will act as a cautionary note to other investigators who currently use or are considering using porcine eyes in research or as a model in surgical training and who may be unaware of the conventional handling procedures in pig abattoirs.

Application of enterobacterial repetitive intergenic consensus-polymerase chain reaction to trace the fate of generic Escherichia coli within a high capacity pork slaughter line

The dissemination of enteric contaminants (generic Escherichia coli and Salmonella) associated with pork carcasses and contact surfaces within a high capacity (6,000 carcasses per day) pork slaughter line was evaluated. Sponge samples were taken periodically from the holding area floor and carcasses at different points in the line throughout an 8.75 h production period. E. coli levels within the holding area were high (ca. 6 log cfu 100 cm(-2)) during the initial phase of processing and did not significantly increase throughout the activity period. In the course of dehairing carcasses, the levels of E. coli were significantly (p<0.05) reduced by scalding but increased during the scraping process. A combination of polishing and triple singeing reduced E. coli populations and the bacterium was only recovered sporadically on eviscerated carcasses. The E. coli populations associated with the slaughter line had a low diversity considering the large number of carcasses processed. In Visit I, the 665 E. coli isolates typed using ERIC-PCR could be grouped into 41 genotypes. In Visit II, 141 genotypes were identified among the 855 E. coli isolates tested. This would suggest that contamination on incoming pigs was of only minor significance compared to that present within the slaughterhouse environment. The holding area was shown to act as a reservoir for endemic E. coli genotypes that could be systematically transferred throughout the dressing line on carcasses. Indeed, the majority of genotypes could be re-isolated throughout the 8.75-h processing period. E. coli isolated from carcasses within the evisceration area could be traced to up-stream operations. The holding area and scraper operation were found to be the most important sites of cross-contamination. Fourteen genotypes recovered (primarily within the holding area) on Visit I were re-isolated on Visit II. Despite the presence of endemic E. coli populations, Salmonella was recovered from only two sites (holding area floor and a carcass within the cooler) on a single occasion. The two Salmonella recovered were genetically distinct (similarity index=22%) suggesting that they originated from different sources and were not part of an endemic population. The study has further illustrated the utility of molecular typing of generic E. coli isolates to establish the dynamics of enteric contamination within pork slaughter lines. However, the extent to which the distribution of E. coli can be extrapolated to that of Salmonella remains uncertain.

Attachment strength to pork skin and resistance to quaternary ammonium salt and heat of Escherichia coli isolates recovered from a pork slaughter line

The aim of this study was to determine whether the attachment strength to pork skin, quaternary ammonium salt resistance, and thermal inactivation kinetics (at 65 degrees C) of a range of Escherichia coli isolates could be correlated with their temporal stability (persistence) within a pork slaughter line. The genetic lineage of the E. coli isolates was determined using enterobacterial repetitive intergenic consensus-PCR. The genotypes were divided into transient and endemic populations based on the number of times they were recovered within and across sampling visits made to a pork slaughterhouse. No significant variation in the D-value at 65 degrees C (0.27 to 0.51 min) was observed among the genotypes tested. However, differences in D-values were found for 100 ppm quaternary ammonium salt (3.0 to 6.0 min). All of the E. coli genotypes attached strongly to pork skin, and a high proportion of cells were irreversibly bound (39 to 42% of the initial inoculum). However, variation among genotypes was found with respect to loose attachment (21 to 33% of inoculated cells). No correlation between persistence of E. coli genotypes within the slaughter line and attachment strength or quaternary ammonium salt resistance was found. Variation in either physiological attribute could not be predicted based on genetic lineage. Additional or alternative factors may contribute to the ability of E. coli populations to become endemic within pork processing facilities. More studies should be conducted to elucidate the underlying factors that promote the formation of endemic populations of E. coli and other enteric bacteria (e.g., Salmonella) within slaughter lines.

An investigation of the thermal inactivation of Staphylococcus aureus and the potential for increased thermotolerance as a result of chilled storage

AIMS

The aims of this study were; (i) to provide thermal inactivation data for Staphylococcus aureus; (ii) to examine the kinetics, including decimal reduction times (D-value) and rate constants (k), that describe the thermal inactivation of Staph. aureus and to compare two different methods of calculating D-values and (iii) to determine whether or not chilled storage would toughen these microorganisms resulting in increased thermotolerance.

METHODS AND RESULTS

Isolates of Staph. aureus recovered from domestic refrigerators were grown in shaken culture for 8 h at 37 degrees C, recovered and washed by centrifugation and combined to form a cocktail of five strains. Samples from this cocktail were (a) heat treated at 50, 55 and 60 degrees C or (b) held under simulated domestic refrigeration conditions for 72 h and then heat treated as above. The numbers of Staph. aureus in heat treated and chill held, heat treated samples were enumerated by direct selective plating onto Baird Parker Agar (BPA) and recovery plating on Tryptone Soya Agar (TSA) subsequently overlaid with BPA. D-values were obtained using two different methods both of which may be used when the thermal inactivation follows first order kinetics. In the first method D-values are obtained by plotting the Log(10) of the surviving cells against time and using the equation D = -1/slope. The second method uses the rate constant (k) which is obtained from the slope of a plot of ln N/N(0)vs time and D is obtained using the equation D = 2.303 k(-1). D(50), D(55) and D(60) values ranged from 94.3 to 127.9 min, 13 to 21.7 min and 4.8 to 6.5 min. Prechilling did not enhance thermal resistance. The method of calculation did not affect the D-values obtained because the thermal inactivation of Staph. aureus in this study followed first order kinetics with r(2) values of 0.91-0.99.

CONCLUSIONS

The thermal inactivation of Staph. aureus in tryptone soya broth (TSB) follows first order kinetics and in general chilling of these bacteria does not increase the resistance to thermal destruction during subsequent thermal processes such as cooking.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides much needed data on the thermal resistance of Staph. aureus and validates chilling as a food storage activity which does not cause toughening of the microorganisms to subsequent cooking. However, the data generated strongly suggests that Staph. aureus is more thermotolerant than Listeria monocytogenes and should be used as the target microorganism in designing mild thermal treatments for food, in which case the current recommendations for pasteurization (70 degrees C for 2 min, minimum) should be revised.