Salmonella in Pork Cuttings in Supermarkets and Butchers’ Shops in Denmark in 2002 and 2006

Authors


T. B. Hansen. National Food Institute, Technical University of Denmark, Mørkhøj Bygade 19, DK-2860 Søborg, Denmark. Tel.: +45 35887556; E-mail: tibha@food.dtu.dk

Summary

The prevalence of Salmonella in fresh pork cuttings in Denmark in the years 2002 and 2006 was investigated at retail and compared with the retail supply pattern. A total of 1025 and 3473 samples were taken in 2002 from butcher’s shops and supermarkets, respectively. The corresponding numbers in 2006 were 259 from butchers’ shops and 628 from supermarkets. In 2002, 1.2% of all samples were positive for Salmonella; butchers’ shops and supermarkets had 1.8% and 1.0% positive samples, respectively. The overall prevalence in 2006 was 4.2%, with prevalence of 8.1% and 2.6% for butchers’ shops and supermarkets, respectively. Hence, increases around 3- to 5-fold were found. There was neither observed any parallel increase in Salmonella positive carcasses in Danish slaughterhouses during the study period, nor were any changes in supply routes towards slaughterhouses with higher prevalence observed, which could explain the apparent increase. We hypothesize that hygiene levels and ability to avoid cross-contamination and prevent growth of the organism, in the meat processing chain after slaughter were the most likely responsible factors. Results from this study indicate that the hygiene performance, particularly at retail, has a significant impact on the occurrence of Salmonella. This implies that there is no direct link between slaughterhouse Salmonella surveillance data and the level of Salmonella contamination at retail. To improve risk assessment of Salmonella in fresh pork meat, this study underlines the need for comprehensive retail data.

Impacts

  • • In this study, we hypothesize that the hygiene performance at retail level has a significant impact on the occurrence of pathogenic bacteria, such as Salmonella, in fresh pork meat.
  • • Our results indicate that hygiene performance varies considerably among butchers’ shops and supermarkets, which confounds a direct link between slaughterhouse surveillance data and level of Salmonella contamination in fresh pork meat at retail.
  • • This observation underlines a need for comprehensive retail data for improving risk assessment of Salmonella in fresh pork meat.

Introduction

Salmonella is a zoonotic agent that is spread from production animals via contaminated meat to the consumer. Salmonella is associated with pig production in many countries and pork meat is a substantial source of human Salmonella infections (Anonymous, 2006b). Close to 25 million slaughter pigs are produced annually in Denmark, of which 15% are distributed to the domestic market. Around 10 large export-authorized slaughterhouses under the Danish Meat Association (DMA) account for more than 95% of the production, whereas the remaining 5% is produced under a smaller slaughter association named Danske Slagtermestres Landsforening (DSL), which includes the small- and medium-sized slaughterhouses (Landbrug og Fødevarer, 2009).

In Denmark, pork meat accounts for 5–8% of all Salmonella cases (Anonymous, 2006a). The Danish Salmonella control programme aims at reducing the consumer risk from Danish fresh pork. The programme includes Salmonella surveillance of fresh meat at the slaughterhouse, which in 2002 and 2006 included testing of almost 37 000 and 28 000 carcasses, respectively (Anonymous, 2002a, 2006a). In 2002, the surveillance at slaughter showed that carcasses had an overall Salmonella prevalence of 1.7% (Anonymous, 2002a) with 1.8% positive carcasses from the large export-authorized slaughterhouses, 1.1% from medium-sized export-authorized slaughterhouses and 0.6% from the small-sized slaughterhouses in 2001/2002 (The National Food Administration in Denmark, personal communication, 2009). In 2006, the overall Salmonella carcass prevalence had decreased to 0.9% (Anonymous, 2006a), with 1.0% of the carcasses from the large export-authorized slaughterhouses and 0.9% from the small- and medium-sized slaughterhouses being positive (The National Food Administration in Denmark, personal communication, 2009). As small- and medium-sized slaughterhouses are assumed to deliver up to 40–60% of fresh pork at retail (Christensen et al., 2009), the impact of a low Salmonella prevalence from these sources on consumer safety may be significant.

Because of the differences observed in Salmonella positive carcasses, depending on type of slaughterhouse, we hypothesized that the prevalence of Salmonella contamination at retail would be influenced by the choice of supplier by retailers. The objective of the study was to test our hypothesis by conducting a retail survey, which identified suppliers of fresh pork cuttings for retail butchers’ shops and supermarkets, and determined the prevalence and concentration of Salmonella in pork cuttings. This information would be of high value for risk assessment of Salmonella in pork as well as input to the public food control system.

Materials and Methods

Sample collection

Meat samples were collected by officials from the regional food control centres in Denmark in connection to routine inspections throughout the years 2002 and 2006. Approximately 50% and 14% of all food retail stores in Denmark were selected for the investigation in 2002 and 2006, respectively. Exclusion criteria were not applied other than the same store only could be sampled once.

Samples consisted of at least 200 g fresh pork cuts from butchers’ shops and supermarkets. In each store, three cuttings, which originated from the shoulder, middle and hind part, respectively, were collected. Samples were transported under cooling at 5 ± 2°C in closed containers to the laboratories of the regional food control centres where they were kept at 2 ± 1°C for up to 24 h until microbiological analysis. For samples collected on Fridays, the storage time could, however, be up to 3 days.

Questionnaire

Both in 2002 and in 2006, each sample was accompanied by a questionnaire, which ensured registration of sample information. In particular, the information included: (i) which part of the carcass the cutting originated from, (ii) the country of origin, (iii) the type of retailer and (iv) the authorization number (Id-number) of the retail supplier of pork meat. The authorization number identifies the supplier and specifies its approval for export or domestic markets, and its organizational affiliation.

Microbiological analyses

Fifty gram of pork meat were prepared according to established bacteriological practice as described in International Standard – ISO 6579: 2002(E) (Anonymous, 2002b), with convenient modifications. In short, the sample was mixed with 200 ml buffered peptone water (BPW) and homogenized for 2 min in a filter bag using a stomacher. From this sample suspension, 100 ml of the fluids were placed in a sealed container and kept refrigerated at 2 ± 1°C for later quantitative detection of Salmonella if the qualitative detection was positive. The remaining of the sample suspension, 100 ml of the fluids plus the meat in the filter, were added to 125 ml BPW and used immediately for qualitative detection of Salmonella. Each of the suspensions was considered to represent 25 g meat. After the non-selective pre-enrichment in BPW at 37°C for 16–24 h, 0.1 ml BPW culture was applied to Modified Semisolid Rappaport-Vassiliadis medium (MSRV) (Oxoid CM910 or equivalent) and incubated at 41–42°C for 18–24 h. From the edge of the swarming zone, colony material was streaked onto the indicative medium xylose–lysin–desoxycholate agar (XLD) followed by incubation at 37°C for 18–24 h. At least two typical colonies were selected for verification. For a random subset of Salmonella positive samples, the serotype was established according to the Kaufmann and White scheme (Popoff and Minor, 1997).

For Salmonella-positive samples, the sample suspension was recovered from the refrigerator, mixed thoroughly and analysed semi-quantitatively to determine the concentration of Salmonella. The mixture was split into aliquots of 10, 1, 0.1 and 0.01 ml corresponding to 2.5, 0.25, 0.025 and 0.0025 g meat, respectively. For the aliquots of 10 ml, 15 ml BPW was added, whereas for the aliquots of 1 ml or below, BPW was added to obtain a total volume of 10 ml. Each of these aliquots was analysed for Salmonella following the principles described above.

Data analysis

As it is difficult to get companies, including slaughterhouses, to disclose their customer segmentation and supply volumes, we chose to identify the retail supply patterns on the basis of questionnaire information from the retailers.

The prevalence of Salmonella positive samples was reported from the proportion of 25-g-samples that tested positive. The likelihood ratio test compared differences in prevalence between types of cutting, types of retailer and types of supplier. In cases with less than five observations, the Fisher’s exact test was used. All statistically significant differences were reported at the < 0.05 level. Exact 95% confidence intervals (95% CI) were calculated based on standard methods for binomial data (Armitage and Berry, 1999).

Results

Salmonella prevalence

A total of 1025 and 3473 samples of fresh pork cuttings were taken in 2002 from butchers’ shops and supermarkets, respectively; Salmonella was detected in 1.2% of all samples (Table 1). For butcher’s shops, the Salmonella prevalence was 1.8%, accounting for 35% of all positive samples, whereas the corresponding prevalence for supermarkets was 1.0%. In 2006, the survey comprised 259 samples from butchers’ shops and 628 from supermarkets. Compared to 2002, the overall prevalence in pork cuttings in 2006 had increased significantly to 4.2% (< 0.001). The largest increase was observed for meat from butchers’ shops, where 8.1% positive samples were detected. The prevalence had increased to 2.6% in meat from supermarkets (Table 1).

Table 1. Salmonella in fresh pork cuttings collected in butchers’ shops and supermarkets in Denmark in 2002 and 2006
Type of retailer20022006
No. samplesNo. positivesPrevalence (%)No. samplesNo. positivesPrevalence (%)
  1. Numbers in parentheses are 95% confidence intervals.

Butchers’ shops1025181.8 (1.0–2.8)259218.1 (5.1–12.1)
Supermarkets3473341.0 (0.7–1.4)628162.6 (1.5–4.1)
Total4498521.2 (0.9–1.5)887374.2 (3.0–5.7)

Regardless of type of retailer and year of investigation, the prevalence of Salmonella-positive samples was always highest in cuttings originating from the shoulder part (Table 2). In 2002, there was a significantly ( 0.034) lower prevalence of Salmonella in cuttings originating from the hind part of the carcass, as compared with the middle and shoulder parts (Table 2).

Table 2. Salmonella in cuttings originating from shoulder, middle and hind parts collected at retail in Denmark in 2002 and 2006
Type of retailerCutting from20022006
No. samplesNo. positivesPrevalence (%)No. samplesNo. positivesPrevalence (%)
  1. Numbers in parentheses are 95% confidence intervals.

Butchers’ shopsShoulder32492.8 (1.3–5.2)7379.6 (4.0–18.8)
Middle41071.7 (0.7–3.5)9466.4 (2.4–13.4)
Hind29120.7 (0.1–2.5)8489.5 (4.2–17.9)
Unknown080
SupermarketsShoulder1206181.5 (0.9–2.4)16063.8 (1.4–8.0)
Middle1237121.0 (0.5–1.7)22662.7 (1.0–5.7)
Hind103040.4 (0.1–1.0)22541.8 (0.5–4.5)
Unknown0170

Serotyping was conducted for approximately 33% of the Salmonella isolates. In 2002, 12 isolates coming from 11 different stores comprised seven S. Typhimurium, three S. Ohio, one S. Infantis and one S. Orion. In 2006, nine S. Typhimurium (DT120, DT104b, DT104, DT66), eight S. Idikan and one S. 4,12:i:- were found in samples coming from seven different stores.

The concentration of Salmonella in fresh pork cuttings was determined in 2002. Among 52 Salmonella-positive samples, 39 samples contained 0.04–0.4 Salmonella per gram, seven contained 0.4–4 Salmonella per gram, five contained 4–40 Salmonella per gram and a single sample contained more than 40 Salmonella per gram (Fig. 1). Butchers’ shops tended to have a higher fraction of samples with concentrations above 0.4 CFU/g than samples from supermarkets (Fig 1), however, this difference was not statistically significant (= 0.085).

Figure 1.

 Concentration of Salmonella in 52 positive samples of pork cuttings found at retail in Denmark in 2002. Black bars are samples from butchers’ shops; white bars are samples from supermarkets.

Retail supply routes

For each pork sample, the supplier of the meat was registered. The supplier could be established for 4372 (97%) and 825 (93%) of the samples in 2002 and 2006, respectively. The suppliers were divided into four categories: (i) slaughterhouses without authorization for cutting (delivering half carcasses), (ii) slaughterhouses authorized for both slaughter and cutting, (iii) specialized cutting plants and (iv) minor wholesalers.

In 2002, 57% of all retail samples originated from meat supplied from slaughterhouses authorized for both slaughter and cutting, 32% of the samples originated from meat which was distributed through specialized cutting plants and 9% of the samples came from meat supplied directly from slaughterhouses without authorization for cutting (Fig. 2). The remaining 2% were distributed to retail through minor wholesalers. In 2006, this pattern had changed significantly (P < 0.001). The most pronounced changes were a 2.3-fold decrease in samples traced back to slaughterhouses without authorization for cutting and a 2.8-fold increase in samples traced back to minor wholesalers (Fig. 2).

Figure 2.

 Retail supply routes of fresh pork cuttings in Denmark in 2002 and 2006.

Combining the supplier category with the size of the suppliers resulted in the distribution of supply routes presented in Table 3. In 2002 as well as 2006, butchers’ shops traded more frequently with small- and medium-sized suppliers (76% and 74% of samples, respectively) compared to supermarkets (12% and 9% of samples, respectively). For supermarkets, the majority of the samples in 2002 (83%) as well as in 2006 (78%) originated from the large DMA members. The most noticeable change from 2002 to 2006 was observed for butchers’ shops. In 2002, a significantly (P < 0.001) larger proportion of the samples originated from half carcasses delivered directly from slaughter from small- and medium-sized slaughterhouses, whereas in 2006 a significantly (P < 0.001) larger proportion came through small- and medium-sized cutting plants.

Table 3.   Distribution of the retail samples of fresh pork cuttings collected in butchers’ shops and supermarkets in Denmark in 2002 and in 2006 according to retail supplier
Retail suppliersn (%)
20022006
Butchers’ shopsSuper-marketsButchers’ shopsSuper-markets
  1. DMA, Danish Meat Association; DSL, Danske Slagtermestres Landsforening.

  2. aIdentified countries were Cyprus, Ireland, Spain and the Republic of the Congo.

Slaughterhouse without cutting plant
 Large enterprises (DMA)1 (<1)2 (<1)0 (0)0 (0)
 Small- and medium-sized enterprises (DSL)261 (25)146 (4)34 (13)0 (0)
Cutting plant associated with the slaughterhouse
 Large enterprises (DMA)145 (14)1729 (50)37 (14)292 (47)
 Small- and medium-sized enterprises (DSL)436 (42)178 (5)153 (59)59 (9)
Specialized cutting plant:
 Large enterprises (DMA)41 (4)1163 (33)11 (4)197 (31)
 Small- and medium-sized enterprises (DSL)90 (9)106 (3)3 (1)0 (0)
Minor wholesalers13 (1)61 (2)7 (3)32 (5)
Importa1 (<1)2 (<1)2 (1)3 (<1)
Unknown37 (4)86 (2)12 (5)45 (7)
Total samples1025  3473  259  628  

Throughout the study, a total of eight samples were of imported meat (Table 3), representing less than 0.2% of all samples. These samples were not brought into analysis.

Discussion

In 2006, we found 4.2% of 887 retail pork cuttings were positive for Salmonella compared with 1.2% of 4498 samples in 2002. Other reports on Salmonella in pork at retail have shown comparable levels. Zhao et al. (2001) isolated Salmonella in 3.3% of 209 samples in Washington D.C in 1999/2000, and in Italy, 4.9% of 3182 samples contained Salmonella in 2001/2002 (Busani et al., 2005). In a more recent study in Ireland, 2.6% of pork samples from retail were positive for Salmonella in 2007 (Prendergast et al., 2009). These findings underline the importance of retail pork as a potential vehicle for consumer exposure to Salmonella.

Our investigations showed that S. Typhimurium was the predominant serotype in retail pork cuttings in Denmark. Similar results were found in Ireland (Prendergast et al., 2009) and Belgium (Ghafir et al., 2005; Delhalle et al., 2009). These findings cohere with data from the EU community summary report, 2005 and 2008 (Anonymous, 2006b, 2010), which showed that S. Typhimurium dominated in pigs and pork, and that pork was likely to be the dominating source of human S. Typhimurium cases in EU at present.

The present study showed that pork meat from butchers’ shops carried Salmonella significantly more frequently than pork meat from supermarkets. A similar trend was observed by Miranda et al. (2009) who found a 5-fold higher Salmonella prevalence in pork meat from butchers’ shops compared with pork meat from supermarkets. In contrast, Prendergast et al. (2009) observed no significant difference between the occurrences of Salmonella at the two types of retail outlet; however, these authors found significantly higher Enterobacteriaceae levels in pork cuttings from butchers’ shops compared with supermarkets. Salmonella belong to the family Enterobacteriaceae and Enterobacteriaceae counts in pork have previously been used as an indicator for presence of Salmonella (Ghafir et al., 2008; Prendergast et al., 2008). In accordance, Prendergast et al. (2009) found a significant association between high numbers of Enterobacteriaceae and occurrence of Salmonella in pork samples. As high counts of Enterobacteriaceae may indicate poor hygiene, it is possible that the difference between butchers’ shops and supermarkets can be explained by differences in hygiene levels. This conclusion is supported by a Danish retail investigation of minced pork meat conducted from 2001 to 2002 (Hansen et al., 2007). They showed that minced meat samples from butchers’ shops had significantly higher levels of enterococci as well as higher prevalence of Salmonella than samples from supermarkets.

Our initial hypothesis, that the occurrence of Salmonella in pork at retail would depend significantly on the category of the meat supplier, was not supported by the present study. The tracing back of the samples to the supplying slaughterhouses showed that the majority of pork cuttings purchased in the butchers’ shops came from the small- and medium-sized enterprises which, according to the national surveillance programme, had lower prevalence of Salmonella-positive carcasses in the year of investigation compared with the large ones. The majority of samples in supermarkets were traced back to large enterprises. Thus, the difference in Salmonella occurrence between the two types of retailers did not correlate with the difference in positive carcasses at their respective suppliers. Also imported pork meat could be excluded as explanation for the difference between Salmonella carriage in samples from butchers’ shops and supermarkets, as only very few samples were of foreign origin.

Salmonella concentration is of particular consumer safety concern because even low concentrations can multiply to hazardous levels if the right conditions emerge (Berends et al., 1997), for example, by temperature abuse. So far, however, only few quantitative data have been reported on pork. A Belgian study found that 1.3% of Salmonella-positive cutting meat samples were contaminated with at least 10 Salmonella per gram and 7.4% with 1–10 Salmonella per gram (Ghafir et al., 2005). A German retail study found that Salmonella concentrations were lower than one per gram in 90% of the minced pork meat samples that were tested positive (Sinell et al., 1990). In a more recent Belgian study, the mean Salmonella concentration in pork cuts was estimated to −2.64 ± 1.76 log CFU/g (Delhalle et al., 2009) and in Ireland Prendergast et al. (2009) observed concentrations ranging from <0.30 to 2.10 MPN/g in pork chops and pieces. In the present study, the Salmonella concentration was determined semi-quantitatively in the 52 positive samples of Danish origin found in 2002. Concentrations varied between 0.04 and >40 CFU/g. A higher fraction of the samples from butchers’ shops tended to contain above 0.4 CFU/g than samples from supermarkets. The same trend was statistically significant for minced meat samples in Danish retail in 2002 (Hansen et al., 2007) altogether supporting a hypothesis of poorer hygiene performance in butchers’ shops than in supermarkets in Denmark. As similar data are reported internationally, this is presumably not only a Danish phenomenon.

Interestingly, our data suggested that meat from the forepart has a higher consumer risk compared to the hind part, as the Salmonella prevalence was significantly lower in the latter. This may be explained by the slaughter routine of hanging the carcass with the head down, which could lead to concentration of contaminated fluid at the forepart.

We observed that occurrence of Salmonella in pork cuttings at retail in Denmark in 2006 was significantly higher than in 2002. As our data was from two disjoint years, the increase could simply be a peak in year-over-year variation. A similar retail investigation, to be carried out in 2010, will substantiate whether the increase between 2002 and 2006 should be considered a peak or a part of an increasing trend.

The increase in occurrence of Salmonella in pork cuttings was observed for both types of retail outlets. In butchers’ shops, a 4.6-fold increase to 8.1% was observed and in supermarkets a 2.6-fold increase to 2.6% was observed. As the rise could not be ascribed a rise in Salmonella carcass prevalence at slaughter a worsening in hygiene levels post-slaughter becomes the most probable explanation.

Conclusion

Several factors may have contributed to the increased Salmonella contamination of pork cuttings observed at retail between 2002 and 2006 in Denmark. Although there was a change in supply route, there was no indication of increased contamination at slaughter, which could explain the observed increase of Salmonella in retail pork. This indicated that the increased Salmonella carriage was a result of a poorer hygiene performance through cutting and retail in 2006 compared to 2002. To substantiate or reject this hypothesis, further investigations will be carried out at cutting plants and retail in Denmark in 2010 and 2011. Furthermore, our understanding of consumer risk from Salmonella in fresh pork would benefit significantly from a more comprehensive understanding of the microbial ecology in meat at retail.

Acknowledgements

The data included in this work were kindly made available by the Danish Veterinary and Food Administration for which we are thankful. The INTERREG IIIC project, PromSTAP, financially supported the processing and statistical analysis of the data. In respect to this, the collaboration with Prof. Dr Brigitte Petersen and Dr Judith Kreyenschmidt, both from Institute of Animal Science, University of Bonn, is highly acknowledged.

Ancillary