Lack of transmission of foodborne and food-poisoning pathogens across packaging materials employed in retail butchers’ shops

Introduction

Six packaging materials including (i) barrier film used for vacuum-packing raw meat, (ii) plastic bags used for loose packing of raw meats, (iii) laminate bags used for packing of cooked chicken, (iv) plastic carrier bags, (v) cling film and (vi) tin foil, which are commonly employed in retail butchers’ shops for the carriage of raw and cooked meats (beef, poultry, lamb and pork) were examined in relation to their ability to allow transmission of ten food-poisoning and one foodborne organism across these materials. There was no evidence of transmission of any pathogen across any material tested suggesting that the packaging materials maintained their structural integrity, by not allowing the transmission of these pathogens.

Foodborne illnesses continue to remain a major cause of morbidity in Western societies, accounting for 76 million cases per annum, of which 325 000 cases require hospitalisation and 5200 deaths each year in the USA (CDC, 2005). It has been further reported, of these cases and deaths, that known pathogens account for approximately 14 million illnesses, 60 000 hospitalisations and 1800 deaths annually (CDC, 2005). Therefore, it is an important function in the prevention of such illness that the sources of these pathogens are identified and appropriate control measures established in an attempt to reduce their incidence.

Raw meats (beef, poultry, pork, etc.) have been identified as a major reservoir of food-poisoning organisms, particularly Campylobacter spp. and Salmonella spp., which are the most frequent causes of acute bacterial enteritis in the Western world (Dincer & Baysal, 2004). Various studies have indicated the importance of avoidance of cross-contamination of raw contaminated meats with cooked and other ready-to-eat foodstuffs (Harrison et al., 2001; Kusumaningrum et al., 2004). In domestic environments, meats are usually purchased from retail butchers’ premises or from large supermarket outlets and may be transported home along with other ready-to-eat foodstuffs, with the potential for cross-contamination of pathogens originating in the drip component from raw meats in the shopping basket.

Therefore, the aim of this study was to examine the potential permeation and transmission of several food-poisoning bacterial pathogens and one foodborne pathogen, through six types of packaging materials commonly employed in such retail outlets to wrap/pack raw and cooked meat products.

Materials and methods

Ten food-poisoning and one foodborne organisms were employed in this study, as detailed in Table 1. All isolates were reference strains, obtained from the National Culture Type Collection, Colindale, London, UK, with the exception of Campylobacter coli, Clostridium perfringens, Vibrio parahaemolyticus and Yersinia enterocolitica, which were well-characterised wild-type isolates. In order to prepare the inoculant, all isolates were cultured individually on Colombia agar base (CM331; Oxoid Ltd, Basingstoke, England), supplemented with defibrinated horse blood 5% [v/v] (Oxoid) at 37 °C under aerobic conditions, with the exception of C. coli and C. jejuni, which were incubated under microaerophilic conditions (CampyGen, Oxoid, UK) and C. perfringens, which was incubated under anaerobic conditions (Anaerobic Pack; Oxoid, UK). Six packaging materials [(i) plastic barrier film used for vacuum-packing raw meat, (ii) plastic bags used for packing of raw meats, (iii) laminate (paper + waxed inner surface) bags used for packing of cooked chicken, (iv) plastic carrier bags, (v) plastic cling film and (vi) tin foil] were examined, which are routinely employed in retail butchers’ shops, for the packing of raw and cooked meats. Packaging materials were cut aseptically into rectangular strips approximately 45× 35 mm and transferred to the surface of sterile plates of Columbia agar as described above, ensuring that all air bubbles had been removed from beneath the material and allowing complete contact with the surface of the agar medium. Cells were harvested separately in 0.1% [w/v] peptone saline and 25 μL was inoculated onto the surface of each test material, to give an inoculant density, as detailed in Table 1, following the enumeration in triplicate on the spiral plater (Don Whitley Scientific Ltd, Shipley, UK) and colony counter. Separate plates were prepared for the Campylobacter and Clostridium isolates examined, because of the different incubation conditions required for these organisms, as described above. Plates were incubated upright for 32 h at 37 °C and the test material removed in order to examine for the presence of growth on the medium surface. Control plates were inoculated in a similar manner with the absence of test materials and incubated, as described above. The experiment was carried out in duplicate.

Results

Each test material was challenged with between log₁₀ 4.52–7.06 CFU of inoculant, as detailed in Table 1. In all control plates, cultures were positive on examination, whereas in the case of all six test materials, no growth was observed with any of the eleven organisms examined, indicating that these materials did not allow the transmission of bacteria through the material to the surface of the medium.

Discussion

There have been few studies examining the potential transmission of food-poisoning and foodborne bacterial pathogens across packaging materials associated with the packing of raw and cooked meats in retail butchers’ premises. One previous study by Harrison et al. (2001) in the UK demonstrated that on examination of the external surface of packaging materials of raw chicken originating from retail premises, 3% were positive for campylobacters and none were positive for salmonellae. However, this study did not suggest either the origin or the mechanisms as to how the external surface of the packaging material became positive for campylobacters. At least three potential mechanisms may be hypothesised, either individually or in combination, to account for the external surface of the packaging material being positive and these include (i) the external surface may become contaminated by preparation of the pack on contaminated work surfaces, (ii) the external surface may become contaminated by migration of contaminated drip through the tying/neck of the bag, because of capillary action, as we have commonly observed this phenomenon (unpublished observation) and/or (iii) transmission of the pathogen across the packaging material itself.

In this study, we wished to examine the hypothesis that pathogens may permeate and translocate through commonly employed packaging materials used in the carriage of raw and cooked meats. Presently, there are a wide variety of packaging materials used in retail butchers’ shops, including co-extruded, laminated, polypropylene, cellophane, boilable films, in a variety of packing modes, including vacuum packed, modified atmosphere packaging, bags, draw string, zip lock, tamper-evident and tape closure features.

Overall, this study was unable to demonstrate the transmission of eleven pathogens across six different packaging materials, commonly used to pack raw and cooked meats from retail butchers’ shops. Therefore, it was encouraging to note that these materials do not appear to allow the transmission of such pathogens in a raw meat packaging system from the inside to the outer external surface, therefore lowering the microbial burden of pathogens on the external surface of such packaging. However, given the potential for the outer surface of such materials to become contaminated through either being processed on contaminated work surfaces and/or through leaking of contaminated drip through the tying/neck of the bag, advice to retailers and consumers alike should be to avoid packing raw meats along with ready-to-eat, processed or foodstuffs that do not require any further cooking, thus avoiding the potential for cross-contamination between raw and cooked foods in the domestic shopping basket.

Acknowledgments

The authors wish to acknowledge with thanks the valuable help and assistance given by Mr Alan Murphy, in relation to provision of bacterial cultures employed in this study. This study was funded by the Research & Development Office, Department of Health & Public Safety, Northern Ireland [Infectious Disease, Recognised Research Group (RRG) 9.9].

References