The effect of MAP on the growth and survival of Escherichia coli O157:H7 and Staphylococcus aureus in chilled minced beef

Abstract

With modern culture moving towards convenience in terms of fresh produce, especially with meat products, science needs to constantly evolve to serve these customer needs. These needs however can sometimes be implemented too hastily without the proper assessment done for factors such as food safety. One such improvement is the use of modified atmosphere packaged minced meat. This form allows the minced meat to be kept for much longer than normal without freezing the product, providing the fast pasted consumer both the convenience of having fresh, unfrozen meat as well as an added shelf life. MAP works by disrupting the atmosphere within these packages, retarding the growth of the spoilage micro-organisms, thus causing them to require a longer time span to reach spoilage numbers. The problem however arises that although most of these techniques are tried and trusted on the products’ spoilage organisms, it does not take into consideration the effect MAP and the altered spoilage organism communities would have on a pathogen that might be present on the products. This study thus aimed to assess the effect of both factors on Escherichia coli O157:H7 (phase 1) and Staphylococcus aureus (phase 2) in minced meat kept at 5°C as well as trying to identify the major affecting factors. The two subject organisms were each inoculated into 2 different types of MAP packs and a non-atmosphere modified PVC overlaid minced meat sample at two differing concentrations of 105 and 102 to also assess the impact of high andlow initial pathogen presence. These packs were then analysed over a time period of 16 days to track the changing minced meat environment. APC, Pseudomonads, LAB and Enterobacteriaceae counts were all investigated along with the pathogenic counts. Apart from colony enumeration, the colour of the minced meat samples were also taken to determine the effect that these parameters have on the appearance of the product, as colour is often the first sensorial characteristic that determines the purchase of fresh meat products. pH was determined to ascertain the environmental changes occurring in the product and whether groups such as the LAB would change the environment to better suit their needs. Finally the atmospheric makeup was also measured to determine the effect of the MAP system and the change occurring in a closed system that could be attributed to the growth and respiration of the bacterial communities present. Apart from the main aim of the study, two additional studies were performed that arose during the planning and analyses of the two primary phases. Firstly the use of a quarter versus a full plate enumeration was studied to determine its accuracy as well as possibility of use in full studies to aid enumeration and decrease time and financial input. Here a direct comparison was done between the two techniques after which they were compared and assessed in their functionality for both homogenous and heterogeneous community enumeration on selective and non-selective media. The other secondary study focussed on the use of new technology for both the enumeration and tracking of genetically modified organisms in a variety of different environments. Here a bioluminescent imaging system was used on a genetically modified strain of E. coli to track its spread through minced meat, packaged either in a MAP or PVC overlaid pack, over 48 hours in an accelerated shelf life study. Enumeration of said organism was also undertaken whereby the intensity of emitted light would correspond to a defined count, enabling rapid enumeration of samples, whether overgrown or not. Copyright