Meijun ZHU
Ph.D., D. Sci.
Department of Animal Science
University of Wyoming
Laramie, WY 82071
U.S.A.
Listeria monocytogenes is one of the most frequent pathogens causing food borne diseases that annually accounts for ∼2,500 cases (meningitis, encephalitis, sepsis, fetal death, premature birth) and 504 deaths (Mead et al., 1999). L. monocytogenes is a gram-positive, non-sporeforming, highly mobile, rod-type, facultative anaerobic bacterium (Farber and Peterkin, 1991). It can grow from 0 to 42 ºC (Ralovich, 1992) and the pH range for growth is between 4.5 and 9.6 (Seelinger and Jones, 1986). It tolerates salt and nitrite (McClure et al., 1997). It is widely present in natural environment such as soil and sewage, the intestinal tract of infected animals, human and animal faeces, processing environments and catering facilities (Farber and Peterkin, 1991; Autio et al., 2000; Gillespie et al., 2000; Beresford et al., 2001). They can form biofilm on different surfaces.
Due to its ubiquity in the environment, it is a challenge to prevent the transmission of L. monocytogenes from raw animal products to meat processing environment and to ready-to-eat (RTE) meats (Beresford et al., 2001). Further, growth at refrigerated temperature and vacuum conditions, and resistance to salt and nitrite make L. monocytogenes a unique safety threat to RTE meats, which usually have long shelf-life and consumed without further heating.
There have been three well-publicized outbreaks of listeriosis involving RTE meat products. A multistate outbreak in 1998 to 1999 was linked to frankfurters and deli meats and caused 101 cases and 21 deaths [Centers for Disease Control (CDC), 1999]. In 2000, a multistate outbreak involving deli turkey meat resulted in 29 cases, 4 deaths and 3 miscarriages or still births (CDC, 2000). Then, a multistate outbreak of L. monocytogenes infections in the Northeast United States was attributed to the consumption of sliceable turkey deli meat. There were 46 confirmed cases, 7 deaths and 3 still births or miscarriages associated with this outbreak (CDC, 2002). These reports clearly implicate that RTE meat products, especially poultry products, are important sources of listeriosis. The recall of 26 million pounds of turkey meat in 2002 indicates the economic consequences of RTE meats contaminated with L. monocytogenes (U.S. Department of Health and Human Services, 2002). Therefore, a successful strategy is urgently needed to prevent the contamination and subsequent proliferation of L. monocytogenes in RTE turkey meat products.
An important source of L. monocytogenes contamination in turkey meat products is the live turkey. L. monocytogenes may enter the packing plant at low levels in the intestine of recently infected birds, survive in bio-films, and ultimately contribute to both environmental and RTE product contamination. Dietary vitamin E supplementation was tested for its effectiveness in accelerating the gut clearance of L. monocytogenes in experimentally infected adult turkeys. Results showed that vitamin E supplementation 200 IU vitamin E/kg feed was effective in boosting immune response and accelerating the elimination of inoculated L. monocytogenes in live turkeys (Zhu et al., 2003a). Therefore, vitamin E supplement can be used to reduce the possible pathogen contamination in turkey and improve safety of turkey products.
Although L. monocytogenes can be killed during the thermal processing of RTE meats (Lemaire et al., 1989), post processing contamination of RTE meat with L. monocytogenes during slicing and packaging is difficult to avoid, due to its ubiquitous nature. Thus, it is necessary to have strategies to control or eliminate those products contaminated with L. monocytogenes in RTE meat. Irradiation is an effective post-packaging intervention technology to ensure microbiological safety of RTE meat products.
Although quite a number of studies were conducted to determine the effect of gamma-irradiation on the survival of L. monocytogenes, little work have been done with e-beam irradiation in RTE meats, which was examined in this study. Irradiation (1.0 to 2.5 kGy) greatly reduced L. monocytogenes and aerobic plate counts in commercial RTE turkey ham and breast roll. The D10 value，radiation dose that results in 90% reduction of viable L. monocytogenes, in breast roll and ham were 0.52 and 0.47 kGy, respectively, indicating that about 2.5 kGy are needed to achieve a 5-log reduction for L. monocytogenes in RTE turkey meat. Despite its effectiveness in eliminating pathogens and spoilage microorganisms, irradiation also causes quality changes (Nam & Ahn, 2003; Ahn & Lee, 2004). To evaluate this, turkey ham and breast rolls were irradiated at 1.0 or 2.0 kGy and stored at 4 °C for up to 14 days. Quality characteristics were analyzed weekly.
Results showed that up to 2.0 kGy irradiation had limited effects on colour and oxidation of vacuum-packaged commercial turkey ham, though a significant increase in redness was detected for turkey rolls. Irradiation has a significant influence on odour/flavour in vacuum-packaged turkey ham and breast rolls. Both sensory panelists and volatile analysis showed that there were significant changes in sulfur-related odour/flavour in RTE turkey products by 2.0kGy irradiation (Zhu et al., 2003b; 2004a). To avoid quality changes, therefore, only low-dose irradiation is recommended for RTE meat products. Unfortunately, some L. monocytogens survives low-dose irradiation, and proliferates and causes a health hazard during refrigerated storage. Thus additional barriers, such as adding preservatives, are necessary in order to ensure the microbial safety of products following low-dose irradiation.
A study was conducted to evaluate the effect of antimicrobials on the survival and growth of L. monocytogenes following electron-beam irradiation.
Six antimicrobial additive treatments that include no preservatives (control), 0.1% potassium benzoate (PB), 2% sodium lactate (SL), 0.1% potassium benzoate plus 2% sodium lactate (PB + SL), 2% sodium lactate plus 0.1% sodium diacetate (SL + SDA), and 0.1% potassum benzoate, 2% sodium lactate and 0.1% sodium diacetate (PB + SL + SDA) were formulated into RTE turkey ham and breast rolls.
Results showed that a combination of physical and chemical antimicrobial intervention offered better inhibitory effect on the growth of L. monocytogenes in RTE turkey meat than that of single intervention (Zhu et al., 2005). A dose of 1.0 kGy was effective in suppressing the growth of L. monocytogenes in turkey rolls/hams formulated with PB + SL or SL + SDA for about six weeks when stored at 4 °C. No growth of L. monocytogenes after irradiation was occurred during 42 days of storage for 2.0 kGy irradiated RTE turkey rolls/hams formulated with 0.1% PB + 2% SL, 2% SL+ 0.1% SDA or 0.1% PB + 2% SL+ 0.1 %SDA, and 1.0 kGy irradiated turkey rolls/hams with 0.1% PB + 2% SL + 0.1% SDA. Thus, from the microbiology safety point of view, PB+SL and SL+SDA antimicrobial treatments combined with 1.0 kGy or 2.0 kGy irradiation were effective treatments to control post-packaging L. monocytogenes contamination in RTE turkey meats. The remaining question is the possible effects of antimicrobials combined with irradiation on the quality of RTE turkey meats. This information is needed to assess the feasibility of using antimicrobials and irradiation as hurdles to ensure L. monocytogenes safety in RTE meats. Results showed that 1.0 kGy irradiation has no significant effect on the sensory characteristics of RTE turkey meats. There was no significant difference in sensory characteristics among 1.0 kGy-irradiated RTE turkey meats with different antimicrobials (Zhu et al., 2004b; 2005). However, the addition of PB in RTE turkey meats produced a high amount of benzene after irradiation, suggesting benzoate salt is not a good antimicrobial to be used in products for irradiation (Zhu et al., 2004b; 2005). These results indicate that including 2% SL plus 0.1% SDA in formulation combined with 1.0 or 2.0 kGy irradiation is a promising technology to ensure the safety of RTE turkey rolls and hams from L. monocytogenes.
In summary, vitamin E supplementation can reduce the possible L. monocytogenes in live birds. One kGy irradiation has no significant effect on the sensory characteristics of RTE turkey meats. Including 2% SL plus 0.1% SDA in formulation combined with 1.0 or 2.0 kGy irradiation is a promising technology to ensure the safety of RTE turkey rolls and hams from L. monocytogenes.
References
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Zhu, M., Wesley, I. V., Nannapaneni, R., Cox, M., Mendonca, A., Johnson, M. G., Ahn, D. U. 2003a. The role of dietary vitamin E in experimental Listeria monocytogenes infections in turkeys. Poult Sci 82, 1559-1564.
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Zhu, M. J., Mendonca, A., Min, B., Lee, E.J., Nam, K.C., Park, K., Du, M., Ahn, D.U. 2004b. Electronic-beam irradiation and antimicrobial on the quality of ready-to-eat turkey breast roll. J Food Sci 69: C382-387.
Zhu, M. J., Mendonca, A., Ismail, H. A., Du, M., Lee, E. J., Ahn, D.U. 2005. Impact of antimicrobial ingredients and irradiation on the survival of Listeria monocytogenes and the quality of ready-to-eat turkey ham. Poult Sci 84: 613-620.
From Proceedings of the “Midwest Poultry Federation Convention”, St. Paul, Minnesota, U.S.A.
