Submitted to: Food Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/23/2006
Publication Date: 1/3/2007
Citation: Schultze, K.K., Linton, R.H., Cousin, M.A., Luchansky, J.B., Tamplin, M.L. 2007. Effect of pre-inoculation growth conditions and fat levels on thermal inactivation of a serotype-4b strain of listeria monocytogenes in frankfurter slurries. Food Microbiology. 24(4):352-361. Interpretive Summary: Each year in the US, Listeria monocytogenes (LM) is responsible for approximately 2,500 cases of listeriosis and 500 deaths. Risk assessment shows that the consumption of ready-to-eat (RTE) non-reheated frankfurters is associated with a high risk of listeriosis, on slurry a per serving and per annum basis. Frankfurter processors rely on many intervention steps to control LM contamination. In 2003, interventions became mandatory with the US Department of Agriculture Food Safety Inspection Service (USDA-FSIS) interim rule for RTE meat and poultry products, requiring processors to include interventions and/or sanitation measures to prevent contamination of RTE products with LM. Since minimal thermal treatments are often used to reduce or eliminate LM in refrigerated RTE foods and the heat resistance of LM is high compared to other vegetative foodborne bacteria, researchers have focused on factors that influence microbial heat resistance. More recently, there has been interest in the effect of sodium lactate and sodium diacetate on the heat resistance of LM since they are being used frequently in frankfurters to control the growth of LM during storage. However, there has been very limited research to date that has examined how various factors influence heat resistance of LM cells in frankfurter systems containing these acidulants. The present study focused on understanding factors that can affect the ability of the post-processing pasteurization intervention step to reduce LM populations from two different frankfurter systems; a frankfurter-slurry system and on actual frankfurter surfaces. Comparison of death rates at 60 degrees C (D-60, minutes) for cells heated in frankfurter slurry showed that the level of fat in the frankfurter heating media significantly (P<0.05) influenced the death rate of LM, in that cells heated in 8.5% fat broth had a significantly higher (P<0.05) D-60 value (2.2 minutes) than those heated in 11% fat (1.0 minutes) and 23% fat slurry (0.9 minutes). The level of fat in the medium used to grow LM before inoculating the frankfurter surfaces did not significantly affect the death rates. Death rates were consistently higher on frankfurter surfaces compared to similar treatments done in frankfurter slurry. On frankfurter surfaces, a 2.3- to 5.1-log10 reduction could be achieved after 15 min. The time necessary to achieve a 3.0-log10 reduction using post-processing pasteurization of frankfurters in a hot water bath at 60 degrees C approximately doubled for cells grown in 23% fat frankfurter broth compared to those heated in 8.5% fat frankfurter slurry. These results show that processing conditions used to reduce LM levels in frankfurters must consider the pre-inoculation growth conditions and the formulation, particularly the %fat, on the subsequent thermal inactivation of L. monocytogenes associated with frankfurters.
Technical Abstract: Pre-inoculation growth conditions and fat levels were evaluated for effects on the heat resistance of Listeria monocytogenes (LM) in a formulated frankfurter slurry and on frankfurter surfaces. Comparison of linear inactivation rates (D-values) for cells heated in frankfurter broth showed that growth conditions significantly (P<0.05) affected thermal resistance. The average D-60 values for the five growth media tested from the most resistant to the least heat resistant were: Tryptic Soy Slurry with 0.6% Yeast Extract (TSBYE) (2.2 min) and 8.5% fat slurry (2.2 min), 23% fat slurry (1.7 min) and 11% fat slurry (1.7 min), and TSYBE with quaternary ammonia compounds added (TSBYE + Q) (1 min). Fat level in the frankfurter heating media also significantly (P<0.05) affected the thermal death rate of LM. Cells heated in 8.5% fat broth had a significantly higher (P<0.05) D-60 value (2.2 min) than those heated in 11% fat (1.0 min) and 23% fat slurry (0.9 min). Growth media (TSBYE, 8.5 % fat slurry, and TSBYE + Q), and fat level (15 % and 20%), however, were not significant factors (P>0.05) affecting inactivation rates on frankfurter surfaces. Inactivation rates were consistently higher on frankfurter surfaces compared to similar treatments done in frankfurter slurry. On frankfurter surfaces, a 2.3- to a 5.1-log10 reduction could be achieved after 15 min depending on the type of frankfurter surface. The time necessary to achieve a 3.0-log10 reduction using post-processing pasteurization of frankfurters in a hot water bath at 60 degree C approximately doubled for cells grown in TSBYE and heated in 23% fat frankfurter slurry (10.8 min) versus cells grown and heated in 8.5% fat frankfurter slurry (19.6 min).