Submitted to: Journal of Food Protection
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/24/2008
Publication Date: 10/1/2008
Citation: Ukuku, D.O., Jin, Z.T., Zhang, H.Q. 2008. Membrane damage and viability loss of Escherichia coli K-12 and Salmonella entertidis in liquid egg by thermal death time disk treatment. Journal of Food Protection. 71(10):1988-1995. Interpretive Summary: Information on the mechanisms of bacteria inactivation by heat using the Thermal-Death-Time (TDT) disk method is limited and is urgently needed by the regulatory agencies. In order to understand the mechanism of inactivation of bacteria by heat using the TDT disk processing, liquid egg white and liquid whole egg were inoculated with Escherichia coli K-12 (ATCC 23716) and Salmonella entritidis at 10,000,000 CFU/ml. The samples were immersed in water bath at room temperature (~23 C), 54, 56, 58 and 60 C for 0 to 240 seconds and the surviving and injured populations were investigated. The surviving and injured population of bacteria were determined using selective and non-selective agar. Leakage of intracellular bacteria ATP and UV-absorbing substances due to injury or membrane damage were determined. Heat treatment at 54 C did not cause much reduction in the populations of E. coli and Salmonella but resulted in sublethal injury for both bacteria. Treatment at 56 C and above, at 120 seconds, caused a lot of injury to the bacterial cells and at 180 seconds, both E. coli and Salmonella were not detected. A higher population of injured bacteria was determined at 56 C than at 54 and 58 C. Damage to the bacterial membrane caused by heat led to the leakage of ATP and UV-absorbing substances, which led to the death of E. coli and Salmonella.
Technical Abstract: Data on mechanisms of bacteria inactivation by heat using the Thermal-Death-Time (TDT) disk method are limited and is urgently needed by the regulatory agencies. In order to understand the mechanism of inactivation of bacteria by heat using the TDT disk processing, liquid egg white and liquid whole egg were inoculated with Escherichia coli K-12 (ATCC 23716) and Salmonella entritidis at 7.0 log CFU/ml. One milliliter liquid egg sample was added to the TDT disk and then immersed in water bath at room temperature (~23 C), 54, 56, 58 and 60 C for 0 to 240 s. Surviving population and bacterial injury were investigated using selective Tryptic Soy Agar containing 3 % NaCl (TSA3), 5% NaCl (TSA5), Sorbitol MacConky Agar (SMAC) and Xylose Lysine Sodium Tetradecylsufate (XLT4) and non-selective Tryptic Soy Agar (TSA). Leakage of intracellular ATP and UV-absorbing substances of bacteria due to membrane damage/injury were determined. Plating of E. coli K-12 and S. entritidis in liquid egg from TDT disks left at room temperature for up 240 s on nonselective and selective did not reveal significant reduction in numbers. When the treatment temperature was increased to 54 C or above, a significant sublethal injury occurred in E. coli and Salmonella populations at 120 s. At 180 s, populations for both bacteria were below detection (< 10 CFU/ml). Membrane damage caused by heat led to the leakage of intracellular ATP and UV-absorbing substances of E. coli and Salmonella. A correlation coefficient (R = 0.93) between treatment and leakage of UV materials was determined. These results suggest that injury/damage of bacterial membrane and leakage of intracellular active substances led to the inactivation of the bacteria in liquid egg.