A Flow-Injection System for Studying Heat Inactivation of Listeria monocytogenes and Salmonella enteritidis in Liquid Whole Egg

Abstract

A flow-injection system was devised to mimic continuous flow-through pasteurization systems for laboratory thermal inactivation studies. Air bubbles were introduced into the sample stream to create separate moving segments (plugs) of liquid stream during pasteurization while residence time was adjusted by a combination of pump speed and column length. The method was used to obtain thermal inactivation data for Listeria monocytogenes Scott A and Salmonella enteritidis ATCC 13076 in liquid whole eggs at different temperatures and heating times. Thermal inactivation of L. monocytogenes using the capillary tube method (Zcap = 7.3°C) gave results comparable to those obtained with the flow-injection system (Zflow = 7.2°C). The flow-injection system also was used to examine thermal inactivation of S. enteritidis (SE) grown in either tryptic soy broth (TSB) or egg yolk medium (EYM) before inoculation into liquid whole egg (LWE). D-values were obtained by regression analysis and the data showed that SE grown in EYM gave D-values 15 to 120% higher than those obtained for SE grown in TSB. Thermal inactivation studies performed with S. enteritidis grown in commercial broth media may therefore inaccurately represent thermal resistance of S. enteritidis grown in liquid or shell raw egg as may occur in egg-associated outbreaks. The continuous flow-injection system described herein may be adapted to study continuous flow pasteurization processes not easily examined by the traditional capillary tube method.

Thermal resistance of Salmonella enteritidis and Escherichia coli K12 in liquid egg determined by thermal-death-time disks

“…Breeuwer et al (2003) reported that S. Senftenberg had a higher D 60 value (2.4 min) with a higher z value (5.7°C) than S. enteritidis. Using a flow-injection system, Muriana et al (1996) found that S. enteritidis grown in egg yolk medium before inoculation into liquid whole egg had higher D values than those grown in tryptic soy broth. When a five-strain cocktail of S. enteritidis was inoculated into liquid whole egg (pH 7.36) and a capillary tube method was used, D 58 value of 1.50 min was observed by Michalski et al (1999).…”

“…This is probably due to the products' different pH, water activity, and nature of constituents (Michalski, Brackett, Hung, & Ezeike, 1999). For instance, S. enteritidis may not be able to tolerate the high pH found in egg white as opposed to the neutral pH of whole egg (Palumbo, Beers, Bhaduri, & Palumbo, 1996), and S. ente- ritidis can utilize the nutritious proteins and lipids found in egg yolk to help stabilize the bacterial cell membrane against heat inactivation (Muriana, Hou, & Singh, 1996).…”

Reducing Risk of Salmonellosis through Egg Decontamination Processes

“…Muriana et al [39] states that exposure to temperatures over 70 °C for less than 1.5 s results in a 6 log reduction of S. Enteritidis in liquid eggs.…”

Scientific Opinion on the public health risks of table eggs due to deterioration and development of pathogens

“…Many experimental studies on heat resistance have been carried out on S. Enteritidis because of the health and food safety concerns in the egg and egg product sector, and, more recently, on Listeria monocytogenes, as it is known to exhibit greater thermal resistance than S. Enteritidis (Palumbo et al, 1995;Muriana et al, 1996;Palumbo et al, 1996;Schuman and Sheldon, 2003;Li et al, 2005;Monfort et al, 2012). It remains difficult to compare the studies, because heat resistance depends on, among other factors, the strain, the culture conditions, the inoculation size and the equipment.…”