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.