Evaluation of Glass Capillary Tube and TDT Disk Methods for Determining Thermal Inactivation Kinetics of Salmonella in Liquid Whole Egg

Authors: Gurtler, Joshua; Zhang, Howard; ZHANG, LEI - MICHIGAN STATE UNIVERSITY; RYSER, ELLIOT - MICHIGAN STATE UNIVERSITY; STRATTON, JAYNE - UNIVERSITY OF NEBRASKA

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 4/12/2008
Publication Date: 8/4/2008
Citation: Gurtler,J.,Zhang,H.,Zhang,L.,Ryser,E.,Stratton,J. 2008. Evaluation of Glass Capillary Tube and TDT Disk Methods for Determining Thermal Inactivation Kinetics of Salmonella in Liquid Whole Egg [abstract].International Association for Food Protection, 95th Annual Meeting. Columbus, OH. p.1.

Interpretive Summary:

Technical Abstract: The USDA liquid egg pasteurization standards are based on the thermal inactivation of Salmonella. Glass capillary tubes have been traditionally used to determine thermal death times of microorganisms in liquid foods. A new thermal-death-time (TDT) disk is a reportedly simpler means for evaluating bacterial inactivation in liquids, providing up to one milliliter of sample for microbiological analysis. The goal of this comparative study was to determine the thermal inactivation kinetics of Salmonella in liquid whole egg (LWE) using the glass capillary tube and TDT disk methods. A five-strain-mixture of Salmonella, representative of common egg-related serovars, was inoculated at ca. 8 log CFU/ml into LWE, and injected (0.025 ml per tube) into glass capillary tubes, which were flame-sealed. Inoculated LWE (0.9 ml) was also added to TDT disks. Capillary tubes and TDT disks were immersed in a circulating water bath at 54, 56 and 58C, and temperatures were measured with thermocouples. Two capillary tubes and one TDT disk were removed at each sampling time, immersed in an ice slush bath, and samples were plated for enumeration onto Tryptic soy agar with 1g/L sodium pyruvate and 6g/L yeast extract. Regression analysis was performed, D and Z values were calculated, and significant differences between methods were determined by t-tests. The D values for the capillary tube method were 13.08, 4.49, and 1.19 minutes at 54, 56, and 58C, respectively, and 11.15, 3.41, and 1.11 min., respectively, for the TDT disk method. The Z-values were calculated at 3.99 for the capillary tube method and 3.84 for the TDT disk method. No significant difference (P > 0.05) was found between methods. The capillary tube and TDT disk methods provide equivalent D and Z- values for the thermal inactivation of Salmonella in LWE, although the latter method provides a twenty-fold larger sample volume for microbiological analysis.