Graduate Studies


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A THESIS presented to the Faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the Degree of Master of Science, Major: Food Science and Technology, Under the Supervision of Professor Harshavardhan Thippareddi. Lincoln, Nebraska: July, 2014

Copyright 2014 Daniel A. Unruh


The USDA Food Safety Inspection Service (USDA-FSIS) declared six serogroups of Shiga toxin-producing Escherichia coli (O26, O45, O103, O111, O121, and O145) as adulterants in raw beef manufacturing trimmings, in addition to E. coli O157:H7. The D-values and z-values for the six serogroups, along with two cocktails of STECs (containing one strain representing each serogroup; USDA-ARS and ATCC) and E. coli O157:H7 were determined (separately) in irradiated, raw ground beef. The D-values for the six serogroups of STECs ranged from 28.32 to 45.02 min at 54.4°C, 1.96 to 2.94 min at 60.0°C, and 0.12 to 0.21 min at 65.6°C, respectively. These D-values were similar (p>0.05) to E. coli O157:H7 values determined in this study at the respective temperatures. Higher D-values (p≤0.05) were observed for O45 and O121 compared to E. coli O157:H7 in 73:27 ground beef at 60°C. Overall, the fat content of the ground beef (27 vs. 7%) did not affect (p>0.05) the D-value of the STECs. Salami is a fermented sausage with high salt content and low water activity, and current regulations dictate a heat-step subsequent to fermentation to control E. coli O157:H7. To avoid sensory defects resulting from heat treatment, high pressure processing (HPP) has been identified as a possible intervention. To quantify the actual reductions of E. coli O121:H19, sliced salami was inoculated with cells grown in conditions to simulate the salami enviornment and allow accumulation of compatible solutes (glycine-betaine and trehalose). E. coli O121 was grown in compatible solute-promoting media, harvested during the stationary phase, inoculated onto sliced salami, vacuum packaged, and stored for 9 d. The inoculated salami was high pressure processed after 0, 15, and 30 d refrigerated storage (4°C). HPP of salami inoculated with cells grown in trehalose-promoting conditions showed the lease reduction (p≤0.05) of E. coli O121, followed by TSB and glycine-betaine. Growing cells in trehalose-promoting conditions provides a conservative estimate of the efficacy of HPP as it results in cells with greater high pressure resistance.

Adviser: Harshavardhan Thippareddi