Date of Award
Master of Science
Food Science and Human Nutrition
Aubrey F. Mendonca
The purpose of this investigation was to examine the viability of Listeria monocytogenes Scott A NADC 2045 that endured selected environmental stresses and were then subsequently exposed to freeze-thaw cycles or high hydrostatic pressure. The environmental stresses investigated in relation to freeze-thaw cycle survival include acid shock (HCl, pH 4.0-6.0), alkali shock (NaOH, pH 8.0-11.0), ethanol shock (2.0% -0.5%), oxidative shock (H2O2, 50-500ppm), and acid adaptation. All shock stresses were applied to exponential phase cells whereas non-stressed exponential phase cells served as a control. Freeze-thaw cycles involved freezing at -18yC for 24 h and thawing at 30yC for 7 min. Injury evaluation for all freeze-thaw treatments were performed by comparing colony counts of the pathogen on tryptic soy agar supplemented with 0.6% yeast extract (TSAYE) to counts on modified oxford agar (MOX). All samples were serially diluted (10-fold) in Buffered Peptone Water (BPW) and surface-plated on appropriate agar media. Inoculated agar plates were incubated at 35yC and bacterial colonies were counted at 72 h. Starvation of washed stationary phase cells in physiological saline (0.85% (w/v) NaCl) over 12 days was examined at 2-day intervals for viability and resistance to high hydrostatic pressure. Starvation preparation involved the static growth of L. monocytogenes in tryptic soy broth supplemented with 0.6% yeast extract (TSBYE) and washing these cells twice in 0.85% NaCl. Cells were then suspended in fresh physiological saline and held at 25yC during the starvation period. Pressurization at 400 MPa from 1 to 75 s was achieved using the Food Lab High-Pressure Food Processor (Stansted Fluid Power Ltd, Essex, U.K.). Viability of pressurized L. monocytogenes was examined after serial dilution in 0.1% peptone and plating on TSAYE followed by incubation of 35yC for 48 h. Control cultures were non-starved stationary phase cells. Results for the freeze-thaw cycles and environmental stresses indicate no statistically significance difference in freeze resistance or injury of the stressed pathogen compared to the control. When examining controls, there was a decrease in viability after 1 cycle of 0.5 log CFU/ml and 0.74 log CFU/ml after 4 cycles. This decrease occurred irrespective of any prior environmental stress tested. No statistically significant freeze-thaw injury was found among cells that endured prior stresses or in freeze-thaw treated cells compared to control cells. For starved L. monocytogenes, approximately a 2 log CFU/ml decrease was seen in viability after 2 days of starvation. Viability remained stable for the remaining 10 days. Maximum D-values (at 400 MPa) of 19.88 s, 18.6 s and 18.5 s were observed after 8, 6, and 10 days of starvation, respectively. D-value (at 400 MPa) of the control was 11.85 s. Overall significance of freeze-thaw results for the food industry is that freeze-thaw resistance of L. monocytogenes does not seem to be affected by certain prior environmental stresses on this pathogen. Reductions after 4 freeze-thaw cycles in the controls were 0.74 Log CFU/ml, which represented a significant decrease in viability. Based on the results of the present study, the exposure of L. monocytogenes Scott A to certain environmental stresses does not increase the resistance of this organism to freeze-thaw cycles. Also, starved L. monocytogenes cells developed a higher resistance to high pressure processing compared to non-starved cells. The increased high pressure resistance of starved L. monocytogenes should be considered when aiming to design safe food processing protocols involving high hydrostatic pressure technology.
Devin Kirk Dutilly
Dutilly, Devin Kirk, "Response of Listeria monocytogenes to high hydrostatic pressure or freeze-thaw cycles following exposure to selected environmental stresses" (2011). Graduate Theses and Dissertations. 10355.