Masters Theses

Date of Award

8-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

David A. Golden

Committee Members

Jochen Weiss, F. Ann Draughon

Abstract

Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes suspended in salt solutions Na2PO4, NaCl, NaNO3, NH4Cl, CaCl2, AlCl3, and L. monocytogenes in salt solutions NaCl, NaNO3, AlCl3 at concentrations ranging from 0 to 5% (wt/vol) by high-intensity ultrasound at two different temperature conditions (ice water bath, 40ºC water bath) and three ultrasonic intensity levels (9.49, 21.83, 49.22 W/cm2) was determined. Increases in sonication treatment time, intensity and temperature led to increased lethality of both pathogens. However, cell lethality, as a function of solute concentration, varied depending upon type of salt. CaCl2, NaCl, NaNO3, NaPO4, and NH4Cl had little or no effect on survival of E. coli O157:H7 (< 1 log reduction) regardless of other treatment conditions at 9.49 W/cm2. Concentrations > 0.5% of CaCl2, NaNO3, NaPO4, and NH4Cl adversely affected survival (1.0 - 1.6 log reduction) with treatment of 21.83 W/cm2 in an ice bath, and showed greater inactivation (1.2 - 4.0 log reduction) at 40°C. Treatment of 49.22 W/cm2 showed the greatest impact regardless of other parameters. At 40°C, treatment for 10 minutes at 49.22 W/cm2 led to total inactivation for bacteria suspended in 0.5% NH4Cl, 1% CaCl3, 2% NaCl, 5% NaPO4, and all concentrations of AlCl3. Complete inactivation also occurred in an ice bath for 5% AlCl3, which was shown to be the most effective salt.

NaCl and NaNO3 had little or no effect on survival of L. monocytogenes (< 1 log reduction) regardless of other treatment conditions at 9.49 W/cm2. Concentrations >0.5% of NaCl and AlCl3 adversely affected survival (0.2 -1.6 log reduction) with treatment of 21.83 W/cm2 in an ice bath and showed greater inactivation (0.4 - 5.2 log reduction) at 40°C. Regardless of other treatment parameters, sonication at 49.22 W/cm2 showed the greatest lethal impact. Treatment for 10 minutes at 40°C at 49.22 W/cm2 led to a 4.8 - 5.7 log reduction for cells suspended in all concentrations of NaNO3. Typically, L. monocytogenes was more resistant to ultrasound treatment than E. coli O157:H7. Inactivation of pathogens is attributed to cavitation-induced shear forces, reaction of cavitation-generated hydrogen peroxide with microbial cell wall constituents and electrostatic interactions of dissociated salts with cell membranes.

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