Masters Theses

Date of Award

5-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

David A. Golden

Committee Members

F. A.nn Draughon, J.R. Mount

Abstract

The research portion of this thesis focuses on two main aspects of Escherichia coli 0157:H7. The first investigation was an examination concerning the influence of growth temperature on heat-, lactic acid-, and freeze- induced inactivation and injury of E. coli 0157:H7. Three strains of E. coli 0157:H7 isolated from ground beef, apple cider, and salami were grown at optimum (37°C) and suboptimum (10°C) growth temperatures for 16 hr and 7 days, respectively. Strains were inoculated into 0.1% peptone water (PW) and heated at 52, 54, and 56°C or stored at -20°C for heat or freeze-induced inactivation and injury, respectively. Lactic acid induced inactivation and injury was performed by inoculating and holding test strains. Growth of strains at 10°C as compared with 37°C had a significant impact on reducing (P<0.01) D-values obtained for heating, acid exposure, and freezing, with the exception of the cider strain stored in lactic acid. When strains were cultivated at 10 and 37°C and heated at 54 and 56°C, the salami strain possessed the highest (P<0.01) D-values. When grown at 10°C, the beef strain had the lowest (P<0.01) D-values after heating at 52, 54, and 56°C. The salami strain grown at 10°C had the highest (P<0.01) D-values in all concentrations of lactic acid. After growth at 37°C, the salami strain had the highest (P<0.01) D-values after storage in 0.1 and 0.25% lactic acid, while D-values for salami and beef strains did not differ (P>0.05) when stored in 0.5% lactic acid. The cider strain grown at 37°C had lower (P<0.01) D-values than the beef strain after storage in 0.25 and 0.5% lactic acid. Substantial portions of strain populations were sublethally injured after heat and lactic acid treatments, as evidenced by the inability of injured organisms to form colonies on tryptone soya agar containing 2% NaCl. Strains cultured at 10°C were more susceptible to sublethal injury, as compared with strains cultured at 37°C. Storage of test strains at -20°C for 7 months resulted in a 4 to 6 log CFU/ml reduction in viable population, but induced only minimal sublethal injury. After 5 months at -20°C, strains cultured at 10°C were more sensitive to freeze-inactivation than strains cultured at 37°C. When grown at 10 and 37°C and stored at -20 for 7 months, the cider strain possessed higher (P<0.01) D-values than beef and salami strains. Results of this investigation indicate that the environment from which E. coli 0157:H7 is isolated, as well as growth temperature, can influence resistance of the organism to heat-, lactic acid-, and freeze-stress.

The second investigation was an examination of the influence of package atmosphere on growth and survival of E. coli 0157:H7 stored at 37,10, and 4°C. The salami strain used in the previous study was used for investigation due to its resistance to heat- and lactic acid-induced inactivation and injury. The strain was heated for 10 min at 56°C, as described in the previous study, to achieve approximately 90% injury within the test population. Populations of uninjured and heat-injured E. coli 0157:H7 were inoculated onto brain heart infusion agar containing 0.3% beef extract, packaged in barrier bags in air, 100% CO₂, 100% N₂, 20% CO2/80% N₂, and vacuum and stored at 37, 10, and 4°C. Package atmosphere and inoculum status (i.e., uninjured or heat-injured) influenced (P<0.01) growth and survival of E. coli 0157:H7 stored at all test temperatures. Growth of heat-injured E. coli 0157:H7 was slower (P<0.01) than grew during 10 days of storage at 10°C, while heat-injured populations declined during 20 days of storage at 10°C. Uninjured E. coli 0157:H7 stored at 10°C reached stationary phase growth within approximately 10 days in all packaging atmospheres except CO₂. Populations of uninjured and heat-injured E. coli 0157:H7 declined throughout storage for 20 days at 4°C. Survival of uninjured populations stored at 4°C, as well as heat- injured populations stored at 4 and 10°C, was enhanced in CO₂ atmosphere. Survival of heat-injured E. coli 0157;H7 at 4 and 10°C was not different (P>0.05). Results of this investigation indicate that uninjured and heat-injured E. coli 0157:H7 are able to survive at low temperatures in the modified atmospheres used in this study. Therefore, packaging treatments commonly applied to fresh beef may be ineffective at inhibiting growth and survival of this pathogen.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS