Escherichia coli O157:H7 contaminated ground beef has been linked to major food recalls in recent years, creating the need for improved processing methods. High hydrostatic pressure processing (HHPP) is a reliable non-thermal processing method used to process foods such as oysters, meats, and juices to improve microbiological safety while retaining quality and organoleptic properties. The application of HHPP with pressure cycling to boost the effectiveness of pressure to inactivate E. coli O157:H7 in ground beef was the focus of this research. The effect of the combination of pressure (300-600 MPa), holding time (6-60 min) and temperature (7-45 °C) on inactivating E. coli JM109 and E. coli O157:H7 (ATCC 43895) in ground beef was investigated. Change in the color of ground beef was also determined. Compared to holding time, pressure and temperature had a significant effect on the color of ground beef. Experiments were conducted using single strains and a 6-strain cocktail from 25 °C to 45 °C, at 400 MPa and at pre-determined pressure cycles totaling a holding time of 15 minutes. The reduction for E. coli O157:H7 population increased from 3 to 5 logs as the number of cycles was increased from 1 to 5. The fate of surviving cells, post-processing and during frozen storage, was also determined by plating on selective and non-selective media. It was found that HHPP caused substantial sub-lethal injury leading to further inactivation during frozen storage. The effect of HHPP on the color of ground beef was investigated. Process temperature, as compared to pressure or time, has the most impact on the color change immediately after processing. Higher process temperature (45 °C) resulted in an undesirable color change. Refrigerated or frozen storage had minimal impact on the color of HHPP ground beef. These results suggest that HHPP has the potential to decrease levels significantly or eliminate E. coli O157:H7 from ground beef; especially when pressure cycling is applied. A 5 log reduction in E. coli O157:H7 in ground beef was achieved at 400 MPa at 25 °C with 5 pressure cycles with total time at high pressure of 15 min. Frozen storage at -20 °C had an additional negative effect on survival of E. coli O157:H7.
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Food Science
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Rutgers University Electronic Theses and Dissertations
Rutgers University. Graduate School - New Brunswick
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