Thesis
SURVIVAL AND THERMAL RESISTANCE OF LISTERIA MONOCYTOGENES IN DRY AND HYDRATED NONFAT DRY MILK AND WHOLE MILK POWDER DURING EXTENDED STORAGE
Washington State University
Master of Science (MS), Washington State University
01/2021
DOI:
https://doi.org/10.7273/000004346
Handle:
https://hdl.handle.net/2376/125190
Abstract
Listeria monocytogenes has unique ability to survive in low water activity (a_w) conditions for prolonged time periods and can cause severe health concerns if post-pasteurization contamination occurs in milk powders. The objective of this study was to determine survivability and thermal resistance of L. monocytogenes in dry and hydrated nonfat dry milk (NFDM) and whole milk powder (WMP) for 6 months storage. The hydrated milks were prepared from stored inoculated milk powders only on the day of D- and z-value analyses. This research was designed as two factorial (storage and powder type) randomized complete block design with three replications. Milk powders were spray inoculated with a 3-strain cocktail of L. monocytogenes and dried back to their original a_w. The D- and z-values of L. monocytogenes were conducted every 30th day, starting on day one for dry and hydrated powders (prepared from stored milk powders). The respective samples were transferred into thermal-death-time (TDT) disks, sealed, and placed in the water baths set at 75, 80, and 85℃ for milk powders, and 54, 57, and 60℃ for hydrated milk powders to determine the D-values. Two-way ANOVA at P ≤ 0.05 and Tukey's test was used for statistical analysis to compare D- and z-values of L. monocytogenes at specific temperatures. The overall reduction of L. monocytogenes population in milk powders was ~2.2 log CFU/g, whereas the hydrated milk powders had ~2.0 log CFU/mL reduction of L. monocytogenes. The D70°C, D75°C, and D80°C values of L. monocytogenes in NFDM at day 1 were 13.1, 6.0 and 4.0 min, respectively, and for WMP the D70°C, D75°C, and D80° were 12.0, 6.3 and 3.3 min, respectively. Whereas the D70°C, D75°C, and D80°C values of L. monocytogenes in NFDM at day 180 were 19.2, 10.4 and 3.8 min, respectively, and for WMP the D70°C, D75°C, and D80° were 16.4, 9.1 and 5.2 min, respectively. The z-value of L. monocytogenes in milk powders stayed similar throughout the storage period with overall z-values of 17.9°C.
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Details
- Title
- SURVIVAL AND THERMAL RESISTANCE OF LISTERIA MONOCYTOGENES IN DRY AND HYDRATED NONFAT DRY MILK AND WHOLE MILK POWDER DURING EXTENDED STORAGE
- Creators
- Yaeseol Yang
- Contributors
- Minto Michael (Advisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Food Science, School of
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University
- Number of pages
- 93
- Identifiers
- 99900882929501842
- Language
- English
- Resource Type
- Thesis