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Effect of pH on desiccation survival of Salmonella
Title
Effect of pH on desiccation survival of Salmonella
Creator
Chen, Fangyu
Date
2019
Description
One intrinsic property of foods, pH, can affect Salmonella survival in high-moisture foods, but its effect in low-moisture foods is unknown....
Show moreOne intrinsic property of foods, pH, can affect Salmonella survival in high-moisture foods, but its effect in low-moisture foods is unknown. In this study, the effect of pH on desiccation and persistence of Salmonella was explored using two approaches. First, the pH range that affects survival in low-moisture environments was explored. The effect of acid adaptation in acidic low-moisture environments was also explored. Salmonella Anatum was grown on trypticase soy agar with 0.6% yeast extract (TSAYE). After harvest, cells were divided and one portion treated independently at pH 3, 4, 5, 7, and 8 for 30 min. Both portions were then desiccated on a cellulose filter in a biohazard cabinet (23±2°C) overnight (24±2 h). After desiccation, cells not previously pH treated were resuspended in buffers at pH 3, 4, 5, 7, and 8 for 30 min, and cells previously pH treated were resuspended in buffered peptone water (BPW). All suspensions were plated on TSAYE with ammonium iron citrate and sodium thiosulfate to determine surviving populations. In addition, S. Anatum was grown on TSAYE adjusted to pH 4, 5, 7, and 8. Cells were either harvested with buffer with the same pH of the growth media or with saline then treated at pH 4, 5, 7 and 8. All were desiccated as indicated before. Desiccated cells were stored at 20% RH at 25°C for up to 29 days. To determine the effect of prior acid adaptation on survival in acidic environments, Salmonella Anatum and Salmonella Agona were grown on agar with or without 1% glucose, harvested, then suspended in buffer at pH 4, 5, and 7. Each culture was desiccated on cellulose filters and stored at 30% RH at 25°C for up to 29 days. Harvested cells were also stored in buffers at the same pH held stored at 25oC for the same time periods. In addition, acid-adapted cells were harvested with saline, desiccated and stored as indicated for each type of cells on wheat flakes. Results from pH range finding experiments indicated pH did have an effect on the survival of Salmonella during desiccation. Desiccation prior to treatment will affect survival at different pH levels. However, prior pH adaptation did not result in increased survival under different pH conditions once cells were desiccated. Acid adaptation prior to desiccation at low pH adversely affected survival for S. Agona but not S. Anatum. Survival after desiccation at different pH levels was greater than survival in the same pH buffers. No advantage or differences in survival was observed with a commercial wheat flake product indicating results obtained in a model environment may be reduced or eliminated when food components are present.
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Evaluation of Salmonella Proliferation on Alfalfa Sprouts during Storage at Different Temperatures
Title
Evaluation of Salmonella Proliferation on Alfalfa Sprouts during Storage at Different Temperatures
Creator
Lin, Chih Tso
Date
2020
Description
Sprouts, a low-calorie vegetable rich in nutrition, have been a popular ingredient in many meals in the USA. They are grown either at...
Show moreSprouts, a low-calorie vegetable rich in nutrition, have been a popular ingredient in many meals in the USA. They are grown either at commercial sprout farms or at home and served raw or lightly cooked. However, sprouts are also known as a source of foodborne illness outbreaks. FDA Food Code identifies raw sprouts as a time/temperature control for safety food. However, little information is known about the growth profile of foodborne pathogens in sprouts stored at different temperatures. This study aimed at evaluating the proliferation of Salmonella in alfalfa sprouts during storage at 4, 10, and 25℃ under two different contamination routes: 1) sprouts that were inoculated with Salmonella after harvest and 2) sprouts that were grown from contaminated seeds. Alfalfa sprouts grown from uninoculated seeds and harvested after 5 days of sprouting were divided into 25-g portions. Each portion was inoculated with a cocktail of five Salmonella serovars at levels of 10^1, 10^3 or 10^5 CFU/g prior to storage at 4, 10, or 25℃. Alternatively, sprouts grown for five days from seeds spiked with 1% of seeds previously inoculated with the Salmonella cocktail were divided into 25-g portions and stored at 4, 10, or 25℃. At defined time points (Days 0, 2, 4, 7, 14, and 21), levels of Salmonella and background microflora in stored sprouts were determined by plate count. Alfalfa sprouts appeared fresh during the 21 days of storage at 4 or 10℃ but started to show signs of spoilage after 4 days of storage at 25℃. The total plate counts maintained at a level above 9 log CFU/g throughout 21 d of storage at 4 and 10℃ or during the first 7 d of storage at 25℃. Storing sprouts at 4 or 10℃ could inhibit the proliferation of Salmonella. After 21 d of storage, the Salmonella counts in inoculated sprouts decreased slightly, by 0.88 or 0.93 log units, respectively. For sprouts stored at 25℃, the Salmonella growth profile differed depending on the route of contamination and the level of Salmonella at the start of storage. In sprouts inoculated at levels of 1.41, 2.83, and 4.75 log CFU/g, the Salmonella counts increased to 6.62, 6.86, and 6.68 log units, respectively, during the first 4-7 days of storage. For alfalfa sprouts grown from contaminated seeds, the Salmonella counts remained at a level similar to that in the harvested sprouts (8.16 log CFU/g) during the first 7 d. Results from this study further the understanding of pathogen growth in sprouts and will aid in the development of guidelines for proper storage of sprouts.
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Growth kinetics of Salmonella enterica and Listeria monocytogenes during rehydration of dehydrated corn and subsequent storage
Title
Growth kinetics of Salmonella enterica and Listeria monocytogenes during rehydration of dehydrated corn and subsequent storage
Creator
Mate, Madhuri
Date
2022
Description
Dehydrated vegetables, including corn, are often used in restaurants and retail grocers. They do not support the growth of pathogens as their...
Show moreDehydrated vegetables, including corn, are often used in restaurants and retail grocers. They do not support the growth of pathogens as their moisture content is very low. After rehydration, these food products attain high water activity values suitable with neutral pH for the survival and proliferation of foodborne pathogens, including Salmonella enterica and Listeria monocytogenes. The purpose of the study was to examine the extent to which dehydrated corn supports the growth of S. enterica and L. monocytogenes during rehydration at 5 or 25°C water and following storage at 5, 10, and 25°C temperatures at 1, 3, 5 and 7 d intervals. Fresh corn was dehydrated at 60°C for 24 h. Dehydrated corn was inoculated with a 4-strain cocktail of either S. enterica or rifampicin-resistant L. monocytogenes, resulting in 4 log CFU/g, and held at ambient temperature for 24 h. This corn was then rehydrated using either 5 or 25°C water for 24 h. Throughout rehydration, corn samples were removed at intervals and enumerated. To enumerate S. enterica and L. monocytogenes, the samples were homogenized with BPB and BLEB respectively and cultivated on TSAYE with overlaid XLD or BHIARif200, respectively. Rehydrated corn was then stored at 5, 10, or 25°C and enumerated at intervals 1,3,5 and 7 d. Triplicate samples were assessed at each timepoint and three independent experiments were conducted for each rehydration water temperature. Growth rates were determined by DMFit and statistically analyzed using Student t-test. A p-value ≤0.05 was considered significant. Overall the growth rate of S. enterica was higher when rehydrated in 5°C water temperature and then stored at 25°C and was determined to be 0.61 ± 0.23 log CFU/g per d. This timepoint was also the shortest time required to increase by 1 log which was: 1.64 d, i.e. 39 h. For L. monocytogenes, the 25°C water rehydration showed the fastest growth rate when stored at 25°C. It took only 1.58 d or 37.8 h for 1 log increase in the population. After 5°C water rehydration of corn the highest populations of mesophilic bacteria and yeasts and molds were observed for 25°C storage ranging from 8.43 to 9.39 log CFU/g and 4.75 to 7.87 log CFU/g, respectively. After 25°C water rehydration, the highest population of mesophilic bacteria, 8.88 log CFU/g, was observed at 5°C storage at 5 d; yeasts and molds were 8.70 log CFU/g for 25°C storage on the same day. The results of this study determined that S. enterica and L.monocytogenes could survive and grow in dehydrated plant foods during rehydration and storage, highlighting the need for product assessments for these types of foods.
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Efficacy and Mechanisms of Power Ultrasound-Based Hurdle Technology for Reduction of Pathogens in Fresh Produce
Title
Efficacy and Mechanisms of Power Ultrasound-Based Hurdle Technology for Reduction of Pathogens in Fresh Produce
Creator
Zhou, Xinyi
Date
2023
Description
Minimally processed produce is frequently contaminated with foodborne bacterial pathogens. Power ultrasound is a non-thermal and cost...
Show moreMinimally processed produce is frequently contaminated with foodborne bacterial pathogens. Power ultrasound is a non-thermal and cost-effective technology that can be combined with other chemical sanitization methods. This study investigated the reduction of Listeria monocytogenes and Salmonella Newport on grape tomato, romaine lettuce, and spinach washed with water, chlorine, or peroxyacetic acid alone or in combination with 25 or 40 kHz power ultrasound for 1, 2, or 5 min. Produce items were inoculated with selected pathogens at approximately 10 log CFU/g, air dried for 2 h, and then treated. Combined treatment of ultrasound and sanitizers resulted in 1.44-3.99 log CFU/g reduction of L. monocytogenes and 1.35-3.62 log CFU/g reduction of S. Newport on washed produce items, with significantly higher reductions observed on grape tomato. Synergistic effects were achieved with the combined treatment of power ultrasound coupled with the chemical sanitizers when compared to the single treatments. An additional 0.48-1.40 log CFU/g reduction of S. Newport was obtained with the combined treatment on grape tomato. In general, no significant differences (p<0.05) were observed in pathogen reductions between the selected ultrasound frequencies, sanitizers, or treatment lengths. Results from this study suggest that incorporation of power ultrasound to current treatment can enhance bacterial pathogen reduction on fresh produce surface, but cannot completely eliminate bacterial pathogens. Transcriptomic study revealed significant (|Log2 fold change|<1 and false discovery rate < 0.05) transcriptional changes in L. monocytogenes LS810 in response to the 2 min power ultrasound treatment. The up-regulation of genes encoding TPI, LLO, and PTS indicates increased energy requirements, enhanced virulence, and demand for sugar sources in bacteria. On the other hand, the down-regulation of genes involved in cyclic dimeric GMP hydrolysis and transcriptional regulation suggests a modulation of intracellular signaling, cellular processes, and metabolisms to enhance survival and recovery. The GO and KEGG analysis demonstrated defense mechanisms against ultrasound stress more comprehensively. L. monocytogenes adjusts its metabolism, repairs cell membranes, and conserves energy for survival. These findings enhance our understanding of its adaptation to environmental stress. Results of this study can be used as a start point for optimizing the efficacy of ultrasound-based hurdle treatments for fresh produce disinfection.
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