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Title: VALIDATION OF BAKING TO INACTIVATE SALMONELLA IN HIGH-PROTEIN AND HIGH-FAT MODEL FOODS
Creator: Wang, Wenqian
Date: 2017, 2017-07
Description: Baked food products, such as dry pet foods, undergo changes of temperature and water activity (aw) during forced hot air processes. As one of...
Show moreBaked food products, such as dry pet foods, undergo changes of temperature and water activity (aw) during forced hot air processes. As one of the most thermal resistant microorganisms in low-moisture/intermediate moisture foods, Salmonella’s thermal inactivation kinetics during these processes is not well understood and difficult to predict. The objective of this study was to evaluate thermal inactivation kinetics of Salmonella enterica serovar Agona 447967 in model high-protein (HP) and high-fat (HF) multiple-component foods baked in a laboratory-scale oven, as influenced by oven temperature and relative humidity (RH). Model high-protein and high-fat foods, formulated with wheat flour, soy protein and soy oil, were inoculated with Salmonella Agona to a level of ~9 log CFU/g, and mixed to form a homogenous dough. Dough samples (57 mm diameter x 6 mm thick) were baked (3 samples per dwell time, 6 dwell times per condition) in a lab-scale oven at 120°C (10% RH) and 85°C (20%, 35% RH, 50% RH), respectively. Temperature and aw were measured at the surface and geometric center of the product during baking. Processed samples were collected in sterile bags and immediately cooled in an ice-water bath. Salmonella was enumerated on trypticase soy agar supplemented with yeast extract and incubated at 37°C for 24 h. Similar reductions (p>0.05) of 5.12-, 5.11-, 4.55-, and 4.78-log CFU/g were achieved after 40 min at 120°C/10% RH, 90 min at 85°C/20% RH, 50 min at 85°C/35% RH, 8 min at 85°C/50% RH, respectively, in the high-protein model food. Similar results were achieved in the high-fat matrix. The aw at the geometric center of the product (initially at aw =0.98) did not change appreciably during baking, while the aw at the product surface, the location of least lethality, decreased significantly (p<0.05) during baking; the decreases were more pronounced at lower oven RH. The results indicate that thermal inactivation of Salmonella Agona was driven by temperature and relative humidity in the oven. Higher temperature and higher relative humidity level led to greater Salmonella inactivation.
M.S. in Food Process Engineering, July 2017
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Title: GROWTH, INACTIVATION, AND SURVIVAL OF SALMONELLA ON SESAME SEEDS DURING TAHINI PROCESSING AND REFRIGERATED STORAGE OF TAHINI
Creator: Zhang, Yangjunna
Date: 2016, 2016-05
Description: Salmonella can survive for long period of time in low-moisture foods and cause human illness after consumption of contaminated foods. Recently...
Show moreSalmonella can survive for long period of time in low-moisture foods and cause human illness after consumption of contaminated foods. Recently, sesame seeds and tahini (sesame seeds pasta), have been identified as unusual sources of salmonellosis. Controlling specific steps during tahini processing and storage of tahini may minimize the risk of Salmonella contamination. This study examined the fate of Salmonella in different steps of tahini processing and refrigerated storage of tahini. A four serovar cocktail of Salmonella was used for inoculation of sesame seeds and tahini. Bacterial populations were determined by aerobic plate counts on both selective and non-selective media. Water activity of samples was determined during processing and storage. For the soaking step during tahini processing, unhulled dry sesame seeds with aw of 0.1 were inoculated with Salmonella, held 24 h, and then soaked in water at ambient temperature. Populations were monitored at 0, 18, 22 and 24 h. Salmonella decreased by 2 - 3-log CFU/g during drying and then increased by 5 log CFU/g after rehydration. Separately, inoculated de-hulled seeds with two different initial aw were roasted at three different temperatures (95, 110, and 130 ºC) for 90 min. Both the Salmonella populations and aw were determined at 10 min intervals during roasting. Finally, inoculated sesame seeds were processed into tahini. As a comparison, the same quantity of uninoculated roasted sesame seeds were processed into tahini, and then inoculated post-processing. Tahini was stored at 4 ºC for 17 weeks. Approximately 8 log CFU/g of Salmonella was detected initially in the sesame seeds prior to roasting with a 1-log CFU/g reduction after 20 min at 95 ºC when the aw of seeds decreased quickly. For different roasting temperatures, the aw declined much faster and stopped decreasing during roasting, while populations continue decreasing throughout the whole roasting. To investigate survival of Salmonella during refrigerated storage of tahini, approximately 9 log CFU/g of Salmonella was inoculated onto sesame seeds or into processed tahini without significant reduction throughout 17 weeks (p > 0.05). These results suggest that Salmonella contamination can be an issue at any step of tahini manufacture.
M.S. in Food Safety and Technology, May 2016
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Title: IN-PACKAGE DECONTAMINATION OF SALMONELLA MONTEVIDEO ON BLACK PEPPER USING PULSED LIGHT TECHNOLOGY
Creator: Xu, Xinran
Date: 2016, 2016-07
Description: This study evaluated the efficacy of three pulsed light systems (1.8, 3 and 100 Hz) for inactivation of Salmonella Montevideo on the surface...
Show moreThis study evaluated the efficacy of three pulsed light systems (1.8, 3 and 100 Hz) for inactivation of Salmonella Montevideo on the surface of black pepper. The impact of frequency, treatment time and distance from the lamp on the rate of inactivation was studied. A reduction of 4.21 1og10 CFU/g was achieved with a 35 s treatment at 16.6-cm from lamp at 1.8 Hz. Similarly, reductions of 4.72 CFU/g and 4.70 CFU/g were obtained at 14.1-cm after 35 s treatment at 3 Hz and 100 Hz, respectively. Temperature measurement of packed and unpacked black pepper indicated that packaging material absorbed a small amount of energy. Measurement of surface and internal temperature suggested that temperature below the surface increased far less than surface temperature. For instance, temperature on the surface of the black pepper can go up to 200 °C as measured by the infrared camera within 30 to 40 s treatment. However, the temperature at 1 mm below the surface was up to 100 °C depending upon the distance from the lamp. The color of the black pepper did not change after pulsed light treatment (p>0.05). These results suggest that pulsed light can potentially be used to inactivate pathogens on black pepper surface without adversely affecting its quality.
M.S. in Food Safety and Technology, July 2016
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Title: SURVIVAL AND ATTACHMENT OF SALMONELLA ENTERICA ON SELECT LOW MOISTURE NUT SURFACES
Creator: Zhang, Bo
Date: 2017, 2017-07
Description: Salmonella can contaminate various nuts and lead to human foodborne disease outbreaks and related product recalls in the United States. Nuts...
Show moreSalmonella can contaminate various nuts and lead to human foodborne disease outbreaks and related product recalls in the United States. Nuts have been identified as an increasingly common vector for human salmonellosis. The objective of this study was to evaluate the influence of environmental factors and Salmonella enterica serotypes on the survival and attachment of this pathogen on the surface of low moisture, raw in shell nuts. Two S. enterica strains (Enteritidis and Typhimurium) were individually tested on three different food samples (almonds, hazelnuts and black peppers) at 4°C or 25°C for up to 14 days. The storage relative humidity was maintained at 20±2% throughout the study. Nut samples were collected at 0, 1, 3, 7 and 14 days. The population of bacteria was calculated based on the plate count data. The results showed that S. Typhimurium attached and survived better than S. Enteritidis on almonds, hazelnuts and black peppers at both 4 and 25°C. A lower storage temperature led to a better survival of Salmonella on raw nuts during storage. The surface characteristics of hazelnuts resulted in the least Salmonella attachment compared to almond and black pepper.
M.S. in Food Safety and Technology, July 2017
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Title: GROWTH AND SURVIVAL OF SALMONELLA IN PARTIALLY SPROUTED PUMPKIN, SUNFLOWER, AND CHIA SEEDS SUBSEQUENTLY DRIED FOR DIRECT CONSUMPTION
Creator: Wang, Can
Date: 2017, 2017-05
Description: Products based on dried, partially sprouted seeds have been associated with foodborne salmonellosis. Typically, sprouted seeds have been a...
Show moreProducts based on dried, partially sprouted seeds have been associated with foodborne salmonellosis. Typically, sprouted seeds have been a major concern with respect to foodborne illness, but little is known about the risks associated with dried partial sprouted seed products. Previous studies suggest that low-temperature, highhumidity storage conditions may provide an alternative means to inactivate Salmonella for those processors opposed to traditional thermal processing. The objective of this study was to determine the growth of Salmonella during germination of pumpkin, sunflower, and chia seeds, and the survival of Salmonella during drying and storage under variable temperature and relative humidity (RH) conditions. Pumpkin, sunflower, and chia seeds were dry inoculated at 5 log CFU/g with 4 serotype mixture of Salmonella using a dry transfer method, and then soaked in sterile water at 25 and 37°C for 24 h. Salmonella and total aerobic mesophilic plate counts (APC) were determined on xylose lysine deoxycholate (XLD) and tryptic soy agar supplemented with 0.6% yeast extract (TSAYE), respectively. Seeds using this procedure were drained and dried at 25, 51 and 60°C. The water activity (aw) and microbial population of seeds were monitored during drying. Seeds soaked and dried at 25oC were utilized for subsequently storage studies. Portions of dried chia seeds were ground into powder, then pumpkin, sunflower and chia seeds and chia seeds powder were stored at either 37 or 45oC at three different RH levels. The aw and microbial population of seed samples were monitored during storage. During the seed soaking, Salmonella and APC increased approximately 3 log CFU/g. Some decreases were observed at the end of the growth period, as may be expected after stationary phase. Drying at 60oC to a aw of 0.1 resulted in over 6-log CFU/g loss in bacterial population in partially sprouted pumpkin and sunflower seeds, and 5-log CFU/g loss in partially sprouted chia seeds. However, seeds dried at 25oC bring the aw to 0.5 had only approximately a 1-log CFU/g loss in bacterial population. The highest temperature (45oC) and RH (76%) storage condition had the greatest inactivation effect. At 45oC and 76%RH, Salmonella in pumpkin and sunflower seeds were below detection limit after 4 d and 7 d respectively. Salmonella populations in chia seeds and chia seeds powder were still detectable after 7 d regardless of the treatment condition. Salmonella is capable of survival during the manufacture of partially sprouted seeds including both drying and storage steps. Mild thermal treatment coupled with high RH storage condition holds promise for Salmonella inactivation in pumpkin and sunflower seeds but is less effective for chia seeds and chia seeds powder. Dried, partially sprouted seeds that are used as ingredients or consumed directly without further processing are a high-risk food product. Such products must be treated to eliminate foodborne pathogens that may cause foodborne illness.
M.S. in Food Science, May 2017
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Title: SALMONELLA SURVIVAL DURING INOCULATION AND STORAGE OF DRIED SPICES CONTAINING ANTIMICROBIAL COMPONENTS
Creator: Hu, Chuxuan
Date: 2016, 2016-05
Description: To provide an adequate initial microbial level for spices containing antimicrobial compounds for use in stability testing of foodborne...
Show moreTo provide an adequate initial microbial level for spices containing antimicrobial compounds for use in stability testing of foodborne pathogens, a dry-transfer inoculation was designed and compared with a traditional aqueous inoculation protocol. A five serovar cocktail of Salmonella spp. was prepared at different concentrations (~11 log CFU/mL, ~8 log CFU/mL and ~4 log CFU/mL). It was inoculated onto 1g silica beads and four types of dried ground spices (clove, oregano, ginger, and black pepper). Both spices and beads were dried for 24h at ambient conditions (~22°C). Silica beads were then used as a carrier to inoculate the same four types of spices. The results suggest that dry-transfer of Salmonella via inoculated silica beads provided a greater starting inoculum than aqueous transfer alone and are a viable alternative to aqueous inoculation when spices contain antimicrobial components. This dry-transfer inoculated ground clove with 8.4 log CFU/g and 3.9 log CFU/g initial populations was then used to determine the survival of Salmonella on spices containing antimicrobial components. Salmonella spp. survival on silica beads with 9.2 log CFU/g and 5.9 log CFU/g initial populations and subsequent transfer to ground clove was also examined. Samples were enumerated at selected time point up to 210 d. Results of this study indicate that Salmonella transfer and recovery from beads and in ground clove was not affect by storage and that Salmonella may persist for an extended period of time. The calculation of recovery was, however, affected by detection limits in clove. To improve detection limits an oil extraction was used and compared to the BAM method (detection limit 3.7 log CFU/g). After oil extraction the clove samples were soaked for 1 h (or 24 h for low inoculum concentration) in TSB at 37ᵒC and subsequently enumerated on tryptic soy agar with 0.6% yeast extract and xylose lysine deoxycholate agars. The result demonstrates that the oil extraction detection method holds promise as an alternative method to detect Salmonella in dry spices containing antimicrobial components and can provide a lower detection limit (1.7 log CFU/g).
M.S. in Food Process Engineering, May 2016
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Title: COMPARISON OF THE EFFICACY OF THREE SANITIZERS WITH 20,000 PPM CALCIUM HYPOCHLORITE FOR INACTIVATION OF SALMONELLA ON ARTIFICIALLY CONTAMINATED ALFALFA SEEDS
Creator: Li, Qing
Date: 2015, 2015-12
Description: This study evaluated the efficacy of three alternative sanitizers, acidified sodium chlorite (ASC, also known as Sanova), stabilized oxychloro...
Show moreThis study evaluated the efficacy of three alternative sanitizers, acidified sodium chlorite (ASC, also known as Sanova), stabilized oxychloro-based sanitizer (SOC, also known as Germin-8-or), and Tsunami 100 in comparison with 20,000 ppm Ca(OCl)2 for treatment of alfalfa seeds artificially inoculated with Salmonella. Small-scale experiments were conducted to determine the best treatment conditions for each sanitizer and how the initial Salmonella contamination level on seeds or Salmonella type may have on the efficacy of the sanitizers. Large-scale studies were performed to evaluate the commercial applicability of these sanitizers. As a comparison, seeds were also treated with 20,000 ppm Ca(OCl)2 for 15 min. To determine the optimal treatment condition, ten g of seeds containing 103 CFU/g (seed lot 1) or 105 CFU/g (seed lot 2) of S. Typhimurium expressing green fluorescent protein was treated with different concentrations of ASC, SOC, and Tsunami 100 for different period of treatment time. Results showed that the efficacy of ASC and SOC increased with increasing concentrations (up to 1500 ppm) and treatment time (up to 3 h); however, the germination rates for seeds treated with > 1200 ppm of either sanitizer was < 80%. When seeds from lot 1 were treated with 800 ppm of ASC or SOC for 3 h, the level of Salmonella reduced, respectively, by 2.6 log units or >2.5 log units to an undetectable level, compared with the 1.1 log unit decrease observed in seeds treated with Ca(OCl)2. When seeds from seed lot 2 (5 log units) were treated with 800 ppm of ASC or SOC for 3 h, the level of Salmonella were reduced by 3.8 log units or >4.3 log units to an undetectable level, respectively, compared with the 2 log units decrease observed in seeds treated with Ca(OCl)2. Treatment with 80 ppm of Tsunami 100 from 10 min to 24 h kept the germination rate higher than 85% but only reduced the Salmonella count by 0 to 1.8 log units. Between two seed lots, results of ASC showed that sanitizer could reduce more pathogen when the seeds contained higher initial Salmonella level. Ten g of seeds containing 105 CFU/g of four-strain cocktail of Salmonella was treated with ASC and SOC by optimal treatment condition. SOC and ASC eliminated Salmonella number by 2.8 and 1.6 log units respectively, less effective compare with the treatment of seed lot 1. For large-scale runs, 1 kg of seeds spiked with 1 % (w/w) of inoculated seeds containing 105 CFU/g of S. Typhimurium was treated with SOC and ASC under optimal treatment conditions. Salmonella level in seeds treated with 800 ppm SOC or ASC for 3 h decreased by 3.9 or 3.6 log units, compared with a 3.3 log reduction in seeds treated with 20,000 ppm Ca(OCl)2. The stabilized oxychloro-based sanitizer may provide an alternative seed treatment method for sprout growers.
M.S. in Food Safety and Technology, December 2015
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Title: THERMAL INACTIVATION OF SALMONELLA AGONA IN LOW-MOISTURE FOOD SYSTEMS AS INFLUENCED BY WATER ACTIVITY
Creator: Jin, Yuqiao
Date: 2016, 2016-07
Description: Salmonella can survive in low-moisture, high-protein and high-fat foods for several years. Despite nationwide recalls for Salmonella in low...
Show moreSalmonella can survive in low-moisture, high-protein and high-fat foods for several years. Despite nationwide recalls for Salmonella in low-moisture products, information on survival of Salmonella during high-protein and high-fat food processing is limited. This project evaluated Salmonella enterica serovar Agona 447967 thermal inactivation kinetics in a high-protein and a high-fat matrix using a defined matrix composition, varying water activities and process conditions. A high-protein matrix, composed of 60:6:25 weight ratio of flour: oil: protein, and a high-fat matrix, composed of 60:25:6 weight ratio of flour: oil: protein was studied. Each matrix was inoculated with Salmonella enterica serovar Agona 447967 at activities of 0.5, and 0.9. Samples were packed in aluminum test cells and heat treated over a range of temperatures and time intervals. Survival of Salmonella Agona was detected on trypticase soy agar with 0.6% yeast extract. The average z-values for the high-protein matrix at the water activity (aw) of 0.5 and 0.9 were 9.01ºC, and 7.51ºC, respectively. The average z-values for the high-fat matrix was 11.91ºC at aw 0.5, and 7.08ºC at aw 0.9. Results showed that the z-value at aw 0.5 was significantly different from the z-value at aw 0.9 (p < 0.05) in both the highprotein and high-fat matrices. Critical process factors associated with pathogen destruction were identified during thermal treatments in this project. Results indicated that a correlation existed between temperature and water activity and must be accounted for when predicating inactivation of Salmonella enterica in these model matrices under dynamic process conditions.
M.S. in Food Process Engineering, July 2016
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Title: EFFICACY OF AQUEOUS INOCULATION IN LOW-WATER ACTIVITY MATRIX ON SALMONELLA ENTERICA SEROTYPE AGONA THERMAL RESISTANCE AND STABILITY DURING STORAGE
Creator: Liao, Li
Date: 2016, 2016-07
Description: Methods used to inoculate foods may impact the results of Salmonella survival and thermal resistance studies. Currently, there is little...
Show moreMethods used to inoculate foods may impact the results of Salmonella survival and thermal resistance studies. Currently, there is little information as to the most effective means to inoculate Salmonella into low-moisture food products for use in process validation studies. The objective of this study is to assess the resulting 42 d storage stability and thermal resistance of Salmonella inoculated by liquid addition in food and inert carrier matrices. Salmonella enterica serovar Agona 447967 was inoculated into ground pepper, oat flour, and sand at approximately 9-10 log CFU/g using a hand mixing procedure and stored at 25°C, 32% relative humidity (RH). After a 2 d equilibration, the samples were tested for inoculation homogeneity. Distribution of Salmonella populations proved uniform in black pepper and sand but not in oat flour. Subsequent testing for stability by enumeration with both tryptic soy agar with 0.6% yeast extract (TSAYE) and xylose lysine desoxycholate (XLD) agars was performed weekly with black pepper and sand only. Thermal resistance was determined periodically during storage using aluminum test cells for pepper and glass vial for sand as sample holders. Over 42 d, the average starting Salmonella populations in pepper and sand were 7.88 ± 0.13 and 7.69 ± 0.11 log CFU/g, respectively, and final populations were 6.14 ± 0.06 and 6.52 ± 0.07 log CFU/g, respectively. Population changes were significantly different (P < 0.05). For pepper the thermal resistance did not change significantly during storage (P > 0.05) and were similar to values reported by other researchers using alternative inoculation techniques. However, thermal resistance measured in sand (D90ᵒC = 3.39 ± 0.41 min) was substantially higher than that measured in black pepper (D75ᵒC-value = 4.99 ± 0.21 min) or other literature values. Overall a liquid inoculation protocol proved efficacious for small particles such as ground black pepper or sand but was unable to provide a uniform distribution of cell populations in a powder such as oat flour.
M.S. in Food Safety and Technology, July 2016
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Title: TRANSFER MECHANISM AND TRANSCRIPTOMIC PROFILING OF SALMONELLA IN FRESH-CUT FRUITS
Creator: Chen, Ruixi
Date: 2018, 2018-05
Description: Salmonella has been recognized as an increasing concern for food safety of fresh fruits because of their constant involvement in associated...
Show moreSalmonella has been recognized as an increasing concern for food safety of fresh fruits because of their constant involvement in associated foodborne outbreaks. However, knowledge is relatively limited about the transfer pattern of this particular pathogen from contaminated to uncontaminated fruits, nor do we know about the molecular mechanisms incorporated by it to address the environment of fresh fruits. In this study, we investigated the transfer mechanism of Salmonella enterica serovars Newport and Typhimurium between fresh gala apples and cantaloupes, respectively, via consecutive fresh cuttings and the subsequent survival of the strains in juice and cube fruit products under 4oC for up to 7 days. We also used RNA-seq and bioinformatics approaches to explore the transcriptome of Salmonella Newport in cantaloupe in comparison with 0.1% BPW (control group). Our results demonstrated fresh cutting as a practical way for bacterial transfer from contaminated to up to 4 uncontaminated fruits, and the transfer rate decreased drastically as the number of fresh cuts increased. The relative distribution of Salmonella cells in the 1st un-inoculated cantaloupe sample was significantly higher than the corresponding gala apple sample (p ≤0.05), while the transfer rate to the following un-inoculated samples in the cantaloupe group was visibly lower than the gala apple group. In general, cantaloupe, rather than gala apple, was capable to at least maintain a constant level of bacterial population during the 7 days storage at 4oC, with juice sample being even more supportive than cube sample (p ≤0.05). Moreover, Salmonella Typhimurium had a better adaptation in cantaloupe environment than Salmonella Newport. According to the results of transcriptome analysis, metabolism activities were promoted for essential nutrient requirements including carbon, energy, nitrogen, phosphorus, iron and zinc comparing to the control group, while suppressed for others such as lipid and fatty acids (FDR ≤0.05, |log2 FoldChange| ≥1). The differential regulation of the corresponding genes of the biological processes was possibly achieved in a time much less than 1 hour.
M.S. in Food Safety and Technology, May 2018
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Title: Toward a Comprehensive Atomistic View of Bacterial Outer Membrane Remodeling and Antimicrobial Peptide Susceptibility
Creator: Rice, Amy
Date: 2019
Description: The cell membrane is arguably one of the most important and complex features of a cell, serving to demarcate “self” from “environment” and...
Show moreThe cell membrane is arguably one of the most important and complex features of a cell, serving to demarcate “self” from “environment” and selectively controlling the flow of material in and out of the cell. Bacterial cell membranes are of additional interest since they serve as the first point of contact for antibiotic drugs or other antimicrobial agents. In this work, I use atomistic molecular dynamics simulations to study factors that alter bacterial susceptibility to antimicrobial agents and their environment, with the goal of better understanding how bacteria are able to modulate their sensitivity.First, I present results from a series of simulations of antimicrobial peptides (AMP) interacting with phospholipid bilayers to elucidate the observed interaction differences between arginine and lysine-containing AMPs. Simulations show that the extensive interaction with arginine is due in part to arginine's strong atomic charge distribution, rather than being purely an effect of the greater hydrogen bond capacity. These results highlight the role of charge and hydrogen bond strength in peptide bilayer insertion, and offer potential insights for designing more potent analogues in the future.Next, the effects of bacterial lipopolysaccharide (LPS) modifications are examined, first to understand study how three key modifications observed in S. enterica affect bilayer properties, as well as to examine the role these modifications play in AMP resistance. We hypothesize that defects at the crystalline/liquid-ordered boundary facilitate LL-37 intercalation into the outer membrane, whereas LPS modification protects against this process by having already increased crystallinity and packing efficiency. These results further improve our understanding of outer membrane chemical properties and help elucidate how outer membrane modification systems are able to alter bacterial virulence and susceptibility. Lastly, I investigate the effects of ion type and phosphate charge on four distinct LPS types. Simulations show that bilayer properties are highly influenced by the choice of cation type, ion parameterization, and phosphate group charges. Additional free energy perturbation simulations predict that the protonated LPS state should dominate at physiological pH, in contrast to the deprotonated state utilized by many LPS force fields. Overall, these results reveal inaccuracies in the existing LPS force fields and establish guidelines to better reproduce experimental LPS membrane properties.
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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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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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Title: SALMONELLA SURVIVAL AND TRANSCRIPTOMIC RESPONSE ON FRESH-CUT CANTALOUPE FLESH WITH AND WITHOUT ORGANIC ACID PRETREATMENT
Creator: Zhou, Xinyi
Date: 2020
Description: Outbreaks of Salmonella enterica associated with fresh-cut melons are becoming more frequent in recent years. Antimicrobial activity of...
Show moreOutbreaks of Salmonella enterica associated with fresh-cut melons are becoming more frequent in recent years. Antimicrobial activity of organic acids on fresh-cut melons have been previously studied. However, little is known about the molecular mechanism behind the antimicrobial activity of organic acid.Four strains of S. enterica were utilized: Newport 36796 and 339652, and Typhimurium LT2 and 46249. Both high and low inoculation levels were performed. For low level, each strain was individually cultured and spot-inoculated onto separate 100 g untreated fresh-cut cantaloupe samples resulting in 4 log-CFU/g. For high level, samples were first submerged into 2% citric acid or malic acid for 1 minute or left as untreated control. Cantaloupe were spot inoculated with one of four strains which resulted in 7-log CFU/g. All inoculated samples were air-dried for 1 h then stored at 4°C for 7 d in deli containers. Enumeration was conducted at 0, 1, 3, 5, and 7 d. Duplicate samples were used in each of three independent trials and results were analyzed by Student’s t-test, p≤0.05. Samples for sequencing were prepared using the TruSeq Stranded mRNA kit and run on a MiSeq according to the manufacturer instructions.For low inoculation level, population of all four strains increased significantly from 0 to 3 d. The two cantaloupe outbreak-related strains (Newport 339652 and Typhimurium 46249) increased significantly between 0 and 7 d from 3.44±0.11 to 3.76±0.13 and 3.36±0.12 to 3.78±0.19 log CFU/g, respectively. For high inoculation level, the population on the untreated cantaloupe was 6.55±0.18 log CFU/g at 7 d, whereas it was significantly lower on the citric and malic acid-treated cantaloupes (6.26 ± 0.09 log CFU/g and 6.07 ± 0.18 log CFU/g). After 1 d, S. enterica genes were downregulated up to 437.4-fold compared and upregulated up to 23.2-fold. The notable downregulated genes encoded proteins related to catalyzing metabolism (L-aspartate oxidase) and also related to nutrient uptake (PstC).The results of this study can aid in understanding population dynamics of S. enterica on fresh-cut cantaloupes and efficacy of malic and citric acids. The results can also aid in understanding mechanism underlying S. enterica survival on fresh-cut cantaloupes.
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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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Title: GROWTH KINETICS OF SALMONELLA ENTERICA DURING REHYDRATION OF DEHYDRATED PLANT FOODS AND SUBSEQUENT STORAGE
Creator: Ren, Yuying
Date: 2020
Description: Dehydrated plant foods have low water activities and do not support the growth of pathogenic bacteria like Salmonella enterica. Once...
Show moreDehydrated plant foods have low water activities and do not support the growth of pathogenic bacteria like Salmonella enterica. Once rehydration, the water activities will increase to > 0.92, and along with their neutral pHs, plant foods may be able to support the growth of S. enterica. Therefore, product assessments are required to determine the extent to which these products support growth of S. enterica. The purpose of this study was to determine the growth kinetics of S. enterica during rehydration with 5 or 25 °C water, and subsequent storage of dehydrated potatoes, carrots, and onions at 5, 10, and 25 °C. Fresh plant foods were dehydrated at 60°C (140°F) for 24 h. Dehydrated plant foods were inoculated with 4 log CFU/g of a 4-strain cocktail of S. enterica and dried for 24 h. Samples were rehydrated using 4-volumes of 5 or 25 °C water for 24 h. During rehydration, 30 g of sample was removed and drained for 10 min. Ninety mL of BPB was added to triplicate 10-g samples. Serial dilutions of the homogenate were plated onto TSA overlaid with XLD agar for enumeration of S. enterica. After 24 h rehydration, the remaining samples were drained and stored in containers at 5, 10, and 25°C for 7 d. S. enterica was enumerated at 1, 3, 5, and 7 d. Three independent trials were conducted. Growth kinetics were determined using DMFit and data were statistically analyzed using Student’s t-test (α=0.05). Overall, the growth rates of S. enterica when 5 °C water was used for rehydration were higher than when 25 °C water was used for potatoes and carrots. The highest growth rate of S. enterica was 3.74 log CFU/g per d on potatoes, leading to a 1 log CFU/g increase in S. enterica after only 0.27 d (16 h) which occurred during storage at 25 ℃ after 5℃ water rehydration. The highest growth rate on carrots was 1.98 log CFU/g per d (requiring only 0.51 d to increase 1 log CFU/g) when rehydrated with 5℃ water and stored at 25 ℃. The growth rates were the lowest during the storage of rehydrated onions. S. enterica required 12.5 d to increase 1 log CFU/g (the growth rate was 0.61 log CFU/g per d) when the onions were rehydrated with 25 ℃ water and stored at 25 ℃. The results of this study determined that S. enterica 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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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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