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Juneja V.K.,Microbial Food Safety Research Unit | Marks H.,U.S. Department of Agriculture | Thippareddi H.H.,University of Nebraska - Lincoln
Innovative Food Science and Emerging Technologies | Year: 2010

A predictive dynamic model for Clostridium perfringens spore germination and outgrowth in cooked pork products during cooling is presented. Cooked, ground pork was inoculated with C. perfringens spores and vacuum packaged. For the isothermal experiments, all samples were incubated in a water bath stabilized at selected temperatures between 10 and 51 °C and sampled periodically. For dynamic experiments, the samples were cooled from 54.4 to 27 °C and subsequently from 27 to 4 °C for different time periods, designated as x and y hours, respectively. The growth models used were based on a model developed by Baranyi and Roberts (1994), which incorporates a constant, referred to as the physiological state constant, q0. The value of this constant captures the cells' history before the cooling begins. To estimate specific growth rates, data from isothermal experiments were used, from which a secondary model was developed, based on a particular form of Ratkowsky's 4-parameter equation. Using the data from dynamic experiments and the Ratkowsky model, an optimal value of q0 (=0.01375) was derived minimizing the mean square error of predictions. However, using this estimate, the model had a tendency to over-predict relative growth when there was observed small amounts of relative growth, and under-predict relative growth when there was observed large relative growth. To provide more fail-safe estimates, rather than using the derived value of q0, a value of 0.04 is recommended. The predictive model with this value of q0 would provide more fail-safe estimates of relative growth and could aid producers and regulatory agencies with determining disposition of products that were subjected to cooling deviations. Industrial relevance: Safe time/temperature for cooling of cooked pork is very important to guard against the pathogen in cooked products. Predictive model will assist industry to determine compliance with regulatory performance standards and to ensure microbiological safety of cooked products. Source

Bhaduri S.,Microbial Food Safety Research Unit | Phillips J.G.,U.S. Department of Agriculture
Zoonoses and Public Health | Year: 2011

The growth kinetics of virulence plasmid-bearing Yersinia pseudotuberculosis (YPST) in sterile ground beef were studied at temperatures ranging from 0 to 30°C. In irradiated sterile ground beef, YPST replicated from 0 to 30°C, with corresponding growth rates (GR) ranging from 0.023 to 0.622logCFU/h at 0-25°C, and the GR was 0.236logCFU/h at 30°C. The maximum population densities (MPD) ranged from 8.7 to 11.0logCFU/g. The growth and MPD of YPST were reduced significantly at 30°C. Models for GR and MPD of YPST in raw ground beef (RGB) as a function of storage temperatures were produced and displayed acceptable bias and accuracy. The models were validated with rifampicin-resistant YPST (rif-YPST) in sterile ground beef stored at 4, 10 and 25°C. The observed GR and MPD were within 95% of the predicted values. When compared to non-sterile retail ground beef, the growth of rif-YPST was not inhibited and displayed similar GR at 0, 10 and 25°C and MPDs as sterile ground beef at 10 and 25°C. Moreover, there was no loss of virulence plasmid in YPST during its growth in ground beef indicating that RGB contaminated with virulence plasmid-bearing YPST could cause disease due to refrigeration failure, temperature (10-25°C) abuse, and if the meat was not properly cooked. © 2009 Blackwell Verlag GmbH. Source

He Y.,U.S. Department of Agriculture | Chen C.-Y.,Microbial Food Safety Research Unit
Food Microbiology | Year: 2010

Campylobacter jejuni is an important foodborne gastrointestinal pathogen and highly sensitive to environmental stresses. Research has shown that changes in culturability, cell morphology, and viability occur when C. jejuni cells are subjected to stresses. In this study, real-time PCR, ethidium monoazide (EMA) in combination with real-time PCR (EMA-PCR), BacLight bacterial viability staining, and agar plate counting methods were used to quantitatively analyze viable, stressed, and dead C. jejuni strain 81-176. The real-time PCR assay provides highly sensitive and specific quantification of total genome copies of C. jejuni culture in different growth phases. Our results also reveal that real-time PCR can be used for direct quantification of Campylobacter genome release into Phosphate Buffered Saline (PBS) as an indicator of cell lysis. Using EMA-PCR, we obtained a dynamic range of greater than 3 logs for differentiating viable vs. dead cells. The viability and morphological characteristics of the stressed cells after one-week incubation at 25 °C, in air, and under nutrient-poor conditions were investigated. Our results indicated that, over 99% of the stressed cells were converted from the spiral to the coccoid form and became non-culturable. However, more than 96% of the coccoid cells retained their membrane integrity as suggested by both the BacLight staining and EMA-PCR analyses. Thus, to detect C. jejuni under stress conditions, conventional culturing method in conjunction with EMA-PCR or BacLight staining might be a more appropriate approach. Source

Sheen S.,Microbial Food Safety Research Unit | Hwang C.-A.,Microbial Food Safety Research Unit
Food Microbiology | Year: 2010

Microbial cross-contamination either at home or production site is one of the major factors of causing contamination of foods and leading to the foodborne illness. The knowledge regarding Escherichia coli O157:H7 surface transfer on ready-to-eat (RTE) deli meat and the slicer used for slicing different RTE products are needed to ensure RTE food safety. The objectives of this study were to investigate and to model the surface cross-contamination of E. coli O157:H7 during slicing operation. A five-strain cocktail of E. coli O157:H7 was inoculated directly onto a slicer's round blade rim area at an initial level of ca. 4, 5, 6, 7 or 8 log CFU/blade (ca. 3, 4, 5, 6 or 7 log CFU/cm2 of the blade edge area), and then the RTE deli meat (ham) was sliced to a thickness of 1-2 mm. For another cross-contamination scenario, a clean blade was initially used to slice ham which was pre-surface-inoculated with E. coli O157:H7 (ca. 4, 5, 6, 7 or 8 log CFU/100 cm2 area), then, followed by slicing un-inoculated ham. Results showed that the developed empirical models were reasonably accurate in describing the transfer trend/pattern of E. coli O157:H7 between the blade and ham slices when the total inoculum level was ≥5 log CFU on the ham or blade. With an initial inoculum level at ≤4 log CFU, the experimental data showed a rather random microbial surface transfer pattern. The models, i.e., a power equation for direct-blade-surface-inoculation, and an exponential equation for ham-surface-inoculation are microbial load and sequential slice index dependent. The surface cross-contamination prediction of E. coli O157:H7 for sliced deli meat (ham) using the developed models were demonstrated. The empirical models may provide a useful tool in developing the RTE meat risk assessment. Source

Hwang C.-A.,Microbial Food Safety Research Unit | Sheen S.,Microbial Food Safety Research Unit
Food Microbiology | Year: 2011

This study examined the growth characteristics of Listeria monocytogenes as affected by a native microflora in cooked ham at refrigerated and abuse temperatures. A five-strain mixture of L. monocytogenes and a native microflora, consisting of Brochothrix spp., isolated from cooked meat were inoculated alone (monocultured) or co-inoculated (co-cultured) onto cooked ham slices. The growth characteristics, lag phase duration (LPD, h), growth rate (GR, log10 cfu/h), and maximum population density (MPD, log10 cfu/g), of L. monocytogenes and the native microflora in vacuum-packed ham slices stored at 4, 6, 8, 10, and 12 °C for up to 5 weeks were determined. At 4-12 °C, the LPDs of co-cultured L. monocytogenes were not significantly different from those of monocultured L. monocytogenes in ham, indicating the LPDs of L. monocytogenes at 4-12 °C were not influenced by the presence of the native microflora. At 4-8 °C, the GRs of co-cultured L. monocytogenes (0.0114-0.0130 log10 cfu/h) were statistically but marginally lower than those of monocultured L. monocytogenes (0.0132-0.0145 log10 cfu/h), indicating the GRs of L. monocytogenes at 4-8 °C were reduced by the presence of the native microflora. The GRs of L. monocytogenes were reduced by 8-7% with the presence of the native microflora at 4-8 °C, whereas there was less influence of the native microflora on the GRs of L. monocytogenes at 10 and 12 °C. The MPDs of L. monocytogenes at 4-8 °C were also reduced by the presence of the native microflora. Data from this study provide additional information regarding the growth suppression of L. monocytogenes by the native microflora for assessing the survival and growth of L. monocytogenes in ready-to-eat meat products. © 2010. Source

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