Cheng-An Hwang

Degradation of Ochratoxin A by Acinetobacter calcoaceticus

Microorganisms were screened for their ability to degrade ochratoxin A (OTA). Among test microorganisms, Acinetobacter calcoaceticus was found to degrade OTA. The degradation of OTA by A. calcoaceticus was studied in an ethanol-minimal salts medium with an initial OTA concentration of 10 (μg/ml at 25 and 30°C. Under these conditions, A. calcoaceticus was able to degrade OTA with an initial concentration of OTA of 10 (μg/ml. The average amounts of OTA removed by A. calcoaceticus in medium with an initial OTA concentration of 10 (μg/ml were 0.1005 and 0.0636 (μg/ml/h at 25 and 30°C, respectively. Ochratoxin A was degraded significantly by A. calcoaceticus during and after the log phase of cell growth at both incubation temperatures. It is postulated that degradation of OTA by A. calcoaceticus yielded a less toxic ochratoxin α.

Modeling the survival of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella Typhimurium during fermentation, drying, and storage of soudjouk-style fermented sausage☆

This study quantified and modeled the survival of Escherichia coli O157:H7, Listeria monocytogenes and Salmonella Typhimurium in soudjouk-style fermented sausage during fermentation, drying, and storage. Batter prepared from ground beef (20% fat), seasonings, starter culture, and dextrose was separately inoculated with a multi-strain mixture of each pathogen to an initial inoculum of ca. 6.5 log(10) CFU/g in the batter. The sausages were subsequently fermented at 24 degrees C with a relative humidity (RH) of 90% to 95% for 3 to 5 days to ca. pH 5.2, pH 4.9 or pH 4.6, then dried at 22 degrees C to a(w) 0.92, a(w) 0.89, or a(w) 0.86, respectively, and then stored at 4, 21, or 30 degrees C for up to 60 days. Lethality of the three pathogens was modeled as a function of pH, a(w) and/or storage temperature. During fermentation to pH 5.2 to pH 4.6, cell reductions ranged from 0 to 0.9 log(10) CFU/g for E. coli O157:H7, 0.1 to 0.5 log(10) CFU/g for L. monocytogenes, and 0 to 2.2 log(10) CFU/g for S. Typhimurium. Subsequent drying of sausages of pH 5.2 to pH 4.6 at 22 degrees C with 80% to 85% RH for 3 to 7 days to a(w) of 0.92 to a(w) 0.86 resulted in additional reductions that ranged from 0 to 3.5 log(10) CFU/g for E. coli O157:H7, 0 to 0.4 log(10) CFU/g for L. monocytogenes, and 0.3 to 2.4 log(10) CFU/g for S. Typhimurium. During storage at 4, 21, or 30 degrees C the reduction rates of the three pathogens were generally higher (p<0.05) in sausages with lower pH and lower a(w) that were stored at higher temperatures. Polynomial equations were developed to describe the inactivation of the three pathogens during fermentation, drying, and storage. The applicability of the resulting models for fermented sausage was evaluated by comparing model predictions with published data. Pathogen reductions estimated by the models for E. coli O157:H7 and S. Typhimurium were comparable to 67% and 73% of published data, respectively. Due to limited published data for L. monocytogenes, the models for L. monocytogenes would need additional validations. Results of pathogen reductions from this study may be used as a reference to assist manufacturers of soudjouk-style sausages to adopt manufacturing processes that meet the regulatory requirements. The resulting models may also be used for estimating the survival of E. coli O157:H7 and S. Typhimurium in other similar fermented sausage during fermentation and storage.

Mathematical modeling the cross-contamination of Escherichia coli O157:H7 on the surface of ready-to-eat meat product while slicing.

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 slicers 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/cm(2) 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 cm(2) 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 >or=5 log CFU on the ham or blade. With an initial inoculum level at <or=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.

Modeling transfer of Listeria monocytogenes from slicer to deli meat during mechanical slicing.

Listeria monocytogenes has been implicated in several listeriosis outbreaks linked to the consumption of presliced ready-to-eat (RTE) deli meats. The possible contamination of sliced RTE meats by L. monocytogenes during the slicing process has become a public health concern. The objectives of this study were to investigate the transfer phenomena of L. monocytogenes between a meat slicer and ham slices, and to develop empirical models to describe the transfer during slicing. A six-strain cocktail of L. monocytogenes was inoculated onto a slicer blade to an initial level of approximately 3, 6, or 9 log(10) colony-forming units (CFU)/blade (2, 5, or 8 log CFU/cm(2) of the blade edge area), and then the ham was sliced to a thickness of 1 to 2 mm (Case I). As a second cross-contamination scenario (Case II), a clean blade was used to slice ham previously inoculated with L. monocytogenes (3, 6, or 9 log(10) CFU per meat surface of ca. 100 cm(2)) prior to slicing uninoculated ham. The ham slicing rate was maintained at an average of three to four slices per minute for both Case I and II. Although the overall recovery ratio, including slicer surfaces and collected ham slices, was less than 100%, more ham slices were contaminated with L. monocytogenes when the blade was contaminated with higher initial levels of L. monocytogenes. Empirical models were developed to describe the transfer of L. monocytogenes between blade and ham slices. The models may be applied to predict the number of ham slices that may be contaminated by a L. monocytogenes-contaminated slicer during ham slicing operation. However, the models are both microbial load and contamination route dependent, which might limit their applications to certain conditions. This study showed the initial step for the development of surface transfer model and discussed the factors that might need to be considered and included in future study to expand the model applications.

Predictive model for the reduction of heat resistance of Listeria monocytogenes in ground beef by the combined effect of sodium chloride and apple polyphenols

We investigated the combined effect of three internal temperatures (57.5, 60, and 62.5°C) and different concentrations (0 to 3.0 wt/wt.%) of sodium chloride (NaCl) and apple polyphenols (APP), individually and in combination, on the heat-resistance of a five-strain cocktail of Listeria monocytogenes in ground beef. A complete factorial design (3×4×4) was used to assess the effects and interactions of heating temperature, NaCl, and APP. All 48 combinations were tested twice, to yield 96 survival curves. Mathematical models were then used to quantitate the combined effect of these parameters on heat resistance of the pathogen. The theoretical analysis shows that compared with heat alone, the addition of NaCl enhanced and that of APP reduced the heat resistance of L. monocytogenes measured as D-values. By contrast, the protective effect of NaCl against thermal inactivation of the pathogen was reduced when both additives were present in combination, as evidenced by reduction of up to ~68% in D-values at 57.5°C; 65% at 60°C; and 25% at 62.5°C. The observed high antimicrobial activity of the combination of APP and low salt levels (e.g., 2.5% APP and 0.5% salt) suggests that commercial and home processors of meat could reduce the salt concentration by adding APP to the ground meat. The influence of the combined effect allows a reduction of the temperature of heat treatments as well as the salt content of the meat. Meat processors can use the predictive model to design processing times and temperatures that can protect against adverse effects of contaminated meat products. Additional benefits include reduced energy use in cooking, and the addition of antioxidative apple polyphenols may provide beneficial health affects to consumers.

Kinetics of Thermal Destruction of Salmonella in Ground Chicken Containing trans-Cinnamaldehyde and Carvacrol†

We investigated the heat resistance of an eight-strain cocktail of Salmonella serovars in chicken supplemented with trans cinnamaldehyde (0 to 1.0%, wt/wt) and carvacrol (0 to 1.0%, wt/wt). Inoculated meat was packaged in bags that were completely immersed in a circulating water bath and held at 55 to 71°C for predetermined lengths of time. The recovery medium was tryptic soy agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. D-values in chicken, determined by linear regression, were 17.45, 2.89, 0.75, and 0.29 min at 55, 60, 65, and 71°C, respectively (z = 9.02°C). Using a survival model for nonlinear survival curves, D-values in chicken ranged from 13.52 min (D(1), major population) and 51.99 min (D(2), heat-resistant subpopulation) at 55°C to 0.15 min (D(1)) and 1.49 min (D(2)) at 71°C. When the Salmonella cocktail was in chicken supplemented with 0.1 to 1.0% trans-cinnamaldehyde or carvacrol, D-values calculated by both approaches were consistently less at all temperatures. This observation suggests that the addition of natural antimicrobials to chicken renders Salmonella serovars more sensitive to the lethal effect of heat. Thermal death times from this study will be beneficial to the food industry in designing hazard analysis and critical control point plans to effectively eliminate Salmonella contamination in chicken products used in this study.

Modeling the impact of chlorine on the behavior of Listeria monocytogenes on ready-to-eat meats.

Listeria monocytogenes (Lm) continues to pose a food safety hazard in ready-to-eat (RTE) meats due to potential cross-contamination. Chlorine is commonly used to sanitize processing equipment and utensils. However, Lm may survive the treatment and then contaminate food products. The objective of this study was to characterize the behavior of chlorine-exposed Lm on RTE ham during refrigerated storage. A two strain cocktail of Lm serotype 4b was pre-treated with chlorine (0, 25, and 50 ppm) for one hour, and then inoculated onto the surface of RTE ham to obtain an inoculum of about 3.0 log CFU/g. The inoculated ham samples were stored at 4, 8, and 16 °C, and Lm was enumerated periodically during the storage. The growth characteristics (lag time and growth rate) of Lm were estimated using the DMFit software. The results indicated that Lm growth was suppressed by the chlorine treatment. At 4 °C, the lag time of Lm with no (0 ppm) chlorine exposure (4.2 days) was shorter than those exposed to 25 ppm (5.4 days) and 50 ppm (6.8 days). The lag time decreased with the increase of temperature, e.g., at 25 ppm, the lag times were 5.2, 3.8 and 2.6 days for 4, 8 and 16 °C, respectively, and increased with the increase of chlorine concentration, e.g., at 16 °C, the lag times were 1.2, 2.6 and 4.0 days for 0, 25 and 50 ppm, respectively. However, growth rate increased with the increase of temperature and decreased with the increase of chlorine concentration. The lag time and growth rate as a function of chlorine concentration and temperature can be described using a modified Ratkowsky model and a modified Zwietering model, respectively. The results showed that the growth of Lm on RTE ham was delayed by pre-exposure to chlorine (at ≤ 50 ppm). The predictive models developed will contribute to microbial risk assessments of RTE meats.

Growth characteristics of Listeria monocytogenes as affected by a native microflora in cooked ham under refrigerated and temperature abuse conditions.

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, log(10) cfu/h), and maximum population density (MPD, log(10) 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 log(10) cfu/h) were statistically but marginally lower than those of monocultured L. monocytogenes (0.0132-0.0145 log(10) 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.

Evaluating the Effect of Temperature on Microbial Growth Rate—The Ratkowsky and a Bělehrádek‐Type Models

The objective of this paper to conduct a parallel comparison of a new Bělehrádek-type growth rate (with an exponent of 1.5, or the Huang model), Ratkowsky square-root, and Ratkowsky square equations as secondary models for evaluating the effect of temperature on the growth of microorganisms. Growth rates of psychrotrophs and mesophiles were selected from the literature, and independently analyzed with the 3 models using nonlinear regression. Analysis of variance (ANOVA) was used to compare the means of growth rate (μ), estimated minimum temperature (T(min) ), approximate standard errors (SE) of T(min) , model mean square errors (MSE), accuracy factor (A(f) ), bias factor (B(f) ), relative residual errors (δ), Akaike information criterion (AICc), and Bayesian information criterion (BIC). Based on the estimated T(min) values, the Huang model distinctively classified the bacteria into 2 groups (psychrotrophs and mesophiles). No significant difference (P > 0.05) was observed among the means of the μ values reported in the literature or estimated by the 3 models, suggesting that all 3 models were suitable for curve fitting. Nor was there any significant difference in MSE, SE, δ, A(f) , B(f) , AICc, and BIC. The T(min) values estimated by the Huang model were significantly higher than those estimated by the Ratkowsky models, but were in closer agreement with the biological minimum temperatures for both psychrotrophs and mesophiles. The T(min) values estimated by the Ratkowsky models systematically underestimated the minimum growth temperatures. In addition, statistical estimation showed that the mean exponent for the new Bělehrádek-type growth rate model may indeed be 1.5, further supporting the validity of the Huang model.

Effect of salt, smoke compound, and temperature on the survival of Listeria monocytogenes in salmon during simulated smoking processes.

The objectives of this study were to examine and develop a model to describe the survival of Listeria monocytogenes in salmon as affected by salt, smoke compound (phenol), and smoking process temperature. Cooked minced salmon containing selected levels of salt (0%, 2%, 4%, and 6%) and smoke compound (0, 5, 10, and 15 ppm phenol) were inoculated with a 6-strain mixture of L. monocytogenes to an inoculum level of 6.0 log(10) CFU/g. The populations of L. monocytogenes in salmon during processing at 40, 45, 50, and 55 degrees C that simulated cold- and hot-smoking process temperatures were determined, and the effects of salt, phenol, and temperature on the survival of L. monocytogenes in salmon were analyzed and described with an exponential regression. At 40 degrees C, the populations of L. monocytogenes in salmon decreased slightly with inactivation rates of <0.01 log(10) CFU/h, and at 45, 50, and 55 degrees C, the inactivation rates were 0.01 to 0.03, 0.15 to 0.30, and 2.8 to 3.5 log(10) CFU/h, respectively. An exponential regression model was developed and was shown to closely describe the inactivation rates of L. monocytogenes as affected by the individual and combined effects of salt, phenol, and smoking process temperature. Temperature was the main effector in inactivating L. monocytogenes while salt and phenol contributed additional inactivation effects. This study demonstrated the inactivation effects of salt, smoke compound, and temperature on L. monocytogenes in salmon under a smoking process. The data and model can be used by manufacturers of smoked seafood to select concentrations of salt and smoke compound and alternative smoking process temperatures at 40 to 55 degrees C to minimize the presence of L. monocytogenes in smoked seafood.