Shelf life evaluation for ready-to-eat sliced uncured turkey breast and cured ham under probable storage conditions based on Listeria monocytogenes and psychrotroph growth

The growth variability of three Listeria monocytogenes ribotypes in ready-to-eat (RTE) sliced uncured turkey breast and cured ham was studied under storage conditions that RTE foods are likely to encounter. Three product treatments studied were: (1) a control; (2) a formulation subjected to high pressure processing to reduce initial microbial load (HPP); (3) a formulation containing 2.0% potassium lactate and 0.2% sodium diacetate (PL/SD). After separate inoculation with individual L. monocytogenes ribotypes and packaging each treatment under air and vacuum, the packages were stored at 4, 8, or 12 degrees C and the counts of L. monocytogenes and psychrotrophic bacteria (PPC) were determined for several weeks. The Baranyi model was used to estimate lag times and growth rates. Significant effect of strain difference was noted in both sliced products (P<0.05). In the absence of antimicrobials (HPP and control), the growth rate (GR) of L. monocytogenes strains increment from 4 to 8 degrees C and from 8 to 12 degrees C was approximately 10 and 2 fold, respectively. The addition of PL/SD was effective in restricting the growth of L. monocytogenes and PPC at 4 degrees C, but at 8 and 12 degrees C significant growth was observed (more than 100-fold increase) (P<0.05). In PL/SD samples, vacuum packaging slowed down the onset and the rate of growth of L. monocytogenes at 12 degrees C in sliced ham and at 8 and 12 degrees C in sliced turkey breast. Generally, the time to increase by 2-logs was greater in control samples than as observed in HPP-treated samples. When antimicrobials were present, the current results showed that L. monocytogenes was able to grow more than 100-fold within the typical quality-based shelf life of 60 to 90 days at 8 and 12 degrees C. The findings of this study should be useful in setting the duration of a safety-based shelf life for RTE sliced meat and poultry foods.     

Use of natural antimicrobials to improve the control of Listeria monocytogenes in a cured cooked meat model system

Concern about nitrite in processed meats has increased consumer demand for natural products manufactured without nitrite or nitrate. Studies on commercial meat products labeled as "Uncured" and "No-Nitrite-or-Nitrate-Added" have shown less control of nitrite in these products and greater potential growth of bacterial pathogens. To improve the safety of the "naturally cured" meats, several natural ingredients were studied in a cured cooked meat model system (80:20 pork, 10% water, 2% salt, and 150 or 50 ppm ingoing sodium nitrite) that closely resembled commercial frankfurters to determine their inhibitory effect on Listeria monocytogenes. Results showed that cranberry powder at 1%, 2% and 3% resulted in 2-4 log cfu/g less growth of L. monocytogenes compared to the control with nitrite alone (P<0.05). Other natural compounds, such as cherry powder, lime powder and grape seed extract, also provided measureable inhibition to L. monocytogenes when combined with cranberry powder (P<0.05).     

Shelf life establishment of a sliced, cooked, cured meat product based on quality and safety determinants

In the present study, the distribution of the shelf life of cooked, cured meat products based on lactic acid bacteria growth and the distribution of the time to cause health risks based on Listeria monocytogenes growth were studied. Growth models, developed and validated on cooked meat products, were used to predict the growth of microorganisms. Temperature data were obtained from retail and home refrigerators. Distribution predictions were conducted by two approaches (time-temperature profiles and Monte Carlo simulation). Time-temperature profiles were more appropriate to be used, because Monte Carlo simulation overestimated the growth of L. monocytogenes. Shelf life was greatly influenced by storage temperature, but initial microbial load had a smaller effect. The expiration date of cooked meat products might be based on only the growth of the spoilage microorganisms, and only when product contamination with L. monocytogenes cell concentrations is high does a product fraction pose health risks for consumers. Sensitivity analysis confirmed that storage temperature and temperature variability were the most important factors for the duration of shelf life. Distributions of shelf life and time to cause health risks give valuable information on the quality and safety of cooked meat products and may be used as practical tools by meat processors.     

Development and validation of a dynamic growth model for Listeria monocytogenes in fluid whole milk

Listeria monocytogenes, a psychrotrophic microorganism, has been the cause of several food-borne illness outbreaks, including those traced back to pasteurized fluid milk and milk products. This microorganism is especially important because it can grow at storage temperatures recommended for milk (< or =7 degrees C). Growth of L. monocytogenes in fluid milk depends to a large extent on the varying temperatures it is exposed to in the postpasteurization phase, i.e., during in-plant storage, transportation, and storage at retail stores. Growth data for L. monocytogenes in sterilized whole milk were collected at 4, 6, 8, 10, 15, 20, 25, 30, and 35 degrees C. Specific growth rate and maximum population density were calculated at each temperature using these data. The data for growth rates versus temperature were fitted to the Zwietering square root model. This equation was used to develop a dynamic growth model (i.e., the Baranyi dynamic growth model or BDGM) for L. monocytogenes based on a system of equations which had an intrinsic parameter for simulating the lag phase. Results from validation of the BDGM for a rapidly fluctuating temperature profile showed that although the exponential growth phase of the culture under dynamic temperature conditions was modeled accurately, the lag phase duration was overestimated. For an alpha0 (initial physiological state parameter) value of 0.137, which corresponded to the mean temperature of 15 degrees C, the population densities were underpredicted, although the experimental data fell within the narrow band calculated for extreme values of alpha0. The maximum relative error between the experimental data and the curve based on an average alpha0 value was 10.42%, and the root mean square error was 0.28 log CFU/ml.     

Modelling the growth of Listeria monocytogenes in dynamic conditions

A recurrent neural network for the prediction of Listeria monocytogenes growth under pH and a(w) variable conditions was developed. The use of this model offered the possibility to take into account the consequences of the variations of the factors on L. monocytogenes growth. The effects of solutions, such as NaCl, acetic acid and NaOH, and their interactions on the response of L. monocytogenes cells were studied. Furthermore, the results showed the capacity of the recurrent neural network to predict growths carried out in different experimental conditions without using those used for its elaboration.     

A study on the kinetic behavior of Listeria monocytogenes in ice cream stored under static and dynamic chilling and freezing conditions

The kinetic behavior of Listeria monocytogenes in 2 commercial ice cream products (A and B) that were inoculated and stored under static chilling (4 to 16°C), static freezing (−5 to −33°C), dynamic chilling, and dynamic chilling-freezing conditions was studied, simulating conditions of the aging process and of normal or abuse conditions during distribution and storage. The ice cream products A and B had different compositions but similar pH (6.50 and 6.67, respectively) and water activity (0.957 and 0.965, respectively) values. For both chilling and freezing conditions, the kinetic behavior of the pathogen was similar in the 2 products, indicating that the pH and water activity, together with temperature, were the main factors controlling growth. Under chilling conditions, L. monocytogenes grew well at all temperatures tested. Under freezing conditions, no significant changes in the population of the pathogen were observed throughout a 90-d storage period for either of the inoculum levels tested (10³ and 10⁶ cfu/g). Growth data from chilled storage conditions were fitted to a mathematical model, and the calculated maximum specific growth rate was modeled as a function of temperature by using a square root model. The model was further validated under dynamic chilling and dynamic chilling-freezing conditions by using 4 different storage temperature scenarios. Under dynamic chilling conditions, the model accurately predicted the growth of the pathogen in both products, with 99.5% of the predictions lying within the ± 20% relative error zone. The results from the chilling-freezing storage experiments showed that the pathogen was able to initiate growth within a very short time after a temperature upshift from freezing to chilling temperatures. This indicates that the freezing conditions did not cause a severe stress in L. monocytogenes cells capable of leading to a significant “additional” lag phase during the subsequent growth of the pathogen at chilling conditions. As a result, the application of the model at chilling-freezing conditions resulted in satisfactory performance, with 98.3% of the predictions lying within the ± 20% relative error zone. The present study provides useful data for understanding the behavior of L. monocytogenes in ice cream stored under single or combined chilling and freezing conditions. In addition, the study showed that such data can be expressed in quantitative terms via the application of mathematical models, which can be used by the dairy industry as effective tools for predicting the behavior of the pathogen during the manufacture, distribution, and storage of ice cream products.     

Effect of packaging and storage conditions on development of warmed-over flavour in sliced, cooked meat

Summary The development of warmed-over flavour (WOF) in a pilot-scale experiment was followed by sensory evaluation and three objective methods (thiobarbituric acid reactive substances, relative hexanal content, and fluorescent lipid oxidation products) during 14 days of chill storage at 4° C of sliced, cooked lean beef using three different packaging conditions: (1) in air in a polyethylene foil (PE); (2) in 99% vacuum in a laminate foil with low oxygen transmission rate (VAC); (3) in a modified atmosphere (30% CO₂/70% N₂) in a laminate foil with low oxygen transmission rate (MAP). Each of the objective methods correlated well with the sensory evaluation. The sensory quality of the meat packed in PE was clearly inferior to the VAC and MAP packed meat, having less meat taste and a perceptible degree of WOF when reheated after only 1 day of storage, increasing to an unacceptable level within 3 days of storage. In contrast, the sensory quality of the VAC and MAP packed meat remained high throughout the storage period. Irrespective of packaging method, no effect of light during the chill storage period on the development of WOF was detected. MAP packaging for precooked beef was tested on a larger scale in a senior citizen food service system. The results showed that in a practical application of MAP the major problem in avoiding WOF was the achievement of a sufficiently low residual oxygen content in the packages.

---

Einfluß der Verpackung und Lagerungsverhältnisse auf die Entwicklung des Aufwärmgeschmacks von in Scheiben gekochtem Rindfleisch

Zusammenfassung In einem Modellversuch mit in Scheiben geschnittenem gekochtem Rindfleisch wurde die Entwicklung des Aufwärmgeschmacks mit sensorischer Beurteilung und drei objektiven Methoden (Thiobarbitursäure-reaktive Substanzen, relativer Gehalt von Hexanal und fluorescierenden Fettoxidationsprodukten) während 14tägiger Kühllagerung bei 4 °C untersucht. Das Fleisch wurde auf drei verschiedene Weisen verpackt: (1) mit Luft in Polyäthylenfolie (PE); (2) in 99% Vakuum in laminierter Folie mit niedriger Sauerstoffdurchlässigkeit (VAC); (3) in modifizierter Atmosphäre (30% CO₂/70% N₂) in laminierter Folie mit niedriger Sauerstoffdurchlässigkeit (MAP). Jede objektive Methode korrelierte gut mit der sensorischen Beurteilung. Die sensorische Qualität des PE-Fleisches war merkbar geringer als die des in VAC und MAP verpackten Fleisches. Es hatte nach nur einem Tag Kühllagerung einen geringen Fleischgeschmack und einen merklichen Aufwärmgeschmack, der nach drei Tagen auf ein unakzeptables Niveau zugenommen hatte. Demgegenüber war die sensorische Qualität des in VAC und MAP verpackten Fleischs sehr gut über die ganze Lagerungszeit. Beleuchtung hatte, ungeachtet der Verpackungsmethode, auf die Entwicklung des Aufwärmgeschmacks keinen Einfluß. Bei modifizierter Atmosphärenverpackung von gekochtem Fleisch in größerem Ausmaß in einer Küche zur Verköstigung von älteren Menschen ergab es sich, daß das größte Problem, den Aufwärmgeschmack zu vermeiden, ein ausreichend kleiner Restsauerstoffgehalt in den Verpackungen war.

     

Protective cultures inhibit growth of Listeria monocytogenes and Escherichia coli O157:H7 in cooked, sliced, vacuum- and gas-packaged meat

Contamination of cooked meat products with Listeria monocytogenes poses a constant threat to the meat industry. The aim of this study was therefore to investigate the use of indigenous lactic acid bacteria (LAB) as protective cultures in cooked meat products. Cooked, sliced, vacuum- or gas-packaged ham and servelat sausage from nine meat factories in Norway were inoculated with 10(3) cfu/g of a mixture of three rifampicin resistant (rif-mutant) strains of L. monocytogenes and stored at 8 degrees C for four weeks. Growth of L. monocytogenes and indigenous lactic acid flora was followed throughout the storage period. LAB were isolated from samples where L. monocytogenes failed to grow. Five different strains growing well at 3 degrees C. pH 6.2, with 3% NaCl, and producing moderate amounts of acid were selected for challenge experiments with the rif-resistant strains of L. monocytogenes. a nalidixic acid/streptomycin sulphate-resistant strain of Escherichia coli O157:H7 and a mixture of three rif-resistant strains of Yersinia enterocolitica O:3. All five LAB strains inhibited growth of both L. monocytogenes and E. coli O157:H7. No inhibition of Y. enterocolitica O:3 was observed. A professional taste panel evaluated cooked, sliced, vacuum-packaged ham inoculated with each of the five test strains after storage for 21 days at 8 degrees C. All samples had acceptable sensory properties. The five LAB strains hybridised to a 23S rRNA oligonucleotide probe specific for Lactobacillus sakei. These indigenous LAB may be used as protective cultures to inhibit growth of L. monocytogenes and E. coli O157:H7 in cooked meat products.     

Effect of packaging and storage temperature on the survival of Listeria monocytogenes inoculated postprocessing on sliced salami

The survival of postprocess Listeria monocytogenes contamination on sliced salami, stored under the temperatures associated with retail and domestic storage, was investigated. Sliced salami was inoculated with low and high concentrations of L. monocytogenes before being packaged under vacuum or air. Survival of L. monocytogenes was determined after storage of sausages for 45 or 90 days for low or high sample inocula, respectively, at 5, 15, and 25 degrees C. All survival curves of L. monocytogenes were characterized by an initial rapid inactivation within the first days of storage, followed by a second, slower inactivation phase or "tailing." Greater reduction of L. monocytogenes was observed at the high storage temperature (25 degrees C), followed by ambient (15 degrees C) and chill (5 degrees C) storage conditions. Moreover, vacuum packaging resulted in a slower destruction of L. monocytogenes than air packaging, and this effect increased as storage temperature decreased. Although L. monocytogenes numbers decreased to undetectable levels by the end of the storage period, the time (in days) needed for this reduction and for the total elimination of the pathogen decreased with high temperature, aerobic storage, and high inoculum. Results of this study clearly indicated that the kinetics of L. monocytogenes were highly dependent on the interaction of factors such as storage temperature, packaging conditions, and initial level of contamination (inoculum). These results may contribute to the exposure assessment of quantitative microbial risk assessment and to the establishment of storage-packaging recommendations of fermented sausages.     

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, 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.     

Survival and growth of Listeria species in a model ready-to-use vegetable product containing raw and cooked ingredients as affected by storage temperature and acidification

Summary The survival and growth of Listeria monocytogenes and L. innocua strains inoculated onto cooked sweet corn and fresh bean sprouts packed individually, and as components of a combination product, were examined at refrigeration and mild abuse temperatures. Growth rates were both temperature and vegetable dependent. Maximal growth rates (1.14 ± 0.1log CFU/day) were identified on cooked sweet corn at 12 °C. The inclusion of cooked sweet corn did not significantly increase ( P > 0.05) the growth rate or final Listeria population density of bean sprouts stored at 8 °C and 12 °C. The sensory quality of bean sprouts was relatively temperature independent for the initial 48 h of storage, but was maximized (4 days shelf life) at 3 °C. Acidification of sweet corn to pH 5, particularly with citric acid, slowed Listeria growth and could be an additional hurdle to supplement temperature in maintaining the safety of minimally processed vegetable combination products.     

A predictive model that evaluates the effect of growth conditions on the thermal resistance of Listeria monocytogenes

A predictive model for Listeria monocytogenes was developed using cells grown in different pH and milkfat levels before subsequent thermal inactivation in identical pH and milkfat conditions. Inactivation of the cells used combinations of temperature (55, 60, 65 degrees C), pH (5.0, 6.0, 7.0), and milkfat (0%, 2.5%, 5.0%) in a complete 3 x 3 x 3 factorial design with each test done in triplicate. A modified Gompertz equation was used to model nonlinear survival curves with the following three parameter estimates: A for the shouldering region, B for the maximum death rate, and C for the tailing region. All treatment sets were analyzed together in a regression model using the modified Gompertz equation. There was good confidence in the overall model when it was used to predict values for the entire data set. The correlation of determination, R2, between the observed log surviving fraction (LSF) of cells from each of the conditions studied in the experiment, for the overall model was 0.811. For the A and B parameter estimates, temperature or milkfat alone, and the interaction of temperature and milkfat significantly (p < 0.05) affected the shouldering region and maximum death rate of a survival curve, respectively. These results were compared to a previously published predictive model, generated for cells grown under optimum conditions (pH 7.0, 0% milkfat), where pH was the only significant (p < 0.05) factor affecting the shoulder region. These results suggested that the conditions of the growth environment had an important impact on survival curve shape and the estimates of the predictive model. Specifically, there were more factor interactions involving temperature and milkfat level. These growth factors affected the shoulder region and maximum rate of death of the survival curve when cells were grown in identical medium conditions to which they were heated. Differences related to shouldering and inactivation rates for cells grown in different conditions may have important and practical importance for estimating inactivation of L. monocytogenes. This study provides some evidence on the importance of growing conditions when evaluating microbial heat resistance.     

Incidence of Clostridium perfringens in commercially produced cured raw meat product mixtures and behavior in cooked products during chilling and refrigerated storage

A total of 445 whole-muscle and ground or emulsified raw pork, beef, and chicken product mixtures acquired from industry sources were monitored over a 10-month period for vegetative and spore forms of Clostridium perfringens. Black colonies that formed on Shahidi-Ferguson perfringens (SFP) agar after 24 h at 37 degrees C were considered presumptive positive. Samples that were positive after a 15-min heat shock at 75 degrees C were considered presumptive positive for spores. Of 194 cured whole-muscle samples, 1.6% were positive; spores were not detected from those samples. Populations of vegetative cells did not exceed 1.70 log10 CFU/g and averaged 1.56 log10 CFU/g. Of 152 cured ground or emulsified samples, 48.7% were positive, and 5.3% were positive for spores. Populations of vegetative cells did not exceed 2.72 log10 CFU/g and averaged 1.98 log10 CFU/g; spores did not exceed 2.00 log10 CFU/g and averaged 1.56 log10 CFU/g. Raw bologna (70% chicken), chunked ham with emulsion, and whole-muscle ham product mixtures were inoculated with C. perfringens spores (ATCC 12916, ATCC 3624, FD1041, and two product isolates) to ca. 3.0 log10 CFU/g before being subjected either to thermal processes mimicking cooking and chilling regimes determined by in-plant temperature probing or to cooking and extended chilling regimes. Populations of C. perfringens were recovered on SFP from each product at the peak cook temperatures, at 54.4, 26.7, and 7.2 degrees C, and after up to 14 days of storage under vacuum at 4.4 degrees C. In each product, populations remained relatively unchanged during chilling from 54.4 to 7.2 degrees C and declined slightly during refrigerated storage. These findings indicate processed meat products cured with sodium nitrite are not at risk for the growth of C. perfringens during extended chilling and cold storage.     

Production of mortadella: behavior of Listeria monocytogenes during processing and storage conditions

The presence of Listeria monocytogenes in ready-to-eat meat products is cause of concern to the food industry as well as to health authorities. Studies were conducted to evaluate the presence of L. monocytogenes in mortadellas acquired at retail stores and to evaluate the fate of two levels of a L. monocytogenes pool spiked in two different formulations of the product, cooked under commercial conditions and stored at refrigeration (7°C) and room temperature (25°C). Among the samples collected at different retail stores, 26.7% harboured L. monocytogenes. Regarding to the fate of L. monocytogenes in mortadella, periodically, samples were taken and the surviving L. monocytogenes cells in the spiked products were counted by the MPN procedure. Populations of <0.35 MPN/g of L. monocytogenes were found in these samples. It could be concluded that the heat treatment was effective to reduce 3-log of L. monocytogenes independent of formulation or storage conditions.     

Evaluation of a challenge testing protocol to assess the stability of ready-to-eat cooked meat products against growth of Listeria monocytogenes

Challenge testing of ready-to-eat (RTE) foods with Listeria monocytogenes is recommended to assess the potential for growth. The present study was undertaken to evaluate a protocol for challenge testing applied to RTE cooked meat products. In order to choose L. monocytogenes strains with a representative behaviour, initially, the variability of the response of multiple L. monocytogenes strains of human and food origin to different stress and growth conditions was established. The strains were not inhibited in their growth at moderate acid pH (5.25) and the four strains tested in particular showed a similar acid-adaptive response. Growth of the various strains under four different combined stress conditions indicated that no L. monocytogenes strain had consistently significant longer or shorter lag phase or higher or lower maximum specific growth rates. The effect of choice of strain and history (pre-incubation temperature 7 or 30 degrees C) on growth of L. monocytogenes under optimum conditions (Brain Heart Infusion, BHI) and modified BHI simulating conditions of cooked ham and paté was studied. In general, all four L. monocytogenes strains behaved similarly. In BHI, no difference in lag phase was observed for the cold-adapted and standard inoculum, whereas in BHI adjusted to ham and paté conditions, a ca. 40-h reduction of the lag phase was noted for the cold-adapted inoculum. Subsequently, microbial challenge testing of L. monocytogenes in modified atmosphere packaged sliced cooked ham and paté was performed. A mixed inoculum of four L. monocytogenes strains and an inoculum level of ca. 1-10 cfu/g was used. On vacuum packed sliced cooked ham, the concentration of 100 cfu/g, the safety limit considered as low risk for causing listeriosis, was exceeded after 5 days whereas ca. 10(5) cfu/g were obtained after 14 days when also LAB spoilers reached unacceptable numbers (ca. 10(7) cfu/g) whether standard or cold-adapted inoculum was used. The concentration of sodium lactate determined the opportunities for growth of L. monocytogenes in paté. If growth of L. monocytogenes in paté was noticed, the threshold of 100 cfu/ml was crossed earlier for the cold-adapted inoculum compared to the standard inoculum.     

Prevalence and concentration of Listeria monocytogenes in sliced ready-to-eat meat products in the Hellenic retail market

The aim of this work was to estimate the prevalence and concentration of Listeria monocytogenes in packaged precut (slices or cubes) ready-to-eat (RTE) meat products available in the Hellenic retail market. Samples of these RTE meat products (n = 209) were taken from local supermarkets during a 3-month period and analyzed for the presence of L. monocytogenes with an automated enzymatic qualitative immunoassay followed by biochemical confirmation of positive results. The concentration of the pathogen in the positive samples was also determined. Seventeen samples (8.1%) were positive for L. monocytogenes. Eight (47.1%) of these 17 samples were from the same manufacturer; 36.4% of the products tested from this manufacturer were positive for L. monocytogenes. When bacon samples were not considered, the estimated prevalence of L. monocytogenes in sliced RTE meat products was much lower (3.1%). The L. monocytogenes populations in all positive samples were low, < or = 10 CFU/g. In 64.7% of the L. monocytogenes-positive samples, other Listeria species, including L. innocua and L. welshimeri, were also present at <10 to 690 CFU/g. These results indicate that L. monocytogenes is present in low numbers but is in a considerable proportion of the packaged precut RTE meat products that are sold in the Hellenic retail market. Cooked ham and bacon cut in cubes were the sample types most often contaminated with L. monocytogenes. The higher level of handling (e.g., cutting) associated with these products may further increase the risk of contamination with L. monocytogenes.     

Inhibition of Listeria monocytogenes growth in cured ready-to-eat meat products by use of sodium benzoate and sodium diacetate

The effect of sodium benzoate (0.08 to 0.25%) in combination with different concentrations of sodium diacetate (0.05 to 0.15%) and NaClI (0.8 to 2%) and different finished product moisture (55 to 75%) on the growth of Listeria monocytogenes in ready-to-eat meat products was evaluated using a central composite design over 18 weeks of storage at 4 degrees C. The effects of these factors on time to growth were analyzed using a time-to-failure regression method. All main effects were significant except product moisture, which was significant when included in the two- and three-way interactions (P < 0.05). Sodium benzoate was more effective (lengthening time to growth) when used with increasing concentrations of sodium diacetate and salt and decreasing finished product moisture. The model indicated that low-moisture products, e.g., bologna or wieners, could have time-to-growth values longer than 18 weeks if they were formulated with 0.1% sodium benzoate and 0.1% sodium diacetate. Time to growth in high-moisture products, e.g., ham or cured turkey breast at 75% moisture, was predicted to be much shorter for the same basic formulation (0.1% sodium benzoate and 0.1% sodium diacetate). Consequently, high-moisture ready-to-eat products in which sodium benzoate is limited to 0.1% (current standard for generally recognized as safe) may need additional ingredients to effectively inhibit growth of L. monocytogenes.     

Bayesian synthesis of a pathogen growth model: Listeria monocytogenes under competition

The Bayesian synthesis method is applied to data from two studies of Listeria monocytogenes grown in broth monocultures to draw inferences about the joint distribution of two Baranyi growth model parameters-lag time and maximum specific growth rate. The resultant joint distribution is then combined with prior distributions for the initial and maximum pathogen density parameters under competitive growth conditions. Finally, the pathogen growth model is updated using the Sampling/Importance Resampling (SIR) algorithm with data on L. monocytogenes growth in competition with natural microflora in fish. Although the latter data provide no information on the stationary phase to directly estimate the maximum pathogen density parameter, combining them with relevant prior information provides a means to characterize L. monocytogenes growth in a food with mixed microbial populations. Based on a specified tolerance for L. monocytogenes growth, the updated model provides a storage time limit for fish held at 5 degrees C, pH 6.8, 43% CO(2), 57% N(2).     

Quantifying the robustness of a broth-based model for predicting Listeria monocytogenes growth in meat and poultry products

Given the importance of Listeria monocytogenes as a risk factor in meat and poultry products, there is a need to evaluate the relative robustness of predictive growth models applied to meat products. The U.S. Department of Agriculture-Agricultural Research Service Pathogen Modeling Program is a tool widely used by the food industry to estimate pathogen growth, survival, and inactivation in food. However, the robustness of the Pathogen Modeling Program broth-based L. monocytogenes growth model in meat and poultry application has not, to our knowledge, been specifically evaluated. In the present study, this model was evaluated against independent data in terms of predicted microbial counts and covered a range of conditions inside and outside the original model domain. The robustness index was calculated as the ratio of the standard error of prediction (root mean square error of the model against an independent data set not used to create the model) to the standard error of calibration (root mean square error of the model against the data set used to create the model). Inside the calibration domain of the Pathogen Modeling Program, the best robustness index for application to meat products was 0.37; the worst was 3.96. Outside the domain, the best robustness index was 0.40, and the worst was 1.22. Product type influenced the robustness index values (P < 0.01). In general, the results indicated that broth-based predictive models should be validated against independent data in the domain of interest; otherwise, significant predictive errors can occur.     

Modeling the growth boundary of Listeria monocytogenes in ready-to-eat cooked meat products as a function of the product salt, moisture, potassium lactate, and sodium diacetate concentrations

A central composite response surface design was used to determine the time to growth of Listeria monocytogenes as a function of four continuous variables: added sodium chloride (0.8 to 3.6%), sodium diacetate (0 to 0.2%), potassium lactate syrup (60% [wt/wt]; 0.25 to 9.25%), and finished-product moisture (45.5 to 83.5%) in ready-to-eat cured meat products. The design was repeated for ready-to-eat uncured meat products giving a fifth categorical variable for cure status. Products were stored at 4 degrees C. The results were modeled using a generalized regression approach. All five main effects, six two-factor interactions, and two quadratic terms were statistically significant. The model was used to show the boundary between growth and no-growth conditions at 4 degrees C using contour plots of time to growth. It was validated using independent challenge studies of cured and uncured products. Generally, the model predicted well, particularly for cured products, where it will be useful for establishing conditions that prevent the growth of L. monocytogenes. For uncured products, there was good agreement overall between predicted and observed times to growth, but the model is less thoroughly validated than for cured products. The model should initially only be used for screening of formulations likely to prevent growth of Listeria monocytogenes in uncured products, with recommendations subject to confirmation by challenge studies.