Growth of Clostridium perfringens from spore inocula in cooked cured beef: development of a predictive model

Predictive modeling and probabilistic risk assessment of Clostridium perfringens in hamburgers and sandwiches

This study aimed to develop a mathematical model for the survival of Clostridium perfringens in hamburgers and sandwiches and to evaluate their microbial risk. The primary model was developed in hamburgers using 4 strains of C. perfringens at 5, 10, 15, 25 and 37 °C, and the kinetic parameters of the primary model were fitted well with the Weibull model (R ² ≥ 0.95). The secondary model was developed and validated in hamburgers and sandwiches using the Davey model, which was evaluated by B _f , A _f , and RMSE values within the acceptable range. A probabilistic risk model was developed and simulated using @Risk program to estimate the probability of infection (P _inf ) of C. perfringens based on the data on prevalence (n = 100), time, temperature, and consumption of hamburgers and sandwiches (150.00 ± 20.96 g). Based on the simulation model, the mean C. perfringens exposure dose was 0.00976 CFU/g, and the estimated mean P _inf was 1.78 × 10^-13, which was very low in comparison with the current available data. The proposed model and the result can thus be useful to establish risk management options and microbial criteria for C. perfringens contamination in hamburgers and sandwiches.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-021-01000-z.

Assessing the Performance of Clostridium perfringens Cooling Models for Cooked, Uncured Meat and Poultry Products

Heat-resistant spores of Clostridium perfringens may germinate and multiply in cooked meat and poultry products when the rate and extent of cooling does not occur in a timely manner. Therefore, six cooling models (PMP 7.0 broth model; PMIP uncured beef, chicken, and pork models; Smith-Schaffner version 3; and UK IFR ComBase Perfringens Predictor) were evaluated for relative performance in predicting growth of C. perfringens under dynamic temperature conditions encountered during cooling of cooked, uncured meat and poultry products. The predicted growth responses from the models were extensively compared with those observed in food. Data from 188 time-temperature cooling profiles (176 for single-rate exponential cooling and 12 for dual-rate exponential cooling) were collected from 17 independent sources (16 peer-reviewed publications and one report) for model evaluation. Data were obtained for a variety of cooked products, including meat and poultry slurries, ground meat and poultry products with and without added ingredients (e.g., potato starch, sodium triphosphate, and potassium tetrapyrophosphate), and processed products such as ham and roast beef. Performance of the models was evaluated using three sets of criteria, and accuracy was defined within a 1- to 2-log range. The percentages of accurate, fail-safe, or fail-dangerous predictions for each cooling model differed depending on which criterion was used to evaluate the data set. Nevertheless, the combined percentages of accurate and fail-safe predictions based on the three performance criteria were 34.66 to 42.61% for the PMP 7.0 beef broth model, 100% for the PMIP cooling models for uncured beef, uncured pork and uncured chicken, 80.11 to 93.18% for the Smith-Schaffner cooling model, and 74.43 to 85.23% for the UK IFR ComBase Perfringens Predictor model during single-rate exponential chilling. Except for the PMP 7.0 broth model, the other five cooling models (PMIP, Smith-Schaffner, and UK IFR ComBase) are useful and reliable tools that food processors and regulatory agencies can use to evaluate the safety of cooked or heat-treated uncured meat and poultry products exposed to cooling deviations or to develop customized cooling schedules.

Effect of oxygen stress on growth and survival of Clostridium perfringens, Campylobacter jejuni, and Listeria monocytogenes under different storage conditions

This study investigated the growth and survival of three foodborne pathogens (Clostridium perfringens, Campylobacter jejuni, and Listeria monocytogenes) in beef (7% fat) and nutrient broth under different oxygen levels. Samples were tested under anoxic (<0.5%), microoxic (6 to 8%), and oxic (20%) conditions during storage at 7 °C for 14 days and at 22 °C for 5 days. Two initial inoculum concentrations were used (1 and 2 log CFU per g of beef or per ml of broth). The results show that C. perfringens could grow in beef at 22 °C, with an increase of approximately 5 log under anoxic conditions and a 1-log increase under microoxic conditions. However, C. perfringens could not survive in beef held at 7 °C under microoxic and oxic storage conditions after 14 days. In an anoxic environment, C. perfringens survived in beef samples held at 7 °C, with a 1-log reduction. A cell decline was observed at 2 log under these conditions, with no surviving cells at the 1-log level. However, the results show that C. jejuni under microoxic conditions survived with declining cell numbers. Significant increases in L. monocytogenes (5 to 7 log) were observed in beef held at 22 °C for 5 days, with the lowest levels recovered under anoxic conditions. L. monocytogenes in refrigerated storage increased by a factor of 2 to 4 log. It showed the greatest growth under oxic conditions, with significant growth under anoxic conditions. These findings can be used to enhance food safety in vacuum-packed and modified atmosphere-packaged food products.

Risk assessment for Clostridium perfringens in ready-to-eat and partially cooked meat and poultry products

An assessment of the risk of illness associated with Clostridium perfringens in ready-to-eat and partially cooked meat and poultry products was completed to estimate the effect on the annual frequency of illnesses of changing the allowed maximal 1-log growth of C. perfringens during stabilization (cooling after the manufacturing heat step). The exposure assessment modeled stabilization, storage, and consumer preparation such as reheating and hot-holding. The model predicted that assuming a 10- or 100-fold increase from the assumed 1-log (maximal allowable) growth of C. perfringens results in a 1.2- or 1.6-fold increase of C. perfringens-caused illnesses, respectively, at the median of the uncertainty distribution. Improper retail and consumer refrigeration accounted for approximately 90% of the 79,000 C. perfringens illnesses predicted by the model at 1-log growth during stabilization. Improper hot-holding accounted for 8% of predicted illnesses, although model limitations imply that this is an underestimate. Stabilization accounted for less than 1% of illnesses. Efforts to reduce illnesses from C. perfringens in ready-to-eat and partially cooked meat and poultry products should focus on retail and consumer storage and preparation methods.

Modeling the growth of Salmonella in raw poultry stored under aerobic conditions

The presence of Salmonella in raw poultry is a well-recognized risk factor for foodborne illness. The objective of this study was to develop and validate a mathematical model that predicts the growth of Salmonella in raw poultry stored under aerobic conditions at a variety of temperatures. One hundred twelve Salmonella growth rates were extracted from 12 previously published studies. These growth rates were used to develop a square-root model relating the growth rate of Salmonella to storage temperature. Model predictions were compared to growth rate measurements collected in our laboratory for four poultry-specific Salmonella strains (two antibiotic-resistant and two nonresistant strains) inoculated onto raw chicken tenderloins. Chicken was inoculated at two levels (10(3) CFU/cm2 and < or = 10 CFU/cm2) and incubated at temperatures ranging from 10 to 37 degrees C. Visual inspection of the data, bias and accuracy factors, and comparison with two other published models were used to analyze the performance of the new model. Neither antibiotic resistance nor inoculum size affected Salmonella growth rates. The presence of spoilage microflora did not appear to slow the growth of Salmonella. Our model provided intermediate predicted growth rates when compared with the two other published models. Our model predicted slightly faster growth rates than those observed in inoculated chicken in the temperature range of 10 to 28 degrees C but slightly slower growth rates than those observed between 30 and 37 degrees C. Slightly negative bias factors were obtained in every case (-5 to -3%); however, application of the model may be considered fail-safe for storage temperatures below 28 degrees C.

Predictive model for Clostridium perfringens growth in roast beef during cooling and inhibition of spore germination and outgrowth by organic acid salts

Spores of foodborne pathogens can survive traditional thermal processing schedules used in the manufacturing of processed meat products. Heat-activated spores can germinate and grow to hazardous levels when these products are improperly chilled. Germination and outgrowth of Clostridium perfringens spores in roast beef during chilling was studied following simulated cooling schedules normally used in the processed-meat industry. Inhibitory effects of organic acid salts on germination and outgrowth of C. perfringens spores during chilling and the survival of vegetative cells and spores under abusive refrigerated storage was also evaluated. Beef top rounds were formulated to contain a marinade (finished product concentrations: 1% salt, 0.2% potassium tetrapyrophosphate, and 0.2% starch) and then ground and mixed with antimicrobials (sodium lactate and sodium lactate plus 2.5% sodium diacetate and buffered sodium citrate and buffered sodium citrate plus 1.3% sodium diacetate). The ground product was inoculated with a three-strain cocktail of C. perfringens spores (NCTC 8238, NCTC 8239, and ATCC 10388), mixed, vacuum packaged, heat shocked for 20 min at 75 degrees C, and chilled exponentially from 54.5 to 7.2 degrees C in 9, 12, 15, 18, or 21 h. C. perfringens populations (total and spore) were enumerated after heat shock, during chilling, and during storage for up to 60 days at 10 degrees C using tryptose-sulfite-cycloserine agar. C. perfringens spores were able to germinate and grow in roast beef (control, without any antimicrobials) from an initial population of ca. 3.1 log CFU/g by 2.00, 3.44, 4.04, 4.86, and 5.72 log CFU/g after 9, 12, 15, 18, and 21 h of exponential chilling. A predictive model was developed to describe sigmoidal C. perfringens growth curves during cooling of roast beef from 54.5 to 7.2 degrees C within 9, 12, 15, 18, and 21 h. Addition of antimicrobials prevented germination and outgrowth of C. perfringens regardless of the chill times. C. perfringens spores could be recovered from samples containing organic acid salts that were stored up to 60 days at 10 degrees C. Extension of chilling time to > or =9 h resulted in >1 log CFU/g growth of C. perfringens under anaerobic conditions in roast beef. Organic acid salts inhibited outgrowth of C. perfringens spores during chilling of roast beef when extended chill rates were followed. Although C. perfringens spore germination is inhibited by the antimicrobials, this inhibition may represent a hazard when such products are incorporated into new products, such as soups and chili, that do not contain these antimicrobials, thus allowing spore germination and outgrowth under conditions of temperature abuse.

Development of an integrated model for heat transfer and dynamic growth of Clostridium perfringens during the cooling of cooked boneless ham

Numerous small meat processors in the United States have difficulties complying with the stabilization performance standards for preventing growth of Clostridium perfringens by 1 log10 cycle during cooling of ready-to-eat (RTE) products. These standards were established by the Food Safety and Inspection Service (FSIS) of the US Department of Agriculture in 1999. In recent years, several attempts have been made to develop predictive models for growth of C. perfringens within the range of cooling temperatures included in the FSIS standards. Those studies mainly focused on microbiological aspects, using hypothesized cooling rates. Conversely, studies dealing with heat transfer models to predict cooling rates in meat products do not address microbial growth. Integration of heat transfer relationships with C. perfringens growth relationships during cooling of meat products has been very limited. Therefore, a computer simulation scheme was developed to analyze heat transfer phenomena and temperature-dependent C. perfringens growth during cooling of cooked boneless cured ham. The temperature history of ham was predicted using a finite element heat diffusion model. Validation of heat transfer predictions used experimental data collected in commercial meat-processing facilities. For C. perfringens growth, a dynamic model was developed using Baranyi's nonautonomous differential equation. The bacterium's growth model was integrated into the computer program using predicted temperature histories as input values. For cooling cooked hams from 66.6 degrees C to 4.4 degrees C using forced air, the maximum deviation between predicted and experimental core temperature data was 2.54 degrees C. Predicted C. perfringens growth curves obtained from dynamic modeling showed good agreement with validated results for three different cooling scenarios. Mean absolute values of relative errors were below 6%, and deviations between predicted and experimental cell counts were within 0.37 log10 CFU/g. For a cooling process which was in exact compliance with the FSIS stabilization performance standards, a mean net growth of 1.37 log10 CFU/g was predicted. This study introduced the combination of engineering modeling and microbiological modeling as a useful quantitative tool for general food safety applications, such as risk assessment and hazard analysis and critical control points (HACCP) plans.

Development of a model to predict growth of Clostridium perfringens in cooked beef during cooling

The objective of this work was to develop a new model to predict the growth of Clostridium perfringens in cooked meat during cooling. All data were collected under changing temperature conditions. Individual growth curves were fit using DMFit. Germination outgrowth and lag (GOL) time was modeled versus temperature at the end of GOL using conservative assumptions. Each growth curve was used to estimate a series of exponential growth rates at a series of temperatures. The squareroot model was used to describe the relationship between the square root of the average exponential growth rate and effective temperature. Predictions from the new model were in close agreement with the data used to create the model. When predictions from the model were compared with new observations, fail-dangerous predictions were made a majority of the time. When GOL time was predicted exactly, many fail-dangerous predictions shifted toward the fail-safe direction. Two important facts regarding C. perfringens should impact future modeling research with this organism and may have broader food safety policy implications: (i) the normal variability in the response of the organism from replicate to replicate may be quite large (1 log CFU) and may exceed the current U.S. Food Safety Inspection Service performance standard, and (ii) the accuracy of the GOL time model has a profound influence upon the overall prediction, with small differences in GOL time prediction (approximately 1 h) having a very large effect on the predicted final concentration of C. perfringens.

Growth potential of Clostridium perfringens during cooling of cooked meats

Many meat-based food products are cooked to temperatures sufficient to inactivate vegetative cells of Clostridium perfringens, but spores of this bacterium can survive, germinate, and grow in these products if sufficient time, temperature, and other variables exist. Because ingestion of large numbers of vegetative cells can lead to concomitant sporulation, enterotoxin release in the gastrointestinal tract, and diarrhea-like illness, a necessary food safety objective is to ensure that not more than acceptable levels of C. perfringens are in finished products. As cooked meat items cool they will pass through the growth temperature range of C. perfringens (50 to 15 degrees C). Therefore, an important step in determining the likely level of C. perfringens in the final product is the estimation of growth of the pathogen during cooling of the cooked product. Numerous studies exist dealing with just such estimations, yet consensual methodologies, results, and conclusions are lacking. There is a need to consider the bulk of C. perfringens work relating to cooling of cooked meat-based products and attempt to move toward a better understanding of the true growth potential of the organism. This review attempts to summarize observations made by researchers and highlight variations in experimental approach as possible explanations for different outcomes. An attempt is also made here to identify and justify optimal procedures for conducting C. perfringens growth estimation in meat-based cooked food products during cooling.

Validation of a polynomial regression model: the thermal inactivation of Bacillus subtilis spores in milk

AIMS

The predicted survival of Bacillus subtilis 168 spores from a polynomial regression equation was validated in milk.

METHODS AND RESULTS

Bias factor suggested as an index of model performance was used to validate the polynomial model predictions in ultrahigh temperature (UHT) treated and sterilized whole and skim milk. Model predictions were fail safe, predicting higher D-values (decimal reduction times) in buffer than actually noted in milk.

CONCLUSIONS

The D-values for spores were lower in milk as compared with those predicted in potassium phosphate buffer contrary to the popular expectation of better spore survival in complex food systems. The Bias factor, a quantitative measure of the model performance, indicated that on average the model predictions exceed the observations by 40% in the case of whole milk and by 60% in the case of skim milk.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present work is an attempt to ascertain the extent of reliability that one can safely place in polynomial model predictions, without compromising on the safety or palatability of foods where it is eventually intended to be applied. The work has also highlighted the differences in the thermal inactivation pattern of spores in buffer and in milk with a possible influence of the various constituents of milk. The work will assist the dairy industry to better design thermal processes to ensure longer shelf life of dairy foods.

Influence of NaCl content and cooling rate on outgrowth of Clostridium perfringens spores in cooked ham and beef

The effect of NaCl concentration and cooling rate on the ability of Clostridium perfringens to grow from spore inocula was studied with the use of a process that simulates the industrial cooking and cooling of smoked boneless ham and beef roasts. NaCl was added to ground cooked hams A and B (which were commercially obtained) to obtain levels of 2.4, 3.1, 3.6, and 4.1% (wt/wt) and 2.8, 3.3, 3.8, and 4.3% (wt/wt), respectively, and to raw ground beef to obtain levels of 0, 1, 2, 3, and 4% (wt/wt). Ham C, a specially formulated, commercially prepared product, was supplemented with NaCl to obtain levels of 2.0, 2.5, 3.0, and 3.5%. The samples were inoculated with a three-strain mixture of C. perfringens spores to obtain concentrations of ca. 3 log10 CFU/g. Portions of meat (5 g each) were spread into thin layers (1 to 2 mm) in plastic bags, vacuum packaged, and stored at -40 degrees C. Thawed samples were heated at 75 degrees C for 20 min and subsequently cooled in a programmed water bath from 54.4 to < or = 8.5 degrees C in 15, 18, or 21 h. For the enumeration of C. perfringens, samples were plated on tryptose-sulfite-cycloserine agar and incubated in an anaerobic chamber at 37 degrees C for 48 h. Population densities for cooked ham and beef increased as cooling time increased, and NaCl exerted a strong inhibitory effect on the germination and outgrowth of C. perfringens. For beef, while 3% NaCl completely arrested growth, pathogen numbers increased by > or = 3, 5, and 5 log10 CFU/g in 15, 18, and 21 h, respectively, when the NaCl level was <2%. C. perfringens did not grow during cooling for 15, 18, or 21 h in ham samples containing > or = 3.1% NaCl. Results obtained in this study suggest that a 15-h cooling time for cooked ham, which is normally formulated to contain >2% NaCl, would yield an acceptable product (with an increase of <1 log10 CFU/g in the C. perfringens count); however, for beef containing <2% NaCl, C. perfringens populations may reach levels high enough to cause illness.

Growth and heat resistance kinetic variation among various isolates of Salmonella and its application to risk assessment

The abilities of cells of a particular type of bacteria to leave lag phase and begin the process of dividing or surviving heat treatment can depend on the serotypes or strains of the bacteria. This article reports an investigation of serotype-specific differences in growth and heat resistance kinetics of clinical and food isolates of Salmonella. Growth kinetics at 19 degrees C and 37 degrees C were examined in brain heart infusion broth and heat resistance kinetics for 60 degrees C were examined in beef gravy using a submerged coil heating apparatus. Estimates of the parameters of the growth curves suggests a small between-serotype variance of the growth kinetics. However, for inactivation, the results suggest a significant between-serotype effect on the asymptotic D-values, with an estimated between-serotype CV of about 20%. In microbial risk assessment, predictive microbiology is used to estimate growth and inactivation of pathogens. Often the data used for estimating the growth or inactivation kinetics are based on measurements on a cocktail--a mixture of approximately equal proportions of several serotypes or strains of the pathogen being studied. The expected growth or inactivation rates derived from data using cocktails are biased, reflecting the characteristics of the fastest growing or most heat resistant serotype of the cocktail. In this article, an adjustment to decrease this possible bias in a risk assessment is offered. The article also presents discussion of the effect on estimating growth when stochastic assumptions are incorporated in the model. In particular, equations describing the variation of relative growth are derived, accounting for the stochastic variations of the division of cells. For small numbers of cells, the expected value of the relative growth is not an appropriate "representative" value for actual relative growths that might occur.

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.