Predicting heat inactivation of Listeria monocytogenes under nonisothermal treatments

Modeling the combined resistance to microwave treatments and salt conditions of Escherichia coli and Staphylococcus aureus

In the present study, the efficiency of the combined effect of microwave irradiation treatments together with salt concentration was assessed against Escherichia coli and Staphylococcus aureus. Microbial survival has been modeled through a one-step Weibull equation considering the non-isothermal profiles during the heating treatments. Three sodium chloride concentrations 0.5%, 3.5%, and 8.5% (w/v) treated under three microwave power levels (450, 600, and 800 W) were studied. Predictive models were validated using the determination coefficient (R2), root mean squared error and the acceptable prediction zone with external data obtained from ultra high temperature milk. The results obtained suggested that increasing microwave power levels and decreasing salt concentrations led to a higher microbial inactivation, being the δ values (time for achieving a first decimal reduction) for E coli of 19.57 s at 800 W and 0.5% NaCl. In contrast, experimental data of S aureus showed a higher variability since it presented more resistance to the microwave treatments. The results obtained and generated models can be used as decision-making tools to set specific guidelines on microwave treatments for assuring food safety.

Synergistic Effects of Butyl Para-Hydroxybenzoate and Mild Heating on Foodborne Pathogenic Bacteria

ABSTRACT

Although high-temperature heat treatments can efficiently reduce pathogen levels, they also affect the quality and nutritional profile of foods and increase the cost of processing. The food additive butyl para-hydroxybenzoate (BPB) was investigated for its potential to synergistically enhance thermal microbial inactivation at mild heating temperatures (54 to 58°C). Four foodborne pathogenic bacteria, Cronobacter sakazakii, Salmonella enterica Typhimurium, attenuated Escherichia coli O157:H7, and Listeria monocytogenes, were cultured to early stationary phase and then subjected to mild heating at 58, 55, 57, and 54°C, respectively, in a model food matrix (brain heart infusion [BHI]) containing low concentrations of BPB (≤125 ppm). The temperature used with each bacterium was selected based on the temperature that would yield an approximately 1- to 3-log reduction over 15 min of heating in BHI without BPB in a submerged coil system. The inclusion of BPB at ≤125 ppm resulted in significant enhancement of thermal inactivation, achieving 5- to >6-log reductions of the gram-negative strains with D-values of <100 s. A 3- to 4-log reduction of L. monocytogenes was achieved with a similar treatment. No significant microbial inactivation was noted in the absence of mild heating for the same time period. This study provides additional proof of concept that low-temperature inactivation of foodborne pathogens can be realized by synergistic enhancement of thermal inactivation by additives that affect microbial cell membranes.

HIGHLIGHTS

Variability in the heat resistance of Listeria monocytogenes under dynamic conditions can be more relevant than that evidenced by isothermal treatments

Heterogeneity in the response of microbial cells to environmental conditions is inherent to every biological system and can be very relevant for food safety, potentially being as important as intrinsic and extrinsic factors. However, previous studies analyzing variability in the microbial response to thermal treatments were limited to data obtained under isothermal conditions, whereas in the reality, environmental conditions are dynamic. In this article we analyse both empirically and through mathematical modelling the variability in the microbial response to thermal treatments under isothermal and dynamic conditions. Heat resistance was studied for four strains of Listeria monocytogenes (Scott A, CECT 4031, CECT 4032 and 12MOB052), in three different matrices (buffered peptone water, pH 7 Mcllvaine buffer and semi-skimmed milk). Under isothermal conditions, between-strain and between-media variability had no impact in the heat resistance, whereas it was very relevant for dynamic conditions. Therefore, the differences observed under dynamic conditions can be attributed to the variability in the ability for developing stress acclimation. The highest acclimation was observed in strain CECT 4031 (10-fold increase of the D-value), while the lowest acclimation was observed in strain CECT 4032 (50% increase of the D-value). Concerning the different media, acclimation was higher in buffered peptone water and semi-skimmed milk than in Mcllvaine buffer of pH 7.0. To the knowledge of the authors, this is the first research work that specifically analyses the variability of microbial adaptation processes that take place under dynamic conditions. It highlights that microbial heat resistance under dynamic conditions are sometimes determined by mechanisms that cannot be observed when cells are treated in isothermal conditions (e.g. acclimation) and can also be affected by variability. Consequently, empirical evidence on variability gathered under isothermal conditions should be extrapolated with care for dynamic conditions.

A comprehensive review on recent developments of radio frequency treatment for pasteurizing agricultural products

Recent pathogen incidents have forced food industry to seek for alternative processes in postharvest pasteurization of agricultural commodities. Radio frequency (RF) heating has been used as one alternative treatment to replace chemical fumigation and other conventional thermal methods since it is relatively easy to apply and leaves no chemical residues. RF technology transfers electromagnetic energy into large bulk volume of the products to provide a fast and volumetric heating. There are two types of RF technology commonly applied in lab and industry to generate the heat energy: free running oscillator and 50-Ω systems. Several reviews have been published to introduce the application of RF heating in food processing. However, few reviews have a comprehensive summary of RF treatment for pasteurizing agricultural products. The objective of this review was to introduce the developments in the RF pasteurization of agricultural commodities and to present future directions of the RF heating applications. While the recent developments in the RF pasteurization were presented, thermal death kinetics of targeted pathogens as influenced by water activity, pathogen species and heating rates, non-thermal effects of RF heating, combining RF heating with other technologies for pasteurization, RF heating uniformity improvements using computer simulation and development of practical RF pasteurization processes were also focused. This review is expected to provide a comprehensive understanding of RF pasteurization for agricultural products and promote the industrial-scale applications of RF technology with possible process protocol optimization purposes.

Structural and Functional Changes of Groundwater Bacterial Community During Temperature and pH Disturbances

In this study, we report the characteristics of a microbial community in sampled groundwater and elucidate the effects of temperature and pH disturbances on bacterial structure and nitrogen-cycling functions. The predominant phyla of candidate OD1, candidate OP3, and Proteobacteria represented more than half of the total bacteria, which clearly manifested as a "low nucleic acid content (LNA) bacteria majority" type via flow cytometric fingerprint. The results showed that LNA bacteria were more tolerant to rapid changes in temperature and pH, compared to high nucleic acid content (HNA) bacteria. A continuous temperature increase test demonstrated that the LNA bacterial group was less competitive than the HNA bacterial group in terms of maintaining their cell intactness and growth potential. In contrast, the percentage of intact LNA bacteria was maintained at nearly 70% with pH decrease, despite a 50% decrease in total intact cells. Next-generation sequencing results revealed strong resistance and growth potential of phylum Proteobacteria when the temperature increased or the pH decreased in groundwater, especially for subclasses α-, β-, and γ-Proteobacteria. In addition, relative abundance of nitrogen-related functional genes by qPCR showed no difference in nitrifiers or denitrifiers within 0.45 μm-captured and 0.45 μm-filterable bacteria due to phylogenetic diversity. One exception was the monophyletic anammox bacteria that belong to the phylum Planctomycetes, which were mostly captured on a 0.45-μm filter. Furthermore, we showed that both temperature increase and pH decrease could enhance the denitrification potential, whereas the nitrification and anammox potentials were weakened.

Mathematical modelling of the stress resistance induced in Listeria monocytogenes during dynamic, mild heat treatments

Modelling of stress acclimation induced by thermal inactivation of Listeria monocytogenes under dynamic conditions is analyzed in this work. A mathematical model that separates the effect of the instantaneous temperature from the one of stress acclimation, was used. The model was trained using isothermal inactivation experiments, and one biphasic dynamic treatment with a heating rate of 1 °C/min and a holding phase of 60 °C. These experiments were performed in laboratory media (Tryptic Soy Broth; TSB). The model parameters estimated through these experiments (D₅₅=12.87±0.82min, z=4.58±0.04°C, a=0.11±0.01min^-1, E=0.50±0.01°C and c=1.23±0.03) were successfully used to predict the microbial inactivation for another seven inactivation profiles, with and without a holding phase. Moreover, similar experiments were performed using milk as heating media, obtaining a good agreement between the model predictions and the empirical observations. The results of this study are compatible with the hypothesis that L. monocytogenes is able to develop a physiological response during dynamic treatments that increases its thermal resistance. Also, that the model used can be used to predict microbial inactivation of this microorganism taking into consideration stress acclimation.

Relevance of the Induced Stress Resistance When Identifying the Critical Microorganism for Microbial Risk Assessment

Decisions regarding microbial risk assessment usually have to be carried out with incomplete information. This is due to the large number of possible scenarios and the lack of specific data for the problem considered. Consequently, risk assessment studies are based on the information obtained with a small number of bacterial cells which are considered the most heat resistant and/or more capable of multiplying during storage. The identification of the most resistant strains is usually based on D and z-values, normally estimated from isothermal experiments. This procedure omits the potential effect that the shape of the dynamic thermal profile applied in industry has on the microbial inactivation. One example of such effects is stress acclimation, which is related to a physiological response of the cells during sub-lethal treatments that increases their resistance. In this article, we use a recently published mathematical model to compare the development of thermal resistance for Escherichia coli K12 MG1655 and E. coli CECT 515 using inactivation data already published for these strains. Based only on the isothermal experiments, E. coli K12 MG1655 would be identified as more resistant to the thermal treatment than the CECT 515 strain in the 50-65°C temperature range. However, we conclude that stress acclimation is strain (and/or media)-dependent; the CECT 515 strain has a higher capacity for developing a stress acclimation than K12 MG1655 (300% increase of the D-value for CECT 515, 50% for K12 MG1655). It, thus, has the potential to be more resistant to the thermal treatment than the K12 MG1655 strain for some conditions allowing acclimation. A methodology is proposed to identify for which conditions this may be the case. After calibrating the model parameters representing acclimation using real experimental data, the applicability of the proposed approach is demonstrated using numerical simulations, showing how the CECT 515 strain can be more resistant for some heating profiles. Consequently, the most resistant bacterial strain to a dynamic heating profile should not be identified based only on isothermal experiments (D- and z-value). The relevance of stress acclimation for the treatment studied should also be evaluated.

High Heating Rates Affect Greatly the Inactivation Rate of Escherichia coli

Heat resistance of microorganisms can be affected by different influencing factors. Although, the effect of heating rates has been scarcely explored by the scientific community, recent researches have unraveled its important effect on the thermal resistance of different species of vegetative bacteria. Typically heating rates described in the literature ranged from 1 to 20°C/min but the impact of much higher heating rates is unclear. The aim of this research was to explore the effect of different heating rates, such as those currently achieved in the heat exchangers used in the food industry, on the heat resistance of Escherichia coli. A pilot plant tubular heat exchanger and a thermoresistometer Mastia were used for this purpose. Results showed that fast heating rates had a deep impact on the thermal resistance of E. coli. Heating rates between 20 and 50°C/min were achieved in the heat exchanger, which were much slower than those around 20°C/s achieved in the thermoresistometer. In all cases, these high heating rates led to higher inactivation than expected: in the heat exchanger, for all the experiments performed, when the observed inactivation had reached about seven log cycles, the predictions estimated about 1 log cycle of inactivation; in the thermoresistometer these differences between observed and predicted values were even more than 10 times higher, from 4.07 log cycles observed to 0.34 predicted at a flow rate of 70 mL/min and a maximum heating rate of 14.7°C/s. A quantification of the impact of the heating rates on the level of inactivation achieved was established. These results point out the important effect that the heating rate has on the thermal resistance of E. coli, with high heating rates resulting in an additional sensitization to heat and therefore an effective food safety strategy in terms of food processing.

Modeling of Clostridium perfringens vegetative cell inactivation in beef-in-sauce products: a meta-analysis using mixed linear models

The aim of the present study was to predict Clostridium perfringens vegetative cell inactivation during the final reheating step of two beef-in-sauce products prepared and distributed in a French hospital for exposure in risk assessment. In order to account for variability according to experts and international organization recommendations, published data were used to estimate the thermal inactivation parameters of a probabilistic model. Mixed effects models were proposed to describe variability on D(ref) the decimal reduction time at temperature T(ref). Many models differing by their description of variability on D(ref) were tested. Based on goodness-of-fit and parsimony of the model, the one including three random effects was chosen. That model describes random effects of vegetative cell culture conditions, strains and other uncontrolled experimental factors. In order to check the ability of the model to predict inactivation under dynamic thermal conditions, model validation was carried out on published non isothermal data. This model was then used to predict C. perfringens vegetative cell inactivation using temperature profiles inside beef-in-sauce products registered in a French hospital and to explore control measures easier to apply than French regulations.

An integral model of microbial inactivation taking into account memory effects: power-law memory kernel

In this article, we propose an alternative framework for the description of non-log-linear thermal inactivation of microorganisms. The proposed framework generalizes classical views by explicitly taking into account memory effects, such as those often associated with cumulative cell damage or progressive cell adaptation. Within this general framework, specialized memory models can be easily accommodated to describe different modes of microbial response to previous thermal stresses. In this introductory study, the advantages and limitations of the simplest nontrivial memory model, the power-law memory model, were explored. Our results indicate that for isothermal treatments the assumption of power-law memory leads to a simple solution that is known to describe a large number of non-log-linear survival curves. For nonisothermal treatments, the power-law memory model leads to predictions that agree well with experimental data. This research may lead to new insights into predictive microbiology with a new appreciation for the importance of memory effects.

Dynamic model of heat inactivation kinetics for bacterial adaptation

The Weibullian-log logistic (WeLL) inactivation model was modified to account for heat adaptation by introducing a logistic adaptation factor, which rendered its "rate parameter" a function of both temperature and heating rate. The resulting model is consistent with the observation that adaptation is primarily noticeable in slow heat processes in which the cells are exposed to sublethal temperatures for a sufficiently long time. Dynamic survival patterns generated with the proposed model were in general agreement with those of Escherichia coli and Listeria monocytogenes as reported in the literature. Although the modified model's rate equation has a cumbersome appearance, especially for thermal processes having a variable heating rate, it can be solved numerically with commercial mathematical software. The dynamic model has five survival/adaptation parameters whose determination will require a large experimental database. However, with assumed or estimated parameter values, the model can simulate survival patterns of adapting pathogens in cooked foods that can be used in risk assessment and the establishment of safe preparation conditions.

Stress-adaptive responses by heat under the microscope of predictive microbiology

AIMS

In previous studies the microbial kinetics of Escherichia coli K12 have been evaluated under static and dynamic conditions (Valdramidis et al. 2005, 2006). An acquired microbial thermotolerance following heating rates lower than 0.82 degrees C min(-1) for the studied micro-organism was observed. Quantification of this induced physiological phenomenon and incorporation, as a model building block, in a general microbial inactivation model is the main outcome of this work.

METHODS AND RESULTS

The microbial inactivation rate observed (k(obs)) under time-varying temperature conditions is studied and expressed as a function of the heating rate (dT/ dt). Hereto, a model building block related to the microbial physiology (k(phys)) under stress conditions is developed. Evaluation of the performance of the developed mathematical approach depicts that physiological adaptation is an essential issue to be considered when modelling microbial inactivation.

CONCLUSIONS

Consideration, at a mathematical level, of microbial responses resulting in physiological adaptations contribute to the reliable quantification of the safety risks during food processing.

SIGNIFICANCE AND IMPACT OF THE STUDY

By taking into account the physiological adaptation, the microbiological evolution during heat processing can be accurately assessed, and overly conservative or fail dangerous food processing designs can be avoided.

Modeling the effect of prior sublethal thermal history on the thermal inactivation rate of Salmonella in ground turkey

Traditional models for predicting the thermal inactivation rate of bacteria are state dependent, considering only the current state of the product. In this study, the potential for previous sublethal thermal history to increase the thermotolerance of Salmonella in ground turkey was determined, a path-dependent model for thermal inactivation was developed, and the path-dependent predictions were tested against independent data. Weibull-Arrhenius parameters for Salmonella inactivation in ground turkey thigh were determined via isothermal tests at 55, 58, 61, and 63 degrees C. Two sets of nonisothermal heating tests also were conducted. The first included five linear heating rates (0.4, 0.9, 1.7, 3.5, and 7.0 K/min) and three holding temperatures (55, 58, and 61 degrees C); the second also included sublethal holding periods at 40, 45, and 50 degrees C. When the standard Weibull-Arrhenius model was applied to the nonisothermal validation data sets, the root mean squared error of prediction was 2.5 log CFU/g, with fail-dangerous residuals as large as 4.7 log CFU/g when applied to the complete nonisothermal data set. However, by using a modified path-dependent model for inactivation, the prediction errors for independent data were reduced by 56%. Under actual thermal processing conditions, use of the path-dependant model would reduce error in thermal lethality predictions for slowly cooked products.

Predicting microbial heat inactivation under nonisothermal treatments

The aim of this study was to develop an equation that accurately predicts microbial heat inactivation under nonisothermal treatments at constantly rising heating rates (from 0.5 to 5 degrees C/min) in media with different pH values (4.0 or 7.4). The survival curves of all bacteria (Enterococcus faecium, Escherichia coli, Listeria monocytogenes, Salmonella Senftenberg 775W, Salmonella Typhimurium, and Staphylococcus aureus) tested under isothermal treatments were nearly linear. For the most heat-resistant microorganism (E. faecium), the estimated DT-values at pH 7.4 were at least 100 times those of the second most thermotolerant microorganism (Salmonella Senftenberg 775W). The heat resistance of E. faecium was up to 30 times lower at pH 4.0 than at pH 7.4. However, E. faecium was still the most heat-resistant microorganism under nonisothermal treatments at both pH values. Inactivation under nonisothermal conditions was not accurately estimated from heat resistance parameters of isothermal treatments when microbial adaptation or sensibilization occurred during the heating up lag phases. The under-prediction of the number of survivors might be greater than 15 log CFU within the nonisothermal treatment conditions investigated. Therefore, the nonisothermal survival curves of the most heat-resistant microorganisms were fitted with the following equation: log S(t) = -(t/delta)P. This equation accurately described the survival curves of all the bacteria tested. We observed a linear relationship between the log of the scale parameter (delta) and the log of the heating rate. A p value characteristic of each microorganism and pH tested was calculated. Two equations capable of predicting the inactivation rate of all bacteria tested under nonisothermal treatments at pH 7.4, 5.5, or 4.0 were developed. The model was evaluated in skim milk and apple juice. The results of this study could be used to help minimize public health risks and to extend the shelf life of those foods requiring long heating up lag phases during processing.

Effect of a previous heat shock on the thermal resistance of Listeria monocytogenes and Pseudomonas aeruginosa at different pHs

In this work we study the effect of heat shocks of various durations up to 60 min, at different temperatures between 35 and 45 degrees C, in media of pH 4.0, 5.5 and 7.4 on the heat resistance of Listeria monocytogenes and Pseudomonas aeruginosa. The pattern of survival curves after heat treatment did not change with the application of a previous heat shock. However, the kinetics of inactivation was different for the two microorganisms studied. Whereas the inactivation of L. monocytogenes was similar to an exponential function of heating time and therefore straight survival curves were obtained, survival curves corresponding to P. aeruginosa showed convex profiles. All survival curves obtained in this investigation were fitted to Weibull-based Mafart equation: log(10)S(t)=-(t / delta)(p). The magnitude of the heat shock induced thermotolerance increased with treatment medium pH. At pH 7.4 the increase in heat tolerance depended on the duration and temperature of the heat shock. On the contrary, at pH 5.5 and pH 4.0, the heat-shock temperature did not exert any effect. The observed maximum delta values increased 2.3, 4.0 and 9.3 fold for L. monocytogenes, and 1.3, 2.1 and 8.4 fold for P. aeruginosa, at pH 4.0, 5.5 and 7.4, respectively. This research has proven that Mafart equation allows studying and quantifying the effect of heat shocks on bacterial heat resistance.

Induced thermotolerance under nonisothermal treatments of a heat sensitive and a resistant strain of Staphylococcus aureus in media of different pH

AIMS

The aim was to assess the induced thermotolerance under nonisothermal treatments of two strains of Staphylococcus aureus in media of different pH.

METHODS AND RESULTS

Staphylococcus aureus ATCC 25923 was more heat resistant than S. aureus ATCC 13565 at any pH investigated under isothermal conditions. At pH 7.4, the D58 value of the resistant strain was approx. 30 times greater. Both strains showed a higher heat resistance at pH 4.0 than at pH 7.4. In contrast, under nonisothermal treatments (0.5-2 degrees C min(-1)), both strains were more heat resistant when treated at pH 7.4 than at pH 4.0 due to heat adaptation at the higher pH. At the slowest heating up rate tested at pH 7.4, the initially heat-sensitive strain nearly reached the thermotolerance of the heat-resistant strain.

CONCLUSIONS

The induced thermotolerance under nonisothermal treatments depended on the treatment medium pH and the microbial strain tested. The induced thermotolerance in a sensitive strain can be greater than in a heat-resistant strain, showing similar resistance under nonisothermal conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work shows data of interest about mechanisms of microbial resistance and adaptation to heat. Moreover, it contributes to the development of more adequate combined processes for food preservation.

Predicting heat inactivation of Staphylococcus aureus under nonisothermal treatments at different pH

The aim was to assess whether heat resistance data obtained from isothermal treatments allow the estimation of survivors of Staphylococcus aureus under nonisothermal conditions and to find a model that accurately predicts its heat inactivation at constantly rising heating rates (0.5-9 degrees C/min) in media of different pH (4.0-7.4). S. aureus showed a higher heat resistance under isothermal treatments at pH 4.0 than at pH 5.5-7.4. However, under nonisothermal treatments S. aureus increased its heat resistance at pH 5.5-7.4 and became more thermotolerant than at pH 4.0. Estimations of survival curves under nonisothermal treatments obtained from heat resistance parameters of isothermal treatments did not adequately fit experimental values. Whereas the number of survivors was much higher than estimated at pH 5.5-7.4, that obtained at the slower heating rates at pH 4.0 was lower. An equation based on the Weibullian-like distribution (log10 S(t) = (t/delta)p) accurately described survival curves obtained under nonisothermal conditions. A nonlinear relationship was observed among the scale parameter (delta) and the heating rate which allowed the development of two equations capable of predicting the inactivation rate of S. aureus under nonisothermal treatments. This study might contribute to prevent public health risks in foods requiring long heating lag phases during their processing.

Combined pressure-thermal inactivation kinetics of Bacillus amyloliquefaciens spores in egg patty mince

Bacillus amyloliquefaciens is a potential surrogate for Clostridium botulinum in validation studies involving bacterial spore inactivation by pressure-assisted thermal processing. Spores of B. amyloliquefaciens Fad 82 were inoculated into egg patty mince (approximately 1.4 x 10(8) spores per g), and the product was treated with combinations of pressure (0.1 to 700 MPa) and heat (95 to 121 degrees C) in a custom-made high-pressure kinetic tester. The values for the inactivation kinetic parameter (D), temperature coefficient (zT), and pressure coefficient (zP) were determined with a linear model. Inactivation parameters from the nonlinear Weibull model also were estimated. An increase in process pressure decreased the D-value at 95, 105, and 110 degrees C; however, at 121 degrees C the contribution of pressure to spore lethality was less pronounced. The zP-value increased from 170 MPa at 95 degrees C to 332 MPa at 121 degrees C, suggesting that B. amyloliquefaciens spores became less sensitive to pressure changes at higher temperatures. Similarly, the zT-value increased from 8.2 degrees C at 0.1 MPa to 26.8 degrees C at 700 MPa, indicating that at elevated pressures, the spores were less sensitive to changes in temperature. The nonlinear Weibull model parameter b increased with increasing pressure or temperature and was inversely related to the D-value. Pressure-assisted thermal processing is a potential alternative to thermal processing for producing shelf-stable egg products.

Evaluating microbial safety of slow partial-cooking processes for bacon: use of a predictive tool based on small-scale isothermal meat inoculation studies

The objective of this study was to develop a predictive tool for evaluating the safety of slow cooking of pork products and identifying associated critical limits. Small-scale (25 g) ground pork isothermal inoculation studies were done to determine Salmonella serovars, Escherichia coli O157:H7, and Staphylococcus aureus estimated critical times (time until growth reached a predefined increase of concern). Estimated critical time values ranged from 720 min at 21 degrees C (S. aureus) to 120 min at 40.6 degrees C (E. coli O157:H7) and were used to develop a multiple-temperature-interval predictive tool for non-isothermal processes. To test predictions, cured-pumped pork bellies were inoculated with Salmonella serovars, E. coli O157:H7, and S. aureus, subjected to slow partial cooking, and quantitatively analyzed for pathogens. Processes lasted 6 to 18 h, with the product interior temperature within the 21 to 46 degrees C range for 263 to 1080 min (high-humidity processes) and 217 to 921 min (low-humidity processes). Growth of Salmonella serovars (>0.3 log), E. coli O157:H7 (>0.3 log), and S. aureus (>1.3 log) in the pork belly interior was predicted for 10, 14, and 5 of 18 trials, respectively. The tool was fail-safe, because pathogen growth, relative to time zero, did not occur anytime regardless of whether it was predicted. For the pork belly surface, the tool performed similarly. Estimated critical time values obtained by fitting the Baranyi equation to isothermal experiment data were also determined and, if used in the predictive tool, would result in even more conservative predictions. Our study substantiates the safety of the tested bacon slow partial-cooking processes and the potential usefulness of our isothermal-based tool in process safety evaluation.