Modeling the heat inactivation of foodborne pathogens in milk powder: High relevance of the substrate water activity

A meta-analysis of microbial thermal inactivation in low moisture foods

Microbial thermal inactivation in low moisture foods is challenging due to enhanced thermal resistance of microbes and low thermal conductivity of food matrices. In this study, we leveraged the body of previous work on this topic to model key experimental features that determine microbial thermal inactivation in low moisture foods. We identified 27 studies which contained 782 mean D-values and developed linear mixed-effect models to assess the effect of microorganism type, matrix structure and composition, water activity, temperature, and inoculation and recovery methods on cell death kinetics. Intraclass correlation statistics (I2) and conditional R2 values of the linear mixed effects models were: E. coli (R2-0.91, I2-83%), fungi (R2-0.88, I2-85%), L. monocytogenes (R2-0.84, I2-75%), Salmonella (R2-0.69, I2-46%). Finally, global response surface models (RSM) were developed to further study the non-linear effect of aw and temperature on inactivation. The fit of these models varied by organisms from R2 0.88 (E. coli) to 0.35 (fungi). Further dividing the Salmonella data into individual RSM models based on matrix structure improved model fit to R2 0.90 (paste-like products) and 0.48 (powder-like products). This indicates a negative relationship between data diversity and model performance.

Practice and Progress: Updates on Outbreaks, Advances in Research, and Processing Technologies for Low-moisture Food Safety

Large, renowned outbreaks associated with low-moisture foods (LMFs) bring to light some of the potential, inherent risks that accompany foods with long shelf lives if pathogen contamination occurs. Subsequently, in 2013, Beuchat et al. (2013) noted the increased concern regarding these foods, specifically noting examples of persistence and resistance of pathogens in low-water activity foods (LWAFs), prevalence of pathogens in LWAF processing environments, and sources of and preventive measures for contamination of LWAFs. For the last decade, the body of knowledge related to LMF safety has exponentially expanded. This growing field and interest in LMF safety have led researchers to delve into survival and persistence studies, revealing that some foodborne pathogens can survive in LWAFs for months to years. Research has also uncovered many complications of working with foodborne pathogens in desiccated states, such as inoculation methods and molecular mechanisms that can impact pathogen survival and persistence. Moreover, outbreaks, recalls, and developments in LMF safety research have created a cascading feedback loop of pushing the field forward, which has also led to increased attention on how industry can improve LMF safety and raise safety standards. Scientists across academia, government agencies, and industry have partnered to develop and evaluate innovate thermal and nonthermal technologies to use on LMFs, which are described in the presented review. The objective of this review was to describe aspects of the extensive progress made by researchers and industry members in LMF safety, including lessons-learned about outbreaks and recalls, expansion of knowledge base about pathogens that contaminate LMFs, and mitigation strategies currently employed or in development to reduce food safety risks associated with LMFs.

Interlaboratory Evaluation of Enterococcus faecium NRRL B-2354 as a Salmonella Surrogate for Validating Thermal Treatment of Multiple Low-Moisture Foods

ABSTRACT

This multi-institutional study assessed the efficacy of Enterococcus faecium NRRL B-2354 as a nonpathogenic Salmonella surrogate for thermal processing of nonfat dry milk powder, peanut butter, almond meal, wheat flour, ground black pepper, and date paste. Each product was analyzed by two laboratories (five independent laboratories total), with the lead laboratory inoculating (E. faecium or a five-strain Salmonella enterica serovar cocktail of Agona, Reading, Tennessee, Mbandaka, and Montevideo) and equilibrating the product to the target water activity before shipping. Both laboratories subjected samples to three isothermal treatments (between 65 and 100°C). A log-linear and Bigelow model was fit to survivor data via one-step regression. On the basis of D80°C values estimated from the combined model, E. faecium was more thermally resistant (P < 0.05) than Salmonella in nonfat dry milk powder (DEf-80°C, 100.2 ± 5.8 min; DSal-80°C, 28.9 ± 1.0 min), peanut butter (DEf-80°C, 133.5 ± 3.1 min; DSal-80°C, 57.6 ± 1.5 min), almond meal (DEf-80°C, 34.2 ± 0.4 min; DSal-80°C, 26.1 ± 0.2 min), ground black pepper (DEf-80°C, 3.2 ± 0.8 min; DSal-80°C, 1.5 ± 0.1 min), and date paste (DEf-80°C, 1.5 ± 0.0 min; DSal-80°C, 0.5 ± 0.0 min). Although the combined laboratory D80°C for E. faecium was lower (P < 0.05) than for Salmonella in wheat flour (DEf-80°C, 9.4 ± 0.1 min; DSal-80°C, 10.1 ± 0.2 min), the difference was ∼7%. The zT values for Salmonella in all products and for E. faecium in milk powder, almond meal, and date paste were not different (P > 0.05) between laboratories. Therefore, this study demonstrated the impact of standardized methodologies on repeatability of microbial inactivation results. Overall, E. faecium NRRL B-2354 was more thermally resistant than Salmonella, which provides support for utilizing E. faecium as a surrogate for validating thermal processing of multiple low-moisture products. However, product composition should always be considered before making that decision.

HIGHLIGHTS

Recent developments in low-moisture foods: microbial validation studies of thermal pasteurization processes

Outbreaks associated with low-moisture foods (e.g., wheat flour, nuts, and cereals) have urged the development of novel technologies and re-validation of legacy pasteurization process. For various thermal pasteurization processes, they share same scientific facts (e.g., bacterial heat resistance increased at reduced water activity) and guidelines. However, they also face specific challenges because of their different heat transfer mechanisms, processing conditions, or associated low-moisture foods' formulations. In this article, we first introduced the general structural for validating a thermal process and the shared basic information that would support our understanding of the key elements of each thermal process. Then, we reviewed the current progress of validation studies of 7 individual heating technologies (drying roasting, radiofrequency-assisted pasteurization, superheated steam, etc.) and the combined treatments (e.g., infrared and hot air). Last, we discussed knowledge gaps that require more scientific data in the future studies. We aimed to provide a process-centric view point of thermal pasteurization studies of low-moisture foods. The information could provide detailed protocol for process developers, operators, and managers to enhance low-moisture foods safety.

Survival of Escherichia coli O157:H7 during Moderate Temperature Dehydration of Plant-Based Foods

The effect of moderate-temperature (≤60 °C) dehydration of plant-based foods on pathogen inactivation is unknown. Here, we model the reduction of E. coli O157:H7 as a function of product-matrix, a_w, and temperature under isothermal conditions. Apple, kale, and tofu were each adjusted to a_w 0.90, 0.95, or 0.99 and inoculated with an E. coli O157:H7 cocktail, followed by isothermal treatment at 49, 54.5, or 60.0 °C. The decimal reduction time, or D-value, is the time required at a given temperature to achieve a 1 log reduction in the target microorganism. Modified Bigelow-type models were developed to determine D-values which varied by product type and a_w level, ranging from 3.0–6.7, 19.3–55.3, and 45.9–257.4 min. The relative impact of a_w was product dependent and appeared to have a non-linear impact on D-values. The root mean squared errors of the isothermal-based models ranged from 0.75 to 1.54 log CFU/g. Second, we performed dynamic drying experiments. While the isothermal results suggested significant microbial inactivation might be achieved, the dehydrator studies showed that the combination of low product temperature and decreasing a_w in the pilot-scale system provided minimal inactivation. Pilot-scale drying at 60 °C only achieved reductions of 3.1 ± 0.8 log in kale and 0.67 ± 0.66 log in apple after 8 h, and 0.69 ± 0.67 log in tofu after 24 h. This illustrates the potential limitations of dehydration at ≤60 °C as a microbial kill step.

Invited review: Stress resistance of Cronobacter spp. affecting control of its growth during food production

Members of the Cronobacter genus include food-borne pathogens that can cause infections in infants, with a mortality rate as high as 40 to 80%. The high fatality rate of Cronobacter and its isolation from numerous types of food, especially from powdered infant formula, demonstrate the serious nature of this organism. The source tracking of Cronobacter spp. and the analysis of high-frequency species from different sources are helpful for a more targeted control. Furthermore, the persistence during food processing and storage may be attributed to strong resistance of Cronobacter spp. to environment stresses such as heat, pH, and desiccation. There are many factors that support the survival of Cronobacter spp. in harsh environments, such as some genes, regulatory systems, and biofilms. Advanced detection technology is helpful for the strict monitoring of Cronobacter spp. In addition to the traditional heat treatment, many new control techniques have been developed, and the ability to control Cronobacter spp. has been demonstrated. The control of this bacteria is required not only during manufacture, but also through the selection of packaging methods to reduce postprocessing contamination. At the same time, the effect of inactivation methods on product quality and safety must be considered. This review considers the advances in our understanding of environmental stress response in Cronobacter spp. with special emphasis on its implications in food processing.

Heating of milk powders at low water activity to 95°C for 15 minutes using hot air-assisted radio frequency processing achieved pasteurization

Salmonella persistence in milk powders has caused several multistate foodborne disease outbreaks. Therefore, ways to deliver effective thermal treatment need to be identified and validated to ensure the microbial safety of milk powders. In this study, a process of hot air-assisted radio frequency (HARF) followed by holding at high temperatures in a convective oven was developed for pasteurization of milk powders. Heating times were compared between HARF and a convection oven for heating milk powders to a pasteurization temperature, and HARF has been shown to considerably reduce the come-up time. Whole milk powder (WMP) and nonfat dry milk (NFDM) were inoculated with a 5-serotype Salmonella cocktail and equilibrated to a water activity of 0.10 to simulate the worst case for the microbial challenge study. After heating the sample to 95°C using HARF, followed by 10 and 15 min of holding in the oven, more than 5 log reduction of Salmonella was achieved in WMP and NFDM. This study validated a HARF-assisted thermal process for pasteurization of milk powder based on previously collected microbial inactivation kinetics data and provides valuable insights to process developers to ensure microbial safety of milk powder. This HARF process may be implemented in the dairy industry to enhance the microbial safety of milk powders.

Antibacterial composition of bioautographic fractions, characteristics, and stability of Carica papaya seed extract

The present work aimed to evaluate the potential of Carica papaya seed extract (CPSE) as an antibacterial agent against Salmonella Enteritidis, Bacillus cereus, Vibrio vulnificus, and Proteus mirabilis. The bioautography of the CPSE on thin-layer chromatography (TLC) plates was performed, followed by fractionation of the CPSE by column chromatography using hexane:ethyl acetate (1:1) eluent. Minimum Inhibitory Concentration (MIC), toxicity, composition, and stability of the crude, fractions, and sub-fractions of the CPSE were evaluated. The bioautographic fractions of the CPSE at MIC of 5.63 mg/mL had shown that hexane:ethyl acetate (1:1) fraction and its sub-fraction 3C (Rf = 0.94 ± 0.03) demonstrated the equivalent MIC value (5.63 mg/mL) with the crude CPSE. However, the hexane:ethyl acetate (1:1) fraction and sub-fraction 3C had higher toxicity (LC50 = 1.797 ± 0.305 and 0.332 ± 0.059 mg/mL, respectively) than the crude CPSE (LC50 = 5.505 ± 0.718 mg/mL). Thus, only the crude CPSE was subjected to stability study. The dominant cis-vaccenic acid in sub-fraction 3C demonstrated the lowest MIC against B. cereus (1.41 mg/mL), P. mirabilis (1.41 mg/mL), and S. Enteritidis (0.70 mg/mL) in its pure form. Hence, these results signified the potency of the cis-vaccenic acid as an antibacterial compound from the CPSE. The stability study of the crude CPSE solution showed that at MIC of 5.63 mg/mL, the crude CPSE solution acted as a potent antibacterial agent in acidic condition (pH 4), water activity (Aw) < 0.950, and temperature < 40°C.

Dynamic modelling of Legionella pneumophila thermal inactivation in water

A predictive mathematical model describing the effect of temperature on the inactivation of Legionella pneumophila in water was developed. Thermal inactivation of L. pneumophila was monitored under isothermal conditions (51 - 61°C). A primary log-linear model was fitted to the inactivation data and the estimated D values ranged from 0.23 to 25.31 min for water temperatures from 61 to 51°C, respectively. The effect of temperature on L. pneumophila inactivation was described using a secondary model, and the model parameters z value and D_ref (D-value at 55°C) were estimated at 5.54°C and 3.47 min, respectively. The developed model was further validated under dynamic temperature conditions mimicking various conditions of water thermal disinfection in plumbing systems. The results indicated that the model can satisfactorily predict thermal inactivation of the pathogen at dynamic temperature environments and effectively translate water temperature profiles to cell number reduction. The application of the model in combination with effective temperature monitoring could provide the basis of an integrated preventive approach for the effective control of L. pneumophila in plumbing systems.

Thermal Resistance of Single Strains of Shiga Toxin-Producing Escherichia coli O121:H19 and O157:H7 Based on Culture Preparation Method and Osmolyte-Reduced Water Activity

ABSTRACT

Bacterial exposure to stress, such as reduced water activity (aw), can increase thermal resistance. Pathogen thermal resistance studies on low-aw foods use a variety of methods to inoculate food, as well as strategies to reduce aw, which can influence observations. This study investigated effects of culture preparation method and osmolyte-induced aw on thermal resistance of two Shiga toxin-producing Escherichia coli (STEC) strains (O121:H19 and O157:H7) challenged with isothermal conditions, determining D- and z-values for each isolate (56, 59, and 62°C). Tryptic soy broth (TSB) and agar (lawn cultures) were compared. D-values of broth cultures were significantly and consistently larger than those of lawn cultures, and O121 was significantly more resistant than O157, but only at 56°C (P < 0.05). To compare potential effects of aw on STEC thermal resistance, cells were suspended in osmolyte solutions with varying aw: high (TSB, aw 0.99), intermediate (61% glycerol or 26% NaCl, aw 0.75), and low (82% glycerol, aw 0.5). In most instances, STEC strains in high-aw broth exhibited greater heat resistance compared to reduced-aw solutions, with the exception of the glycerol intermediate-aw solution (aw 0.75). Magnitudes varied with strain and temperature. The z-values of lawn cultures were significantly lower than those of broth cultures (P < 0.05), but there were few differences between high-aw and reduced-aw samples. There were no significant differences of z-values based on strain type. These results highlight that thermal resistance can be affected by culture preparation and that osmolyte-induced changes to aw influence thermal inactivation of STEC by varying magnitudes. These results emphasize the challenges of extrapolating results from laboratory inactivation kinetic experiments to determine the inactivation of low-aw foods, especially those considered dry in nature.

HIGHLIGHTS

Survivability of Salmonella Pathogens and Physicochemical Characteristics of Powder Goat Milk Stored under Different Storage Treatment Regimens

Survivability of Salmonella pathogens in commercial powdered goat milk (PGM) under different storage treatments was investigated using three batches of PGM products stored at two temperatures (4 °C and 25 °C) and ten storage periods (0, 3, 7, 14, 21, 30, 60, 90, 120 and 180 days). A cocktail of three Salmonella serotypes (Salmonella agona, Salmonella enteritidis and Salmonella tennessee) was inoculated to the PGM samples and then survival of Salmonella counts was enumerated in the inoculated and non-inoculated control groups. Results showed that the initial Salmonella counts were 7.103 Log CFU (colony forming unit)/g at both temperatures. At the first 3 days, the viable Salmonella counts were reduced about 0.94 and 1.40 Log CFU/g at 4 °C and 25 °C, respectively, where the same levels were sustained for 14 days. Further reductions continued and at the end of 180 days storage, Salmonella survivability was 1.15 Log CFU/g higher at 4 °C than at 25 °C under the same water activity condition. As the storage period advanced, viable pathogen counts were gradually decreased. The pH of samples stored at 4 °C for 0 and 4 month were higher than those stored at 25 °C except for 2 months, while no differences were found in water activity (aw) between treatments of the PGM products. With regard to physicochemical characteristics, the samples stored at 25 °C showed higher POV (peroxide value) values than those stored at 4 °C for 2 and 4 month periods, indicating that the rate of lipid oxidation in the PGM was elevated by a higher storage temperature and a longer storage period. The basic nutrient compositions of the experimental PGM were similar to those reported in recent studies. Oleic acid (C18:1) was the highest, caprylic acid (C8:0) was the second highest, and behenic acid (C22:0) was the lowest concentration among all fatty acids identified in the PGM samples. Most of the fatty acid concentrations tended to decrease with advanced storage periods. This research indicates that the survivability of Salmonella pathogens in the PGM products stored at 4 °C for 180 days was higher than those stored at 25 °C under the same aw condition.

Thermal inactivation of Salmonella, Shiga toxin-producing Escherichia coli, Listeria monocytogenes, and a surrogate (Pediococcus acidilactici) on raisins, apricot halves, and macadamia nuts using vacuum-steam pasteurization

Salmonella, Shiga toxin-producing Escherichia coli (STEC), and Listeria monocytogenes have been isolated from low water activity foods (LWAF), where they may survive for extended periods. The ready-to-eat nature of many LWAF, such as dried fruits and nuts, warrants effective post-harvest thermal treatment for the reduction of pathogens such as low-temperature, saturated steam, also known as vacuum-assisted steam pasteurization. The objective of this study was to determine reductions of Salmonella, STEC, L. monocytogenes, and a possible surrogate (Pediococcus acidilactici) on dried apricot halves, whole macadamia nuts, and raisins after treatment with vacuum-assisted steam at three temperatures (62 °C, 72 °C, or 82 °C) and multiple time intervals. Bacterial inactivation was variable between commodities, with higher temperatures and longer times necessary to achieve comparable reductions of pathogens on apricot halves and macadamia nuts compared to raisins. Reductions of the tested pathogens were comparable; therefore, one species was not more resistant than the others. Pathogens were reduced by 5-log CFU/g on apricot halves after 20 min at 72 °C and after 5 min at 82 °C. Longer treatment times were necessary to achieve reductions of each pathogen on macadamia nuts. Pathogens were reduced by nearly 5 log CFU/g on macadamia nuts after 38 min at 72 °C (4.6-6.5 log CFU/g) and after 12 min at 82 °C (4.9-5.7 log CFU/g). Reductions of pathogens on raisins were achieved at lower temperatures than necessary for the other foods. A 5-log reduction for each of the pathogens (CFU/g) on raisins occurred after 20 min at 62 °C and after 5 min at 72 °C. Overall, the reductions of the pathogens exceeded those of P. acidilactici on both the dried fruits and macadamia nuts. Statistically significant differences, indicating greater confidence as a conservative surrogate, were observed at lower treatment temperatures. Inactivation kinetics were modeled for each pathogen on each food type and temperature. Bacterial survival was best described by the Weibull model for raisins and macadamia nuts, while the Gompertz model best described reductions on apricot halves according to Akaike information criterion (AIC) and root-mean-square error (RMSE) evaluations. Water activity and moisture content were increased due to the treatments, which could be addressed through implementation of drying steps. Thermal inactivation kinetic models and 5-log reduction parameters can help food processors design and evaluate similar vacuum-assisted steam interventions to comply with FSMA regulations and preventive control plans. However, results or model predictions should not be extrapolated to assume the safety of other types of foods.

Functional characteristics and microbiological viability of foam-mat dried Bambara groundnut (Vigna subterranea) yogurt from reconstituted Bambara groundnut milk powder

The functional, nutritional, and physical characteristics of foam-mat dried Bambara groundnut (Vigna subterranea) yogurt were investigated. Bambara groundnut powdered yogurt (BGNPY) was produced using Bambara groundnut milk powder (BGNMP) and Bambara groundnut milk (BGNM). BGNMP was reconstituted with water (1:5). The reconstituted BGN milk (BGNM-R) and original nonreconstituted BGNM (BGNM-NR) were inoculated with normal yogurt culture while held at a temperature of 45°C and incubated for 24 hr at 35°C. The BGN yogurts were dried employing the foam-mat drying process with gum arabic (6%) and methylcellulose (0.5%) as foaming agents and dried at 50°C for 24 hr. The BGN powdered yogurt from reconstituted milk (BGNPY-RM) and BGN powdered yogurt from nonreconstituted milk (BGNPY-NRM) were evaluated for functional, nutritional, thermal, and physical characteristics. Water absorption (1.27 and 1.31 g/g) and water solubility (73.3. and 71.22 100/g) index of the powdered yogurts did not differ significantly, while a significant (p < 0.05) difference was observed for the Tg of BGNPY-R and BGNPY-NR. Nutrient composition of BGNPY-R and BGNPY-NR had no significant (p > 0.05) difference, while ash differed significantly (p < 0.05). Particle size and particle size distribution of BGNPY-R and BGNPY-NR had no significant (p > 0.05) difference. Probiotic viability of BGNPY-R (7.2 log cfu/ml) remained above the minimum recommended dosage (6 log cfu/ml).

Influence of water activity on the heat resistance of Salmonella enterica in selected low-moisture foods

Low-moisture foods (LMF with water activity, a_w < 0.85) including pet foods and black pepper powder have consistently been associated with foodborne disease caused by Salmonella enterica. Increased heat resistance and prolonged survival at low-moisture conditions, however, remain major challenges to achieve effective inactivation of Salmonella in low-moisture foods. At low water activity (a_w) conditions, heat resistance of Salmonella is greatly enhanced when compared to high a_w conditions. This study aimed to quantify the effect of a_w on the heat resistance of Salmonella enterica in pet food pellets and black pepper powder. Pet food pellets were inoculated with two strains of heat resistant S. enterica and black pepper powder was inoculated with a 5-strain cocktail of Salmonella. Both inoculated food samples were equilibrated at 0.33, 0.54, and 0.75 a_w in controlled humidity chambers. Inoculated pet food pellets and black pepper powder in closed aluminum cells were heat treated at specific temperatures for selected times. The results showed that the Weibull model fitted well the inactivation data. At a specific temperature, the rate of inactivation increased with the increase in the a_w from 0.33 to 0.75, and the 3-log reduction times decreased for Salmonella in both food samples with the increase in a_w. Water adsorption isotherms of pet food pellets and black pepper powder at initial and treatment temperatures were developed to understand the change in a_w during heat treatments. The change in a_w during heat treatment was dependent on the type of food matrix, which possibly influenced the thermal inactivation of Salmonella in pet food pellets and black pepper powder. The quantitative analysis of heat reduction of Salmonella with respect to a_w aids in selection of the appropriate initial a_w to develop effective heat treatment protocols for adequate reduction of Salmonella in pet foods and black pepper powder.

Effect of Talc as a Dry-Inoculation Carrier on Thermal Resistance of Enterococcus faecium NRRL B-2354 in Almond Meal

HIGHLIGHTS

E. faecium was more thermally resistant in dry- than in wet-inoculated almond meal. Presence of talc affected thermal resistance of E. faecium in almond meal. Use of dry inoculum carriers for thermal validation studies requires further work.

Quantifying the Responses of Three Bacillus cereus Strains in Isothermal Conditions and During Spray Drying of Different Carrier Agents

Spray drying is a widely used method for producing milk powder. This process is not aimed to cause microbial inactivation, thus sporeforming bacteria may be abundant in the microbiota of milk powder. The first aim of this study was to determine the inactivation kinetics parameters in capillary tubes of three Bacillus cereus strains (436, B63, 540) in three menstrua (whole milk, phosphate buffer, and talc suspension) at 90, 100, and 110°C. D-values for B. cereus in the three menstrua were not significantly different at the highest tested temperature (p > 0.05). Thus, talc was chosen as a carrier agent to allow the recovery of B. cereus from spray dried materials given its low interference on inactivation kinetics. B. cereus spores were also inoculated in whole milk and skim milk following spray drying at 95, 105, and 110°C (outlet temperature). After the spray drying runs, B. cereus spores were counted and the number of decimal reductions (γ) calculated. A correlation between the small diameter of the particles with the survival of spores of three B. cereus strains was found, and B. cereus 436 presented consistently the lowest γ no matter temperature and a carrier agent. The highest γ was found when talc powder was used, which suggest that this carrier agent does not protect B. cereus spores during spray drying. Spray drying of milk can lead to up to 4 γ (strain 540) of B. cereus spores but depending on the strain less than one γ (strain 436) could be observed. This study contributes to the knowledge on the microbiology of low water activity foods by providing novel findings regarding the fate of three B. cereus strains to different spray drying conditions. Acknowledging the variability of inactivation of B. cereus during spray drying is key in the current context of food safety in which the quantification of effects of unit operations must be known for the validation of processes and development of more robust formulations.

Cellular Injuries in Cronobacter sakazakii CIP 103183T and Salmonella enterica Exposed to Drying and Subsequent Heat Treatment in Milk Powder

Because of the ability of foodborne pathogens to survive in low-moisture foods, their decontamination is an important issue in food protection. This study aimed to clarify some of the cellular mechanisms involved in inactivation of foodborne pathogens after drying and subsequent heating. Individual strains of Salmonella Typhimurium, Salmonella Senftenberg, and Cronobacter sakazakii were mixed into whole milk powder and dried to different water activity levels (0.25 and 0.58); the number of surviving cells was determined after drying and subsequent thermal treatments in closed vessels at 90 and 100°C, for 30 and 120 s. For each condition, the percentage of unculturable cells was estimated and, in parallel, membrane permeability and respiratory activity were estimated by flow cytometry using fluorescent probes. After drying, it was clearly observable that the percentage of unculturable cells was correlated with the percentage of permeabilized cells (responsible for 20–40% of the total inactivated bacteria after drying), and to a lesser degree with the percentage of cells presenting with loss of respiratory activity. In contrast, the percentages of unculturable cells observed after heat treatment were strongly correlated with the loss of respiratory activity and weakly with membrane permeability (for 70–80% of the total inactivated bacteria after heat treatment). We conclude that cell inactivation during drying is closely linked to membrane permeabilization and that heat treatment of dried cells affects principally their respiratory activity. These results legitimize the use of time–temperature scales and allow better understanding of the cellular mechanisms of bacterial death during drying and subsequent heat treatment. These results may also allow better optimization of the decontamination process to ensure food safety by targeting the most deleterious conditions for bacterial cells without denaturing the food product.