Survival of Salmonella senftenberg 775W to current liquid whole egg pasteurization treatments

Effects of Sequential Combination of Moderate Pressure and Ultrasound on Subsequent Thermal Pasteurization of Liquid Whole Egg

As an alternative to commercial whole egg thermal pasteurization (TP), the sequential combination of moderate pressure (MP) and/or ultrasound (US) pre-treatments prior to a shorter TP was evaluated. The use of US alone or in combination with MP or TP resulted in an inactivation that was far from that of commercial TP. Nevertheless, when these three technologies were combined (MP-US-TP, 160 MPa/5 min-50% amplitude/1 min-60 °C/1.75 min), a safety level comparable to that of commercial TP was established. This was likely due to a decrease in the thermal resistance of Salmonella Senftenberg 775/W caused by MP and US pre-treatments. Regarding liquid whole egg (LWE) properties, using raw LWE as a reference, TP and MP treatments each decreased protein solubility (7-12%), which was accompanied by a viscosity increment (41-59%), whereas the US-only and MP-US-TP treatments improved protein solubility (about 4%) and reduced viscosity (about 34%). On average, all treatments lowered the emulsifying properties of LWE by 35-63%, with the MP-US-TP treatment having a more dramatic impact than commercial TP. In addition, the US-only, MP-only, and MP-US-TP treatments had the greatest impact on the volatile profile of LWE, lowering the concentration of the total volatile components. In comparison to commercial TP, LWE treated with MP-US-TP exhibited greater protein solubility (19%), lower viscosity (56%), and comparable emulsifying stability, but with a decreased emulsifying capacity (39%) and a lower total volatile compounds content (77%). Considering that a combined treatment (MP-US-TP) is lethally equivalent to commercial TP, but the latter better retained the quality properties of raw LWE, including volatiles, the application of MP followed by US pre-treatments before a shorter TP did not demonstrate significant advantages on quality parameters in comparison to commercial TP.

Experimentally Implementing the Linear Nonisothermal Equation for Simultaneously Obtaining D- and z-Values of Salmonella Senftenberg in Skim Milk with a Differential Scanning Calorimeter

ABSTRACT

For bacteria with log-linear thermal inactivation kinetics in food, D-values are obtained in multiple isothermal inactivation experiments at different temperatures, and the z-value is obtained from these D-values. In a previous work, the cumulative lethality integral was mathematically solved in closed form when temperature in the food increased linearly with time. The solution revealed that each nonisothermal experiment could yield both D- and z-values, eliminating the need for getting multiple D-values to get a z-value. The present study reports on the first experimental implementation of this method of obtaining D- and z-values for Salmonella Senftenberg suspended in skim milk for which a differential scanning calorimeter (DSC) provided the required constant heating rate. The resulting D- and z-values were compared with those obtained from an isothermal method with capillary tubes. No significant differences in z-values were found between the two methods. The D-values also agreed but only after correcting the nonisothermal value for temperature lag in the DSC caused by the large sample size required. A 5 K/min heating rate was used in this comparison. Other rates were also investigated: 1, 3, 7.5, and 10 K/min. Although D- and z-values should be independent of DSC heating rate, heating rates of 1 and 10 K/min yielded values that were significantly different from the others; therefore, these rates cannot be recommended for use in this nonisothermal method.

HIGHLIGHTS

Effect of moderate thermal treatments on the inactivation of a strain of Listeria monocytogenes and physicochemical properties of soursop pulp

Soursop (Annona muricata L.) is a commercially important tropical fruit, whether fresh or processed as a pasteurized or frozen pulp used to prepare juice, drinks, nectar, ice cream, popsicles, and desserts. Besides preserving quality, another preoccupation in the processing of fruit pulps is product safety. Several studies show the association between pulp processing and the development of various microorganisms; however, few have focused on the association between L. monocytogenes and the pulp of sour fruits. The objective was to evaluate the effect of moderate thermal treatments on the inactivation of L. monocytogenes and the physicochemical properties in soursop pulp in order to determine the best processing conditions that will allow to maintain quality as well as to achieve an adequate level of safety. Thermal inactivation kinetics were obtained for L. monocytogenes inoculated in soursop pulp at five levels of temperature (50, 52.5, 55, 57.5, and 60 ℃) and different exposure times (0-60 min). The survival curves did not suggest a log-linear relationship, and were, consequently, fitted to the modified Gompertz equation. The results indicated that the modified Gompertz equation provided an acceptable goodness of fit. Five-log₁₀ cycles reductions of L. monocytogenes were achieved at 50 ℃/60 min, 52.5 ℃/16 min, 55 ℃/10 min, 57.5 ℃/5 min, and 60 ℃/1.25 min. These 5-log₁₀ treatments applied to the soursop pulp indicated that the soursop pulp showed changes in the color parameters and a decrease in the content of total sugars, reducing sugars, ascorbic acid, total phenols, and pH.

Nanoemulsified D-Limonene Reduces the Heat Resistance of Salmonella Senftenberg over 50 Times

Salmonella Senftenberg is a pathogen agent causative of foodborne disease and it is considered the most heat-resistant serovar within this genus. Food industries use heat treatment and chemical antimicrobials in order to eliminate this microorganism in food, but consumers prefer natural antimicrobials as essential oils and their components. This study evaluates the combined effect of thermal treatments and different concentrations of D-limonene nanoemulsion on the inactivation of Salmonella (S.) Senftenberg. The results showed an important effect of the nanoemulsified D-limonene on the heat resistance of S. Senftenberg. The δ_{50 °C} value was reduced by 85%, 96% and 98% when 0.1, 0.5 and 1 mM of nanoemulsified D-limonene was added to the heating medium. The effect was kept along all the heating temperatures researched and the shape of the survival curves did not change with the addition of the antimicrobial. The results obtained in this research could be very useful for food industries for optimizing or improving heat treatments applied to food.

The locus of heat resistance (LHR) mediates heat resistance in Salmonella enterica, Escherichia coli and Enterobacter cloacae

Enterobacteriaceae comprise food spoilage organisms as well as food-borne pathogens including Escherichia coli. Heat resistance in E. coli was attributed to a genomic island called the locus of heat resistance (LHR). This genomic island is also present in several other genera of Enterobacteriaceae, but its function in the enteric pathogens Salmonella enterica and Enterobacter cloacae is unknown. This study aimed to determine the frequency of the LHR in food isolates of E. coli, and its influence on heat resistance in S. enterica and Enterobacter spp. Cell counts of LHR-positive strains of E. coli, S. enterica and E. cloacae were reduced by less than 1, 1, and 4 log (cfu/mL), respectively, after exposure to 60 °C for 5 min, while cell counts of LHR-negative strains of the same species were reduced by more than 7 log (cfu/mL). Introducing an exogenous copy of the LHR into heat-sensitive enteropathogenic E. coli and S. enterica increased heat resistance to a level that was comparable to LHR-positive wild type strains. Cell counts of LHR-positive S. enterica were reduced by less than 1 log(cfu/mL) after heating to 60 °C for 5 min. Survival of LHR-positive strains was improved by increasing the NaCl concentration from 0 to 4%. Cell counts of LHR-positive strains of E. coli and S. enterica were reduced by less than 2 log (cfu/g) in ground beef patties cooked to an internal core temperature of 71 °C. This study indicates that LHR-positive Enterobacteriaceae pose a risk to food safety.

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.

Salmonella enterica: survival, colonization, and virulence differences among serovars

Data indicate that prevalence of specific serovars of Salmonella enterica in human foodborne illness is not correlated with their prevalence in feed. Given that feed is a suboptimal environment for S. enterica, it appears that survival in poultry feed may be an independent factor unrelated to virulence of specific serovars of Salmonella. Additionally, S. enterica serovars appear to have different host specificity and the ability to cause disease in those hosts is also serovar dependent. These differences among the serovars may be related to gene presence or absence and expression levels of those genes. With a better understanding of serovar specificity, mitigation methods can be implemented to control Salmonella at preharvest and postharvest levels.

Combination of pulsed electric fields, mild heat and essential oils as an alternative to the ultrapasteurization of liquid whole egg

The production of microbiologically safe liquid whole egg (LWE) by industrial ultrapasteurization is restricted by the high thermal sensitivity of LWE components. This research proposes an alternative treatment based on the application of pulsed electric fields (PEF) and mild heat, in the presence of natural essential oils (EOs) or their individual components (ICs). The obtained results indicate that the successive application of PEF (25kV/ and 100kJ/kg) followed by heat (60°C during 3.5') to LWE added with 200μL/L of lemon EO would reach 4log10cycles of inactivation of Salmonella Senftenberg 775W and Listeria monocytogenes, when any of these barriers acting alone inactivated less than 1.5log10cycles of either bacteria. Therefore, the synergism between lemon EO and the successive application of PEF and heat would provide a safety level similar to that of ultrapasteurization treatment for Salmonella Senftenberg 775W and L. monocytogenes, but at a lower temperature. To a lesser extent, synergism with the successive application of PEF and heat was also observed in the presence of 200μL/L of carvacrol, citral, (+)-limonene, or mandarin EO, reaching about 3.5log10cycles of inactivation in Salmonella Senftenberg and 3.0log10cycles in L. monocytogenes, respectively. A sensory test on LWE containing 200μL/L of each additive in the form of omelets and sponge cakes revealed that this concentration of mandarin EO, lemon EO, or (+)-limonene did not decrease the sensory acceptability of the LWE-containing products, and lemon EO and mandarin EO even increased the hedonic acceptability of sponge cakes. In conclusion, this process could be applied in the food industry to obtain microbiologically safe LWE, which could be used to produce egg-based products without decreasing (and even increasing) their sensory appeal.

Combined effect of nisin, carvacrol and a previous thermal treatment on the growth of Salmonella enteritidis and Salmonella senftenberg

The aim of this study was to evaluate the combined effect of a previous mild heat treatment (15 min at 55 ) with the use of antimicrobials, nisin and carvacrol, on the growth of Salmonella enteritidis and Salmonella senftenberg. Natural antimicrobials, alone or combined with a previous mild heat treatment, affected the growth of these two serovars in Tryptone Soy Broth at 37 . Increasing concentrations of carvacrol had a significant effect on both growth rate and lag phase duration of both strains. The time to reach stationary phase was almost doubled in the case of S. enteritidis when a concentration of 0.77 mM in carvacrol was added. For S. senftenberg the effect was smaller. The effect of nisin and of heat, applied individually, was lower for both microorganisms. A combination of 1.2 µM nisin with 0.77 mM carvacrol significantly delayed the growth of heat treated cells, compared to the control without antimicrobials, showing additive effects.

Predictive thermal inactivation model for the combined effect of temperature, cinnamaldehyde and carvacrol on starvation-stressed multiple Salmonella serotypes in ground chicken

We investigated the combined effect of three internal temperatures (60, 65 and 71.1 °C) and four concentrations (0.0, 0.1, 0.5 and 1% vol/wt) of two natural antimicrobials on the heat resistance of an eight-strain cocktail of Salmonella serovars in chicken meat. A complete factorial design (3×4×4) was used to assess the effects and interactions of heating temperature and the two antimicrobials, carvacrol and cinnamaldehyde. The 48 variable combinations were replicated to provide a total of 96 survivor curves from the experimental data. Mathematical models were then developed to quantify the combined effect of these parameters on heat resistance of starved Salmonella cells. The theoretical analysis shows that the addition of plant-derived antimicrobials overcomes the heat resistance of starvation-stressed Salmonella in ground chicken meat. The influence of the antimicrobials allows reduced heat treatments, thus reducing heat-induced damage to the nutritional quality of ground-chicken products. Although the reported omnibus log-linear model with tail and the omnibus sigmoid model could represent the experimental survivor curves, their discrepancy only became apparent in the present study when lethality times (D-values and t7.0) from each of the models were calculated. Given the concave nature of the inactivation curves, the log-linear model with tail greatly underestimates the times needed to obtain 7.0 log lethality. Thus, a polynomial secondary model, based on the sigmoid model, was developed to accurately predict the 7.0-log reduction times. The three-factor predictive model can be used to estimate the processing times and temperatures required to achieve specific log reductions, including the regulatory recommendation of 7.0-log reduction of Salmonella in ground chicken.

Relative prevalence of Salmonella Sofia on broiler chickens pre- and postprocessing in Australia

A survey was conducted to determine the relative prevalence of Salmonella serovars on whole chicken carcasses before and after processing in 3 Australian poultry abattoirs. Ninety and 180 whole chicken carcasses were tested for Salmonella serovars before and after processing, respectively. Each carcass was subjected to a buffered peptone water rinse according to Australian Standard methodologies and Salmonella prevalence was determined using Australian Standard methodologies. After isolation, Salmonella isolates were serotyped and results were analyzed to determine the relative percentage of each serovar at both processing points. Salmonella Sofia was shown to significantly increase its relative prevalence (P < or = 0.05) after processing and proved to be the dominant serovar accounting for 45/89 (51%) isolations before processing and 51/69 (74%) isolations after processing. The reasons for the increased relative prevalence of Salmonella Sofia are currently unknown and require further investigation but may involve factors related to prevalence and numbers on chickens and the ability of Salmonella Sofia to respond to environmental stressors and attach to surfaces.

Inactivation of Salmonella enterica serovar Senftenberg 775W in liquid whole egg by ultrahigh pressure homogenization

Two batches of samples of liquid whole egg were inoculated with a load of approximately 3 and 7 log CFU/ml, respectively, of Salmonella enterica serovar Senftenberg 775W and submitted to different ultrahigh pressure homogenization (UHPH) treatments at 150, 200, and 250 MPa. The inlet temperature of the samples was 6 degrees C. Counts of viable and injured Salmonella cells were obtained 2 h after the UHPH treatments and after 5, 10, 15, and 20 days of storage at 4 degrees C. The level of pressure applied influenced the lethality attained significantly (P < 0.05). In the samples with an initial load of approximately 7 log CFU/ml, the highest lethality value of 3.2 log CFU/ml was obtained at 250 MPa, and it is similar to those values reported in other surveys for thermal pasteurization with this same Salmonella strain. When the initial load was approximately 3 log CFU/ml, total inactivation was apparently obtained after the 250-MPa treatment (2.7 log CFU/ml). After 10 days of storage at 4 degrees C, Salmonella counts decreased in UHPH-treated samples, and colonies were not observed in tryptone soy agar and yeast extract medium. Nevertheless, presence of viable Salmonella cells was detected with the VIDAS Salmonella immunoassay method during the entire storage period. These results encourage further investigation of UHPH processing of liquid whole egg, assaying the possibility of using higher pressures and fluid inlet temperatures.

Chemical markers for the evaluation of raw material hygienic quality in egg products

The aim of this research was to study uracil and lactic and acetic acids as chemical markers for hygienic quality evaluation of raw material in liquid pasteurized egg products. Uracil, absent in sound whole eggs, was formed in raw and pasteurized egg products as a consequence of high microbial contamination (>10(6) cfu/g) after a sufficient lag time, remaining stable at 4 degrees C but disappearing after 7 days of storage at 25 degrees C. Both lactic and acetic acids, starting from initial values of 1-7 mg/kg dry matter, presented trends similar to that of uracil; however, acetic acid never decreased during the storage of raw egg products. With few exceptions, all three metabolites were produced by Enterobacter cloacae, Escherichia coli, Morganella morganii, Serratia liquefaciens, Aeromonas hydrophyla, Pseudomonas fluorescens, Enterococcus avium, and Enterococcus faecalis, separately inoculated in whole egg samples. Uracil seems to be the most sensible marker, with a suggested limit corresponding to the detectable level.

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.

Inactivation of Salmonella serovars in liquid whole egg by heat following irradiation treatments

Salmonella is a frequent contaminant on eggs and is responsible for foodborne illnesses in humans. Ionizing radiation and thermal processing can be used to inactivate Salmonella in liquid whole egg, but when restricted to doses that do not affect egg quality, these technologies are only partially effective in reducing Salmonella populations. In this study, the effect of ionizing radiation in combination with thermal treatment on the survival of Salmonella serovars was investigated. Of the six Salmonella serovars tested, Salmonella Senftenberg was the most resistant to radiation (Dgamma = 0.65 kGy) and heat (D(55 degrees C) = 11.31 min, z = 4.9 degrees C). Irradiation followed by thermal treatment at 55 or 57 degrees C improved the pasteurization process. Radiation doses as low as 0.1 kGy prior to thermal treatments synergistically reduced the D(55 degrees C) and D(57 degrees C) of Salmonella Senftenberg 3.6- and 2.5-fold, respectively. The D(55 degrees C) and D(57 degrees C) of Salmonella Typhimurium were reduced 2- and 1.4-fold and those of Salmonella Enteritidis were reduced 2- and 1.6-fold, respectively. Irradiation prior to thermal treatment would enable the reduction of heat treatment times by 86 and 30% at 55 and 57 degrees C, respectively, and would inactivate 9 log units of Salmonella serovars.

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.

Inactivation of Salmonella Senftenberg 775W by ultrasonic waves under pressure at different water activities

The inactivation of Salmonella Senftenberg 775W by ultrasonic waves (20 kHz, 117 microm) under pressure (175 kPa) treatments at sublethal (manosonication; MS) and lethal temperatures (manothermosonication; MTS) in media of different water activities has been investigated. Heat decimal reduction time values increased up to eighteen fold when the water activity was decreased from >0.99 to 0.93 at 65 degrees C, but hardly increased the MS resistance. In reduced water activity media (a(w) of 0.96 and 0.93) a synergistic lethal effect was observed between heat and ultrasound under pressure, being the inactivation rate of Salmonella Senftenberg 775W three times faster than the expected additive rate considering an effect of both bacterial lethal processes. An empirical mathematical equation enabled to predict the D(MS) and D(MTS) values obtained at different temperatures and a(w) in the ranges investigated of Salmonella serovars and also the microbial level of inactivation due to the synergistic lethal effect of MTS treatments in media of reduced a(w). This work could be useful for improving sanitation and preservation treatments of foods, especially those in which components protect microorganisms to heat.

Predicting heat inactivation of Listeria monocytogenes under nonisothermal treatments

The aim of this study was to find a model that accurately predicts the heat inactivation of Listeria monocytogenes (ATCC 15313) at constantly rising heating rates (0.5 to 9 degrees C/min) in media of different pH values (4.0 to 7.4). Survival curves of L. monocytogenes obtained under isothermal treatments at any temperature were nearly linear. Estimations of survival curves under nonisothermal treatments obtained from heat resistance parameters of isothermal treatments adequately fit experimental values obtained at pH 4.0. On the contrary, survivors were much higher than estimations at pH 5.5 and 7.4. The slower the heating rate and the longer the treatment time, the greater the differences between the experimental and estimated values. An equation based on the Weibullian-like distribution, log S(t) = (t/delta)p, accurately described survival curves of L. monocytogenes obtained under nonisothermal conditions within the range of heating rates investigated. A nonlinear relationship was observed between the scale parameter (delta) and the heating rate, which allowed the development of an equation capable of predicting the inactivation rate of L. monocytogenes under nonisothermal treatments at pH 5.5 and 7.4. The model predictions were a good fit to the measured data independent of the magnitude of the thermotolerance increase. This work might contribute to the increase in safety of those food products that require long heating lag phases during the pasteurization process.