Note: Use of a Distribution of Frequencies Model to Interpret the Tailed Heat Inactivation Curves of Prions

Inactivation of prions responsible of bovine spongiform encephalytis (BSE) by heat is a major concern for the canned food industry in Europe. Experimental data of heat inactivation of prions obtained from the scientific literature have been modelled. Two models were applied to analyse the inactivation curves, the Weibull distribution model and a two-parameter empirical model. Statistical analysis of available data indicated that the Weibull frequency distribution provided the best description of non-linear survival curves. The effect of the temperature on Weibull model parameters ( a and b) was also studied. The shape parameter, b, indicates deviations from linearity in the inactivation curves, and the scale parameter, a, can be considered as a kinetic constant rate for the inactivation. The values of the b parameter were on the order of 0.15-0.2 which meant that these curves had a strong upper concavity. The a values obtained ranged from 1.8 × 10−3 to 2.3 × 10−6 for temperatures between 100 and 160°C. These results indicate that temperature has a relevant effect on the inactivation of prions, although it remains a considerable fraction of prions with infective capacity. The Weibull frequency distribution model appears as a useful and convenient model because the a parameter could allow a quantitative comparison of the inactivation of prions at different temperatures or under different conditions.

Combined Effect of Nisin, Carvacrol and Thymol on the Viability of Bacillus Cereus Heat-Treated Vegetative Cells

The influence of mild heat pretreatment on the bactericidal action of nisin, carvacrol and thymol on stationary and exponential phase cells of two Bacillus cereus strains was studied. Carvacrol or thymol concentrations of 0.3 mmol/L had no bactericidal effect on unheated, and a minor effect on heated B. cereus cells either in stationary or exponential growth phases. Nisin (0.15 mg/mL) alone decreased the viable counts in all cases (unheated and heated cells in stationary and exponential phase) although the two strains tested showed different sensitivity to this natural antimicrobial between them. When carvacrol or thymol were combined with nisin, it resulted in a greater loss of viability of heated cells than when nisin was applied alone. There was a synergistic effect of nisin and both essential oils with a mild heat pretreatment on the viability of B. cereus cells. This study pointed out the potential use of nisin alone and in combination with carvacrol or thymol applied after a mild heat pretreatment for preservation of minimally processed foods.

One-step global parameter estimation of kinetic inactivation parameters for Bacillus sporothermodurans spores under static and dynamic thermal processes

Bacillus sporothermodurans produces highly heat-resistant endospores, that can survive under ultra-high temperature. High heat-resistant sporeforming bacteria are one of the main causes for spoilage and safety of low-acid foods. They can be used as indicators or surrogates to establish the minimum requirements for heat processes, but it is necessary to understand their thermal inactivation kinetics. The aim of the present work was to study the inactivation kinetics under both static and dynamic conditions in a vegetable soup. Ordinary least squares one-step regression and sequential procedures were applied for estimating these parameters. Results showed that multiple dynamic heating profiles, when analyzed simultaneously, can be used to accurately estimate the kinetic parameters while significantly reducing estimation errors and data collection.

Application of artificial neural networks to describe the combined effect of pH and NaCl on the heat resistance of Bacillus stearothermophilus

A model for prediction of bacterial spore inactivation was developed. The influence of temperature, pH and NaCl on the heat resistance of Bacillus stearothermophilus spores was described using low-complexity, black box models based on artificial neural networks. Literature data were used to build and train the neural network, and new experimental data were used to evaluate it. The neural network models gave better predictions than the classical quadratic response surface model in all the experiments tried. When the neural networks were evaluated using new experimental data, also good predictions were obtained, providing fail-safe predictions of D values in all cases. The weights and biases values of neurons of the neural network that gave the best results are presented, so the reader can use the model for their own purposes. The use of this non-linear modelling technique makes it possible to describe more accurately interacting effects of environmental factors when compared with classical predictive microbial models.

The apoplastic antioxidant system in Prunus: response to long-term plum pox virus infection

This work describes, for the first time, the changes taking place in the antioxidative system of the leaf apoplast in response to plum pox virus (PPV) in different Prunus species showing different susceptibilities to PPV. The presence of p-hydroxymercuribenzoic acid (pHMB)-sensitive ascorbate peroxidase (APX) (class I APX) and pHMB-insensitive APX (class III APX), superoxide dismutase (SOD), peroxidase (POX), NADH-POX, and polyphenoloxidase (PPO) was described in the apoplast from both peach and apricot leaves. PPV infection produced different changes in the antioxidant system of the leaf apoplast from the Prunus species, depending on their susceptibility to the virus. In leaves of the very susceptible peach cultivar GF305, PPV brought about an increase in class I APX, POX, NADH-POX, and PPO activities. In the susceptible apricot cultivar Real Fino, PPV infection produced a decrease in apoplastic POX and SOD activities, whereas a strong increase in PPO was observed. However, in the resistant apricot cultivar Stark Early Orange, a rise in class I APX as well as a strong increase in POX and SOD activities was noticed in the apoplastic compartment. Long-term PPV infection produced an oxidative stress in the apoplastic space from apricot and peach plants, as observed by the increase in H2O2 contents in this compartment. However, this increase was much higher in the PPV-susceptible plants than in the resistant apricot cultivar. Only in the PPV-susceptible apricot and peach plants was the increase in apoplastic H2O2 levels accompanied by an increase in electrolyte leakage. No changes in the electrolyte leakage were observed in the PPV-inoculated resistant apricot leaves, although a 42% increase in the apoplastic H2O2 levels was produced. Two-dimensional electrophoresis analyses revealed that the majority of the polypeptides in the apoplastic fluid had isoelectric points in the range of pI 4-6. The identification of proteins using MALDI-TOF (matrix-assisted laser desorption/ionization-time of flight) and peptide mass fingerprinting analyses showed the induction of a thaumatin-like protein as well as the decrease of mandelonitrile lyase in peach apoplast due to PPV infection. However, most of the selected polypeptides showed no homology with known proteins. This fact emphasizes that, at least in Prunus, most of the functions of the apoplastic space remain unknown. It is concluded that long-term PPV infection produced an oxidative stress in the leaf apoplast, contributing to the deleterious effects produced by PPV infection in leaves of inoculated, susceptible Prunus plants.

Estimation of the non-isothermal inactivation patterns of Bacillus sporothermodurans IC4 spores in soups from their isothermal survival data

The isothermal survival curves of the heat resistant spores of Bacillus sporothermodurans IC4, in the range of 117-125 degrees C, were determined in chicken, mushroom and pea soups by the capillary method. They were all non-linear with a noticeable upper concavity and could be described by the equation log(10) [N(t)/N(0)]=-b(T)t(n) with a fixed power, n, of the order of 0.7-0.8. The temperature dependence of b(T) could be described by the equation b(T)=log(e)[1+exp[k(T-T(c))]], where T(c) is the temperature where intensive inactivation starts and k is the slope of b(T) at temperatures well above T(c). They were 121-123 degrees C and 0.2-0.4 degrees C(-1), respectively, depending on the soup. These parameters were used to estimate the survival curves of the spores in two non-isothermal heat treatments using the procedure originally proposed by Peleg and Penchina [Crit. Rev. Food Sci. Nutr. 40 (2000) 159]. The results were compared with experimental survival curves, determined by the direct injection method, in another laboratory. There was a general agreement, although not perfect, between the predicted and experimentally observed survival ratios. Also, the isothermal survival parameters, estimated directly from the non-isothermal inactivation data using the model, were in general agreement with those calculated from the isothermal data. This suggests that the heat inactivation patterns of B. sporothermodurans IC4 spores in soups can be at least roughly estimated using the same general survival model, which has until now only been experimentally validated for vegetative bacterial cells.

Combined effect of nisin and carvacrol at different pH and temperature levels on the viability of different strains of Bacillus cereus

The influence of pH and temperature on the bactericidal action of nisin and carvacrol on vegetative cells of different Bacillus cereus strains was studied. The five strains tested showed significant differences in sensitivity towards nisin, at pH 7.0 and 30 degrees C. Carvacrol concentrations of 0.3 mmol l(-1) had no effect on viability of B. cereus cells. When the same carvacrol concentration was combined with nisin, however, it resulted in a greater loss of viability of cells than when nisin was applied alone. The concentration of carvacrol played an important role on the bactericidal effect of nisin and, therefore, on the synergistic action of both compounds combined. At lower pH values (6.30 and 5.75), nisin was more active against B. cereus cells than at pH 7.0 at 30 degrees C, with a different sensitivity of the strains tested. The combined effect of nisin and carvacrol was found to be significantly different at pH 7.0 and 5.75. When the temperature was 8 degrees C, nisin was significantly less active against B. cereus IFR-NL 94-25 than at 30 degrees C, both at pH 7.0 and 6.30. At 8 degrees C, there was a significant increased effect of nisin at lower pH values. Also at this low temperature, a synergistic effect between nisin and carvacrol on B. cereus cells was observed at the pHs tested. This study indicates the potential of nisin and carvacrol at lower pHs to be used for preservation of minimally processed foods.

Risk analysis of the thermal sterilization process. Analysis of factors affecting the thermal resistance of microorganisms

A risk analysis was applied to experimental heat resistance data. This analysis is an approach for processing experimental thermobacteriological data in order to study the variability of D and z values of target microorganisms depending on the deviations range of environmental factors, to determine the critical factors and to specify their critical tolerance. This analysis is based on sets of sensitivity functions applied to a specific case of experimental data related to the thermoresistance of Clostridium sporogenes and Bacillus stearothermophilus spores. The effect of the following factors was analyzed: the type of target microorganism; nature of the heating substrate; pH, temperature; type of acid employed and NaCl concentration. The type of target microorganism to be inactivated, the nature of the substrate (reference or real food) and the heating temperature were identified as critical factors, determining about 90% of the alteration of the microbiological risk. The effect of the type of acid used for the acidification of products and the concentration of NaCl can be assumed to be negligible factors for the purposes of engineering calculations. The critical non-uniformity in temperature during thermobacteriological studies was set as 0.5% and the critical tolerances of pH value and NaCl concentration were 5%. These results are related to a specific case study, for that reason their direct generalization is not correct.

Predictive model to describe the combined effect of pH and NaCl on apparent heat resistance of Bacillus stearothermophilus

The combined effect of pH and NaCl on the apparent thermal resistance of Bacillus stearothermophilus ATCC 12980 spores was studied. Spores were heated at different temperatures (115-125 degrees C) in mushroom substrate, acidified using glucono-delta-lactone to different pH levels (from 5.75 to 6.7), which contained concentrations of NaCl that ranged from 0.5 to 3% (w/v). The recovery medium was acidified to the same pH level and contained the same NaCl concentration as the heating menstruum. A factorial experimental design allowed a predictive model to be developed, which described the combined effect of heating temperature, pH and NaCl on the thermal resistance of B. stearothermophilus spores. Predictions from the model provided a valid description of the data used to generate the model, and agreed with observations from the literature and from an independent experiment performed using asparagus and bean substrates.