Application of a frequency distribution model to describe the thermal inactivation of two strains of Bacillus cereus

Visible-light photoactivated proanthocyanidin and kappa-carrageenan coating with anti-adhesive properties against clinically relevant bacteria

The increase of bacterial resistance to antibiotics is a growing concern worldwide and the search for new therapies could cost billions of dollars and countless lives. Inert surfaces are major sources of contamination due to easier adhesion and formation of bacterial biofilms, hindering the disinfection process. Therefore, the objective of this study was to develop a photoactivatable and anti-adhesive kappa-carrageenan coating using proanthocyanidin as a photosensitizer. The complete reduction (>5-log10 CFU/cm3) of culturable cells of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa pathogens was achieved after 30 min of exposure to visible light (420 nm; 30 mW/cm2) with 5 % (w/v) of the photosensitizer. Cell membrane damage was confirmed by measuring potassium leakage, epifluorescence microscopy and bacterial motility analysis. Overall, visible light irradiation on coated solid surfaces mediated by proanthocyanidin showed no cytotoxicity and inactivated clinically important pathogens through the generation of reactive oxygen species, inhibiting bacterial initial adhesion. The developed coating is a promising alternative for a wide range of applications related to surface disinfection and food biopreservation.

Joint effect of heat, pH and grape extract on Bacillus cereus spores survival in a rice solution

Rice due to its high carbohydrate content, is an ideal medium for Bacillus cereus growth, a spore-producing microorganism. The objective of this study was to determine the antimicrobial activity of a grape extract in combination with heat treatments and different pH against B. cereus spores in a rice solution. The survivor data obtained were fitted to the Weibull survival function, and the values of parameters a and b (scale and shape indexes, respectively) were determined. Results showed that the grape extract affected the survival of B. cereus spores at 90 °C and 95 °C, reaching greater logarithmic reductions in acidic pH values. This behaviour was reflected in a parameter of the Weibull survival function which decreased as the temperature increased and at acidic pH values. In addition, a secondary model was developed by relating the logarithm of a to the independent variables (temperature and pH). A global model relating B. cereus inactivation with temperature and pH was developed, and validated by calculating the accuracy factor. The results demonstrate the usefulness of grape extract as a by-product, which can be used as an additional control measure for rice, especially when combined with mild heat treatments and acidic pH values.

Salt, glucose, glycine, and sucrose protect Escherichia coli O157:H7 against acid treatment in laboratory media

This study investigated the effects of combinations of acetic or malic acid and various solutes (salt, glucose, glycine, or sucrose) on the survival of Escherichia coli O157:H7 in laboratory broth. Additionally, the effectiveness of combining organic acids and various concentrations of salt (0-18%) or sucrose (0-100%) with different water activity values against E. coli O157:H7 were evaluated. For treatment of 1% malic acid, the addition of 3% salt showed synergistic effect. Whereas, when 3% salt, glucose, glycine, or sucrose was added to 1% acetic acid, the solutes antagonized the action of the acid against E. coli O157:H7. Acetic, lactic, or propionic acid combined with salt at either 7 or 9% or sucrose at 60, 80, or 100% resulted in the highest resistance of E. coli O157:H7. From a result of evaluating the membrane fatty acid (MFA) composition of cells, salt or sucrose significantly increased levels of saturated fatty acids (SFAs) or SFAs and cyclopropane fatty acids, respectively. From the results of this study, the addition of solutes and organic compounds may increase the tolerance of E. coli O157:H7 to acetic, lactic, and propionic acid treatments and that the salt or sucrose significantly affects cell MFA composition.

Thermal inactivation and kinetic parameters for Campylobacter jejuni on chicken skin

New Zealand has a higher reported incidence rate of campylobacteriosis than other developed countries. It has been suggested that this may be due to the emergence of heat-resistant strains that can survive normal cooking. To test this, typed Campylobacter strains ST474 and ST48 were inoculated onto slices of chicken skin <18 mm in diameter and 4 mm thick using a pipette, and placed in a special aluminium cell, which was heated to a predetermined temperature (in the range of 56.5 to 65 °C) using a temperature-controlled water bath. Survivor curves were plotted, and GlnaFit software was chosen to fit the experimental data; inactivation parameters were estimated using 1-step and 2-step regression. The D values and z values were in the range of 3-6 s and 8-11 °C, respectively. The D values at 60 and 56 °C were in the range of 12-41 s. These D values are in general agreement with previously published reports. Thus, New Zealand's higher reported rate of campylobacteriosis is possibly due to factors other than the emergence of heat-resistant strains.

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.

Comparative evaluation of approaches to modelling kinetics of microbial thermal death as in the case of Alicyclobacillus acidoterrestris

Microbial death kinetics modelling is an integral stage of developing the food thermal sterilisation regimes. At present, a large number of models have been developed. Their properties are usually being accepted as adequate even beyond boundaries of experimental microbiological data zone. The wide range of primary models existence implies the lack of universality of each ones. This paper presents a comparative assessment of linear and nonlinear models of microbial death kinetics during the heat treatment of the Alicyclobacillus acidoterrestris spore form. The research allowed finding that single-phase primary models (as adjustable functions) are statistically acceptable for approximation of the experimental data: linear – the Bigelow’ the Bigelow as modified by Arrhenius and the Whiting-Buchanan models; and nonlinear – the Weibull, the Fermi, the Kamau, the Membre and the Augustin models. The analysis of them established a high degree of variability for extrapolative characteristics and, as a result, a marked empirical character of adjustable functions, i.e. unsatisfactory convergence of results for different models. This is presumably conditioned by the particularity and, in some cases, phenomenology of the functions themselves. Consequently, there is no reason to believe that the heat treatment regimes, developed on the basis of any of these empirical models, are the most effective. This analysis is the first link in arguing the necessity to initiate the research aimed at developing a new methodology for determining the regimes of food thermal sterilisation based on analysis of the fundamental factors such as ones defined spore germination activation and their resistance to external impact.

Combined pre-treatments effects on zucchini (Cucurbita pepo L.) squash microbial load reduction

Freezing vegetables requires pre-treatments to reduce microbial load and destroy enzymes that impair the frozen product quality. So far blanching has been the most effective pre-treatment, preferred by the food industry, despite its severity: heating up to temperatures close to 100 °C for 1-3 min causes sensory and texture changes in most horticultural products. Alternative blanching treatments, using UV-C radiation combined with milder thermal treatments or with thermosonication, may improve the quality of the final frozen vegetables. Zucchini (Cucurbita pepo L.), the vegetable under study, has an availability in fresh restricted to a season, needing therefore to be often frozen to be used throughout the year. In this study, its surface was first inoculated with two vegetable contaminants, Enterococcus faecalis and Deinococcus radiodurans cells, which are resistant, respectively, to high temperatures and to radiation and then submitted to several blanching treatments, single or combined, and the effect on these microorganisms reduction was evaluated. As single treatments, water blanching (the control treatment, as it is the blanching treatment traditionally used) was applied up to 180 s at temperatures ranging from 65 to 90 °C, and UV-irradiation applied in continuous. As combined pre-treatments, water blanching combined with UV-C (continuous or in pulses), and thermosonication (20 kHz at 50% of power) combined with UV-C pulses were also studied. The continuous UV-C radiation incident irradiance was 11 W/m² up to 180 s, and the pulses at incident radiance of 67 W/m², lasting 3.5 s each (35 pulses). Mathematical modeling of bacterial reduction data was carried out using the Bigelow, the Weibull and Weibull modified models, and estimation of their respective kinetic parameters proved that the latter models presented a better fit below 75 °C. The best results proved to be the combination of water blanching at temperatures as low as 85 °C during <2 min with 25 pulses of UV-C (incident irradiance of 67 W/m²) or thermosonication at 90 °C also combined with UV-C pulses, both resulting in 3 log reductions of both microorganisms under study. These results proved to overcome what industry is requiring so far (a 2 log microbial reduction in 3 min), hence minimizing quality changes of frozen zucchini.

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.

Modeling the isothermal inactivation curves of Listeria innocua CECT 910 in a vegetable beverage under low-temperature treatments and different pH levels

Thermal inactivation kinetics of Listeria innocua CECT 910 inoculated in a vegetable beverage at three pH conditions (4.25, 4.75, and 5.20), four levels of temperature (50, 55, 60, 65℃), and different treatment times (0-75 min) were obtained. Survival curves did not follow a log-linear relationship and consequently were fitted to various mathematical models: Weibull, Geeraerd, Cerf with shoulder, and the modified Gompertz equation. Results indicated that the best model for the treatment conditions was the modified Gompertz equation, which provides the best goodness-of-fit and the lowest Akaike information criterion value. Sensitivity analysis indicated that the most influential factors affecting the final microbial load were temperature and time in the case of the higher temperature level (65℃) and time in the case of the lower temperature level (50℃).

Effects of high hydrostatic pressure and temperature increase on Escherichia coli spp. and pectin methyl esterase inactivation in orange juice

The aim of this study was to evaluate the effect of high hydrostatic pressure treatment combined with moderate processing temperatures (25 ℃-50 ℃) on the inactivation of Escherichia coli O157: H7 (ATCC 700728), E. coli K12 (ATCC 23716), and pectin methyl esterase in orange juice, using pressures of 250 to 500 MPa with times ranging between 1 and 30 min. Loss of viability of E. coli O157:H7 increased significantly as pressure and treatment time increased, achieving a 6.5 log cycle reduction at 400 MPa for 3 min at 25 ℃ of treatment. With regard to the inactivation of pectin methyl esterase, the greatest reduction obtained was 90.05 ± 0.01% at 50 ℃ and 500 MPa of pressure for 15 min; therefore, the pectin methyl esterase enzyme was highly resistant to the treatments by high hydrostatic pressure. The results obtained in this study showed a synergistic effect between the high pressure and moderate temperatures in inactivating E. coli cells.

A new chemically defined medium for the growth and sporulation of Bacillus cereus strains in anaerobiosis

A new chemically defined liquid medium, MODS, was developed for the aerobic growth and anaerobic growth and sporulation of Bacillus cereus strains. The comparison of sporulation capacity of 18 strains of B. cereus has shown effective growth and spore production in anaerobiosis..

Inactivation kinetics of spores of Bacillus cereus strains treated by a peracetic acid-based disinfectant at different concentrations and temperatures

The purpose of this study was to assess the effect of a commercial peracetic acid-based disinfectant against spores of Bacillus cereus, to identify the most influential factor for the final number of microorganisms after different disinfection procedures, and to evaluate the nature of the inactivation kinetics. The spores of four different strains of B. cereus (DSM 318, 4312, 4313, and 4384) were treated with five different disinfectant concentrations (0.25%, 0.5%, 1.0%, 1.5%, and 2.0% [w/v]) at three different temperatures (10°C, 15°C, and 20°C) with or without protein load. A higher temperature and PES 15/23 concentration resulted in a higher inactivation. Inactivation of B. cereus strain 4312 was around 2 log₁₀ cycles at 10°C and around 7 log₁₀ at 20°C (conc=1% [w/v] PAA; t=60 min; without protein). The protein load at higher concentrations did not significantly reduce the efficacy of the disinfectant (p>0.05). This article indicates the applicability of the Weibull model to fit the B. cereus disinfectant survival curves. A Monte Carlo simulation was used to carry out a sensitivity analysis, which revealed the most influential factors affecting the final number of microorganisms after the disinfection process.

Relevant factors affecting microbial surface decontamination by pulsed light

Pulsed Light (PL) uses intense flashes of white light rich in ultraviolet (UV) light for decontamination. A log-reduction higher than 5 was obtained in one flash and at fluences lower than 1.8J/cm(2) on spores of a range of spore-forming bacteria, of vegetative cells of non-spore-forming bacteria and on yeasts spread on agar media. Vegetative cells were more sensitive than spores. The inactivation by PL of Bacillus subtilis, B. atrophaeus, B. cereus, Geobacillus stearothermophilus, and Aspergillus niger spores sprayed on polystyrene was similar. The inactivation by PL of B. subtilis and A. niger spores sprayed on glass was slightly lower than on polystyrene. No alteration of the spore structures was detected by scanning electron microscopy for both PL treated B. subtilis and A. niger spores. The inactivation of spores of B. subtilis, B. atrophaeus, B. cereus and B. pumilus by PL or by continuous UV-C at identical fluences was not different, and was much higher by PL for A. niger spores. The increase in the input voltage of the lamps (which also increases the UV-C %) resulted in a higher inactivation. There was no correlation between the resistance to heat and the resistance to PL. The relative effect of UV-C radiations and light thermal energy on PL inactivation was discussed.

Development of associations and kinetic models for microbiological data to be used in comprehensive food safety prediction software

The objective of this study was to use an existing database of food products and their associated processes, link it with a list of the foodborne pathogenic microorganisms associated with those products and finally identify growth and inactivation kinetic parameters associated with those pathogens. The database was to be used as a part of the development of comprehensive software which could predict food safety and quality for any food product. The main issues in building such a predictive system included selection of predictive models, associations of different food types with pathogens (as determined from outbreak histories), and variability in data from different experiments. More than 1000 data sets from published literature were analyzed and grouped according to microorganisms and food types. Final grouping of data consisted of the 8 most prevalent pathogens for 14 different food groups, covering all of the foods (>7000) listed in the USDA Natl. Nutrient Database. Data for each group were analyzed in terms of 1st-order inactivation, 1st-order growth, and sigmoidal growth models, and their kinetic response for growth and inactivation as a function of temperature were reported. Means and 95% confidence intervals were calculated for prediction equations. The primary advantage in obtaining group-specific kinetic data is the ability to extend microbiological growth and death simulation to a large array of product and process possibilities, while still being reasonably accurate. Such simulation capability could provide vital ''what if'' scenarios for industry, Extension, and academia in food safety.

Inactivation kinetics of inoculated Escherichia coli O157:H7, Listeria monocytogenes and Salmonella Poona on whole cantaloupe by chlorine dioxide gas

The objectives of this study were to examine inactivation kinetics of inoculated Escherichia coli O157:H7, Listeria monocytogenes and Salmonella Poona inoculated onto whole cantaloupe and treated with ClO(2) gas at different concentrations (0.5, 1.0, 1.5, 3.0 and 5.0 mg l(-1)) for different times (0, 2.0, 4.0, 6.0, 8.0 and 10.0 min). The effect of ClO(2) gas on the quality and shelf life of whole cantaloupe was also evaluated during storage at 22 degrees C for 12 days. A 100 microl inoculation of each targeted organism was spotted onto the surface (5 cm(2)) of cantaloupe rind (approximately 8-9 log CFU 5 cm(-2)) separately, air dried (60 min), and then treated with ClO(2) gas at 22 degrees C and 90-95% relative humidity for 10 min. Surviving bacterial populations on cantaloupe surfaces were determined using a membrane transferring method with a non-selective medium followed by a selective medium. The inactivation kinetics of E. coli O157:H7, L. monocytogenes and S. Poona were determined using nonlinear kinetics (Weibull model). A 3 log CFU reduction of E. coli O157:H7, L. monocytogenes and S. Poona were achieved with 5.0 mg l(-1) ClO(2) gas for 5.5, 4.2 and 1.5 min, respectively. A 5l og CFU reduction of S. Poona was achieved with 5.0 and 3.0 mg l(-1) ClO(2) gas for 6 and 8 min, respectively. A 4.6 and 4.3 log reduction was achieved after treatment with 5.0 mg l(-1) ClO(2) gas at 10 min for E. coli O157:H7 and L. monocytogenes, respectively. Treatment with 5.0 mg l(-1) ClO(2) gas significantly (p<0.05) reduced the initial microflora (mesophilic bacteria, psychrotrophic bacteria, and yeasts and molds) on cantaloupe by more than 2 log CFU cm(-2) and kept them significantly (p<0.05) lower than the untreated control during storage at 22 degrees C for 12 days. Treatment with ClO(2) gas did not significantly (p>0.05) affect the color of whole cantaloupe and extended the shelf life to 9 days compared to 3 days for the untreated control, when stored at ambient temperature (22 degrees C).

Survival of Campylobacter jejuni in mineral bottled water according to difference in mineral content: application of the Weibull model

The aim of the study was to examine the hypothesis proposed by Evans et al. [2003. Hazards of healthy living: bottled water and salad vegetables as risk factors for Campylobacter infection. Emerg. Infect. Dis. 9(10), 1219-1225] that mineral bottled water accidentally contaminated by Campylobacter jejuni would represent a risk factor for Campylobacter infection. Culturability of C. jejuni cells inoculated in low- and high-mineral bottled water during storage at 4 degrees C in the dark was performed by surface plating and modelled using the Weibull model. The loss of C. jejuni culturability observed in all conditions tested was shown to be dependent on strain, preculture condition and water composition. Following inoculation of C. jejuni, the rapid loss of culturability was not correlated to complete cell death as the passage into embryonated eggs enabled recovery of cells from the viable but non-culturable state. In conclusion, the sanitary risk associated with contaminated bottled water cannot be excluded although it is presumably low. Culture conditions, strain and water type must be taken into account in the evaluation of the risk factors as they influence significantly Campylobacter survival in water.

Modelling the effect of high pressure on the inactivation kinetics of a pressure-resistant strain of Pediococcus damnosus in phosphate buffer and gilt-head seabream (Sparus aurata)

AIMS

The aim of this research was to: (i) determine the inactivation pattern of a pressure-resistant strain of Pediococcus damnosus by high hydrostatic pressure in phosphate buffer (pH 6.7) and gilt-head seabream using the linear, biphasic and Weibull models; and (ii) validate the applicability of the Weibull model to predict survival curves at other experimental pressure levels.

METHODS AND RESULTS

A pressure-resistant strain of P. damnosus was exposed to a range of pressures (500, 550, 600 and 650 MPa) in phosphate buffer (pH 6.7) and gilt-head seabream for up to 8 min at ambient temperature (23 degrees C). Inactivation kinetics were described by the linear, biphasic and Weibull models. Increasing the magnitude of the pressure applied resulted in increasing levels of inactivation. Pronounced tailing effect was observed at pressures over 600 MPa. The Weibull and biphasic models consistently produced better fit than the linear model as inferred by the values of the root mean squared error, coefficient of determination (R2) and accuracy factor (A(f)). The scale factor (b) of the Weibull model was linearly correlated with pressure (P) treatment in the whole pressure range. Substituting the b parameter in the initial Weibull function and calculating the shape factor (n) by linear interpolation, high pressure (P) was directly incorporated into the model providing reasonable predictions of the survival curves at 570 and 630 MPa. Comparison between the survival curves in phosphate buffer and gilt-head seabream showed a clear protective effect of the food matrix on the resistance of the micro-organism, especially at 500 and 550 MPa.

CONCLUSIONS

The Weibull and biphasic models were more flexible to describe the survival curves of P. damnosus in the experimental pressure range, taking also into account the tailing effect that could not be included in the linear model. The Weibull model could also give reasonable predictions of the survival curves at other experimental pressures in both pressure menstrua. As the food matrix has a protective effect in microbial inactivation, the development of accurate mathematical models should be done directly on real food to avoid under- or over-processing times.

SIGNIFICANCE AND IMPACT OF THE STUDY

The development of accurate models to describe the survival curves of micro-organisms under high hydrostatic pressure treatment would be very important to the food industry for process optimisation, food safety and extension of the applicability of high pressure processing.