Predictive growth modeling of Yersinia enterocolitica in fresh kimchi cabbage brassica pekinensis as a function of storage temperature

We developed a predictive growth model of Yersinia enterocolitica for fresh Kimchi cabbages as a function of storage temperature (5-20 °C). The Baranyi equation used for primary modeling at these storage temperatures was suitable as a model for obtaining lag time (LT) and specific growth rate (SGR) (R² = 0.97-0.98). As the temperature increased, the growth of Y. enterocolitica tended to increase, with SGR values of 0.33, 0.40, 0.60 and 0.68 log colony-forming units/h at 8, 11, and 15 °C, and LT values of 5.63, 3.54, 2.23 and 1.09 h, respectively. The secondary model was determined by the non-linear regression analysis. The suitability of the modeling results for the SGR and LT value was verified by determining the mean square error (<0.01), bias factor (0.919-0.999), and accuracy factor (1.032-1.136). The predicted models can be used to predict the growth of Y. enterocolitica in Kimchi cabbage at various temperatures and as an effective tool for maintaining the safe level of Y. enterocolitica in the production, processing, and distribution of fresh agricultural products.

Alternative culture medium design for biomass production of autochthonous meat starter Latilactobacillus sakei sp. sakei ACU-2

The autochthonous strain Latilactobacillus sakei sp. sakei ACU-2 was selected as a meat starter culture for dry sausage production. Transferring this strain from laboratory scale to industry requires an increase in biomass production, while lowering process costs. In this study, a combination of techniques was applied in order to optimize the culture medium composition to enhance biomass production of L. sakei ACU-2. One variable at a time experiments, Plackett-Burman design, and mixture design were performed to fulfill the strain nutritional requirements. Eventually, the optimized formulation contained 19.46 g/L yeast extract; 8.28 g/L whey protein concentrate; 2.26 g/L soy peptone; 30 g/L cerelose; 1 g/L Tween 80; 5 g/L sodium acetate; 0.2 g/L magnesium sulfate and 0.05 g/L manganese sulfate. When L. sakei ACU-2 was cultivated in a bioreactor using the alternative medium, an enhancement of 75.5% of biomass production was achieved, in comparison to its growth in the commercial de Man, Rogosa, and Sharpe medium. Furthermore, a reduction of 62-86% of the cost was also attained. These results support a promising large-scale application of the designed medium for high biomass yields of the starter culture at minor costs.

Atomic Force Microscopy Investigation of the Contributions of Listeria monocytogenes Cell-Wall Biomacromolecules to Their Adherence and Mechanics

In this work, the contributions of the pathogenic Listeria monocytogenes cell-wall biomacromolecules to the bacterial mechanics and adhesion to a model inert surface of silicon nitride in water were investigated by atomic force microscopy. Chemical ethylenediaminetetraacetic acid (EDTA) and biological enzymatic trypsin treatments of cells were performed to partially or totally remove the bacterial cell-wall proteins and carbohydrates. Removal of 48.2% proteins and 29.2% of carbohydrates from the cell-wall of the bacterium by the EDTA treatment resulted in a significant decrease in the length of the bacterial cell-wall biomacromolecules and an increase in the rigidity of the bacterial cells as predicted from fitting a model of steric repulsion to the force-distance approach data and classic Hertz model to the indentation-force data, respectively. In comparison, removal of almost all the cell-wall proteins (99.5% removal) and 8.6% of cell-wall carbohydrates by the trypsin treatment resulted in an increase in the elasticity of the bacterial cells, an increase in the extension of the cell-wall biomacromolecules, and a significant decrease in their apparent grafting densities. In addition, adhesion strength of native-untreated L. monocytogenes to silicon nitride in water decreased by 30% on average after the EDTA treatment and further decreased by 60% on average after the trypsin treatment, showing a positive correlation with the% removal of cell-wall proteins by the EDTA and trypsin treatments, respectively.

Predictive Modeling for the Growth of Aeromonas hydrophila on Lettuce as a Function of Combined Storage Temperature and Relative Humidity

This study developed predictive growth models of Aeromonas hydrophila on lettuce as a function of combined storage temperature (15-35°C) and relative humidity (RH, 60-80%) using a polynomial equation. The primary model of specific growth rate, lag time, and maximum population density showed a good fit (R2 ≥ 0.95) with a Gompertz equation. A secondary model was obtained using a quadratic polynomial equation. The appropriateness of the secondary model was verified by mean square error (0.0001-0.8848), bias factor (Bf = 0.962-1.009), and accuracy factor (Af = 1.002-1.104). The newly developed secondary models for A. hydrophila could be incorporated into the tertiary modeling program to predict the growth of A. hydrophila as a function of combined temperature and RH. The developed model may be useful to predict potential A. hydrophila growth on lettuce, which is important for food safety purpose during the overall food chain of lettuce from farm to table. It could offer reliable and useful information of growth kinetics for the quantification of microbial risk assessment of A. hydrophila on lettuce.

Application of response surface methodology to tailor the surface chemistry of electrospun chitosan-poly(ethylene oxide) fibers

Chitosan is a promising biocompatible polymer for regenerative engineering applications, but its processing remains challenging due to limited solubility and rigid crystalline structure. This work represents the development of electrospun chitosan/poly(ethylene oxide) blend nanofibrous membranes by means of a numerical analysis in order to identify and tailor the main influencing parameters with respect to accessible surface nitrogen functionalities which are of importance for the biological activity as well as for further functionalization. Depending on the solution composition, both gradient fibers and homogenous blended fiber structures could be obtained with surface nitrogen concentrations varying between 0 and 6.4%. Response surface methodology (RSM) revealed chitosan/poly(ethylene oxide) ratio and chitosan molecular weight as the main influencing factors with respect to accessible nitrogen surface atoms and respective concentrations. The model showed good adequacy hence providing a tool to tailor the surface properties of chitosan/poly(ethylene oxide) blends by addressing the amount of accessible chitosan.

Modeling the Combined Effects of Temperature, pH, and Sodium Chloride and Sodium Lactate Concentrations on the Growth Rate of Lactobacillus plantarum ATCC 8014

Nowadays, microorganisms with probiotic or antimicrobial properties are receiving major attention as alternative resources for food preservation. Lactic acid bacteria are able to synthetize compounds with antimicrobial activity against pathogenic and spoilage flora. Among them, Lactobacillus plantarum ATCC 8014 has exhibited this capacity, and further studies reveal that the microorganism is able to produce bacteriocins. An assessment of the growth of L. plantarum ATCC 8014 at different conditions becomes crucial to predict its development in foods. A response surface model of the growth rate of L. plantarum was built in this study as a function of temperature (4, 7, 10, 13, and 16°C), pH (5.5, 6.0, 6.5, 7.0, and 7.5), and sodium chloride (0, 1.5, 3.0, 4.5, and 6.0%) and sodium lactate (0, 1, 2, 3, and 4%) concentrations. All the factors were statistically significant at a confidence level of 90%  (p<0.10). When temperature and pH increased, there was a corresponding increase in the growth rate, while a negative relationship was observed between NaCl and Na-lactate concentrations and the growth parameter. A mathematical validation was carried out with additional conditions, demonstrating an excellent performance of the model. The developed model could be useful for designing foods with L. plantarum ATCC 8014 added as a probiotic.

A model for the influences of soluble and insoluble solids, and treated volume on the ultraviolet-C resistance of heat-stressed Salmonella enterica in simulated fruit juices

This study was conducted to determine the effects of intrinsic juice characteristics namely insoluble solids (IS, 0-3 %w/v), and soluble solids (SS, 0-70 °Brix), and extrinsic process parameter treated volume (250-1000 mL) on the UV-C inactivation rates of heat-stressed Salmonella enterica in simulated fruit juices (SFJs). A Rotatable Central Composite Design of Experiment (CCRD) was used to determine combinations of the test variables, while Response Surface Methodology (RSM) was used to characterize and quantify the influences of the test variables on microbial inactivation. The heat-stressed cells exhibited log-linear UV-C inactivation behavior (R² 0.952 to 0.999) in all CCRD combinations with D_UV-C values ranging from 10.0 to 80.2 mJ/cm². The D_UV-C values obtained from the CCRD significantly fitted into a quadratic model (P < 0.0001). RSM results showed that individual linear (IS, SS, volume), individual quadratic (IS² and volume²), and factor interactions (IS × volume and SS × volume) were found to significantly influence UV-C inactivation. Validation of the model in SFJs with combinations not included in the CCRD showed that the predictions were within acceptable error margins.

Prediction and optimization of the laccase-mediated synthesis of the antimicrobial compound iodine (I2)

An artificial neural network (ANN) and genetic algorithm (GA) were applied to improve the laccase-mediated oxidation of iodide (I(-)) to elemental iodine (I2). Biosynthesis of iodine (I2) was studied with a 5-level-4-factor central composite design (CCD). The generated ANN network was mathematically evaluated by several statistical indices and revealed better results than a classical quadratic response surface (RS) model. Determination of the relative significance of model input parameters, ranking the process parameters in order of importance (pH>laccase>mediator>iodide), was performed by sensitivity analysis. ANN-GA methodology was used to optimize the input space of the neural network model to find optimal settings for the laccase-mediated synthesis of iodine. ANN-GA optimized parameters resulted in a 9.9% increase in the conversion rate.

Combined effects of chlorine and thiamine dilauryl sulfate on reduction of Listeria monocytogenes in chicken breast and development of predictive growth models

The inhibitory effect of chlorine (50, 100, and 200 mL/kg) and thiamine dilauryl sulfate (TDS: 100, 500, and 1,000 mg/kg) on Listeria monocytogenes in chicken breast was investigated. Also, predictive growth models as a function of chlorine and TDS concentration, and storage temperature (4, 10, and 15°C) were developed using a polynomial model. Listeria monocytogenes counts were significantly (P < 0.05) different in samples treated with sterile distilled water and combinations of chlorine and TDS. The maximum reduction effect was 0.5 log cfu/g by combined treatment of 200 mL/kg chlorine and 1,000 mg/kg TDS. The largest synergistic effect was 0.38 log cfu/g by combined treatment of 100 mL/kg chlorine and 1,000 mg/kg TDS. The primary models that were developed to obtain the specific growth rates (SGR) and lag time (LT) had good fitness (R(2) > 0.91) determined by the reparameterized Gompertz equation. The secondary polynomial models were calculated by nonlinear regression analysis. In the validation of the developed models, the bias factor (Bf) and accuracy factor (Af) for SGR were 0.54 and 1.84, respectively, whereas those for LT were 0.97 and 1.04, respectively. In quality analysis, chlorine and TDS did not change the color or texture of chicken breast meat during storage at 4°C for 7 d. Thus, our findings indicate that a combined treatment of 100 mL/kg chlorine and 1,000 mg/kg TDS appears to an effective method into reduce L. monocytogenes in broiler carcasses with no negative effects on color and textural quality. The predictive models were in good agreement with the validation and may be used to predict L. monocytogenes growth in chicken breast.

Inhibitory effect of chlorine and ultraviolet radiation on growth of Listeria monocytogenes in chicken breast and development of predictive growth models

The inhibitory effect of chlorine (50, 100, and 200 mg/kg) was investigated with and without UV radiation (300 mW·s/cm(2)) for the growth of Listeria monocytogenes in chicken breast meat. Using a polynomial model, predictive growth models were also developed as a function of chlorine concentration, UV exposure, and storage temperature (4, 10, and 15°C). A maximum L. monocytogenes reduction (0.8 log cfu, cfu/g) was obtained when combining chlorine at 200 mg/kg and UV at 300 mW·s/cm(2), and a maximum synergistic effect (0.4 log cfu/g) was observed when using chlorine at 100 mg/kg and UV at 300 mW·s/cm(2). Primary models developed for specific growth rate and lag time showed a good fitness (R(2) > 0.91), as determined by the reparameterized Gompertz equation. Secondary polynomial models were obtained using nonlinear regression analysis. The developed models were validated with mean square error, bias factor, and accuracy factor, which were 0.0003, 0.96, and 1.11, respectively, for specific growth rate and 7.69, 0.99, and 1.04, respectively, for lag time. The treatment of chlorine and UV did not change the color and texture of chicken breast after 7 d of storage at 4°C. As a result, the combination of chlorine at 100 mg/kg and UV at 300 mW·s/cm(2) appears to an effective method into inhibit L. monocytogenes growth in broiler carcasses with no negative effects on color and textural quality. Based on the validation results, the predictive models can be used to accurately predict L. monocytogenes growth in chicken breast.

The role of growth temperature in the adhesion and mechanics of pathogenic L. monocytogenes: an AFM study

The adhesion strengths of pathogenic L. monocytogenes EGDe to a model surface of silicon nitride were quantified using atomic force microscopy (AFM) in water for cells grown under five different temperatures (10, 20, 30, 37, and 40 °C). The temperature range investigated was chosen to bracket the thermal conditions in which L. monocytogenes survive in the environment. Our results indicated that adhesion force and energy quantified were at their maximum when the bacteria were grown at 30 °C. The higher adhesion observed at 30 °C compared to the adhesion quantified for bacterial cells grown at 37, 40, 20, and 10 °C was associated with longer and denser bacterial surface biopolymer brushes as predicted from fitting a model of steric repulsion to the approach distance-force data as well from the results of protein colorimetric assays. Theoretically predicted adhesion energies based on soft-particle DLVO theory agreed well with the adhesion energies computed from AFM force-distance retraction data (r(2) = 0.94); showing a minimum energy barrier to adhesion at 30 °C.

Atomic force microscopy investigations of heterogeneities in the adhesion energies measured between pathogenic and non-pathogenic Listeria species and silicon nitride as they correlate to virulence and adherence

Atomic force microscopy (AFM) was used to probe heterogeneities in adhesion energies measured between pathogenic and non-pathogenic species of Listeria and silicon nitride in water at four levels. Adhesion energies were quantified on individual bacterial cells (cell level), bacterial cells that belonged to an individual Listeria strain but varied in their cultures (strain level), bacterial cells that belonged to an individual Listeria species but varied in their strain type (species level) and on bacterial cells that belonged to the Listeria genus but varied in their species type (genus level). To quantify heterogeneities in the adhesion energies, a heterogeneity index (HI) was defined based on quantified standard errors of mean. At the cell level, spatial variations in the adhesion energies were not observed. For the strain, species, and genus levels, the HI increased with increased adhesion energies. At the species level, the HI increased with strain virulence.

The role of the pH conditions of growth on the bioadhesion of individual and lawns of pathogenic Listeria monocytogenes cells

The work of adhesion that governs the interactions between pathogenic Listeria monocytogenes and silicon nitride in water was probed for individual cells using atomic force microscopy and for lawns of cells using contact angle measurements combined with a thermodynamic-based harmonic mean model. The work of adhesion was probed for cells cultured under variable pH conditions of growth that ranged from pH 5 to pH 9. Our results indicated that L. monocytogenes cells survived and adapted well to the chemical stresses applied. For all pH conditions investigated, a transition was observed in the generation time, physiochemical properties, biopolymer grafting density and bioadhesion for cells cultured in media adjusted to pH 7 of growth. In media with pH 7, the generation time for the bacterial cells was lowest, the specific growth rate constant was highest, the cells were the most polar, cells displayed the highest grafting density of surface biopolymers and the highest bioadhesion to silicon nitride in water represented in terms of the work of adhesion. When compared, the work of adhesion values quantified between silicon nitride and lawns of L. monocytogenes cells were linearly correlated with the work of adhesion values quantified between silicon nitride and individual L. monocytogenes cells.

Application of response surface methodology to optimize pressurized liquid extraction of antioxidant compounds from sage (Salvia officinalis L.), basil (Ocimum basilicum L.) and thyme (Thymus vulgaris L.)

The present study optimized pressurized liquid extraction (PLE) conditions using Dionex ASE® 200, USA to maximize the antioxidant activity [Ferric ion Reducing Antioxidant Power (FRAP)] and total polyphenol content (TP) of the extracts from three spices of Lamiaceae family (sage, basil and thyme). Optimal conditions with regard to extraction temperature (66-129 °C) and solvent concentration (32-88% methanol) were identified using response surface methodology (RSM). For all three spices, results showed that 129 °C was the optimum temperature with regard to antioxidant activity. Optimal methanol concentrations with respect to the antioxidant activity of sage and basil extracts were 58% and 60% respectively. Thyme showed a different trend with regard to methanol concentration and was optimally extracted at 33%. Antioxidant activity yields of the optimal PLE were significantly (p < 0.05) higher than solid/liquid extracts. Predicted models were highly significant (p < 0.05) for both total phenol (TP) and FRAP values in all the spices with high regression coefficients (R(2)) ranging from 0.651 to 0.999.

Variations in the Nanomechanical Properties of Virulent and Avirulent Listeria monocytogenes

Atomic force microscopy (AFM) was used to quantify both the nanomechanical properties of pathogenic (ATCC 51776 & EGDe) and non-pathogenic (ATCC 15313 & HCC25) Listeria monocytogenes strains and the conformational properties of their surface biopolymers. The nanomechanical properties of the various L. monocytogenes strains were quantified in terms of Young's moduli of cells. To estimate Young's moduli, the classic Hertz model of contact mechanics and a modified version of it that takes into account substrate effects were used to fit the AFM nanoindentation-force measurements collected while pushing onto the bacterial surface biopolymer brush. When compared, the classic Hertz model always predicted higher Young's moduli values of bacterial cell elasticity compared to the modified Hertz model. On average, the modified Hertz model showed that virulent strains are approximately twice as rigid (88.1 ± 14.5 KPa) as the avirulent strains (47.3 ± 7.6 kPa). To quantify the conformational properties of L. monocytogenes' strains surface biopolymers, two models were used. First, the entropic-based, statistical mechanical, random walk formulation, the wormlike chain (WLC) model was used to estimate the elastic properties of the bacterial surface molecules. The WLC model results indicated that the virulent strains are characterized by a more flexible surface biopolymers as indicated by shorter persistence lengths (L(p) = 0.21 ± 0.08 nm) compared to the avirulent strains (L(p) = 0.24 ± 0.14 nm). Second, a steric model developed to describe the repulsive forces measured between the AFM tip and bacterial surface biopolymers indicated that the virulent strains are characterized by crowded and longer biopolymer brushes compared to those of the avirulent strains. Finally, scaling relationships developed for grafted polyelectrolyte brushes indicated L. monocytogenes strains' biopolymer brushes are charged. Collectively, our data indicate that the conformational properties of the bacterial surface biopolymers and their surface densities play an important role in controlling the overall bacterial cell elasticity.

Determination of optimal growth parameters for the bioincising fungus Physisporinus vitreus by means of response surface methodology

AIM

To evaluate the influence of water activity (a(w)), temperature and pH on the radial growth and lag phase of Physisporinus vitreus (E-642), a basidiomycete was used in the biotechnological process of bioincising.

METHODS AND RESULTS

Radial growth was monitored for 20 days on malt extract agar medium. Five levels of a(w) (0.998, 0.982, 0.955, 0.928, 0.892) were combined with three incubation temperatures (10, 15, 20 degrees C) and three pH values (4, 5, 6). Data analyses showed a highly significant effect of a(w) and temperature (P < 0.0001) and a significant effect of pH (P < 0.05). The radial growth rate and lag phase of P. vitreus were very sensitive to a(w) reduction. Although P. vitreus was able to grow at all the selected temperatures and pH values, the lag phase increased with decreasing a(w) and growth became inhibited at a(w) = 0.955. Optimal conditions for growth of P. vitreus were a(w) = 0.998, 20 degrees C and pH 5. The response surface model provided reliable estimates of these growth parameters and confirmed a greater dependence on a(w) than on temperature or pH under in vitro conditions.

CONCLUSIONS

Low levels of a(w) can prevent growth of P. vitreus, so wood moisture content should be adjusted accordingly.

SIGNIFICANCE AND IMPACT OF THE STUDY

Implementation of these results should contribute towards the optimization and efficiency of bioincising.