Isolation and Identification of the Antibacterial Compounds Produced by Maillard Reaction of Xylose with Phenylalanine or Proline.

Maillard reaction products (MRPs) of xylose with phenylalanine and xylose with proline exhibit high antibacterial activity. However, the active antibacterial compounds in MRPs have not yet been identified or isolated. This study aimed to isolate the active compounds in the two antibacterial MRPs. The organic layer of the MRP solution was separated and purified using silica gel chromatography and high-performance liquid chromatography. The chemical structures of the isolated compounds were determined by mass spectrometry and nuclear magnetic resonance spectroscopy. The compounds inhibited the growth of Bacillus cereus and Salmonella Typhimurium at 25 °C for 7 days at a concentration of 0.25 mM. Furthermore, the isolated compounds inhibited the growth of naturally occurring microflora of lettuce and chicken thighs at 25 °C for 2 days at a concentration of 0.5-1.0 mM. The antibacterial compounds found in MRPs demonstrated a wide range of effectiveness and indicated their potential as alternative preservatives.

Antibacterial Effect of Melanoidins Derived From Xylose and Phenylalanine Against Bacillus cereus and Clostridium perfringens.

Melanoidins produced from the combination of D-xylose and L-phenylalanine have been reported to exhibit strong antibacterial effects. This study investigated the influence of environmental factors, such as temperatures (10, 15, 20, 25, 30, 35, 40, and 45°C), pH (5.5, 6.0, 6.5, 7.0, 7.5, and 8.0), and water activity (aw: 0.99, 0.96, and 0.93), on the antibacterial effect of the melanoidins produced from the combination of D-xylose with L-phenylalanine against Bacillus cereus and Clostridium perfringens in culture media. Furthermore, freeze-dried powdered melanoidin was used to determine the minimum concentration for growth inhibition, to compare the antibacterial effect of the melanoidin with conventional food preservatives. The liquid melanoidins significantly inhibited the growth of B. cereus (up to 4 log CFU/mL at the maximum) and C. perfringens (up to 6.5 log CFU/mL at the maximum) regardless of the incubation temperatures. However, the remarkable difference between the presence and absence of the melanoidins was demonstrated in the range of 20-35°C as 4 log-cycle lower in B. cereus and 2 log-cycle lower in C. perfringens than those without the melanoidins. The antibacterial effect of the melanoidin on B. cereus was not influenced by pH from 5.5 to 7.0, which exhibited 2-3 log-cycle lower viable counts than those without the melanoidin. Only one log-cycle difference between with and without the melanoidin was shown in C. perfringens growth under the pH range of 5.5-7.0. Although there was no significant difference in the growth of B. cereus between three aw conditions, the melanoidin exhibited a significant antibacterial effect at aw 0.99 demonstrating 4 log-cycle lower viable numbers than those without the melanoidin. Minimum inhibitory concentration of the melanoidin powder for B. cereus and C. perfringens was 7 mg/mL and 15 mg/mL, respectively, regardless of the kind of foods. Furthermore, the melanoidin exhibited comparable antibacterial effect on B. cereus and C. perfringens to potassium sorbate and sodium benzoate under the same concentration as the minimum inhibitory concentration of the melanoidin, demonstrating 2 log-cycle reduction during 3 days of incubation period at 25°C. The results presented here suggest that the xylose- and phenylalanine-based melanoidin demonstrates the possibility to be an alternative food preservative.

Non-destructive quality classification of rice taste properties based on near-infrared spectroscopy and machine learning algorithms.

The taste quality of rice is determined by protein and amylose percentages, with low levels indicating high-quality taste in Japan. However, accurate non-destructive screening remains a challenge for the industry. We explored the use of machine learning models and near-infrared spectra to classify rice taste quality. Three models were optimized using 796 brown rice samples from Hokkaido, Japan, produced between 2008 and 2016, and tested on 278 distinct samples from the same region produced between 2017 and 2019. Logistic regression and support vector machine models outperformed the partial least-squares discriminant analysis model, achieving high accuracy (94%), f1-score (90%), average precision (0.94), and low classification error (4%) and allowing accurate non-destructive classification of rice quality. These results not only improve rice quality, post-harvest technology, and producer output in Japan but also could enhance quality control processes and foster the production of high-quality products for other agricultural goods and food commodities worldwide.

Effect of water activity on the mechanical glass transition and dynamical transition of bacteria.

The purpose of this study was to clarify the glass-transition behavior of bacteria (Cronobacter sakazakii) as a function of water activity (aw). From the water sorption isotherm (298 K) for C. sakazakii, monolayer water content and monolayer aw were determined to be 0.0724 g/g-dry matter and 0.252, respectively. Mechanical relaxation was investigated at 298 K. In a higher aw range of over 0.529, the degree of mechanical relaxation increased with an increase in aw. From the effect of aw on the degree of mechanical relaxation, the mechanical awc (aw at which mechanical glass transition occurs at 298 K) was determined to be 0.667. Mean-square displacement of atoms in the bacteria was investigated by incoherent elastic neutron scattering. The mean-square displacement increased gradually with an increase in temperature depending on the aw of samples. From the linear fitting, two or three dynamical transition temperatures (low, middle, and high Tds) were determined at each aw. The low-Td values (142-158 K) were almost independent from aw. There was a minor effect of aw on the middle Td (214-234 K) except for the anhydrous sample (261 K). The high Td (252-322 K) largely increased with the decrease in aw. From the aw dependence of the high Td, the dynamical awc was determined to be 0.675, which was almost equivalent to the mechanical awc. The high Td was assumed to be the glass-transition temperature (Tg), and anhydrous Tg was estimated to be 409 K. In addition, molecular relaxation time (τ) of the bacteria was calculated as a function of aw. From the result, it is suggested that the progress of metabolism in the bacterial system requires a lower τ than approximately 6 × 10-5 s.

Inhibitory effect of d-Tryptophan on the spoilage potential of Shewanella baltica and Pseudomonas fluorescens and its potential application in salmon fillet preservation.

In this study, we found the inhibitory effect of d-Tryptophan (d-Trp) on the bacterial growth and spoilage potential of P. fluorescens and S. baltica. The effectiveness of d-Trp in bacterial growth inhibition was highly dependent on environmental NaCl concentrations. The growth/no growth models were developed to clarify the relationship between salt and d-Trp concentrations for optimal growth-inhibitory effect. It showed that higher levels of NaCl (>3.5%) with d-Trp (>15 mM) are required for growth inhibition. The d-Trp treatment combined with NaCl also retarded the psychrotrophic growth and reduced the respiratory active bacteria at low temperature, indicating that the inhibitory effect of d-Trp might be associated with the tolerance to osmotic and cold stress. Furthermore, the treatment d-Trp combined NaCl (artificial seawater) on salmon fillets was investigated. During refrigerated storage, the d-Trp treated salmon fillet either inoculated with S. baltica or P. fluorescen maintained remained a relatively low level of total volatile basic nitrogen (TVBN) (below 30 mg N/100 g during 8-days storage) and delayed increase of low total viable bacteria counts (TVC) with a longer lag phase of 2 days.

Non-destructive assessment of amylose content in rice using a quality inspection system at grain elevators.

Amylose content is an important determinant of rice quality. Its accurate, non-destructive assessment is a challenge to the industry. We examined whether the automatic quality inspection system accurately measured the amylose content. Brown and milled rice models were calibrated with 902 samples produced between 2008 and 2017 in Hokkaido, Japan, and validated individually by samples collected in 2018 (n = 33) and 2019 (n = 71) from several grain elevators within the region. Models were developed by processing the automatic system data comprising of a near-infrared spectrometer, a visible light grain segregator, combined analysis of chemometrics, and by merging low and ordinary amylose variety validation results. The lower standard error of prediction (<0.52%) and a higher ratio of performance deviation (>4.0) in both models enabled accurate non-destructive assessment of amylose content. Hence, the automatic quality inspection system is a useful tool for meeting the demands of higher quality and palatability of rice.

Competitive growth kinetics of Campylobacter jejuni, Escherichia coli O157:H7 and Listeria monocytogenes with enteric microflora in a small-intestine model.

AIMS: The biological events occurring during human digestion help to understand the mechanisms underlying the dose-response relationships of enteric bacterial pathogens. To better understand these events, we investigated the growth and reduction behaviour of bacterial pathogens in an in vitro model simulating the environment of the small intestine. METHODS AND RESULTS: The foodborne pathogens Campylobacter jejuni, Listeria monocytogenes and Escherichia coli O157:H7 were cultured with multiple competing enteric bacteria. Differences in the pathogen's growth kinetics due to the relative amount of competing enteric bacteria were investigated. These growth differences were described using a mathematical model based on Bayesian inference. When pathogenic and enteric bacteria were inoculated at 1 log CFU per ml and 9 log CFU per ml, respectively, L. monocytogenes was inactivated over time, while C. jejuni and E. coli O157:H7 survived without multiplying. However, as pathogen inocula were increased, its inhibition by enteric bacteria also decreased. CONCLUSIONS: Although the growth of pathogenic species was inhibited by enteric bacteria, the pathogens still survived. SIGNIFICANCE AND IMPACT OF THE STUDY: Competition experiments in a small-intestine model have enhanced understanding of the infection risk in the intestine and provide insights for evaluating dose-response relationships.

Experimentally observed Campylobacter jejuni survival kinetics in chicken meat products during model gastric digestion tended to be lower than model predictions.

Campylobacter jejuni-related foodborne diseases are mainly attributed to the consumption of undercooked chicken meat and cross-contaminated produce. This study aimed to develop a survival kinetics model, based on the Weibull model, for predicting foodborne C. jejuni survival during gastric digestion in a model stomach. We previously confirmed that C. jejuni can survive temperatures up to 62 °C; therefore, certain types of grilled chicken skewers (yakitori) were examined for C. jejuni survival during simulated gastric digestion. C. jejuni survival on a chicken thigh following grilling was examined to confirm the foods for digestion experiments. Further, C. jejuni survival during model digestion was investigated through simultaneous digestion of raw chicken and cross-contaminated iceberg lettuce. The model stomach pH increased from 1.5 to 6.0 immediately after yakitori ingestion and did not decrease below 4.0 within 3 h of digestion. Gastric digestion did not significantly contribute to C. jejuni inactivation (<1.5 log reduction after 3 h digestion). Our model could predict C. jejuni survival kinetics in simulated gastric fluid under varying pH during model digestion. This approach can be used to predict C. jejuni survival rates following digestion to improve food safety and reduce Campylobacter-related disease outbreaks.

Why Does Cronobacter sakazakii Survive for a Long Time in Dry Environments? Contribution of the Glass Transition of Dried Bacterial Cells.

To investigate the mechanism of adaptation of Cronobacter sakazakii to desiccation stress, the present study focused on the glass transition phenomenon of dried bacterial cells, using a thermomechanical technique. The mechanical glass transition temperature (Tg) of dried C. sakazakii cells per se, prepared by different drying methods (air drying and freeze-drying) and with different water activity (aw) levels (0.43, 0.57, 0.75, and 0.87), were determined. In addition, we investigated the survival of two strains of C. sakazakii (JCM 1233 and JCM 2127) prepared by different drying methods under different storage temperatures (4, 25, and 42°C) and aw conditions (0.43 and 0.87). While the Tg of the air-dried C. sakazakii cells increased as the aw decreased, the freeze-dried C. sakazakii cells showed an unclear aw dependency of the Tg. Air-dried C. sakazakii cells showed a higher Tg than freeze-dried C. sakazakii cells at an aw of <0.57. Freeze-dried C. sakazakii cells were more rapidly inactivated than air-dried cells regardless of the difference in aw and temperature. The difference between the Tg and storage temperature was used as an index that took into consideration the differences in the drying methods and aw levels. As the difference between the Tg and storage temperature increased to >20°C, the dried C. sakazakii cells survived stably regardless of the drying method. In contrast, when the difference between the Tg and storage temperature was reduced to <10°C, the viable cell numbers in dried C. sakazakii cells were quickly decreased. Thus, the Tg is a key factor affecting the desiccation tolerance of C. sakazakii. IMPORTANCE The mechanical glass transition temperature (Tg) of dried Cronobacter sakazakii cells varied depending on differences in drying methods and water activity (aw) levels. Because the Tg of the dried bacterial cells varied depending on the drying method and aw, the Tg will play an important role as an operational factor in the optimization of dry food processing for controlling microbial contamination in the future. Furthermore, the differences between the Tg and storage temperature were introduced as an integrated index for survival of bacterial cells under a desiccation environment that took into consideration the differences in the drying methods and aw levels. As the difference between the Tg and storage temperature decreased to <10°C, the dried C. sakazakii cells were inactivated quickly, regardless of the drying methods. The relationship between Tg and storage temperature will contribute to understanding the desiccation tolerance of bacterial cells.

Antibacterial Properties of Melanoidins Produced from Various Combinations of Maillard Reaction against Pathogenic Bacteria.

Novel melanoidins are produced by the Maillard reaction. Here, melanoidins with high antibacterial activity were tested by examining various combinations of reducing sugars and amino acids as reaction substrates. Twenty-two types of melanoidins were examined by combining two reducing sugars (glucose and xylose) and eleven l-isomers of amino acids (alanine, arginine, glutamine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, and valine) to confirm the effects of these melanoidins on the growth of Listeria monocytogenes at 25°C. The melanoidins produced from the combination of d-xylose with either l-phenylalanine (Xyl-Phe) or l-proline (Xyl-Pro), for which absorbance at 420 nm was 3.5 ± 0.2, completely inhibited the growth of L. monocytogenes at 25°C for 48 h. Both of the melanoidins exhibited growth inhibition of L. monocytogenes which was equivalent to the effect of nisin (350 IU/mL). The antimicrobial spectrum of both melanoidins was also investigated for 10 different species of bacteria, including both Gram-positive and Gram-negative species. While Xyl-Phe-based melanoidin successfully inhibited the growth of Bacillus cereus and Brevibacillus brevis, Xyl-Pro-based melanoidin inhibited the growth of Salmonella enterica Typhimurium. However, no clear trend in the antimicrobial spectrum of the melanoidins against different bacterial species was observed. The findings in the present study suggest that melanoidins generated from xylose with phenylalanine and/or proline could be used as potential novel alternative food preservatives derived from food ingredients to control pathogenic bacteria. IMPORTANCE Although the antimicrobial effect of melanoidins has been reported in some foods, there have been few comprehensive investigations on the antimicrobial activity of combinations of reaction substrates of the Maillard reaction. The present study comprehensively investigated the potential of various combinations of reducing sugars and amino acids. Because the melanoidins examined in this study were produced simply by heating in an autoclave at 121°C for 60 min, the targeted melanoidins can be easily produced. The melanoidins produced from combinations of xylose with either phenylalanine or proline exhibited a wide spectrum of antibiotic effects against various pathogens, including Listeria monocytogenes, Bacillus cereus, and Salmonella enterica Typhimurium. Since the antibacterial effect of the melanoidins on L. monocytogenes was equivalent to that of a nisin solution (350 IU/mL), we might expect a practical application of melanoidins as novel food preservatives.

A New Dose-Response Model for Estimating the Infection Probability of Campylobacter jejuni Based on the Key Events Dose-Response Framework.

Understanding the dose-response relationship between ingested pathogenic bacteria and infection probability is a key factor for appropriate risk assessment of foodborne pathogens. The objectives of this study were to develop and validate a novel mechanistic dose-response model for Campylobacter jejuni and simulate the underlying mechanism of foodborne illness during digestion. Bacterial behavior in the human gastrointestinal environment, including survival at low pH in the gastric environment after meals, transition to intestines, and invasion to intestinal tissues, was described using a Bayesian statistical model based on the reported experimental results of each process while considering physical food types (liquid versus solid) and host age (young adult versus elderly). Combining the models in each process, the relationship between pathogen intake and the infection probability of C. jejuni was estimated and compared with reported epidemiological dose-response relationships. Taking food types and host age into account, the prediction range of the infection probability of C. jejuni successfully covered the reported dose-response relationships from actual C. jejuni outbreaks. According to sensitivity analysis of predicted infection probabilities, the host age factor and the food type factor have relatively higher relevance than other factors. Thus, the developed "key events dose-response framework" can derive novel information for quantitative microbiological risk assessment in addition to dose-response relationship. The framework is potentially applicable to other pathogens to quantify the dose-response relationship from experimental data obtained from digestion. IMPORTANCE Based on the mechanistic approach called the key events dose-response framework (KEDRF), an alternative to previous nonmechanistic approaches, the dose-response models for infection probability of C. jejuni were developed considering with age of people who ingest pathogen and food type. The developed predictive framework illustrates highly accurate prediction of dose (minimum difference 0.21 log CFU) for a certain infection probability compared with the previously reported dose-response relationship. In addition, the developed prediction procedure revealed that the dose-response relationship strongly depends on food type as well as host age. The implementation of the key events dose-response framework will mechanistically and logically reveal the dose-response relationship and provide useful information with quantitative microbiological risk assessment of C. jejuni on foods.

Rapid detection and enumeration of aerobic mesophiles in raw foods using dielectrophoresis.

The concept of dielectrophoresis (DEP), which involves the movement of neutral particles by induced polarization in nonuniform electric fields, has been exploited in various biological applications. However, only a few studies have investigated the use of DEP for detecting and enumerating microorganisms in foodstuffs. Therefore, we aimed to evaluate the accuracy and efficiency of a DEP-based method for enumerating viable bacteria in three raw foods: freshly cut lettuce, chicken breast, and minced pork. The DEP separation of bacterial cells was conducted at 20 V of output voltage and 6000 to 9000 kHZ of frequency with sample conductivity of 30-70 µS/cm. The accuracy and validity of the DEP method for enumerating viable bacteria were compared with those of the conventional culture method; no significant variation was observed. We found a high correlation between the data obtained using DEP and the conventional aerobic plate count culture method, with a high coefficient of determination (R2 > 0.90) regardless of the food product; the difference in cell count data between both methods was within 1.0 log CFU/mL. Moreover, we evaluated the efficiency of the DEP method for enumerating bacterial cells in chicken breasts subjected to either freezing or heat treatment. After thermal treatment at 55 °C and 60 °C, the viable cell counts determined via the DEP method were found to be lower than those obtained using the conventional culture method, which implies that the DEP method may not be suitable for the direct detection of injured cells. In addition to its high accuracy and efficiency, the DEP method enables the determination of viable cell counts within 30 min, compared to 48 h required for the conventional culture method. In conclusion, the DEP method may be a potential alternative tool for rapid determination of viable bacteria in a variety of foodstuffs.

Effects of glass transition and hydration on the biological stability of dry yeast.

The purpose of this study was to determine the effects of glass transition and hydration on the storage stability of baker's dry yeast (Saccharomyces cerevisiae). The glass transition temperature (Tg ) of the yeast decreased with increase in water activity (aw ), and aw at which glass transition occurs at 25 °C was determined as the critical aw (awc ). From mechanical relaxation measurements at 25 °C, the yeast exhibited a large mechanical relaxation above the awc , and the degree of mechanical relaxation increased gradually with increasing aw . This behavior corresponded to a gradual increase in molecular mobility with increasing aw in the rubbery liquid state. Freezable water was observed from aw ≥0.810, and the proportion of freezable water increased with increasing aw . Examination of the effect of aw on the residual biological activity of yeast samples stored at 25 °C for 30 days revealed maximum residual biological activity at aw  = 0.225 to 0.432. In the lower aw range, the residual biological activity decreased because of oxidation of lipids. In the higher aw range, the residual biological activity decreased gradually with increasing aw . The yeast samples maintained a relatively high residual biological activity, because they could maintain relatively low molecular mobility even in the rubbery liquid state, as suggested by their mechanical relaxation behavior. At aw ≥0.809, residual activity decreased to a negligible value. This could be explained by the appearance of secondary hydrate water (freezable water). Hydrate water protects yeast cells from lipid oxidation but reduces the Tg . As a result, the yeast cells are stabilized maximally only at the awc . PRACTICAL APPLICATION: Although the growth rate of yeast cells becomes negligible below a certain aw , the biological activity of dry yeast decreases gradually during storage. The fact that dry yeast can be maximally stabilized at the awc is practically useful as a criterion for controlling storage stability. In addition, it was found that a remarkable reduction in the molecular mobility, which is otherwise ordinarily increased due to the glass-to-rubber transition, is prevented in yeast. It is possible that the crystallization of amorphous sugar can be prevented by yeast extract. The suggested effect is expected to result in enhanced quality of carbohydrate-based foods.

Modeling Invasion of Campylobacter jejuni into Human Small Intestinal Epithelial-Like Cells by Bayesian Inference.

Current approaches used for dose-response modeling of low-dose exposures of pathogens rely on assumptions and extrapolations. These models are important for quantitative microbial risk assessment of food. A mechanistic framework has been advocated as an alternative approach for evaluating dose-response relationships. The objectives of this study were to investigate the invasion behavior of Campylobacter jejuni, which could arise as a foodborne illness even if there are low counts of pathogens, into Caco-2 cells as a model of intestinal cells and to develop a mathematical model for invading cell counts to reveal a part of the infection dose-response mechanism. Monolayer-cultured Caco-2 cells and various concentrations of C. jejuni in culture were cocultured for up to 12 h. The numbers of C. jejuni bacteria invading Caco-2 cells were determined after coculture for different time periods. There appeared to be a maximum limit to the invading bacterial counts, which showed an asymptotic exponential increase. The invading bacterial counts were higher with higher exposure concentrations (maximum, 5.0 log CFU/cm2) than with lower exposure concentrations (minimum, 0.6 log CFU/cm2). In contrast, the ratio of invading bacteria (number of invading bacteria divided by the total number of bacteria exposed) showed a similar trend regardless of the exposure concentration. Invasion of C. jejuni into intestinal cells was successfully demonstrated and described by the developed differential equation model with Bayesian inference. The model accuracy showed that the 99% prediction band covered more than 97% of the observed values. These findings provide important information on mechanistic pathogen dose-response relationships and an alternative approach for dose-response modeling.IMPORTANCE One of the infection processes of C. jejuni, the invasion behavior of the bacteria in intestinal epithelial cells, was revealed, and a mathematical model for prediction of the cell-invading pathogen counts was developed for the purpose of providing part of a dose-response model for C. jejuni based on the infection mechanism. The developed predictive model showed a high accuracy of more than 97% and successfully described the C. jejuni invading counts. The bacterial invasion predictive model of this study will be essential for the development of a dose-response model for C. jejuni based on the infection mechanism.

Describing the Individual Spore Variability and the Parameter Uncertainty in Bacterial Survival Kinetics Model by Using Second-Order Monte Carlo Simulation.

The objective of this study was to separately describe the fitting uncertainty and the variability of individual cell in bacterial survival kinetics during isothermal and non-isothermal thermal processing. The model describing bacterial survival behavior and its uncertainties and variabilities during non-isothermal inactivation was developed from survival kinetic data for Bacillus simplex spores under fifteen isothermal conditions. The fitting uncertainties in the parameters used in the primary Weibull model was described by using the bootstrap method. The variability of individual cells in thermotolerance and the true randomness in the number of dead cells were described by using the Markov chain Monte Carlo (MCMC) method. A second-order Monte Carlo (2DMC) model was developed by combining both the uncertainties and variabilities. The 2DMC model was compared with reduction behavior under three non-isothermal profiles for model validation. The bacterial death estimations were validated using experimentally observed surviving bacterial count data. The fitting uncertainties in the primary Weibull model parameters, the individual thermotolerance heterogeneity, and the true randomness of inactivated spore counts were successfully described under all the iso-thermal conditions. Furthermore, the 2DMC model successfully described the variances in the surviving bacterial counts during thermal inactivation for all three non-isothermal profiles. As a template for risk-based process designs, the proposed 2DMC simulation approach, which considers both uncertainty and variability, can facilitate the selection of appropriate thermal processing conditions ensuring both food safety and quality.

Transforming kinetic model into a stochastic inactivation model: Statistical evaluation of stochastic inactivation of individual cells in a bacterial population.

Kinetic models performing point estimation are effective in predicting the bacterial behavior. However, the large variation of bacterial behavior appearing in a small number of cells, i.e. equal or less than 102 cells, cannot be expressed by point estimation. We aimed to predict the variation of Escherichia coli O157:H7 behavior during inactivation in acidified tryptone soy broth (pH3.0) through Monte Carlo simulation and evaluated the accuracy of the developed model. Weibullian fitted parameters were estimated from the kinetic survival data of E. coli O157:H7 with an initial cell number of 105. A Monte Carlo simulation (100 replication) based on the obtained Weibullian parameters and the Poisson distribution of initial cell numbers successfully predicted the results of 50 replications of bacterial inactivation with initial cell numbers of 101, 102, and 103 cells. The accuracy of the simulation revealed that more than 83% of the observed survivors were within predicted range in all condition. 90% of the distribution in survivors with initial cells less than 100 is equivalent to a Poisson distribution. This calculation transforms the traditional microbial kinetic model into probabilistic model, which can handle bacteria number as discrete probability distribution. The probabilistic approach would utilize traditional kinetic model towards exposure assessment.

Predictive Growth Model of Listeria monocytogenes Under Fluctuating Temperature Conditions in Pasteurized Milk by Using Real-Time Polymerase Chain Reaction.

The aim of this study was to evaluate the application of real-time polymerase chain reaction (PCR)-based quantification as a rapid and accurate tool for the monitoring and prediction of Listeria monocytogenes growth in pasteurized milk under constant and fluctuating temperature conditions. The growth of L. monocytogenes was monitored under constant temperature conditions at 4°C, 10°C, 15°C, 20°C, and 35°C. High correlation was obtained between the bacterial growth rate and incubation temperature, where the R2 of the slope of the square root model was calculated to be 0.993 and 0.996 for real-time PCR and the conventional culture method, respectively. Moreover, the obtained maximum specific growth rate (µmax) data plots were correlated with 188 L. monocytogenes µmax data points from the existing model according to ComBase database, with an R2 of 0.961 for real-time PCR and of 0.931 for the conventional culture method. The growth models were examined under three different patterns of fluctuating temperature conditions ranging from 2°C to 30°C. The prediction results fell within ±20% of the relative error zone, showing that real-time PCR quantification could be used for fast, sensitive, and specific bacterial growth monitoring with high-throughput results. Real-time PCR should be considered a promising option and powerful tool for the construction of a bacterial growth prediction model for safety risk analysis in the dairy industry.

Relationship between glass transition temperature, and desiccation and heat tolerance in Salmonella enterica.

Pathogenic bacteria such as Salmonella enterica exhibit high desiccation tolerance, enabling long-term survival in low water activity (aw) environments. Although there are many reports on the effects of low aw on bacterial survival, the mechanism by which bacteria acquire desiccation tolerance and resistance to heat inactivation in low-aw foods remains unclear. We focused on the glass transition phenomenon, as bacteria may acquire environmental tolerance by state change due to glass transition. In this study, we determined the glass transition temperature (Tg) in S. enterica serovars under different aw conditions using thermal rheological analysis (TRA). The softening behaviour associated with the state change of bacterial cells was confirmed by TRA, and Tg was determined from the softening behaviour. Tg increased as the aw decreased in all S. enterica serovars. For example, while the Tg of five S. enterica serovars was determined as 35.16°C to 57.46°C at 0.87 aw, the Tg of all the five serovars increased by 77.10°C to 83.30°C at 0.43 aw. Furthermore, to verify the thermal tolerance of bacterial cells, a thermal inactivation assay was conducted at 60°C for 10 min under each aw condition. A higher survival ratio was observed as aw decreased; this represented an increase in Tg for Salmonella strains. These results suggest that the glass transition phenomenon of bacterial cells would associate with environmental tolerance.

Inactivation kinetics of Bacillus cereus spores by Plasma activated water (PAW).

In recent years, plasma activated water has attracted more attention as a new disinfectant. The purpose of this study was to explore impact of variation of different treatment conditions on the inactivation kinetics of Bacillus cereus spores by PAW. All survival curves showed that the number of spores has decreased rapidly at first, followed by tailing results from the reduction inactivation rate. A linear and two nonlinear models (Weibull and Log-logistic model) were fitted to these data, and Log-logistic model fitted the inactivation of the B. cereus spores best. B. cereus spores in 106 CFU/mL was reduced by 1.62-2.96 log CFU/mL by PAW at 55 °C due to the reactive species generated in PAW. Elevated temperature, lower initial spore concentration, lower bovine serum albumin content, and smaller activation volume of PAW considerably enhanced PAW inactivation of B. cereus spores. These results provide an approach to evaluate the inactivation efficacy of different treatment conditions for PAW.

Combined d-Tryptophan Treatment and Temperature Stress Exert Antimicrobial Activity against Listeria monocytogenes in Milk.

ABSTRACT: d-Tryptophan (d-Trp) has a significant inhibitory effect on growth of gram-negative bacteria under osmotic stress. However, the inhibitory effect of d-Trp on the gram-positive Listeria monocytogenes under chilled and thermal stresses has not been evaluated previously. The effect of d-Trp on L. monocytogenes growth under cold and/or heat stress in milk and cream was dependent on the magnitude of the temperature stress. Low temperatures (4, 7, and 10°C) and treatment with 40 mM d-Trp resulted in significant inhibition of L. monocytogenes growth during the 4-week storage period. Lower temperatures more effectively inhibited growth. When added before thermal processing, 40 mM d-Trp completely inactivated L. monocytogenes (>6-log reduction) heated at 60°C for 25 min or 65°C for 20 min. These results suggest that d-Trp can be used as a preservative for controlling the growth of L. monocytogenes in milk and cream at refrigeration temperatures and could be used to enhance the thermal inactivation of L. monocytogenes.