Modelling spoilage of fresh turbot and evaluation of a time-temperature integrator (TTI) label under fluctuating temperature

Kinetic models were developed to predict the microbial spoilage and the sensory quality of fresh fish and to evaluate the efficiency of a commercial time-temperature integrator (TTI) label, Fresh Check(R), to monitor shelf life. Farmed turbot (Psetta maxima) samples were packaged in PVC film and stored at 0, 5, 10 and 15 degrees C. Microbial growth and sensory attributes were monitored at regular time intervals. The response of the Fresh Check device was measured at the same temperatures during the storage period. The sensory perception was quantified according to a global sensory indicator obtained by principal component analysis as well as to the Quality Index Method, QIM, as described by Rahman and Olley [Rahman, H.A., Olley, J., 1984. Assessment of sensory techniques for quality assessment of Australian fish. CSIRO Tasmanian Regional Laboratory. Occasional paper n. 8. Available from the Australian Maritime College library. Newnham. Tasmania]. Both methods were found equally valid to monitor the loss of sensory quality. The maximum specific growth rate of spoilage bacteria, the rate of change of the sensory indicators and the rate of change of the colour measurements of the TTI label were modelled as a function of temperature. The temperature had a similar effect on the bacteria, sensory and Fresh Check kinetics. At the time of sensory rejection, the bacterial load was ca. 10(5)-10(6) cfu/g. The end of shelf life indicated by the Fresh Check label was close to the sensory rejection time. The performance of the models was validated under fluctuating temperature conditions by comparing the predicted and measured values for all microbial, sensory and TTI responses. The models have been implemented in a Visual Basic add-in for Excel called "Fish Shelf Life Prediction (FSLP)". This program predicts sensory acceptability and growth of spoilage bacteria in fish and the response of the TTI at constant and fluctuating temperature conditions. The program is freely available at http://www.azti.es/muestracontenido.asp?idcontenido=980&content=15&nodo1=30&nodo2=0.     

Sporulation boundaries and spore formation kinetics of Bacillus spp. as a function of temperature, pH and a(w)

Sporulation niches in the food chain are considered as a source of hazard and are not clearly identified. Determining the sporulation environmental boundaries could contribute to identify potential sporulation niches. Spore formation was determined in a Sporulation Mineral Buffer. The effect of incubation temperature, pH and water activity on time to one spore per mL, maximum sporulation rate and final spore concentration was investigated for a Bacillus weihenstephanensis and a Bacillus licheniformis strain.     

Effects of heat-processing regime, pH, water activity and their interactions on the behaviour of Listeria monocytogenes in ground pork. Modelling the boundary of the growth/no-growth areas as a function of pH, water activity and temperature

The influence of four heat‐processing regimes and a storage phase on the behaviour of Listeria monocytogenes in ground pork was studied. The effects of pH and water activity (a_w) were also tested. During the heat process phase, a_w, the heat‐processing regime and its interactions with pH or a_w, had a significant effect on the behaviour of L. monocytogenes. During the storage phase, all parameters tested and their interactions had significant effects. Nevertheless, the area in which the growth of L. monocytogenes was observed at the end of the experiment was not influenced by the heat‐processing regime tested. On the contrary, pH, a_w and their interactions had significant effects on Listeria behaviour. The boundary of the growth area delimited by environmental conditions where growth was higher than 1.0 Log CFU g^?1 from those where growth was lower than this limit was correctly predicted by Augustin's model.     

Modelling the effects of pH, storage temperature and redox potential (Eh) on the survival of bifidobacteria in fermented milk

The effects of storage temperature, pH and E _h on the survival of Bifidobacterium longum and Bifidobacterium animalis ssp. lactis were modelled in a 3 × 4 × 3 factorial design using three storage temperatures (4, 8, 12 °C), four pH (4.0, 4.25, 4.5, 4.75) and three E _h (0, 200, 400 mV) values. Bifidobacterium animalis ssp. lactis showed significantly better survival than B. longum ( P  < 0.0001) in fermented milk under normal environmental conditions. Bifidobacterium longum was more susceptible to acidity and survival was log-linear at low pH values whereas at higher pH pronounced shouldering and tailing effects were apparent. Both individual environmental conditions ( T , pH, E _h) and interactions between E _h and pH affected survival and death was always more rapid in B. longum . Bifidobacterium animalis ssp. lactis generally showed a simple log-linear reduction over time. Temperature, pH and E _h could be manipulated to enhance survival but E _h reduction appears to be the best way to enhance survival without adversely affecting other properties.     

Model for the combined effects of temperature, pH and sodium chloride concentration on survival of Shigella flexneri strain 5348 under aerobic conditions

Shigella is recognized as a major foodborne pathogen; however, relatively few studies have been reported on its growth and survival characteristics, particularly under conditions relevant to food. A fractional factorial design was used to measure the effects and interactions of temperature (4-37 degrees C), pH (2-6) and NaCl (0.5-9%) on survival kinetics of Shigella flexneri strain 5348 in BHI broth. Stationary-phase cells were inoculated into sterile media to give initial populations of 6-7 log(10) CFU/ml and bacterial populations were determined periodically by aerobic plate counts. A total of 267 cultures, representing 83 variable combinations of temperature, pH and NaCl concentration, were analyzed. Survivor curves were fitted from plate count data by means of a two-phase linear model to determine lag times and slopes of the curves, from which decimal reduction times (D-values) and times to a 4-log10 inactivation (t 4D) were calculated. Second order response surface models in terms of temperature, initial pH and NaCl concentration were obtained for the inactivation kinetics parameters of S. flexneri using regression analysis. The use of log10 transformation of the inactivation kinetics parameters yielded models with R2 values of >0.8. These models can provide an estimate of Shigella inactivation. The data obtained suggest that Shigella is resistant to acid and salt and that low pH foods stored at low temperatures may serve as vehicles for gastrointestinal illness.     

Modeling the boundaries of growth of Salmonella Typhimurium in broth as a function of temperature, water activity, and pH

The growth limits of a mixture of five strains of Salmonella Typhimurium in tryptic soy broth were examined at different environmental conditions. The response of the pathogen was monitored in a total of 350 combination treatments of temperature (10 to 35 degrees C), pH (3.76 to 6.44), and water activity (aw, 0.913 to 0.990) for 62 days. No growth/growth (turbidity) data were modeled by logistic polynomial regression. The concordance index of the logistic model was 99.8%, indicating a good fit to the observed data. The minimum pH and aw values that permitted growth were 3.94 and 0.942, respectively, and occurred in the temperature range of 25 to 35 degrees C. At temperatures below this range, the minimum pH and aw allowing growth increased as the temperature decreased. The results showed an abrupt change in the probability of growth close to the boundary with minor changes of the environmental factors. The probabilities predicted by the model were compared with published data on the actual response of Salmonella Typhimurium or other salmonellae serotypes in 50 cases of food products, including salad dressing, mayonnaise, meat, cheese, vegetables, and fruits. The model predicted successfully the response of the pathogen in 90% of the tested cases. The results of the study indicated that the developed model predicts satisfactorily the growth/no growth interface of Salmonella Typhimurium in foods and can provide useful quantitative data for the development of safer food products and processes.     

Modelling the effect of temperature, carbon dioxide, water activity and pH on growth and histamine formation by Morganella psychrotolerans

A mathematical model was developed to predict growth and histamine formation by Morganella psychrotolerans depending on temperature (0-20 degrees C), atmosphere (0-100% CO2), NaCl (0.0-6.0%) and pH (5.4-6.5). Data from experiments with both sterile tuna meat and Luria Bertani broth was used to develop the mathematical growth and histamine formation model. The expanded Logistic model with a growth dampening parameter (m) of 0.7 was used as primary growth model. A primary model for histamine formation during storage was obtained by combining the expanded Logistic growth model with a yield factor (YHis/CFU). 120 maximum specific growth rate (micromax)-values were generated for M. psychrotolerans and used to model the combined effect of the studied environmental parameters. A simple cardinal parameter type secondary model was used to model the effect of the four parameters on micro(max). The maximum population density (log Nmax) was correlated with log (YHis/CFU) and a simple constrained polynomial (quadratic) secondary model was developed for the effect of the environmental conditions on these model parameters. The developed model describes the effect of initial cell concentrations, storage conditions and product characteristics on histamine formation. This is a significant progress compared to previously available models for the effect of storage temperature only.     

Response surface models for effects of temperature, pH, and previous growth pH on growth kinetics of Salmonella Typhimurium in brain heart infusion broth

Response surface models were developed for effects of temperature (15 to 40 degrees C), pH (5.2 to 7.4), and previous growth pH (5.7 to 8.6) on lag time (lambda) and specific growth rate (mu) of Salmonella Typhimurium in brain heart infusion broth (BHIB). Seventy-five growth curves for model development and 30 growth curves for model validation were fit to a two-phase linear growth model to obtain direct estimates of lambda and mu of Salmonella Typhimurium in BHIB. Response surface models for natural logarithm transformations of lambda and mu as a function of temperature, pH, and previous growth pH were obtained by regression analysis. Previous growth pH did not alter (P > 0.05) or interact with temperature or pH to alter subsequent growth kinetics of Salmonella Typhimurium. However, lambda and mu of Salmonella Typhimurium in BHIB were affected (P < 0.05) by linear and quadratic effects of temperature and pH. The models were validated against data not used in their development. Mean absolute relative error of predictions (model accuracy) was 7.8% for lambda and 6.6% for mu. Median relative error of predictions (model bias) was -1.8% for lambda and -2.8% for mu. Results of the current study indicated that the models developed accurately predicted growth kinetics of Salmonella Typhimurium in BHIB within the matrix of factors modeled and that the range of previous growth pH (5.7 to 8.6) investigated did not alter the subsequent growth kinetics of Salmonella Typhimurium in BHIB.     

Effect of pH, NaCl content, and temperature on growth and survival of Arcobacter spp

Growth and survival of six human isolates of the pathogenic Arcobacter spp. in the presence of selected environmental factors were studied. Four strains of Arcobacter butzleri and two strains of Arcobacter cryaerophilus were exposed to pH levels of 3.5 to 8.0. Most strains grew between pH 5.5 and 8.0, with optimal growth of most A. butzleri and A. cryaerophilus strains at pH 6.0 to 7.0 and 7.0 to 7.5, respectively. The 24-h optimal growth range in the presence of NaCl was 0.5 to 1.0% for A. cryaerophilus. However, after 96 h, the optimum was between 0.5 and 2.0% NaCl. The optimum range for growth of A. butzleri strains was 0.09 to 0.5% NaCl after 96 h. The upper growth limits were 3.5 and 3.0% NaCl for A. butzleri and A. cryaerophilus, respectively. Survival at 25 degrees C in up to 5% NaCl was noted for A. butzleri 3556 and 3539 and A. cryaerophilus 3256. Decimal reduction times (D-values) at pH 7.3 in phosphate-buffered saline for three A. butzleri strains were 0.07 to 0.12 min at 60 degrees C, 0.38 to 0.76 min at 55 degrees C, and 5.12 to 5.81 min at 50 degrees C. At pH 5.5, decreased thermotolerance was observed, with D-values of 0.03 to 0.11 min at 60 degrees C, 0.30 to 0.42 min at 55 degrees C, and 1.97 to 4.42 min at 50 degrees C. Calculated z-values ranged from 5.20 to 6.28 degrees C. D-values of a three-strain mixture of A. butzleri in raw ground pork were 18.51 min at 50 degrees C and 2.18 min at 55 degrees C. Mild heat (50 degress C) followed by cold shock (4 or 8 degrees C exposure) had a synergistic lethal effect, reducing more cells than with an individual 50 degrees C treatment or with cold shock temperatures of 12 or 16 degrees C.     

Influence of the conditions of storage and cooking on growth,invasion and survival of Salmonella enteritidis in eggs

Basal studies for the confirmation of sanitary rules in the kitchen were performed, focusing on preventing an outbreak of food poisoning due to eggs contaminated with Salmonella Enteritidis (SE), using hen and quail eggs. SE did not grow at 5 degrees C but grew markedly at 25 degrees C in eggs. The invasion and growth of SE were marked under very humid conditions regardless of whether the eggshell was damaged. The invasion of SE into egg also occurred when eggs were taken in and out of the refrigerator. Moreover, SE was spread immediately to all non-contaminated eggs when SE-contaminated eggs were cracked into a bowl with non-contaminated eggs. In homemade mayonnaise containing 15% vinegar, sterilization took several hours to occur. On a stainless-steel bowl, SE survived for 2 weeks or more. These findings suggest that it is necessary to pay attention to secondary contamination.     

Milk pH as a function of CO2 concentration, temperature, and pressure in a heat exchanger

Raw skim milk, with or without added CO2, was heated, held, and cooled in a small pilot-scale tubular heat exchanger (372 ml/min). The experiment was replicated twice, and, for each replication, milk was first carbonated at 0 to 1 degree C to contain 0 (control), 600, 1200, 1800, and 2400 ppm added CO2 using a continuous carbonation unit. After storage at 0 to 1 degree C, portions of milk at each CO2 concentration were heated to 40, 56, 72, and 80 degrees C, held at the desired temperature for 30 s (except 80 degrees C, holding 20 s) and cooled to 0 to 1 degree C. At each temperature, five pressures were applied: 69, 138, 207, 276, and 345 kPa. Pressure was controlled with a needle valve at the heat exchanger exit. Both the pressure gauge and pH probe were inline at the end of the holding section. Milk pH during heating depended on CO2 concentration, temperature, and pressure. During heating of milk without added CO2, pH decreased linearly as a function of increasing temperature but was independent of pressure. In general, the pH of milk with added CO2 decreased with increasing CO2 concentration and pressure. For milk with added CO2, at a fixed CO2 concentration, the effect of pressure on pH decrease was greater at a higher temperature. At a fixed temperature, the effect of pressure on pH decrease was greater for milk with a higher CO2 concentration. Thermal death of bacteria during pasteurization of milk without added CO2 is probably due not only to temperature but also to the decrease in pH that occurs during the process. Increasing milk CO2 concentration and pressure decreases the milk pH even further during heating and may further enhance the microbial killing power of pasteurization.     

肠炎沙门菌的温度、pH值和水分活度主参数模型的构建

为探究不同环境条件对肠炎沙门菌的生长规律影响,以液体培养基为基质,采用Baranyi模型作为一级模型,主参数模型作为二级模型。建立肠炎沙门菌在不同温度（30～42?℃）、pH值（6.6～8.0）、水分活度（0.976～0.997）环境影响下的主参数模型,基于Gamma概念,研究3?种环境的独立影响。通过决定系数评价一级模型的拟合程度、数学检验值以及拓展检验评价二级模型的有效性。结果显示：Baranyi模型可以很好地拟合肠炎沙门菌的生长（R2＞0.900）。拟合得到的主参数模型参数值分别为：Tmax?44.51?℃、Topt?34.24?℃、pHmax?8.14、pHmin?6.09、pHopt?7.55、aw（min）0.957和aw（opt）0.997,实验范围内随机组验证主参数模型得到的均方误差为2.311×10－5、偏差因子为0.963和准确因子为1.267,均在可接受范围内。本实验构建的主参数模型能有效描述肠炎沙门菌的生长状况,为食品安全保证提供良好的理论依据,可用于减少由致病菌引发的食品安全风险。     

Performance evaluation of a model describing the effects of temperature,water activity,pH and lactic acid concentration on the growth of Escherichia coli

A square root-type model for Escherichia coli growth in response to temperature, water activity, pH and lactic acid was developed by Ross et al. [Int. J. Food Microbiol. (2002).]. Predicted generation times from the model were compared to the literature data using bias and accuracy factors, graphical comparisons and plots of residuals for data obtained from both liquid growth media and foods. The model predicted well for 1025 growth rate estimates reported in the literature after poor quality or unrepresentative data (n=215) was excluded, with a bias factor of 0.92, and an accuracy factor of 1.29. In a detailed comparison to two other predictive modes for E. coli growth, Pathogen Modeling Program (PMP) and Food MicroModel (FMM), the new model generally performed better. The new model consistently gave better predictions than the other models at generation times 相似文献   

Effects of pH, temperature and NaCl concentration on the growth kinetics, proteolytic activity and biogenic amine production of Enterococcus faecalis

In this work, the combined effects of temperature, pH and NaCl concentration on the growth dynamics of Enterococcus faecalis EF37, its proteolytic activity and its production of biogenic amines have been studied. The effects of the selected variables have been analysed using a Central Composite Design. The production of biogenic amines, under the adopted conditions, was found to be mainly dependent on the extent of growth of E. faecalis. Its proteolytic activity was not a limiting factor for the final amine production, because in the system studied (skim milk) an excess of precursors was guaranteed. Quantitatively, the most important biogenic amine produced was 2-phenylethylamine but substantial amounts of tyramine were detected in all the samples. This work confirms that the main biological feature influencing the biogenic amine formation is the extent of growth of microorganisms, like E. faecalis, characterised by decarboxylase activity. In the traditional and artisanal cheeses produced using raw milk, enterococci usually reach levels of 10(7) cells/g. With this perspective, it is important that the presence of biogenic amines due to the activities of these microorganisms is maintained within safe levels, without affecting the positive effects of enterococci on the final organoleptic characteristics of the cheese.     

Effect of pH,concentration, and temperature on the strength of cytoplasmic protein foams

The strength of foams generated from solutions of red clover cytoplasmic protein is dependent on pH, protein concentration, arid temperature. The variation of foam strength as a function of these three parameters has been studied and results indicate that the onset of foam stability occurs over very short concentration and temperature ranges. A phase diagram for these foams is proposed which defines the differences between stable and unstable states in terms of protein concentration and temperature. The results show that above 40°c only unstable foams are generated, irrespective of protein concentration. Below 40°c, stable foams can be generated provided the protein concentration is greater than a certain critical concentration for the system. A possible implication in bloat studies is suggested.     

Application of a transmission model to estimate performance objectives for Salmonella in the broiler supply chain

The aim of the present study was to demonstrate how Performance Objectives (POs) for Salmonella at various points in the broiler supply chain can be estimated, starting from pre-set levels of the PO in finished products. The estimations were performed using an analytical transmission model, based on prevalence data collected throughout the chain in The Netherlands. In the baseline (current) situation, the end PO was set at 2.5% of the finished products (at end of processing) being contaminated with Salmonella. Scenario analyses were performed by reducing this baseline end PO to 1.5% and 0.5%. The results showed the end PO could be reduced by spreading the POs over the various stages of the broiler supply chain. Sensitivity analyses were performed by changing the values of the model parameters. Results indicated that, in general, decreasing Salmonella contamination between points in the chain is more effective in reducing the baseline PO than increasing the reduction of the pathogen, implying contamination should be prevented rather than treated. Application of both approaches at the same time showed to be most effective in reducing the end PO, especially at the abattoir and during processing. The modelling approach of this study proved to be useful to estimate the implications for preceding stages of the chain by setting a PO at the end of the chain as well as to evaluate the effectiveness of potential interventions in reducing the end PO. The model estimations may support policy-makers in their decision-making process with regard to microbiological food safety.     

The effect of ionic strength, polyphosphates type, pH, cooking temperature and preblending on the functional properties of normal and pale, soft, exudative (PSE) pork

Compared to processed meat product made from normal pork, products made from pale soft exudative (PSE) pork have higher cook loss (CL) and weaker texture. In this study interactions between a range of processing conditions (ionic strength, polyphosphate addition, polyphosphate chain length, pH, cooking temperature and time between preparation and cooking), and their effect on the texture [shear stress (SS), true shear strain (TSS)] and CL of gels made from normal and PSE pork were examined. Of the processing conditions studied, ionic strength, polyphosphate addition and polyphosphate chain length affected the functional properties of normal and PSE pork differently. Generally, the functional properties of normal pork were superior to PSE pork, with no combination of conditions making all the functional properties of PSE pork equal to those of normal pork under the same conditions. The combination of conditions that was most effective in reducing the difference between normal and PSE pork was high ionic strength in the presence of added polyphosphate. Under these conditions there was no significant difference in CL between normal and PSE pork, although the texture (SS and TSS) of the PSE pork samples was still inferior.