Biomedical subjects

Gary C Barker

Publications and source records attributed to Gary C Barker.

5 recordsLinked to original sources

PubMed2005

Sedimentation in bidisperse and polydisperse colloids.

We introduce a simple force and flux balance model for sedimentation and creaming in high volume fraction, polydisperse colloidal suspensions. The model is set alongside monodisperse and bidisperse sedimentation data for latex spheres, and we suggest that the broadening of the larger species sedimentation profile observed in the bidisperse case is linked to the particle pressure gradient arising from the smaller species. The model gives a satisfactory qualitative description of real emulsion creaming data, but implies either that the effective droplet radius is larger than the measured droplet radius, or that the effective background viscosity is reduced. Increasing the particle pressure gradient results in interface broadening at short times. We propose that the smallest emulsion droplets contribute to this broadening.

Journal Article↗

PubMed2004

Modeling the prevalence of Bacillus cereus spores during the production of a cooked chilled vegetable product.

In minimally processed vegetable foods, pathogenic spore-forming bacteria pose a significant hazard. As part of a quantitative risk assessment, we used Bayesian belief methods to model the uncertainty and variability of the number of Bacillus cereus spores that can be found in packets of a vegetable puree. The model combines specific information from the manufacturer, experimental data on inactivation of spores, and expert opinion concerning spore concentrations in the raw vegetables and ingredients. Sensitivity analysis revealed that spore contamination of added ingredients contributes most uncertainty to the assessment. The assessment produced a quantitative estimate of the prevalence of B. cereus spores in packets of vegetable puree at the end point of the manufacturing process.

Bacillus cereus↗

PubMed2003

A retail and consumer phase model for exposure assessment of Bacillus cereus.

An exposure assessment is conducted for psychrotrophic and mesophilic Bacillus cereus in a cooked chilled vegetable product. A model is constructed that covers the retail and consumer phase of the food pathway, using the output of a similar model on the industrial process as input. Microbial growth is the predominant process in the model. Variability in time and temperature during transport and storage is included in the model and different domestic refrigerator temperature distributions are compared. As an end point, probable levels of B. cereus colony forming units (cfu) in packages of vegetable purée are predicted at the moment the consumer takes the product from its refrigerator, that is prior to a cooking process. The psychrotrophic strain is predicted to end up above a threshold level of 10(5) cfu/g in 0.9% to 6.3% of the vegetable purée packages, depending on domestic refrigerator temperature. Accounting for spoilage this reduces to 0.3% to 2.4%. Even if the purée is stored at 4 degrees C in the domestic refrigerator and use-by-date (UBD) is respected, the threshold level may be passed. For the mesophilic strain the threshold level is rarely passed, but in contrast to the total viable count, the spore load at the end point is predicted to be higher than in the psychrotrophic strain. Our study illustrates how an exposure assessment model, which may be used in quantitative risk assessment, can integrate expertise in modelling, food processing and microbiology over the food pathway, and thus evaluate food safety, identify gaps in knowledge and compare risk management measures. As important gaps in knowledge, the lack of sporulation and germination models and data, validated non-isothermal growth models and a spoilage model useful for risk assessment are identified. Knowledge of the dose-response relationship is limited and does not allow a full risk assessment. It is shown that exposure can be lowered by lowering domestic refrigerator temperatures, and less so much by monitoring and withdrawing contaminated products at the end of industrial processing.

Bacillus cereus↗

PubMed2002

Characterisation and modelling of oscillatory behaviour related to reuterin production by Lactobacillus reuteri.

During reuterin production by Lactobacillus reuteri in a chemostat, the growth, substrate and metabolite concentrations showed oscillatory behaviour. The sensitivity of L. reuteri towards reuterin was shown to be a possible explanation of the oscillatory behaviour. A deterministic mathematical model consisting of four coupled differential equations describing the concentrations of biomass, glucose, glycerol and reuterin with time was developed. With a set of parameter values determined from batch experiments, the model was able to predict both oscillatory and steady state behaviour in a chemostat by changing the input variables. In a batch system, the model was able to give a satisfactory description of the glucose and glycerol concentrations but not of the biomass and reuterin concentrations. Mathematical modelling of the system was shown to be an effective and systematic approach in exploring a complex biological system.

Aldehydes↗

PubMed1997

Spatial interactions between subsurface bacterial colonies in a model system: a territory model describing the inhibition of Listeria monocytogenes by a nisin-producing lactic acid bacterium.

The effect of spatial separation on interactions between subsurface bacterial colonies was tested using a model system: the inhibition of Listeria monocytogenes by nisin-producing and nisin-non-producing Lactococcus lactis subsp. lactis. Separation distance was controlled by altering the number of inoculum organisms within the agar. Mean separation distance was calculated by determining the mean volume available to each cell at the start of the experiment. Inhibition was assessed by comparing the growth of L. monocytogenes in pure culture with its growth in the presence of Lac. lactis subsp. lactis. Increasing the distance between colonies resulted in an exponential decrease in inhibition. When L. monocytogenes and Lac. lactis subsp. lactis colonies were within 100 microns of each other, the increase in cell numbers per L. monocytogenes colony was only 0.6 c.f.u. (which indicated some cells had become non-viable). This was a log reduction of 3.5 compared to the pure culture control. A separation distance of 1000 microns resulted in a L. monocytogenes colony growth increment of 2.5 x 10(2) c.f.u. per colony, a log reduction of 3.0 compared to the control. Increasing the separation distance to 3000 microns resulted in a L. monocytogenes colony growth increment of 1.3 x 10(6) c.f.u. per colony, a log reduction of 0.9 compared to the control. The effects of nisin and acidity were investigated by using a nisin-non-producing strain of Lac. lactis subsp. lactis and by buffering the medium. Data were obtained for the effect of separation on inhibition, as well as competition between colonies of the same species. The inhibition was mathematically described in terms of a simplified 'territory' model of immobilized bacterial growth. There was a strong qualitative agreement between the mathematical model and the experimental data. It was concluded that the phenomenon of propinquity is of important consideration when modeling and predicting microbial growth within solid food systems.

Antibiosis↗