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The metabolic processes involved in simultaneous production of vitamin B12 and propionic acid by Propionibacterium freudenreichii are very complicated. To further investigate the regulatory mechanisms of this metabolism, a simplified metabolic network was established. The effects of glucose feeding, propionic acid removal, and 5,6-dimethylbenzimidazole (DMB) addition on the metabolic flux distribution were investigated. The results showed that synthesis of propionic acid can be increased by increasing the metabolic flux through the oxaloacetate and methylmalonyl-CoA branches in the early and middle stages of the coupled fermentation. After DMB addition, the synthesis of vitamin B12 was significantly enhanced via increased metabolic flux through the δ-aminolevulinate branch, which promoted the synthesis of uroporphyrinogen III, a precursor of vitamin B12. Therefore, the analysis of metabolic flux at key nodes can provide theoretical guidance for the optimization of P. freudenreichii fermentation processes. In an experimental coupled fermentation process, the concentrations of vitamin B12 and propionic acid reached 21.6 and 50.12 g/L respectively, increased by 105.71% and 73.91% compared with batch fermentation, which provides a new strategy for industrial production.

In the present study, a modified multi-grid fire evacuation model (MG-Fire-Evac) was devised to study the influence of fire-related phenomena such as toxic smoke and heat on occupant evacuating from a burning building. The degree of injury suffered by the occupants is quantified in terms of “health points” (HPs) in the model. When a person’s HP is less than 0.5 or 0.8, he or she is regarded as having succumbed to the fire (becoming a fatality or serious injury). A parameter is defined to represent the degree of influence of the fire on the occupants’ movement, which is used to predict which of three possible kinds of egress is attempted. To decrease the degree of error in the data transferred between the fire simulation and egress model, a finer grid resolution is adopted in the new model. For the scenario examined, simulation results demonstrate that a higher fire load, longer pre-movement time, or narrower exit would all lead to more serious occupant injury in the event of a fire. Based on the MG-Fire-Evac model, a new building fire risk procedure is presented whereby it is possible to predict the death toll and number of serious injuries and to quantitatively evaluate evacuation plans. Compared to the conventional method, the new method is stricter and more scientific. The model and method presented herein will be useful for application to the performance-based design, assessment, and management of building fire safety.