Steam Explosion Sterilization Of Solid State Medium And Fermentation Process Intensification

Solid-state fermentation (SSF) is an eco-friendly process. Solid phase plays a role of supporting framework which is covered by a thin film of liquid phase while gas phase is continuous in solid state medium. Thermal conductivity of air is low and the solid state medium is not uniform, leading to difficult sterilization, nutrients degradation, fermentation heat accumulation, inadequate monitoring and automatic control in SSF. Therefore, SSF industrialization progress is slow. To solve the above problems, a novel sterilization strategy, high-temperature and short-time steam explosion sterilization, was exploited for solid state medium in this thesis. Effects of steam explosion sterilization on medium physicochemical properties and fermentation performance were further investigated. Moreover, a new variable pressure pulsation frequency method in gas double-dynamic solid-state fermentation was invented based on heat balance model to enhance heat transfer during fermentation process. Finally, digital image analysis system was established to monitor SSF process. Based on above research achievements with automatic control program development, SSF industrialization was achieved.The main research results obtained are as follows:(1) A new steam explosion sterilization method for solid state medium was developed. First, effects of steam explosion temperature and residence time on sterilization efficiency were investigated. Steam explosion conditions for complete sterilization were 172℃ for 2 min and 128℃ for 5 min. Steam explosion sterilization kinetics curve exhibited a non-linear behavior, which indicated that steam explosion has multiple targets of action on microbial cells. On one hand, thermal sterilization effect occurred during the high-temperature maintenance stage. On the other hand, microbial cells were damaged by the instant pressure release. These results indicated that steam explosion sterilization improved sterilization efficiency. Second, effects of steam explosion sterilization on nutrients contents were studied. Glucose content in medium under steam explosion sterilization conditions of 172℃ for 2 min,128 ℃ for 5 min were 0.035 g/g dry medium, 0.054 g/g dry medium, which increased by 66.7% and 157.1% respectively compared with that after conventional thermal sterilization (121℃ for 20 min). Results indicated that steam explosion sterilization improved solid state medium nutrition effectively. Finally, sterilization efficiency and steam consumption were compared between steam explosion sterilization and conventional thermal sterilization from industrial scale view. Since temperature was increased and cooling stage was canceled, sterilization time was shortened by 69.8% and steam consumption was reduced by 1.66% in steam explosion sterilization. Based on above research results, steam explosion sterilization industrialization was achieved.(2) Physical structure changes of solid state medium were investigated to reveal effects of steam explosion sterilization on SSF. Water states in solid state medium after steam explosion sterilization and conventional thermal sterilization were compared to clarify medium structure variations. Steam explosion sterilization significantly changed physical structure of solid state medium. T2b fraction (bound water) generated and T21 (capillary water), T22 (lumen water) fraction relaxation times increased in steam explosion sterilized medium, which indicated that steam explosion exposed hydrophilic groups at molecular level and enlarged pores and cavities at three-dimensional structural level. It was interesting to find that pores where capillary water located were the active sites for microbial growth, due to the close relationship among capillary water relaxation time, specific surface area and viable cell number. Therefore, steam explosion sterilization increased the effective contact area for microbial cells on solid state medium, which increased SSF productivity by 2.90 times than that of conventional thermal sterilization.(3) Variable pressure pulsation frequency method in gas double-dynamic solid-state fermentation was invented to enhance heat transfer during fermentation process. Heat balance model integrating pressure pulsation frequency, microbial metabolism, evaporative cooling and heat exchange with air was established. Variable pressure pulsation frequency was then optimized based on the heat balance model. Heat transfer and fermentation performance were investigated under the variable pressure pulsation frequency. The medium temperature increase extent reduced by 40.5% while the fermentation production (cellulase activity) increased by 9.23% under the variable frequency than that under conventional constant frequency. Therefore, the variable frequency was the preferred operation mode than the conventional constant one in gas double-dynamic solid-state fermentation.(4) Digital image analysis system for rapid determination of SSF process parameters was established. Based on the relationship between microbial biomass and average gray value of the medium, microbial biomass was determined by digital image analysis. With advantages of reasonable accuracy and low cost, this on-line and non-destructive method would provide much convenience for microbial biomass rapid determination and process optimization in SSF. Automatic control software for laboratory-scale gas double-dynamic solid-state fermentation bioreactor was developed on MCGS platform. Temperature, humidity and pressure were monitored in real time. Furthermore, automatic control logic diagram for industrial-scale fermentation process was designed to promote SSF industrialization.(5) SSF industrialization was achieved based on the above research achievements. Steam explosion sterilization, variable pressure pulsation frequency strategy and automatic control in gas double-dynamic solid-state fermentation were integrated for ecological fertilizer industrial production, which promote the modern SSF progress.