Towards a Fundamental Understanding of Solid State Bioprocessing

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Abstract

Solid state fermentation and solid state bioprocessing have been recognised for their unique features. However, there are several unsolved challenges which have prevented their widespread adoptions. In order to address these challenges, an investigation has been conducted, aiming at both identifying and addressing the problems. Two common applications of solid state bioprocessing, namely reducing toxins in raw materials and producing generic microbial feedstocks, were studied and are reported in Part A of the thesis (Chapter 3 and Chapter 4). The findings reported in Chapter 3 show that by applying a two stage solid state bioprocess, the major toxin in rapeseed meal can be reduced to a level suitable for animal consumption. The first stage, pre-incubation, greatly assisted toxin reduction in the second stage (fermentation), through the addition of water into the substrate. Although the exact mechanisms were not elucidated, it was clear that water is a critical factor in the process. In Chapter 4, a three stage solid state bioprocess was used to produce a generic microbial feedstock from mixed rapeseed meal and sugarcane bagasse. The main findings from this study were that such processes are highly variable, and that the performance of solid state fermentation is highly influenced by the microscopic environment of the microorganism. This led to the conclusion that a more fundamental understanding of the system is required. To obtain a better fundamental understanding, the effect of water on fungi cultivated on solid substrates (water bioavailability), was investigated in Part B of the thesis which includes theoretical and experimental investigations. The following theory has been developed from the findings. “Fungal growth can only consume water from the surrounding local micro-environment. As germination and growth occur, they result in a modest depletion of water from the immediate vicinity. This creates a driving force for water to migrate from the bulk substrate (usually through diffusion), or to be absorbed from the gas phase, to replenish the water that has been consumed. The driving force within the substrate increases with fungal water consumption, while water absorption from the gas phase is driven both by the gas phase relative humidity and presence/absence of water at the substrate surface. Meanwhile, the resistance to mass transfer is greatly affected by both chemical and physical properties of the substrate. Fungal growth can continue, if, and only if water within their immediate vicinity can be replenished.” Experiments showed that water bioavailability cannot be fully represented by terms such as “water content” and “water activity”, but is affected by the presence of water in the immediate vicinity of the fungus. This is under the influence of substrate chemical properties and consequently affects germination and early growth. It is also affected by mobility of water through the substrate which impacts on growth rate in the absence of gas phase water. Further, if there is water in the gas phase, it can fully compensate reduced water availability in the substrate, and even support growth with no substrate water. Experiments also showed that solid state fermentation may have distinct advantages compared to submerged fermentation. The physical structure of solid substrates allows oxygen and fungal hyphae to penetrate by providing surfaces for oxygen transfer, without forced aeration and without disturbing the natural growth pattern of the fungus. Water can be variously supplied either from the substrate or from the gas phase, and therefore, compared to submerged fermentation, gives greater process flexibility, which may enable water to be used more effectively and the needs for downstream extraction to be reduced. The project presented in this thesis is only the starting point for understanding the effects of bioavailability of water in solid substrates. Hopefully, it can be a pioneering work for gaining a better fundamental understanding of solid state bioprocessing systems, and by gaining this better understanding, can bring a new era to this ancient technology

Keyword(s)

Dynamic vapour sorption; Microbial activity; Solid state fermentation; Water; filamentous fungi

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