Solid-State Cultivation Bioreactors

Abstract

Solid-state cultivation (SSC) involves the cultivation of microorganisms on moist solid particles surrounded by a continuous gas phase. Although SSC has been used for centuries in the production of traditional fermented foods, most biotechnological products are currently produced by submerged culture. However, SSC will become of increasing importance for processing solid residues in biorefineries, especially when filamentous fungi are involved. This chapter presents the basic design and operating principles of the four bioreactor types available for SSC processes: trays, packed beds, rotating (or stirred) drums, and forcefully aerated agitated bioreactors. Control of bed temperature is the main challenge, with the selection of an appropriate bioreactor depending on the specific growth rate of the process organism and also its tolerance of agitation. The bioreactors differ with respect to whether or not the substrate bed is agitated and whether or not the substrate bed is forcefully aerated, offering different combinations of conductive, convective, and evaporative cooling. The best tools for guiding the scale-up of SSC bioreactors are mathematical models that integrate growth kinetics with energy and water balances and which recognize and describe the gradients that occur across the bed during periods of static operation. Due to the complexity of the microscopic-scale phenomena involved in growth of the microorganism on the substrate particles, the kinetic equations used in these models are usually simple empirical equations, such as the logistic equation.