Abstract

Solid Substrate Cultivation (SSC) systems are microbiological processes where growth and product formation are carried out inside a solid medium, generally porous, in absence of excess water. They comprise four interacting phases: a gas phase, a solid insoluble support, a liquid phase containing dissolved substrates and products and a biotic phase formed by the micro-organisms. Although several works have been reported using yeasts and bacteria in SSC, filamentous fungi are by their nature the most adapted to this kind of environment. This is due to their specific capacity to colonize the interparticle spaces of the solid matrix that sustains them and to their ability to grow well at water activities under 0.98. Given the inherent complexity of scaling up heterogeneous solid substrate reactors, SSC has not been developed at the same technological level as Liquid Substrate Cultivation (LSC) processes. In addition, efficient operation of SSC bioreactors is hampered by the lack of reliable and affordable devices to measure relevant operating variables inside the fermenting bed (like pH, water content, dissolved oxygen, biomass, and substrate concentration). Furthermore, the metabolic heat generation and the slow heat transfer inside the bed cause a temperature rise difficult to regulate. However, several advantages over LSC have been identified for SSC processes, like higher productivities, low cost substrates, and differential gene expression. In spite of these advantages, the state of the art in SCC technology is only profitable in small scale processes associated with products of high added value as those of biopharmaceutical origin (antibiotics, drugs, enzymes and organic acids). In our opinion, SSC technology can be significantly improved if the latest developments in process modeling, instrumentation and control are incorporated into the design and operation of SSC bioreactors. Hence, the scope of applicability of SSC processing will be widened extensively, particularly at large production scale.