Intra-particle oxygen diffusion limitation in solid-state fermentation

Oxygen limitation in solid-state fermentation (SSF) has been the topic of m odeling studies, but thus far, there has been no experimental elucidation o n oxygen-transfer limitation at the particle level. Therefore, intra-partic le oxygen transfer was experimentally studied in cultures of Rhizopus oligo sporus grown on the surface of solid, nutritionally defined, glucose and st arch media. The fungal mat consisted of two layers-an upper layer with spar se aerial hyphae and gas-filled interstitial pores, and a dense bottom laye r with liquid-filled pores. During the course of cultivation ethanol was de tected in the medium indicating that oxygen was depleted in part of the fun gal mat. Direct measurement of the oxygen concentrations in the fungal mat during cultivation, using oxygen microelectrodes, showed no oxygen depletio n in the upper aerial layer, but revealed development of steep oxygen conce ntration gradients in the wet bottom layer. Initially, the fungal mat was f ully oxygenated, but after 36.5 hours oxygen was undetectable at 100 mum be low the gas-liquid interface. This was consistent with the calculated oxyge n penetration depth using a reaction-diffusion model. Comparison of the ove rall oxygen consumption rate from the gas phase to the oxygen flux at the g as-liquid interface showed that oxygen consumption of the microorganisms oc curred mainly in the wet part of the fungal mat. The contribution of the ae rial hyphae to overall oxygen consumption was negligible. It can be conclud ed that optimal oxygen transfer in SSF depends on the available interfacial gas-liquid surface area and the thickness of the wet fungal layer. It is s uggested that the moisture content of the matrix affects both parameters an d, therefore, plays an important role in optimizing oxygen transfer in SSF cultures. (C) 2001 John Wiley & Sons, Inc.