Intra‐particle oxygen diffusion limitation in solid‐state fermentation

Abstract

Oxygen limitation in solid-state fermentation (SSF) has been the topic of modeling studies, but thus far, there has been no experimental elucidation on oxygen-transfer limitation at the particle level. Therefore, intra-particle oxygen transfer was experimentally studied in cultures of Rhizopus oligosporus grown on the surface of solid, nutritionally defined, glucose and starch media. The fungal mat consisted of two layers--an upper layer with sparse aerial hyphae and gas-filled interstitial pores, and a dense bottom layer with liquid-filled pores. During the course of cultivation ethanol was detected in the medium indicating that oxygen was depleted in part of the fungal mat. Direct measurement of the oxygen concentrations in the fungal mat during cultivation, using oxygen microelectrodes, showed no oxygen depletion in the upper aerial layer, but revealed development of steep oxygen concentration gradients in the wet bottom layer. Initially, the fungal mat was fully oxygenated, but after 36.5 hours oxygen was undetectable at 100 microm below the gas-liquid interface. This was consistent with the calculated oxygen penetration depth using a reaction-diffusion model. Comparison of the overall oxygen consumption rate from the gas phase to the oxygen flux at the gas-liquid interface showed that oxygen consumption of the microorganisms occurred mainly in the wet part of the fungal mat. The contribution of the aerial hyphae to overall oxygen consumption was negligible. It can be concluded 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 suggested that the moisture content of the matrix affects both parameters and, therefore, plays an important role in optimizing oxygen transfer in SSF cultures.

Insights into the cellular responses to hypoxia in filamentous fungi

“…A total number of 10 5 conidia were point inoculated on AMM-Agar with 10 % (v/v) Gyamerah 1995;Wang et al 2005). Also, in solid-state fermentations of the zygomycete Rhizopus oligosporus, microelectrode measurements have shown that in mature fungal mats, O 2 saturation is only maintained at the colony surface and decreases rapidly to nearly anaerobic levels within less than a few hundred micrometers distance (Oostra et al 2001). This O 2 gradient is exploited by the formation of the aerial hyphae, which function as the fungal respiratory organs, accomplishing around 75 % of the mycelial O 2 uptake in A. oryzae (Rahardjo et al 2002).…”

Monitoring fermentation parameters during phytase production in column-type bioreactor using a new data acquisition system

“…As the fungal mycelium develops on a solid surface, the void spaces between the hyphae can either be fully or partially filled with water. In the former case, it results in severe oxygen limitation causing anaerobic conditions affecting the performance of SSF [27,28]. The phytase production was 10.59% higher with forced air (1.0 N mL g -1 min -1 and 65% of initial moisture), achieving the average of 94.24 U gds -1 [more than without forced air (84.26 U gds -1 )].…”

An Overview on Fungal Cellulases with an Industrial Perspective

“…In SSF, the operating conditions like temperature, pH and moisture content are vital factors influencing the microbial growth and production of cellulase. Availability of oxygen in the open space between substrate particles (i.e., porosity) and generation of heat are the major challenges in SSF, which have to be addressed properly [55,56].…”