Estimation of hyphal tensile strength in production-scale Aspergillus oryzae fungal fermentations

Fragmentation of filamentous fungal hyphae depends on two phenomena: hydrodynamic stresses, which lead to hyphal breakage, and hyphal tensile strength, which resists breakage. The goal of this study was to use turbulent hydrodynamic theory to develop a correlation that allows experimental data of morphology and hydrodynamics to be used to estimate relative (pseudo) tensile strength (σpseudo) of filamentous fungi. Fed‐batch fermentations were conducted with a recombinant strain of Aspergillus oryzae in 80 m3 fermentors, and measurements were made of both morphological (equivalent hyphal length, L) and hydrodynamic variables (specific power input, $\bar \varepsilon$; kinematic viscosity, v). We found that v increased over 100‐fold during these fermentations and, hence, Kolmogorov microscale (λ) also changed significantly with time. In the impeller discharge zone, where hyphal fragmentation is thought to actually take place, λ was calculated to be 700–3500 μm, which is large compared to the size of typical fungal hyphae (100–300 μm). This result implies that eddies in the viscous subrange are responsible for fragmentation. Applying turbulent theory for this subrange, it was possible to calculate σpseudo from morphological and hydrodynamic measurements. Pseudo tensile strength was not constant but increased to a maximum during the first half and then decreased during the second half of each fermentation, presumably due to differences in physiological state. When a literature correlation for hyphal fragmentation rate (kfrag) was modified by adding a term to account for viscosity and tensile strength, the result was better qualitative agreement with morphological data. Taken together, these results imply hyphal tensile strength can change significantly over the course of large‐scale, fed‐batch fungal fermentations and that existing fragmentation and morphology models may be improved if they accounted for variations in hyphal tensile strength with time. © 2002 John Wiley & Sons, Inc. Biotechnol Bioeng 77: 601–613, 2002; DOI 10.1002/bit.10209