The influence of circulation frequency on fungal morphology: a case study considering Kolmogorov microscale in constant specific energy dissipation rate cultures of Trichoderma harzianum

The energy dissipation/circulation function (EDCF) is the product of the specific energy dissipation rate in the impeller swept volume (P/kD³) and the frequency of particle circulation (1/t_c) through that volume. A direct relationship between mycelial fragmentation and EDCF has been reported. However, and although hyphal fragmentation is assumed to occur by hyphae-eddy interaction, Kolmogorov microscale (λ) has not been shown to determine, at least directly, fungal morphology. In this work we studied the influence of λ and EDCF evolution, as well as the individual effects of P/kD³ and 1/t_c, on Trichoderma harzianum cultures in an attempt to elucidate the mechanistic interactions between parameters. T. harzianum cultures, conducted at equivalent yielding P/kD³ conditions, were developed using two different Rushton turbines diameter sets. For the studied conditions, 1/t_c had a greater effect over mycelial clump size and growth rate than P/kD³. Consequently, broth viscosity, and hence Kolmogorov microscale, was a function of impeller diameter, even among cultures operated at equivalent specific energy dissipation rates. The latter could partially explain why Kolmogorv's theory has not been able to fully correlate morphological data, and highlights the importance of 1/t_c on fungal bioprocesses. A theoretical approach to monitor λ in large-scale bioreactors is proposed.