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/kD(3)) 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 (lambda) has not been shown to determine, at least directly, fungal morphology. In this work we studied the influence of lambda and EDCF evolution, as well as the individual effects of P/kD(3) 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/kD(3) 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/kD(3). 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 lambda in large-scale bioreactors is proposed.