EFFECT OF AGITATION INTENSITIES ON FUNGAL MORPHOLOGY OF SUBMERGED FERMENTATION

Both parallel fermentations with Aspergillus awamori (CBS 115.52) and a literature study on several fungi have been carried out to determine a relation between fungal morphology and agitation intensity. The stu died parameters include hyphal length, pellet size, surface structure or so-called hairy length of pellets, and dry mass per-wet-pellet volu me at different specific energy dissipation rates. The literature data from different strains, different fermenters, and different cultivati on conditions can be summarized to say that the main mean hyphal lengt h is proportional to the specific energy dissipation rate according to a power function with an exponent of -0.25 +/- 0.08. Fermentations wi th identical inocula showed that pellet size was also a function of th e specific energy dissipation rate and proportional to the specific en ergy dissipation rate to an exponent of -0.16 +/- 0.03. Based on the e xperimental observations, we propose the following mechanism of pellet damage during submerged cultivation in stirred fermenters. Interactio n between mechanical forces and pellets results in the hyphal chip-off from the pellet outer zone instead of the breakup of pellets. By this mechanism, the extension of the hyphae or hair from pellets is restri cted so that the size of pellets is related to the specific energy dis sipation rate. Hyphae chipped off from pellets contribute free filamen tous mycelia and reseed their growth. So the fraction of filamentous m ycelial mass in the total biomass is related to the specific energy di ssipation rate as well. To describe the surface morphology of pellets, the hyphal length in the outer zone of pellets or the so-called hairy length was measured in this study. A theoretical relation of the hair y length with the specific energy dissipation rate was derived. This r elation matched the measured data well. it was found that the porosity of pellets showed an inverse relationship with the specific energy di ssipation rate and that the dry biomass per-wet-pellet volume increase d with the specific energy dissipation rates. This means that the tens ile strength of pellets increased with the increase of specific energy dissipation rate. The assumption of a constant tensile strength, whic h is often used in literature, is then not valid for the derivation of the relation between pellet size and specific energy dissipation rate . The fraction of free filamentous mycelia in the total biomass appear ed to be a function of the specific energy dissipation in stirred bior eactors. (C) 1997 John Wiley & Sons, Inc.