Co-cultures of Yeasts and Zygomycetes in the Form of Pellets Methods for the Preparation of Pellets and Biocapsules, Their Properties and Applications

Abstract: Many industrially important fungi can grow in the form of small spherical pellets.Such pellets reduce the viscosity and enhances mass transfer rates in culture broths.The pelleted morphology also influences the fungus’s metabolism, directing it tospecific metabolites. The pellets are easily harvested from the broth and recycled.These properties makes pelleted morphology very attractive.The pelleted morphology of four Zygomycetes strains was studies. Several differentnutrient media used by other researchers for achieving pelleted growth was tested.The influence of eight factors on pelletization of Rhizopus sp. in a completely definedmedium was determined using a fractional central composite design and logistic regression.Pelleted growth of all four Zygomycetes was achieved, with very good results for twoRhizomucor sp. strains. A simple medium containing calcium carbonate was foundto induce pelletization with very high reproducibility.Immobilization of yeast cells was attempted in pellets of Rhizomucor. It was foundthat a flocculating yeast can be immobilized inside pellets of fungal mycelium, forming”biocapsules”. This is accomplished by first using a medium that induces pelletizationof the filamentous fungus and does not allow for growth of the yeast. Thepellets are then inoculated into a second medium that induces growth and flocculationof the yeast and inhibits further growth of the filamentous fungus.Non-flocculating yeasts could not be immobilized, suggesting that the flocculin proteinsin the cell wall of flocculating strains are important for proper immobilization.The flocculation and immobilization arises due to expression of several differentFLO-genes and the importance of these genes for successful immobilization isdiscussed.The results demonstrate that the morphology of Zygomycetes can be controlled andthat this may be useful in industrial fermentation. The new immobilization techniquereveals the great importance of flocculation and cell surface hydrophobicity. Usingyeast strains that express certain FLO-genes may be beneficial in fermentation oflignocellulosic hydrolysates.Microscopy techniques were developed that allows for high quality microphotographyof pellets and thin cross-sections of pellets and biocapsules.