Simulation of the compression of pellets out of filamentous microorganisms using DEM

Filamentous microorganisms enable the production of a wide range of industrially relevant substances, such as enzymes or active pharmaceutical ingredients, from renewable side products and waste materials. The microorganisms' growth is characterized by the formation of complex, porous networks (mycelium) of tubular, multi-branched cells (hyphae). The mycelium is increasingly used in textiles, packaging, food and construction materials, in addition to the production of chemical substances. Overall, the mycelium's mechanical behavior is essential to many applications. In submerged cultures, spherical hyphal networks (pellets) are formed. The pellets are subjected to mechanical stress during cultivation, which can lead to structural changes affecting product titer and process conditions. To numerically investigate the mechanical behavior of pellets under normal stresses, the discrete element method (DEM) was used for the first time to simulate pellet compression. Initially, pellet structures were generated using a biological growth model and represented by a flexible fiber model. Force–displacement curves were recorded during compression to investigate the influencing factors. The effects of pellet size, fiber segment length, biological growth and DEM model parameters were studied. A strong influence of the growth parameters on the radial hyphal fraction and thus on the compression force was shown. Furthermore, the mechanical properties of the fiber joints significantly determined the pellet mechanics in the considered compression range. Overall, the simulation approach provides a novel tool for the digital investigation of stress on different mycelia, which may be used in the future to enhance mycelial structures through genetic and process engineering methods.