Characterization of Metarhizium anisopliae spore extraction in a rotary drum using DEM method

The fungus Metarhizium anisopliae is commonly cultivated on rice, and the extraction of its spores remains a critical step with room for optimization in biofactories. Rotary drums are emerging as a promising technology. This study evaluated the effect of drum rotation speed (30 and 60 ​rpm) and particle moisture content (0.250; 0.111; and 0.053 ​kg-w/kg-ds) on the collision forces and frequencies of rice particles during the extraction process. To this end, the Discrete Element Method (DEM) was applied to simulate particle motion and collisions. The simulations were carried out using a rotary drum with a diameter of 20 ​cm and a length of 33 ​cm, equipped with two straight lifters. Spherical particles with structural and material properties referring to conidiated rice particles of different moisture contents were used. The DEM input parameters, restitution as well as static and rolling friction coefficients for particle-particle and particle-wall contacts, were experimentally determined. The experiments indicated that the moisture content of the particles predominantly affects the sliding friction coefficient, while the softness resulting from the cooking of the rice mainly influences the coefficient of restitution. The rolling friction coefficient, in turn, was primarily impacted by the sphericity of the particles. Additionally, the reduction in moisture content, by decreasing the drum's filling degree, promoted the expansion of the active layer, the region associated with more intense particle collisions. The results indicated that, despite the higher collision velocities observed at 60 ​rpm, the frequency of impacts with sufficient energy to overcome adhesive forces did not significantly increase compared to the 30 ​rpm condition. As a major outcome, DEM simulations clearly revealed that the reduction of particle size, associated with decreasing moisture content, resulted in a considerable drop of the filling degree, yielding an expansion of the active layer and therefore enhanced particle mobility. This is an important finding with great interest for the scale-up of continuous spore extraction in rotary drums. • Collisions during M. anisopliae spore extraction were studied using rotating drums and DEM simulations. • Moisture content and drum rotation speed were evaluated as key variables in particle behavior. • Frictional properties for particle–particle and particle–wall contacts were measured at varied moisture levels. • Lower moisture altered friction, drum fill dynamics, and the frequency and intensity of particle collisions. • Higher forces occurred at 60 ​rpm, yet the effective impact frequency remained comparable to that at 30 ​rpm.