In Situ Imaging for Monitoring Biomass Content and Pellet Size in Submerged Filamentous Stirred‐Tank Cultivation

ABSTRACT

Filamentous microorganisms play a central role in industrial biotechnology, where pellet morphology affects mass transfer, rheology, and process performance. However, time‐resolved monitoring of filamentous biomass and pellet size dynamics in stirred‐tank reactors remains challenging due to offline sampling limitations. We present a noninvasive in situ imaging approach to observe biomass‐related light scattering and projected pellet size. A custom‐built camera and ring‐light setup with MATLAB‐based image acquisition and processing was used to quantify image luminosity as a proxy for biomass content and to determine projected object area (POA) and the number of detected objects per image above a POA threshold of 0.5 mm ² . To characterize systematic biases, polyoxymethylene spheres of 1–4 mm diameter served as a model system. Luminosity scaled with object concentration, whereas object detection and apparent size estimation were increasingly affected by object size and concentration. In a bench‐scale cultivation of malic acid‐producing Aspergillus oryzae , image luminosity followed biomass content, while in situ POA analysis revealed increasing projected pellet size coinciding with Zn ²⁺ limitation. These observations differed from ex situ sampling, indicating that image‐based in situ monitoring provides time‐resolved information on biomass formation and pellet size dynamics that may not be captured by offline sampling alone.