Optimizing solid-state fermentation of Trichoderma harzianum in packed-bed bioreactors using beer spent grains and...

Phosphorus (P) availability is a key limiting factor for plant growth, and microbial-based strategies represent a promising approach for developing sustainable agricultural inputs. In this context, this study investigates the cultivation of Trichoderma harzianum using beer spent grains and wheat straw as substrates in a solid-state fermentation (SSF) process in packed bed bioreactors (PBBs) at lab and bench scales. The objective was to evaluate the strain's potential to enhance P availability by promoting medium acidification and the production of phosphatases, indole derivatives, and flavonoids. The optimized process in 0.5 L PBB resulted in fermented solids with high spore content (1.08 × 10¹⁰ spores g^-1 of dry mass, DM), acid phosphatase (ACP) activity (3.7 U g^-1 DM), and phosphate mobilization. Additionally, the strain produced indole derivatives (511.3 µg g^-1 DM) and total flavonoids (2.2 mg g^-1 DM). Air flow and substrate ratio were the most significant factors, highlighting the role of adequate O₂ supply and efficient CO₂ removal in SSF processes. At bench scale (22 L PBB), cultivation also supported high biomass formation, at the order of 1 × 10⁹ spores g^-1 DM, and yielded appreciable amounts of indole derivatives (688 and 498 µg g^-1 DM in batches 1 and 2, respectively). Production of ACP and flavonoids was lower compared with lab scale, likely influenced by a 16.5°C temperature rise above the optimal range, underscoring the need to improve heat dissipation. These findings demonstrate the potential of PBBs for producing multifunctional T. harzianum-based products from agro-industrial residues under solid-state fermentation.