Solid-state fermentation of spent Spirulina platensis biomass by Aspergillus oryzae: Enhancement of antioxidant activity, protein content, and amino acid profile

This study explores spent Spirulina platensis biomass (SPSB) under solid-state fermentation (SSF) through the application of GRAS fungal fermentation system ( A. oryzae) . In this regard, SPSB was evaluated firstly by its viability as a support-substrate for fungal development in terms of pH, water absorption capacity (WAC) and radial growth. Subsequently, SPSB underwent solid-state fermentation (SSF) for 0–120 h, and changes in sugar and soluble protein concentrations, antioxidant capacity, enzymatic activities, organic acid production, and free amino acid release were quantified in the fermented extracts. The results show an increase in the content of reducing sugars (0.61 ± 0.01 g equivalents of glucose per 100 g of fermented biomass, g EG/100 gFB at 96 h) and soluble protein (9.57 ± 0.817 g BSAE/100 gFB, 120 h). Similarly, antioxidant activity increased during SSF, reaching values up to 6.11 ± 1.9-fold higher than those observed at 0 h (Folin–Ciocalteu assay). Cellulase activity remained low throughout the process, reaching only 0.184 FPU/g at 120 h, whereas proteolytic activity increased to 15.27 ± 0.40 U/g, which may have contributed to the generation of low-molecular-weight peptides (<6.5 kDa). An increase in free amino acids, particularly six essential amino acids, was observed. Finally, fungal metabolism led to the accumulation of organic acids, with acetic acid, malic acid, and kojic acid. Overall, A. oryzae enhanced the bioactive profile by increasing antioxidant activity, soluble proteins, free amino acids, and organic acids. This bioprocess represents a promising approach for value-added functional ingredients within a sustainable biorefinery framework.