Biotransformation of Hemp Seed Cake through Solid-State Fermentation for the Development of Functional Food Ingredients

The transition toward sustainable food systems requires innovative strategies to valorize agro-industrial by-products into high-value functional ingredients. Hemp seed cake, a protein-rich co-product of oil extraction, remains largely underutilized despite its promising potential for food applications. This study investigated the use of solid-state fermentation (SSF) with Janthinobacterium lividum to enhance the nutritional and bioactive properties of hemp seed cake. SSF was conducted under previously optimized conditions (temperature: 25 °C, substrate moisture content: 80%, inoculum volume: 4 mL, and fermentation time: 7 days). Biomass composition was evaluated by determining total protein content (Kjeldahl method), ash content (AOAC method), and total phenolic compounds (Folin–Ciocalteu assay). Violacein production was also monitored as a value-added microbial metabolite. Fermentation induced significant biochemical modifications (p < 0.05). Total phenolic content increased markedly from 0.67 to 5.15 g GAE·100 g⁻¹ dry matter, indicating a substantial enrichment in bioactive compounds and a potential improvement in antioxidant properties. Total protein content showed a slight but non-significant increase from 35.00 to 36.36% (+1.36%). Ash content decreased significantly from 6.80 to 1.53%, likely reflecting microbial assimilation of inorganic minerals during fermentation. Violacein production reached 0.99 mg·g⁻¹ dry matter, demonstrating the capacity of SSF to simultaneously generate a high-value bioactive metabolite alongside an upgraded fermented biomass. These findings demonstrate that SSF is an effective strategy for upgrading hemp seed cake into a phenolic-enriched biomass with potential applications as a functional food ingredient, while simultaneously producing value-added violacein. This approach contributes to agro-industrial by-product valorization and supports the development of sustainable food systems aligned with circular bioeconomy principles.