Solid-State Fermentation by Trichoderma Remodels the Metabolome and Enhances the Antioxidant Properties of Tomato Peel and Green Waste

The increasing generation of agro-industrial residues requires the development of sustainable valorization strategies for converting low-value biomasses into high-added-value products within a circular bioeconomy framework. In this study, tomato peels (TP) derived from the tomato-processing industry and green waste (GW) from urban pruning activities were investigated as substrates for solid-state fermentation (SSF) mediated by Trichoderma harzianum, with the aim of evaluating fungal growth, metabolomic remodeling, and the production of antioxidant bioactive compounds. Different substrate formulations containing TP and GW were subjected to SSF for 7 days, and fungal colonization was monitored. The highest fungal colonization was observed in substrates containing high proportions of GW, whereas pure tomato peels showed negligible colonization, indicating a strong substrate-dependent effect on fungal development. UPLC-QToF-MS metabolomic profiling was subsequently performed on the fermented substrate formulations using three independent biological replicates per treatment, whereas the 100% TP formulation, which showed negligible fungal colonization, was excluded from metabolomic analysis. Results revealed marked differences among fermented substrates, and 22 discriminant metabolites significantly enriched in those containing higher proportions of TP were identified. These metabolites mainly included hydroxycinnamic acid derivatives, flavonoids, lignans, phenolic glycosides, and organic acids, such as coumaric acid, hydroxycaffeic acid, cinnamoylglucose, citric acid, and cyanidin glycosides, suggesting fermentation-associated transformation and release of phenolic compounds. Fermented extracts obtained from mixed substrates enriched in TP exhibited the highest antioxidant activity, with DPPH and ABTS radical scavenging capacities reaching up to 63.48 µmol TE/g dw and 119.67 µmol TE/g dry weight, respectively, together with increased total phenolic content. The integration of microbiological, metabolomic, and antioxidant analyses demonstrated that co-fermentation of tomato-processing byproducts with green waste can modulate fermentation outcomes and enhance antioxidant potential. Overall, this study provides new insights into substrate-driven metabolic transformations during Trichoderma-mediated SSF and highlights the potential of mixed agro-industrial residues as sustainable feedstocks for the production of antioxidant-rich extracts with possible nutraceutical and biotechnological applications.