Metabolomics-Guided Fermentation of Tropical Agricultural Byproducts: Linking Microbial Biotransformation to Bioactive Function and Circular Ingredient Development

Agricultural and agro-industrial byproducts from tropical crops represent abundant sources of structurally bound and freely extractable bioactive compounds. However, variability in feedstock composition, the incomplete characterization of microbial transformations, and limited translation beyond chemical antioxidant assays constrain their development as standardized ingredients. This review critically evaluates metabolomics-guided fermentation as a strategy for converting tropical agricultural byproducts into functional and circular products. It covers fruit-processing residues, coffee and cocoa byproducts, cereal bran, cassava-processing residues, coconut and oilseed materials, and selected medicinal-crop tissues. Solid-state, submerged, lactic-acid-bacterial, yeast, filamentous fungal, Bacillus-based, and mixed-culture fermentations are compared. Particular attention is given to cell-wall deconstruction, phenolic release, flavonoid deglycosylation, phenolic-acid conversion, carotenoid stability, peptide generation, fiber modification, organic-acid production, antinutrient reduction, and undesirable metabolite formation. Analytical strategies based on untargeted and targeted LC-MS, GC-MS, NMR, volatilomics, peptidomics, and multiomics integration are assessed, together with experimental design, quality control, annotation confidence, and statistical validation. A substrate–microorganism–process–metabolite–function framework and a six-level evidence-grading system are proposed to distinguish exploratory chemical findings from mechanistically validated and industrially translatable outcomes. Applications in functional foods, nutraceutical ingredients, natural preservatives, animal feed, active packaging, agricultural inputs, and cascade biorefineries are evaluated alongside microbial safety, mycotoxins, biogenic amines, contaminants, regulatory classification, life-cycle assessment, and techno-economic feasibility. The central conclusion is that fermentation can become a programmable platform for tropical byproduct valorization only when metabolite-resolved characterization, causal validation, process standardization, safety, and sustainability are developed as an integrated pipeline.