Valorization of agricultural residues through fermentation: postbiotic preparations, related metabolites and contributions to sustainable food systems and the sustainable development goals

Abstract Agricultural production and food processing generate a high volume of residues, such as peels, pomace, brans and husks, that are often thrown away, burnt or landfilled. As food systems are pushed to both enhance supply and minimise emissions and waste, this adds to environmental pressure. When the residues are converted to postbiotics, i.e. preparations containing non-viable microorganisms or their components, which confer a health benefit on the host, there is an opportunity to add value to waste and potentially reduce dependency on live-culture products and synthetic preservatives while supplying functional ingredients for food, feed and health applications. However, most of these benefits remain untested for agro-waste preparations. This review provides a summary of the current information regarding agricultural waste as fermentation feedstock, the fermentation platforms utilised, the main classes of fermentation-derived metabolites that may be constituents of postbiotic preparations comprising short-chain fatty acids, organic acids, exopolysaccharides, cell-wall components, biotransformed polyphenols, peptides and biosurfactants and how they mediate anti-inflammatory, anti-oxidant and anti-pathogen activities. Evidence from banana peel, citrus peel, apple pomace and cereal bran is summarized, and the potential contribution of such products to food preservation and safety, nutrition security, antimicrobial-resistance mitigation in animal production and circular bioeconomy models is assessed in relation to Sustainable Development Goals (SDGs) 2, 3, 8, 9, 12 and 13. Most of the studies examined fermented, unfermented or fibre fractions of substrates rather than defined preparations of inactivated microorganisms. Hence, they are categorized as supporting evidence on substrates and precursors and distinguished from studies on postbiotic preparations. Key barriers are identified: non-standardised fermentation, strain variability, limited metabolite quantification, scarce human trials, unresolved scale-up constraints of solid-state fermentation (heat and moisture gradients), and the absence of life cycle and techno-economic assessments. Priorities for research, industry and policy are proposed, including regionally relevant feedstocks such as date-palm by-products. Agro-waste postbiotics are a promising but still largely preclinical and pre-commercial pathway towards sustainable food systems. Realizing that potential will require standardized, sustainability-assessed and regulator-ready production systems.