Microbial approaches for sustainable upcycling of food waste and resource recovery

The production of food waste (FW) at current scale has been emerging as a pressing critical global, environmental and socio-economic crisis with nearly one-third of food produced annually, ~ 1.3 billion tons ending up as waste. The present data not only suggests a considerable loss of valuable natural resources viz. water, energy, and land, but also led to significant increase in global greenhouse gases (GHGs) emission along with contributing to large-scale environmental pollution when inadequately managed through unsustainable landfills. This impending backdrop warrants immediate attention; therefore, the upcycling of FW has emerged as a potent transformative strategy to convert bio-waste into high-value added bio-products while simultaneously addressing the climate change, resource inefficiency, and global food insecurity concerns. Upcycling approaches are broadly classified into three groups: biological, chemical, and thermochemical strategies. The later approaches are less eco-friendly and non-sustainable. In contrast, biological strategies are eco-friendly alternatives that primarily emphasize the utilization of living organisms or their enzymes system to transform FW into value-added products with approach of using sustainable technologies. The utilization of a distinct microbiome via a diverse array of microbial-mediated upcycling strategy represents a strong opportunity to encourage a circular economy model for the management and valorisation of FW with resource recovery. The microbial upcycling technologies include anaerobic digestion, microbial enzyme-assisted hydrolysis, fermentation, composting, phototrophic valorization, and the production of microbial protein (referred as single-cell protein). These processes produce biofuels (biogas, biohydrogen, and bioethanol), organic acids (acetic, butyric, formic, propionic, and lactic acid, etc.), bioplastics (polyhydroxyalkanoates and polylactic) and nutraceuticals, while also enhancing soil fertility through compost and biofertilizers. Recent research has advanced microbial metabolic engineering, catalytic system development, and integrated biorefinery models, enhancing the efficiency and selectivity of these convergent pathways. Moreover, life-cycle assessment and techno-economic analysis support the identification of sustainable and scalable solutions. However, significant barriers remain, including heterogeneity of FW composition, high capital and operating costs, process water management and challenges in large-scale deployment. Therefore, the review describes different microbial technologies employed for upcycling of FW and converting it into various value-added bio-products of economic importance. This comprehensive overview also highlights current research progress in microbial upcycling of FW, including key advances, challenges, and future perspectives. Overall, there is a pressing need to commercialize these microbial processes for effective upcycling of FW.