A novel sequential biphasic bioprocess (enzymatic/fermentative) for the valorisation of orange peel waste into products with potential biostimulant and biofertilizer activity

The orange juice manufacturing industry generates significant amounts of waste, consisting of peels, seeds, and pulp, which can pose environmental challenges if not properly managed. This study presents a novel two-phase bioprocess that combines enzymatic hydrolysis and bacterial fermentation. The process was developed to convert citrus waste (CW) into high-value products, achieving complete transformation into agronomic biostimulants and biofertilizers. The first phase involves enzymatic hydrolysis, which breaks down the biopolymeric components of CW—typically of low bioavailability—into simple sugars, peptides, and other metabolites with plant biostimulant properties. The second phase focusses on bacterial fermentation using strains naturally adapted to citrus waste, specifically Bacillus subtilis, which exhibits biofertilizer activities such as nitrogen fixation, phytohormone production, and siderophore secretion. The hydrolysed citrus waste served as the fermentation medium for B. subtilis. This bioprocess effectively manages CW by converting it entirely into biostimulant metabolites while generating biomass with biofertilizing functional activity. Beyond providing an environmentally sustainable solution for managing agricultural by-products, it demonstrates the potential of transforming waste into valuable agricultural resources. This approach creates a closed-loop system in line with the principles of the circular economy and offers new opportunities to advance sustainability and efficient resource management in the agricultural sector.