Circular Valorization of Orange Peel Waste into Alkaline Pectinases Using Polyextremotolerant Bacillus amyloliquefaciens

Introduction
Agro-industrial citrus residues constitute a major environmental burden but also represent promising renewable substrates for sustainable bioprocesses. This study investigated the recycling potential of orange peel waste for low-cost alkaline pectinase production using multifunctional polyextremotolerant bacterial strains.

Methods
Thirty pectinolytic bacterial isolates were recovered from decayed fruits collected in Algeria and screened for alkaline pectinase production. Three high-performing strains were identified as Bacillus amyloliquefaciens through phenotypic and 16S rRNA analyses. Fermentation parameters were optimized using a one-factor-at-a-time approach, and commercial pectin was replaced with different agro-industrial wastes to evaluate substrate valorization efficiency.

Results
Optimal pectinase production was achieved at pH 9.0, 37 °C, and 48 h of submerged fermentation. Among the tested strains, ST9 showed the highest enzymatic activity, reaching 22.4 U/mL under optimized conditions. Orange peel waste significantly enhanced pectinase yield compared with commercial pectin while reducing substrate costs by approximately 99 %. The selected strains also demonstrated remarkable polyextremotolerance, maintaining growth under highly alkaline, saline, and thermal stress conditions. In addition, the isolates exhibited multiple hydrolytic activities, plant-growth-promoting traits, and antimicrobial activity against phytopathogenic fungi and bacteria.

Conclusions
This study demonstrates an efficient waste-to-value strategy for converting citrus-processing residues into high-value alkaline biocatalysts. The obtained results highlight the potential of polyextremotolerant Bacillus amyloliquefaciens strains for sustainable enzyme biomanufacturing and circular bioeconomy applications in agro-industrial waste recycling.