Optimization of keratinase production by <i>Bacillus cereus</i> HD1 using feather waste and its laboratory scale validation in bioreactor with oxygen and carbon supplementation

The global expansion of chicken meat production has generated substantial feather waste, representing 5-7% of total body weight. Conventional disposal methods such as landfilling and open dumping, raise environmental concerns because feathers contain recalcitrant keratin. Nevertheless, chicken feathers are an inexpensive substrate for keratinase production, an enzyme with diverse industrial applications. Most previous studies optimized keratinase in shake flasks, with limited validation in controlled bioreactors. This study optimized keratinase production from <i>Bacillus cereus</i> HD1 using chicken feathers in 250 mL flasks, then validated the process in a 2 L bioreactor under controlled dissolved oxygen (DO). Optimal flask conditions were pH 8, 37 °C, 1% substrate, 0.5 g/L sucrose, and 10% inoculum. Sucrose supplementation enhanced growth and minimized substrate inhibition. During scale-up, maintaining 30% DO yielded the highest keratinase activity of 30 U/mL, exceeding flask production. Controlled aeration improved biomass formation and enzyme synthesis compared with lower or higher oxygen levels. The optimized bioreactor system enhances productivity and offers a sustainable approach for feather waste valorization, reducing pollution and supporting circular bioeconomy strategies aligned with Sustainable Development Goal 12. These findings demonstrate the importance of oxygen regulation for efficient industrial bioprocess development and scalable keratinase production from agroindustrial residues at a commercial scale.