Selective Quenching of Peracetic Acid by Sodium Dithionite Enables Rapid, Non-Thermal Sterilization for Euglena gracilis Cultivation

Peracetic acid (PAA) has strong biocidal activity against bacteria, fungi, and spores, even with short contact times. PAA-mediated sterilization is therefore an attractive method for sterilization of growth media that have heat-labile components or when polymer-based equipment is used. However, residual PAA and co-existing hydrogen peroxide (H₂O₂) can inhibit the growth of cultivated species, necessitating a fast and reliable quenching strategy that does not require rinsing. In contrast to Fe-EDTA-based catalytic decomposition that is strongly influenced by pH, buffers, and organic nitrogen, we demonstrate a fundamentally different, stoichiometric quenching strategy using sodium dithionite that enables instantaneous and selective removal of PAA. Na₂S₂O₄ preferentially reduced PAA over H₂O₂ in a 0.03% PAA solution and achieved complete PAA reduction within 5 s, independent of pH and in the presence of nitrogen compounds. By adjusting the Na₂S₂O₄ dose, PAA could be selectively removed while allowing a small fraction of H₂O₂ to remain. When applied to the cultivation of <i>Euglena gracilis</i>, which tolerates low levels of H₂O₂, the PAA-Na₂S₂O₄-treated medium resulted in greater cell growth and higher paramylon production than autoclaved medium.