Zero-waste strategy by means of valorization of bread waste

To impose the zero-waste strategy through the diverse valorization route, this study used bread waste (BW) as the model compound. To realize these grand technical challenges, BW was enzymatically hydrolyzed to obtain a monomer sugar compound (glucose), and glucose was used as the carbon substrate in heterotrophic cultivation of Euglena gracilis (E. gracilis). Cultivation of E. gracilis using cultivation medium derived from BW was evaluated from a perspective of an economic viability. In addition, the more production of the target compound (paramylon, β-1,3-glucan) stemmed from E. gracilis was achieved (1.93 g L⁻¹d⁻¹; 24% higher productivity than that of control). To approach zero waste disposal, bread waste residue (BWR) derived from enzymatic hydrolysis of BW was valorized into syngas. To offer a greener pyrolysis platform for BWR, CO₂ was used as a raw material. Here in this study, the mechanistic functionality of CO₂ was disclosed. In detail, CO₂ reacted with volatile matters (VMs) evolved from BWR, thereby resulting in CO₂ reduction. Simultaneously, reduced CO₂ also led to oxidation of VMs. Such consecutive gas-phase-reactions (GPRs) played a critical role to enhance CO formation. Lastly, the identified GPRs induced by CO₂ were tried to expedite the reaction kinetics in the presence of 5 wt% of Ni/SiO₂ catalyst. As a result, the molar concentrations of H₂ and CO in gaseous pyrolytic products derived from catalytic pyrolysis in CO₂ environment were 2- and 6-times higher, respectively, than that from pyrolysis without catalyst in N₂ environment.