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, beta-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.

Automated summary

This study converted bread waste into glucose through enzymatic hydrolysis, and used the glucose as a carbon substrate for cultivating Euglena gracilis. Additionally, the bread waste residue was valorized into syngas using CO2 as a raw material and a catalyst. The results showed that, in the presence of CO2 and catalyst, the concentration of H2 and CO in the pyrolytic gas products was significantly higher.