Valorization of Zymomonas mobilis for bioethanol production from waste bread: optimization of the enzymatic hydrolysis and fermentation processes

To investigate the potential for increasing the yield of bioethanol production from bread waste, this study utilized Zymomonas mobilis. Since optimizing production conditions significantly impacts product yield and cost reduction, the enzymatic hydrolysis and fermentation processes were optimized. First, the effect of the α-amylase enzyme on the amount of released sugars was examined. Then, using the response surface methodology (RSM) experimental design, the simultaneous effects of three parameters—amyloglucosidase enzyme dosage (U), solid loading (%), and process time (h)—on the saccharification process were analyzed. The results showed that the maximum reducing sugar of 141 g/l was achieved under optimal conditions of 19.9% solid loading, 11.8 U of amyloglucosidase enzyme, and saccharification time of 26.5 h. In the next phase, the fermentation of the released sugars into bioethanol using Zymomonas mobilis was investigated. The simultaneous effects of four factors—initial sugar concentration (g/l), amount of inoculum (bacterial dry weight), the weight of peptone (g) used in the medium (as a nutrient), and fermentation time (h)—were analyzed using RSM. Data analysis indicated that the highest ethanol production (31.4 g/l) was obtained under the following optimal conditions: initial sugar concentration of 85 g/l, bacterial dry weight of 0.0325 g, peptone weight of 0.36 g, and fermentation time of 27 h. In this process, 80.83% of the initial sugar was consumed by the bacteria, resulting in an ethanol production yield of 0.457. This indicates the efficient performance of Zymomonas mobilis in producing ethanol from bread waste.