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چکیده
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Using food waste as a resource in a circular economy is increasingly seen as a key way to produce sustainable biofuels. Among various waste streams, bread waste—which is plentiful, starchy, and often underused — shows great poten tial. Traditional simultaneous saccharification and fermentation (SSF) methods, however, face issues with compatibility between enzymatic hydrolysis and microbial fermentation, often using yeast strains with low ethanol tolerance. This study proposes a new SSF process employing Zymomonas mobilis, a bacterium known for its high ethanol production, quick sugar absorption, and resilience under stress. Using a Box-Behnken response surface design, we optimized four main factors: pH (4.0–7.0), temperature (29–60 °C), solid loading (1–20% w/v), and enzyme dose (1–15 U). The optimal conditions predicted (pH 5.61, 42.6 °C, 11.5% solid, 10.7 U enzyme) led to 41.1 g/L ethanol — about 96.44% of the theoretical maximum and 96.88% sugar utilization in only 27 h. These findings greatly surpass typical yeast-based SSF results. This indicates that waste bread, combined with Z. mobilis and process tuning, can become an efficient, scalable raw material for industrial bioethanol, promoting circular economy principles and sustainability worldwide.
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