Potential of Recombinant Raw Starch‐Degrading Enzyme from Escherichia coli for Sugar Syrup and Bioethanol Productions Using Broken Rice Powder as Substrate

The recombinant raw starch‐degrading enzyme (LsA175) which harbors the lsa175 gene from Laceyella sacchari LP175 is optimized for expression in Escherichia coli BL21 (DE3). The recombinant strain provides an alternative avenue for enzyme production in a stirrer fermenter with tolerance to shear force and avoids losing stability by protease existing in the native strain. The maximum enzyme production at 4472±112 U g−1 cell dry weight is obtained using the optimum conditions at 22 °C, 1.0 mM IPTG, and 24 h of induction temperature, Isopropyl β‐ d‐1‐thiogalactopyranoside (IPTG) concentration, and induction time, respectively. The enzyme production of 4545 ± 89.8 U g−1 cell dry weight is obtained by batch fermentation in a 2.0 L stirrer fermenter and 5329 ± 86.1 U g−1 cell dry weight by continued glucose feeding with high bacterial cell concentration. The uncooked broken rice powder (200 g L−1) is hydrolyzed by crude recombinant LsA175 (300 U mL−1) at 50 °C for 12 h without the addition of exogenous glucoamylase (GA), yielding 86 ± 1.79 g L−1 reducing sugar. The bioethanol production resulting from modified simultaneous saccharification and fermentation with Kluyveromyces marxianus DMKU‐KS07 at 42 °C gave 71.4±4.35 g L−1 ethanol with 88.24% of theoretical ethanol yield. LsA175 presents engaging performances regarding sugar syrup and bioethanol production, which can diminish the cost of production and energy consumption. Statistical optimization for expression of recombinant raw starch‐degrading enzyme (LsA175) in Escherichia coli is evaluated. This study showed the feasibility of a modified simultaneous saccharification process on raw broken rice powder by using recombinant protein LsA175 without the addition of commercial glucoamylase for bioethanol production, thus reducing energy consumption and operation costs.