초록
<P><B>Abstract</B></P> <P>For commercial lactic acid production, a considerable amount of time (phase II) is required for the depletion of residual sugar to an acceptable range after glucose exhaustion. In this study, the effects of pH, glucoamylase, pullulanase and invertase addition on the degradation of residual sugar were investigated. When pH value in phase II was controlled at 5.4, 5.7, 6.0, 6.3 and 6.5, respectively, pH 6.5 was found to be optimal for residual sugar consumption, and the residual total sugar decreased to 3.74g/L from 5.02g/L under normal conditions. Addition of 125U/mL of glucoamylase and 1.25U/mL of invertase to the medium further reduced the residual sugar, particularly after lactic acid production ending, while pullulanase addition had no effect. Accordingly, we developed a combined strategy that using the unsterilized hydrolysate, adding invertase at the beginning of phase II and maintaining pH at 6.5 in phase II to reduce residual sugar content. Under these conditions, the residual total sugar and reducing sugar decreased to 1.45 and 1.01g/L, respectively, 71.12 and 58.94% lower than these obtained under normal conditions. These improvements will reduce costs and facilitate industrial application.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Increased culture pH and glucoamylase and invertase addition reduced residual sugar. </LI> <LI> Residual sugar components and residual sugar metabolism-related enzymes were identified. </LI> <LI> A combined strategy using unsterilized hydrolysate together with control of high pH and invertase addition was developed. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>