Culture Optimization Strategy for 1-Deoxynojirimycin-producing <Emphasis Type="Italic">Bacillus methylotrophicus</Emphasis> K26 Isolated from Korean Fermented Soybean Paste, <Emphasis Type="Italic">Doenjang</Emphasis>

1-Deoxynojirimycin (1-DNJ) is an α-glucosidase inhibitor that is used for the treatment of type 2 diabetes. In this study, we isolated Bacillus methylotrophicus K26 with α-glucosidase inhibition (AGI) activity from Korean fermented soybean paste (Doenjang) and confirmed that the genome harbored the DNJ biosynthesis genes including gabT1, yktc1, and gutB1 by PCR screening, while 1-DNJ production was confirmed by ultra-performance liquid chromatography–quadrupole time-of-flight–mass spectrometry. To increase 1-DNJ production by B. methylotrophicus K26, culture conditions were optimized with one-factor-ata- time (OFAT) and response surface methodology (RSM) approaches. Screen of 11 carbon and 9 nitrogen sources by the OFAT method identified sucrose and yeast extract as optimal culture components. Sucrose concentration (X₁), yeast extract concentration (X₂), and culture temperature (X₃) were selected as independent variables for central composite design. The coefficient of determination (R²) for the model was 0.927, and the probability value of the regression model was highly significant. RSM predicted the optimal conditions for 1-DNJ production by B. methylotrophicus K26 as sucrose and yeast extract concentrations of 4.61% and 7.03%, respectively, at a temperature of 34°C. Under these conditions, AGI activity was experimentally measured as 89.3%, which was close to the predicted value of 91.9%.