Analysis of metabolites
Samples taken periodically during fermentation were centrifuged (10,000 rpm, 5 min). The supernatants were pipetted and filtered using a 0.45-syringeless PTFE filter, while the cells were washed once with sterile water and stored at -80 °C. The metabolites contained in the supernatant were quantified via HPLC (LC-20AB, Shimadzu, Japan) equipped with a Coregel column (7.8 mm, I.D. × 300 mm Transgenomic, USA) at 80 °C using 5 mM sulfuric acid as the eluent (flow rate at 0.6 mL·min−1; 40 min). A refractive index detector was used.