Fig. 2

a Growth characteristic of yeasts and the feed rate profile during the fed-batch operation without a closed-loop control. The arrows point out the produced ethanol due to overflow metabolism. b Evolution of the dissolved oxygen and the intensity of NADH, tryptophan, flavins, and pyridoxine during the fed-batch operation without a closed-loop control. c Characteristics of the fluorophores in different metabolic states of oxidative and oxidoreductive metabolism. The dotted lines are plotted from the polynomial function based on the state of oxidative metabolism to see the intensity change between these two metabolic states. The small arrow shows the different intensity between the oxidative and oxidoreductive states