AbstractAcetic acid formation in Escherichia coli fermentation has been studied in continuous cultures. Experimental results suggest that the limited capacity of the oxidative metabolism (perhaps the limited capacity of TCA cycle) may be responsible for acetic acid formation. At low growth rates, both anabolic and catabolic requirements may be satisfied by the oxidative metabolism. However, at high growth rates these two demands may exceed the capacity of the oxidative metabolism alone. It is proposed that under these circumstances, E. coli reorganizes the oxidative metabolism to first meet the anabolic requisition and then supply the necessary amount of energy using both the remaining capacity of the oxidative metabolism and acetic acid formation metabolism. Escherichia coli selects acetic acid synthesis as the aerobic energy source because it generates the second largest amount of ATP and NADH2. According to our proposition, acetic acid formation could be reduced by decreasing the anabolic requirement, i.e., reducing glucose uptake, or by increasing the capacity of the oxidative metabolism. These two approaches were experimentally confirmed by observing reduced acetic acid formation by reducing the glucose uptake with a yeast extract addition and enhancing the capacity of oxidative metabolism with a methionine addition.