Enhanced l-lysine production from pretreated beet molasses by engineered Escherichia coli in fed-batch fermentation

Faster sugar consumption rate and low-cost nitrogen source are required for the chemical biosynthesis using molasses. Five pretreatment methods were applied to beet molasses prior to fermentation through engineered Escherichia coli, respectively, and corn steep liquid was used as an organic nitrogen source to replace expensive yeast extract. Furthermore, the effects of different feeding strategy in fed-batch fermentation on l-lysine production were investigated. The experimental results showed that combined tricalcium phosphate, sulfuric acid, and activated carbon pretreatment method (TPSA) pretreatment could improve the sugar consumption rate most greatly, and the initial total sugar concentration of 35 g/L from TPSA-pretreated beet molasses gave the best results with respect to l-lysine production, dry cell weight concentration, and l-lysine yield in batch fermentation. Moreover, a mixture of low-cost corn steep liquid and yeast extract containing equal amount of nitrogen could be used as the organic nitrogen source for effective l-lysine fermentation, and constant speed feeding strategy of TPSA-pretreated beet molasses promoted l-lysine production by engineered E. coli. The TPSA-pretreated beet molasses had a sugar consumption rate of 1.75 g/(L h), and a l-lysine yield of 27.81 % was achieved, compared with the theoretical yield of 62 % by glucose. It was clarified that the pretreatment significantly enhanced the conversion of sugars in beet molasses to l-lysine.