BACKGROUND High-efficiency utilization of sweet sorghum juice can bring significant economic feasibility of l-lactic acid production. However, an expensive nitrogen source increases the capital cost, and the complex composition of sweet sorghum juice could limit the yield. RESULTS In the present study, a bioprocess was developed to enhance the efficiency of l-lactic acid production using purified sweet sorghum juice and cheap nitrogen supply strategies. It was first demonstrated that cheaper soybean hydrolysate could be used as a low-cost nitrogen source for efficient l-lactic acid production by Lactobacillus thermophilus A69, the optimal soybean hydrolysate concentration being 25 g L-1. It was then proved that the efficiency of l-lactic acid production was significantly boosted using sweet sorghum juice purified via clarification treatment. Finally, on combining the cheap nitrogen source and the clarification strategy, 114.6 g L-1 l-lactic acid with a productivity of 2.61 g L-1 h(-1) was produced by A69 strain in a 5 L bioreactor. CONCLUSION This study provides a simple, practical and economic approach to promote l-lactic acid production using sweet sorghum juice as feedstock. (c) 2019 Society of Chemical Industry