Efficient lactic acid production from sweet sorghum juice using Lactococcus lactis IO-1: Optimization and scale-up with low-cost nitrogen sources

• Sweet sorghum juice proves an excellent carbon source for sustainable lactic acid production. • DSY combined with reduced YE cuts nitrogen costs while maintaining high lactic acid yield. • Lactococcus lactis IO-1 achieves 98% of theoretical lactic acid yield from sweet sorghum juice. • Optimized process produces 72.19 g/L lactic acid with 93.30 % sugar consumption. • Successful scale-up to 7.5-L fermenter demonstrates industrial feasibility. Among the low-cost nitrogen sources (dry spent yeast or DSY, rice bran, and soybean meal), DSY was identified as the most suitable supplement for lactic acid fermentation from sweet sorghum juice by Lactococcus lactis IO-1. However, lactic acid concentration (P L ) using DSY was ∼ 22 % lower than the control nitrogen source, yeast extract (YE). Statistical analysis using YE as a control nitrogen source revealed optimal conditions of 74.70 g/L of initial sugar and 15.20 g/L of YE, achieving a P L of 71.95 g/L and a 91.63 % sugar consumption (SC). When DSY (22.61 g/L) containing an equivalent nitrogen content to the optimal YE was tested, P L and SC decreased to 55.13 g/L and 72.33 %, respectively. Increasing DSY to 33.92 g/L (1.5 times) improved lactic acid productivity (Q L ) by ∼ 31 % but did not enhance P L or SC. However, supplementing 33.92 g/L of DSY with 7.60 g/L of YE enhanced lactic acid production by ∼ 19–53 %, achieving a P L of 70.11 g/L, SC of 92.33 %, Q L of 2.34 g/L·h, with a high lactic acid yield (Y L/S ), 1.02 g/g. Scale-up fermentation in a 7.5-L fermenter demonstrated comparable results to those using 15.20 g/L of YE alone. These findings demonstrate that sweet sorghum juice supplemented with DSY and reduced YE is a promising medium for efficient lactic acid production, offering significant cost reduction potential for industrial applications while maintaining high productivity and yield.