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Influence of sulfur dioxide (SO2) and lactic acid (LA) on grain sorghum steeping efficiency in terms of wet-milling characteristics was investigated using response surface analysis. Tested ranges of concentrations in steeping water were 0.050 to 0.278% and 0 to 0.282% for SO2 and LA, respectively. Measured wet-milling variables were steeping solids yield, starch yield, protein yield, fiber/germ yield, starch recovery, protein recovery and protein content of starch. Starch and protein recovery values were highly correlated to each other (r = 0.74). Correlations of these attributes with fiber/germ yield were strongly negative (r = –0.96 and r = –0.70 for starch and protein recovery, respectively). A model comprised of the linear and quadratic terms of SO2 and LA concentrations and the interaction between SO2 and LA was fitted to data for each dependent variable. The model satisfactorily explained starch yield (R2 = 0.84), starch recovery (R2 = 0.85) and fiber/germ yield (R2 = 0.71). Lactic acid was the factor that consistently influenced all significant surfaces (P < 0.01 for starch yield and starch recovery, and P < 0.05 for fiber/germ yield). Maximum stationary points for starch yield and recovery, and a minimum stationary point for fiber/germ yield were observed at similar combinations of SO2 (0.191-0.209%) and LA (1.387-1.415%) concentrations.