Effects of three processing variables: feed composition (% sweet potato), screw speed, and die diameter on available lysine and browning index were investigated following extrusion cooking of mixtures of defatted soy flour and sweet potato flour. Response surfaces for the parameters were generated using a second degree polynomial. Results show that increase in screw speed and a reduction in die diameter enhanced lysine retention. Increase in feed composition, and screw speed increased browning index, but decreases in die diameter and feed composition increased browning index. Optimum extrusion conditions resulting in maximum available lysine and minimum browning index were estimated. (C) 2003 Elsevier Ltd. All rights reserved.