Color of single-screw extruded blends of soy-sweet potato flour --a response surface analysis.

Blends of defatted soy flour and sweet potato flour were extruded in a single-screw extruder run at varying pre-set rotational speeds. Die diameter of the extruder was also varied. A central composite, rotatable nearly orthogonal response surface design was used in studying the interactive effects of extrusion variables on color of the extrudates. Color of raw and processed samples was determined on a DRLANGE Tricolor LFM3 instrument. Measurements were made in duplicates to obtain the CIELAB L*a*b* values. Results showed that whiteness (L*) decreased with increase in sweet potato in the blends during extrusion. Reduction in whiteness (darkening) evidenced in decreased L* values of samples was only affected by increase in sweet potatoes in the blends. Redness (a*) significantly increased as sweet potatoes content increased, which confirmed the decrease in whiteness. Yellowness (b*) also increased considerably, but as a result of the die diameter.