Summary High‐temperature high‐pressure extrusion of sorghum–maize composite flour, of potential for healthy food manufacture, was investigated by factorial experimental design to determine the effect of level of sorghum in dry mix (15–60%); final barrel zone temperature (120–150 °C); total moisture in barrel (21.4–25.8%); total input rate (2.3–6.8 kg h−1); and screw speed (250–450 rpm) on extrudate slowly digestible starch (SDS), phenolic content, antioxidant capacity, protein digestibility, density and expansion ratio. Extrudate SDS increased with increasing sorghum level and decreased as the barrel temperature increased. Total phenolic content and antioxidant capacity were positively associated with sorghum level. Protein digestibility was associated negatively with sorghum level and positively with barrel temperature. Extrudate density was associated positively with total moisture and negatively with barrel temperature and input rate. Sorghum in dry mix, final barrel zone temperature and total moisture in barrel were the three most significant independent variables influencing extrudate dependant variables.

Sorghum is a grain with potential for developing foods with slowly digested starch, of benefit to healthy glucose metabolism. Response surface methodology (RSM) was used to determine the effects of total moisture in barrel (22–25%), final barrel zone temperature (115–140°C), and sorghum flour in dry mix (30–80%) during extrusion of sorghum‐maize flour, on extrudate slowly digested starch (SDS) and expansion ratio. SDS level had a negative linear association (p = 0.007) with final barrel zone temperature and a positive linear association (p < 0.001) with sorghum level. Expansion ratio had a quadratic association with final barrel zone temperature (p = 0.002) and moisture in barrel (p < 0.001). A linear model described the combined effects of extrusion parameters on SDS levels (R2 = 68.23%), while a quadratic model described their effects on expansion ratio (R2 = 75.24%). SDS level was positively associated with both polyphenolic level (r = 0.622, p = 0.001) and antioxidant capacity (r = 0.668, p = 0.001). The validated RSM model indicated that 22% total moisture in barrel, 115 °C final barrel temperature zone, and 74.67% sorghum in dry mix were optimum settings to deliver maximum levels of SDS with adequate expansion ratio. This is the first report of the optimisation of SDS level in a sorghum based extrudate. These findings demonstrate the potential of sorghum for the development of extruded snack foods with elevated levels of both SDS and antioxidant capacity.