Twin-screw extrusion of barley–grape pomace blends: Extrudate characteristics and determination of optimum processing conditions

The barley flour–grape pomace blends were extruded in a 30 mm APV co-rotating twin-screw extruder. Response surface methodology using a central composite design was used to evaluate the effects of independent variables, namely die temperature (140–160 °C), screw speed (150–200 rpm) and pomace level (2–10%, db) on product responses (expansion, bulk density, texture and color). Sensory analysis was carried out for selected extrudates for appearance (color, porosity), taste (bran flavor, bitterness and sweetness), off-odor, texture (hardness, crispness and brittleness) and overall acceptability. Multiple regression equations were obtained to describe the effects of each variable on product responses. The product responses were most affected by changes in temperature, pomace level and to a lesser extent by screw speed. Blends of 2% grape pomace extruded at 160 °C, 200 rpm and 10% grape pomace extruded at 160 °C, 150 rpm had higher preference levels for parameters of appearance, taste, texture and overall acceptability. However, graphical optimization studies resulted in 155–160 °C, 4.47–6.57% pomace level and 150–187 rpm screw speed as optimum variables to produce acceptable extrudates. The results suggest that grape pomace can be extruded with barley flour into an acceptable snack food.