The study was designed to investigate the effects of die temperature and CO2 injection on physicochemical properties of extruded rice with vegetable powder. Moisture content and screw speed were fixed at 25% and 150 rpm, respectively. The first experiment of extrusion condition was adjusted to 80, 110, and 140℃ die temperature with CO2 injection of 0, and 300 mL/min.
The second experiment was compared cold and conventional extrusion. Cold extrusion was adjusted to 80℃ die temperature with CO2 injection of 300 mL/min, and conventional extrusion was adjusted to 140℃ die temperature. Pumpkin, tomato, strawberry, and green tea powder blended individually at 10% with rice flour.
Specific mechanical energy (SME) input decreased as die temperature from 80 to 140℃, and conventional extrusion were lower than cold extrusion. The expansion index increased as die temperature from 80 to 140℃, and CO2 injection from 0 to 300 mL/min. The expansion index of conventional extrusion was higher than cold extrusion. Bulk density of extrudates was decreased in die temperature from 80 to 140℃. Bulk density was decreased with the increase in expansion index. All extrudates showed increased water soluble index (WSI) and water absorption index (WAI) through the extrusion process. WSI increased as die temperature increased. WSI of conventional extrusion was higher than cold extrusion. All extrudates decreased in lightness. Lightness of conventional extrusion was lower than cold extrusion.
DPPH radical-scavenging activity increased as die temperature increased from 80 to 140℃. DPPH radical-scavenging activity of extruded pumpkin and tomato mix by conventional extrusion was higher than cold extrusion. Total phenolic compound of extruded rice with tomato mix increased as increased die temperature. Total phenolic compound in extruded pumpkin, tomato, and strawberry mix by cold extrusion was higher than conventional extrusion. Total carotenoid content decreased at extruded pumpkin while increased at extruded tomato. Tomato mix extrudates of second experiment by cold extrusion were higher lycopene content than conventional extrusion. Anthocyanin content of conventionally extruded strawberry mix was higher than cold extrusion. Total chlorophyll contents decreased through the extrusion process.
In conclusion, die temperature and CO2 injection affect physicochemical properties. CO2 injection affects expansion properties of extrudates, and die temperature affects antioxidant activities. Antioxidant activities by cold and conventional extrusion were not significantly different. Therefore cold extrusion with CO2 injection could be used for value-added puffed snack or cereals with unique texture and expansion properties.