Effect of extrusion conditions on the structural properties of corn extrudates enriched with dehydrated vegetables

Abstract

The present study examines the addition of dehydrated broccoli or olive paste to corn flour for the production of extrudates with increased value and superior quality. Extrudates were prepared using a twin-screw extruder, operated at different conditions, including screw speed (150rpm, 200rpm, 250rpm) and extrusion temperature (140°C, 160°C, 180°C). The moisture content of the raw mixture was regulated in three levels (14%, 16.5%, 19%), whereas the concentration of the added ingredient was adjusted to 4%, 7% and 10% for broccoli and to 4%, 6% and 8% for olive paste. Structural properties and rehydration were investigated with regard to process conditions and material characteristics. Mathematical models were also used to correlate structural properties with process parameters. Regression analysis showed that the increment of moisture content and broccoli or olive paste concentration, as well as the decrement of temperature and screw speed resulted in denser extrudates with lower porosity. Products with 14% moisture content and 4% material concentration that were extruded at the highest screw speeds (250rpm) presented the highest degree of expansion. These conclusions were strengthened with the results obtained from mercury porosimetry and scanning electron microscopy. In addition, moisture diffusion coefficient was calculated from the rehydration kinetics and was correlated with process parameters. Diffusion coefficient decreased with moisture content and material concentration, while it increased with temperature and screw speed, indicating positive relationship with extrudates' porosity.