Effect of extrusion temperature and screw speed on properties of oat and rice flour extrudates.

BACKGROUND: Whole oat and rice flours were mixed to develop instant flours by a high pressure and low mechanical shear extrusion process. The screw profile was designed aiming to obtain an infant food with gelatinized starch and high hydration ability. Response surface methodology was selected to study the impact of operating parameters such as temperature and screw speed (73-186 °C; 109-391 rpm) on physicochemical and pasting properties of the final extruded product. The main challenge of this study was to process high oats content, since they are characterized by high lipid and fiber content, which impact on material processing.

RESULTS: The optimal response was achieved at 170 °C and 350 rpm. The optimal expansion ratio, bulk density, water absorption index, and water solubility index were 2.24, 289.65 kg m-3 , 6.42 g g-1 , and 4.75 g g-1 respectively. Overall, both temperature and screw speed affected the responses studied, except for water absorption index (only screw speed affected this response). Although lipids from oats reduce the expansion ratio of extrudates compared with samples containing higher starch proportions, their lipids protect the starch granules from mechanical degradation when higher screw speed values are used. As a result, both ungelatinized and gelatinized starches may be found in extrudates, which was confirmed by pasting property analyses.

CONCLUSION: High oat content may be efficiently processed by optimizing the extruder conditions (temperature, screw speed, and profile), improving the nutritional properties of the final product. © 2017 Society of Chemical Industry.