Effect of Extrusion Cooking on Functional Properties and in vitro Starch Digestibility of Barley-Based Extrudates from Fruit and Vegetable By-Products

Barley flour and barley flour-pomace (tomato, grape) blends were extruded through a co-rotating twin-screw extruder. The aim of the present study was to investigate the effects of die temperature, screw speed, and pomace level on water absorption index (WAI), water solubility index (WSI), degree of starch gelatinization, and in vitro starch digestibility using a response surface methodology. The selected extrudate samples were examined further using differential scanning calorimetry (DSC) and polarized light microscopy, respectively. The WAI of barley-pomace extrudates was affected by increasing pomace level. Temperature had significant effect on all types of extrudate but screw speed had significant linear effect only on barley and barley-grape pomace extrudates on degree of starch gelatinization. Although no gelatinization peak was detected, an endotherm was observed on all selected extrudates. In general, extrusion cooking significantly increased in vitro starch digestibility of extrudates. However, increasing level of both tomato and grape pomace led to reduction in starch digestibility.