Evaluation of extrudates from sorghum-grape pomace blends by extrusion processing

Abstract

Extrusion cooking is recognized as a smart technology for food processors. It is a high temperature, short-time process and requires few ingredients to create a puffed snack. Grape pomace (GP) is a fiber and polyphenol-rich by-product of wine industry. It has the potential to serve as an important source of insoluble fiber and phenolic antioxidant for functional food development. This study investigated the effects of extrusion parameters and examined the addition of GP into a sorghum and corn-based extruded snack formulation. Different ratio of grape pomace and sorghum were blended with corn starch and extruded using a twin-screw extruder. Response surface methodology was used to investigate the effect of grape pomace level and extrusion processing conditions on the product properties. Five different blends at a level of 0-15% w/w GP were extrusion cooked with varied barrel and die temperature (100-150oC), screw speed (100-250 rpm) and feed moisture content (15-25 % wet basis). Physical and functional properties of the extrudates were analyzed. Multiple regression equations were obtained to describe the effects of each variable on product responses. The expansion ratio, water solubility index, hardness, lightness, and yellowness significantly (P<0.05) decreased with increase in the GP level in the blends. Whereas increasing the GP content in the blends significantly (P<0.05) increased the bulk density, water absorption index, total color change, total phenolic content and antioxidant activity of the extrudates. Screw speed had significant effect on all the properties except crispness of the extrudates. Graphical optimization studies resulted in 140oC extrusion temperature, 5% pomace level, 200 rpm screw speed and 22% moisture content as the optimum variables to produce extrudates with maximum expansion ratio, crispness, phenolic content and lightness; and minimum hardness and bulk density. The results suggest that grape pomace can be combined with sorghum and corn starch to develop an acceptable and nutritional extruded snack food. It can also be concluded that an active interaction between grape pomace and starch occurs during expansion process. As the ingredients in our study are naturally gluten free, the extruded product would appeal to people who suffer from gluten intolerances, allergies and celiac disease.