Structure-Property Relationships of Extruded Starch, 2 Extrusion Products from Native Starch

Abstract

The supermolecular structure and morphology of extruded flat films from several native starch materials of A- and B-crystal type were investigated by wide-angle X-ray scattering and scanning electron microscopy. The degree of crystallinity and crystallite dimensions of both the different starting materials and the extruded films were determined and a scheme of the lattice transformations resulting from extrusion was established. The conditions of structure formation of the extruded starch films were varied in relation to plasticizer composition and extruder zone temperatures. The mechanical properties and biodegradability of the films were also measured. The extruded starches crystallized in the VH polymorph with crystallinities between 33 and 41% and crystallite sizes of up to 35 nm. An increase in crystallite size was found for all starches (sometimes a doubling) with increasing extruder middle zone temperatures from 120 up to 210°C. For extruded potato and maize starches a steep rise in strength and modulus and a drop in elongation was observed above 190°C. Purified amylopectin from maize showed after extrusion the crystalline A-type and small amounts of B polymorph with small crystallites (up to 3 nm) and the best mechanical performance with strengths and moduli of about 20 MPa and 1 500 MPa, respectively, for the present extrusion conditions. Native starch films that include 20 to 30% plasticizer biodegrade rapidly in 25 d consuming 90% of the oxygen needed for complete degradation, as analyzed by the Sapromat test.