Structure-property relationships of extruded starch, 2 - Extrusion products from native starch

The supermolecular structure and morphology of extruded flat films from sev eral native starch materials of A an B crystal type were investigated by wi de-angle X-ray scattering and scanning electron microcopy. The degree of cr ystallinity and crystallite dimensions of both the different starting mater ials and the extruded films were determined and a scheme of the lattice tra nsformations resulting from extrusion was established. The conditions of st ructure formation of the extruded starch films were varied in relation to p lasticizer composition and extruder zone temperatures. The mechanical prope rties and biodegradability of the films were also measured. The extruded st arches crystallized in the V-H polymorph with crystallinities between 33 an d 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 degreesC. For extruded potato and maize starches a steep rise in strength and modulus and a drop in elong ation was observed about 190 degreesC. Purified amylopectin from maize show ed after extrusion the crystalline A-type and small amounts of B polymorph with small crystallites (up to 3 nm) and the best mechanical performance wi th strengths and moduli of about 20 MPa and 1500 MPa, respectively, for the present extrusion conditions. Native starch films that include 20 to 30% p lasticizer biodegrade rapidly in 25 d consuming 90% of the oxygen needed fo r complete degradation, as analyzed by the Sapromat test.