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Extrusion and Characterization of Starch Films¹

¹A contribution of the University of Nebraska Agricultural Research Division, Lincoln 68583. This study was conducted at the Industrial Agricultural Products Center. Mention of a trade name, proprietary products, or company name is for presentation clarity and does not imply endorsement by the authors or the University of Nebraska. ²Department of Biological Systems Engineering, University of Nebraska-Lincoln. ³Department of Statistics, University of Nebraska-Lincoln. ⁴Department of Food Science and Technology, University of Nebraska-Lincoln. ⁵Corresponding author. Phone: +1-402-472-1634. Fax: +1-402-472-6338. E-mail: mhanna1@unl.edu

Starch plasticized with water, glycerol, and stearic acid was extruded and sheeted into films 0.4–0.6 mm thick. The ingredients were extruded in a conical twin-screw extruder at a temperature profile of 50–120–120–120°C and a screw speed of 45 rpm. The effects of glycerol, water, and stearic acid on selected physical and functional properties of the films were studied. The tensile strength, tensile strain at break, and Young's modulus were 0.23–2.91 MPa, 45.79–90.83%, and 2.89–37.94 MPa, respectively. Differential scanning calorimetry thermograms exhibited two glass transitions and multiple melting endotherms, including that of amylose-lipid complexes formed during extrusion. The enthalpy of gelatinization of starch in the extruded films was 0.7–4.1 J/g and was dependent largely on the plasticizer content. Fourier-transform infrared spectra revealed significant interactions between the starch and plasticizer but the peaks shifted to higher wave numbers with increasing glycerol content. During extrusion in the presence of glycerol, the A-type crystalline structure of starch was transformed to B-type. It also was observed that the V_h crystallinity increased with increase in glycerol content due to tight packing of starch chains. The water vapor permeabilities of the starch films were 12.3–19.9 g·mm/hr·m²·kPa.