Macromolecular Changes in Extruded Starch-Films Plasticized with Glycerol, Water and Stearic Acid

Native corn starch, plasticized with water, glycerol and stearic acid, was extruded in a conical twin-screw extruder and sheeted into 0.4-0.6 mm thick films. The effects of extrusion and plasticizers on gelatinization, as well as the molecular and structural changes, in thermoplastic starch were analyzed. The onset and peak gelatinization temperatures of extruded starch varied from 42-46°C and 52.9-56.9°C, respectively, depending on the glycerol content. The enthalpy of gelatinization of extruded thermoplastic starch in excess water varied from 3.6-7.6 J/g, which also increased with plasticizer content. Amylose-lipid complexes were formed during extrusion, and their enthalpies depended on the initial stearic acid and moisture contents. High-performance size-exclusion chromatography (HPSEC) data revealed that the starch underwent fragmentation during extrusion even under highly plasticized conditions, but the degradation was not severe as compared to previous findings. The relative percentages of amylopectin and amylose in native starch were 76.9 and 23.1%, respectively, which were changed to 71.3-76.6% and 23.4-28.7% in the extrudates. The average molecular weights of amylopectin and amylose in the extrudates ranged from 1.55x10⁷-2.07x10⁷ and 4.35x10⁵-7.39x10⁵, respectively. On the other hand, the molecular weights of amylopectin and amylose in native corn starch were observed as 2.27x10⁷ and 4.68x10⁵, respectively. Cross-polarization magical angle spinning (CP/MAS) and high-power decoupling (HP-DEC) nuclear magnetic resonance (NMR) spectra of thermoplastic starch revealed the characteristics of amylomaize starch, confirming HPSEC results that the amylopectin macromolecules underwent fragmentation into amylose-like fractions. In the extrudates, glycerol was found to be less mobile and entrained within the starch network.