Corn starch, subjected to solid-state degradation using gaseous hydrogen chloride, was characterized to investigate the parameter effects on starch properties and further explore the thermal behavior of inter-crystalline amorphous lamellae of starch. The obtained starch granules exhibited A-type polymorph with 75-98 % recovery (w/w). Degreasing moisture content (6-12 %) with increasing duration (1-8 h) and temperature (25-45 °C) progressively decreased the weight-average degree of polymerization to 55-75 anhydroglucose unit and increased the gelatinization temperature range. However, starch degraded at 6 % moisture and 25 °C exhibited much higher polydispersity index than those degraded at 9-12 % moisture, accompanied by the decreased start but comparable onset gelatinization temperatures. This thermodynamic difference was inherent but diminished gradually with increasing degradation temperature. These findings suggest that the degradation at 9-12 % moisture starts from the bulk amorphous region of starch then sequentially reaches its inter-crystalline amorphous lamellae, while the simultaneous degradation of both regions is for starch with 6 % moisture.
Keywords: Degradation; Moisture content; Molecular structure; Starch; Thermal properties.
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