Weakening the integrity of waxy starch granules by selective degradation of amorphous matrices using gaseous hydrogen chloride.

Waxy corn starch with 6-12 % moisture was degraded by gaseous hydrogen chloride (HCl(g)) then characterized chromatographically and microscopically to investigate the relationship between moisture-resulting changes in unit chain properties of starch and its susceptibility to swelling. Starches with the highest degradation extents at different moisture content had recoveries of 76-83 % (w/w) and the weight-average degrees of polymerization of 66-93 anhydroglucose unit. Additionally, the HCl(g)-degraded starches were still of birefringent granules but with internal cavity. When increasing the swelling temperature from 25 to 40 °C, some granules showed enlarged internal cavity, and some were fragmented. Change in granule morphology, accompanied by leaching of constituting polymers, was particularly evident for starch with preferential degradation of B1 chains over B2+ chains as the moisture content decreased from 9 % to 6 %. These findings not only manifest the impact of changes in B2+ and B1 chains on the swelling characteristic of HCl(g)-degraded granules, but also demonstrate an approach for weakening the integrity of granules by selective degradation of amorphous matrices using HCl(g).

Degradation of corn starch with different moisture content by gaseous hydrogen chloride.

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.