Study on melting and crystallization of short-linear chains from debranched waxy starches by in situ synchrotron wide-angle X-ray diffraction

In situ melting and crystallization of short-linear α-1,4-glucan (short-chain amylose, or SCA) from debranched waxy starches were investigated by synchrotron wide-angle X-ray diffraction. Amorphous SCA was prepared by dissolving completely debranched waxy starches in alkaline solution and neutralized by hydrochloric acid. When hydrated with 50% water at 25 °C, all amorphous SCA crystallized immediately and gave a B-type structure. The SCA from debranched waxy potato starch had a longer average chain length and a higher melting temperature but relatively lower crystallinity upon hydration; it was not completely melted at 100 °C and retained its original B-type structure during rapid cooling. In contrast, the SCA from debranched waxy wheat and waxy maize starches had a large portion of low molecular weight fractions, a higher crystallinity upon hydration, and a lower melting temperature. These differences suggest that amylopectin short chains crystallized more readily but their crystals were weaker than those of long chains. After the B-type crystals of hydrated SCA from waxy wheat and waxy maize starches melted, they reformed into the A-type polymorph upon rapid cooling. The thermal properties showed that the A-type polymorph of debranched waxy wheat and waxy maize starches had a higher melting temperature than their B-type structure.

► Synchrotron wide-angle X-ray diffraction was used to study melting and crystallization of debranched starches in situ. ► Debranched waxy wheat, waxy maize, and waxy potato starches were investigated. ► B-type crystalline structure was found when the debranched starches were hydrated with water at 25 °C. ► A-type polymorph was formed by cooling of amorphous debranched waxy wheat and waxy maize starches from 100 °C.