Reversible thermal transition of soluble branched chains from slightly acid-treated potato starch.

The reversible thermal transition of soluble branched starch chains prepared from slightly acid-treated potato starch granules (ATS) was investigated. Potato starch was immersed in 15% sulfuric acid to obtain ATS with a 1% hydrolysis rate. About half of the molecules of ATS, which spontaneously formed large aggregates with a mass of a few million daltons in aqueous solution, was fractionated and soluble branched starch chains with a relative molecular weight (Mr) of 8.91 x 10(4) were obtained. Structural analysis indicated that the soluble branched starch chains consisted of three unit chains with Mr 7,900 and 21 unit chains with Mr 2,700. DSC and FT-IR measurements showed that the soluble branched starch chains underwent a reversible thermal transition, which is considered to be a helix-coil transition, during heating and cooling, but a debranched sample and beta-limit dextrins showed substantially different thermal behavior, indicating the contribution of the ordered structure of the branched chains.