Retrogradation and gel textural attributes of corn starch amylose and amylopectin fractions

An understanding of relationships between starch molecular structure and function could increase the opportunity to develop additional uses for starch. Using a combined aqueous (aq.) leaching and alcohol precipitation technique, six amylose fractions were obtained from regular and two high amylose (hAM) corn starches (50 and 70%). Five amylopectin fractions were obtained from regular and waxy corn starches. After freeze-drying the fractions, high performance size exclusion chromatography (HPSEC) was used to estimate weight-average (M-w) and number-average (M-n) molecular weights, and to determine polydispersity (M-w/M-n) and purity. Increase in fraction reducing power after pullulanase treatment was determined and expressed as a relative index of branching. Amylose-fractions showed varying but similar (P>0.05) M-w (1.40-5.68E + 05). similar polydispersities (2.1-2.9). and branching values (1.93E-04-4.14E-05). Amylose branching increased with decreasing M-w. Amylopectin had varying M-w (2.88-5.43E + 07) and similar polydispersities (1.3-1.5), but different branching values (7.30E-04-1.54E-03), that did not follow any M-w order. Fraction melting temperatures, enthalpies, and gel retrogradation behaviours were determined using differential scanning calorimetry (DSC). No enthalpies were observed for amylose fractions. Amylopectin fractions with high branching values that had intermediate to low M-w (3.75-2.88E + 07), showed marked retrogradation, Enthalpy was correlated neither to branching nor M-w. Gel textural profile analysis (fracturability, adhesive force, cohesiveness, and springiness) showed that for amylose, intermediate M-w, fractions promoted less fracturability, while adhesive force, and stringiness increased with a decrease in M-w. For amylopectin, the intermediate M-w and highly branched fractions reduced fracturability: and promoted adhesive force, and stringiness. Amylopectin fractions of intermediate M-w and high extent of branching underwent increased retrogradation. Amylose gel textural attributes were governed by molecular weight, while molecular weight and branching governed amylopectin gel textural properties. (C) 1998 Academic Press Limited.