Molecular size and mass distributions of native starches using complementary separation methods: Asymmetrical Flow Field Flow Fractionation (A4F) and Hydrodynamic and Size Exclusion Chromatography (HDC-SEC)

Starch consists of a mixture of two alpha-glucans built mainly upon alpha-(1,4) linkages: amylose, an essentially linear polymer, and amylopectin, a branched polymer containing 5-6% of alpha-(1,6) linkages. The aim of the present work was to analyze the structural properties of native starches displaying different amylose-to-amylopectin ratios and arising from different botanical sources, using asymmetrical flow field flow fractionation (A4F) and a combination of hydrodynamic and size-exclusion chromatography (HDC-SEC) coupled with multiangle laser light scattering, online quasi-elastic light scattering, and differential refractive index techniques. The procedure, based upon dimethyl sulfoxide pretreatment and then solubilization in water, generates a representative injected sample without altering the initial degree of polymerization. The amylopectin weight-average molar masses and radii of gyration were around 1.0 x 10(8)-4.8 x 10(8) g mol(-1) and 110-267 nm, respectively. For each starch sample, the hydrodynamic radius (R(H)) distributions and the molar mass distributions obtained from the two fractionation systems coupled with light scattering techniques were analyzed. The size determination scales were extended by means of R(H) calibration curves. HDC-SEC and A4F data could be matched. However, A4F enabled a better separation of amylopectins and therefore an enhanced structural characterization of the starches. The two advantages of this experimental approach are (1) it can directly obtain distributions as a function of both molar mass and size, while taking account of sample heterogeneity, and (2) it is possible to compare the results obtained using the different techniques through the direct application of R(H) distributions.