The effect of freezing-assisted cross-linking on structural and rheological properties of potato starch

Abstract

In order to achieve preparation of cross-linked (CL) potato starch with the maximum degree of substitution, freezing pre-treatment (FS) in different modes as three days freezing (3D), two freezing-thawing cycles (3D + 3D) and 6 days freezing (6D) were conducted. Thereafter, native, frozen and cross-linked starches were characterized for morphological, structural and pasting properties as well as alkaline and intrinsic viscosity. Regarding obtained result, freezing pre-treatment as 3D + 3D was found to be an efficient method to achieve high level of cross-linking than native and other modes of freezing pre-treatments when exposed to POCl3 reagent. The crystallinity (%) and ratio of 1047/1022 cm−1 increased from 38.6 % and 1.112 (native potato starch; NPS) to 41.6 % and 1.269 (cross-linked native potato starch; CL) and 41.3 and 1.292 (cross-linked freeze- thawed starch 3D + 3D + CL) after being treated with POCl3. Data obtained by intrinsic viscosity was in line with the power-law model. Cross-linked starch with POCl3 exhibited the lowest k value and the highest n value, implying lower shear-thinning behavior of cross-linked starch after freezing pre-treatment than CL native starch. To sum up, low peak viscosity (determined by RVA) and intrinsic viscosity (by U-tube viscometer) could also explain the high level of cross-linking and low swelling power of 3D + 3D + CL.