Iodine-binding in Granular Starch: Different Effects of Moisture Content for Corn and Potato Starch
2008
Saibene, Debora | Zobel, Henry F. | Thompson, Donald B. | Seetharaman, Koushik
The ability of iodine to complex with glucan polymers has been mainly used to elucidate the structure of dispersed starch molecules. In a previous publication [D. Saibene, K. Seetharaman, Carbohydr. Polym., 2006, 64, 539-547] we reported the ability of granular corn starches to bind iodine at moisture contents as low as 8%. We presumed that iodine binding in a native granule requires a certain minimal level of mobility of the linear molecules, and that the water content would plasticize the starch, thus increasing the ability to bind iodine by the formation of single helices. The objective of the present study was to investigate iodine complex formation with starch molecules in granular common corn starch (CCS) and potato starch (PS) as a function of water content. CCS and PS granules have approximately similar amylose content (21 and 17% amylose, respectively), but different crystalline structures (A- and B-type, respectively). Variable water contents were achieved using salt solutions at water activities (aw) of 0.33, 0.75 or 0.97, and also using Drierite® (aw less than 0.15). The granular samples were then exposed to iodine vapor before determination of the K/S spectra (the ratio of the absorption and scattering coefficients), and X-ray diffraction patterns. Based on the K/S spectra, at moisture contents in the range of 12-20%, CCS binds iodine more effectively than PS. We suggest that the reason may be related to the greater amount of water associated with the crystalline regions of PS. Iodination more strongly diminished the crystallinity of PS granules than it did in CCS granules. This behavior is consistent with a greater involvement of amylose in the crystalline structures of PS than in CCS.
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